# New Portable Amp - "Pocket Class A" by xrk971 - now available as complete PCB



## stellarelephant (Jul 30, 2017)

Edit 7/30/17: This amp is now available from the XRKAudio Etsy store. PCBs for DIY versions are available too. 
*
"In systems where the utmost in natural reproduction is the goal, simple single-ended Class A circuits are the topologies of choice."  *--Nelson Pass

 Single-Ended (SE) Class A amplification that fits in your pocket!  Yes, it's a real thing now.  Possibly a first in portable listening...?  This little DIY-friendly amp is now being offered as a fully populated and tested PCB, professionally built by the amp's designer himself, known as xrk971 on the diyAudio forums. You can skip all the soldering and just install the board into an Altoids tin, add batteries, and voila!  He just started selling them on eBay...and while I will be jealous if you get one delivered before I finish building my own, I think everybody with ears should know about this amp!

X's eBay listing for the Pocket Class A

 A month ago I started a thread in the DIY forum about this amp project.  I am building two of these amps myself with bare PCBs I got from the original group buy at the diyAudio forums. (There is a second group buy going on now for the DIY-inclined.)  I am really excited about the potential of this amp...

 First pic shows what you get...second two show the amp at home in a tin.

 This ain't your mama's CMOY...no op-amp...two transistors per channel.  "Curiously Good Sound" lol.  
	

	
	
		
		

		
			





 The amp uses some killer high-grade components...*Panasonic OSCON caps, WIMA audio grade film caps, Switchcraft jacks, Alps potentiometer, Zetex power MOSFETs, NXP JFETs, and 1% SMT resistors.*

 Here's my original Head-Fi thread on the DIY version of this amp, where I'll be posting my build and listening impressions when parts arrive: http://www.head-fi.org/t/833106/new-diy-altoids-tin-amp-the-class-a-wintergreen-handwarmer-by-xrk971

 and more info including measurements of the amp here:
http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb.html


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## stellarelephant

Details:
  
*This is the world's first and only pocket sized portable Single-Ended Class A topology headphone amplifier that fits in a mint tin.  This amp uses the highly regarded Single-Ended (SE) Class A topology with zero global feedback.  The audio signal passes only through two field-effect transistors (FETs) for a pure and direct sound that is lively, engaging, with shimmering highs, and deep impactful bass, and a rare musicality usually only heard in high-end tube amplifiers.   Compared to most headphone amps that use opamps containing 10  to 20 transistors, two discrete field effect transistors in a pure Class A mode of operation results in the ultimate sound quality that is transparent, engaging, and never sounds sterile.  *
  
 The circuit was developed using extensive computer simulations and real-world testing and listening.  The circuit was developed on the DIYaudio.com forum by member xrk971 and offered there as a bare PCB group buy (GB).  It is now offered here as a fully-built and tested amp board module that is ready to be installed in a mint tin as a quick DIY project only requiring cutting holes in the mint tin and fitting the PCB and battery wiring harness.   The circuit as designed, provides a superb level of transparency and detail without fatigue, and a presentation that continues to remain engaging song after song.  Designed for the discriminating high-end headphone listener needing the ultimate in portable headphone amplifiers.  This amp is designed to drive headphones with impedances ranging from 30 to 600 ohms with high resolution and dynamic authority.  Through use of a simple and elegant circuit the following specifications were achieved (actual measurements): 

*Ultra-low -126dB noise floor
 Tube-like harmonic distortion profile with only 0.028% THD (1kHz 700mV into 270 ohms) that is predominantly second harmonic (90%) 
 Dynamic range of 94.2dB 
 Stereo Cross-Talk of better than -74dB
 Flat frequency response from 15Hz to 40khz (+/-0.5dB)*

 Despite its small size, when compared to much larger full-size amps, the performance is nothing short of stunning when listening to high resolution sound sources using high-end headphones.  The amp operates in pure Class A mode with a bias current of approximately 50mA, thus two readily available 9v batteries (or rechargeable Li-ion 9v batteries), will permit usage times ranging from 4 to 5 hrs depending on battery capacity. The amp is a joy to listen to and has no audible hiss, noise, or hum even when using 112dB/mW sensitive IEM's.  It is not possible to tell that the amp is even on when the music is not playing.  It can also be used as an excellent line-preamp for your stereo speakers' power amplifier.


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## stellarelephant

*"Tube-like harmonic distortion profile with only 0.028% THD (1kHz 700mV into 270 ohms) that is predominantly second harmonic (90%)"*
  
 This is what really sets this amp apart in my opinion.  It should have that magical sound we call "Euphonic."


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## xrk971

I wanted to thank StellarElephant for posting my amp from DIYaudio over here. I just joined HeadFi now that I am a big headphone user and amp builder. I will be glad to answer any questions about this amp. The group buy number 1 amps (52 PCBs ) were sent out en made last Thursday and amazingly, they have arrived in Europe and we have first sound from member Morde over in DIYA. 

http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-24.html#post4997809

I am listening to it now as I type - testing it out on classical now. Mozart's Marriage of Figaro, Sull'aria is wonderful. Just beautiful magical presentation. 

I have tested it extensively with jazz, pop, deep house, rock, alternative, and EDM. They all sound excellent but what really raised my eyebrows was Infected Mushroom's Pink Nightmares. 

Now rounding it off is opera. Nice nice sounding amp. There indeed is magic in single ended Class A.


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## stellarelephant

Hi X, it's about time you joined Head-Fi! I'm sure your expertise will benefit the community. 

Say, do you fancy taking a stab at giving us a your interpretation of the graph I posted above? This is partly for my own sake...I have a basic understanding of what FFT measurements are, and how harmonic intervals relate in musical instruments, but not really how to read a graph like this. 

How, for instance, do you draw your conclusion of 90% H2 from this reading?

I think your input could help folks like me get a handle on one major "claim to fame" of the SE Class A sound--it's appealing harmonic signature.


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## xrk971

Sure, it's easy.  The graph shows the amplitude of the peaks of the harmonic components.  This is using the de-facto standard of 1kHz excitation signal so we look for second harmonic (H2) at 2kHz and third harmonic (H3) at 3kHz and so forth.  If you look at the amplitude difference between H2 and H3, H3 is about -20dB lower. 20dB is same as factor of 10x. Natural sounding noises or sounds tend to have predominant H2 and are pleasing.  Odd harmonics like H3, H5, H7 tend to be unpleasant. Also, higher order harmonics, odd or even are noted by our subconscious as unatural. The theory is that our brains have evolved to be acutely tuned to odd order harmonics as un-natural sounds equivalent to a high alert for signs of danger.That is the psycho-acoustics behind that anyhow.


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## stellarelephant

xrk971 said:


> 20dB is same as factor of 10x.


 
 A-ha!  Now the 90% H2 statement makes sense.  I am surprised that the graphs are not customarily scaled logarithmically to reflect this, like how FR graphs are.
  


> Originally Posted by *xrk971* /img/forum/go_quote.gif
> 
> Natural sounding noises or sounds tend to have predominant H2 and are pleasing.  Odd harmonics like H3, H5, H7 tend to be unpleasant.


 
 Very cool.  I'm not sure if this is the source of your reasoning or not, but...from a music theory geek perspective:
  
 Second harmonic = one octave above original note
 Third harmonic = one fifth above second harmonic 
  
 So...theoretically, since H2 is one octave above the fundamental tone, it seems like pure H2 in an amplifier should always add consonance (octave harmony) to the music.  Even during a complex song with multi-part harmony, still no problem.  For fun, let's say we're listening to Purple Haze, with its weirdo dominant seventh sharp ninth chord...AKA the "Jimi Hendrix chord"---even with this music, the amp would never add dissonance because the H2 is always reproducing the same exact chord present in the music, just transposed an octave higher.  
  
 When you add in H3, (an interval of one fifth above the H2 tone), well then musically, the shiz hits the fan in terms of harmony.  There is no way that adding a fifth to every single note in Purple Haze is going to sound good.
  
 Of course, I'm ignoring the phenomenon of intermodulation distortion, which adds dissonance to all music containing two or more simultaneous frequencies (by producing tones at the sum and difference of those frequencies).  This happens regardless of harmonic signature.  Add to that the fact that every musical instrument known to man produces harmonics of every order (it would be pretty boring if we used pure sine wave generators to make music, eh?).  And the fact that the Western musical scale is "tempered" so that the intervals of the scale are not quite exactly harmonically related...  All in all, it's a wonder that music sounds good at all, and is one of the most sensorially wonderful experiences that life has to offer!
  
 Anyhow, purely at the level of harmonics, having a H2-dominant sound signature should indeed sound "euphonic."


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## stellarelephant

Wow. I love this amp. Listening impressions below. 

I built my own using a PCB and matched transistors I got from xrk871. I also customized the output caps to taste, to pair with my Sennheiser HD598. More on my specific build here.

I am getting phenomenal sound out of this amp, feeding it lossless music from the auxiliary output of my xDuoo x3 player. The sound of my budget-fi DAP has been lifted into a new realm, and my 598 cans have never sounded better. 

Tested FLAC music includes Daft Punk's RAM, Steve Martin and Edie Brickell's So Familiar, Gregory Porter's Liquid Spirit, Pentatonix's self-titled album, and Stevie Wonder's Talking Book, all ranging in quality from Redbook to HD 24/192. 

*Bass has excellent weight and texture. This is a stunning result for the 598, which tends to sound uncontrolled and wooly down low with lesser amplification. 

Mids are rich and electrifying. Vocals are wonderfully engaging--right in my face--in a beautiful, hyper-real way that is totally addictive. The x3 DAP retrieves great detail, but tends to sound thin and unconvincing alone. This amp gives music the full-bodied warmth and impact I always wanted. 

Treble is not quite as transparent nor as extended as the Class D amp circuits I am used to, but treble detail is still magically present, with a gorgeous euphonic sparkle. 

Overall the sound is full-bodied, detailed, attention-grabbing, and just downright sweet. ...And mine isn't even entirely soldered together yet, let alone burned in!*

After a listening session with this setup, reverting to the cheap op-amp of my DAP sounds lifeless, hollow, tinny, bass-light, and absolutely boring. It has a blacker background and more high treble clarity, but other than that, no contest...I'll never go back!

It ain't hype if its true: the harmonic profile of this SE Class A amp yields a presence and musicality that sounds awesome.


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## xrk971

Hi StellarElephant,
 Great to hear that the sound is incredible. I knew it was special the first time I listened to it - that was the little hand etched PCB version.  Now with a real board, upgraded caps, matched FETs, I know that it must sound even better. I am surprised you say that the DAP direct output has blacker blacks because this amp with -126dB noise floor should add nothing noise wise.  Perhaps it is because you have not mounted it inside a metal tin yet?  There is some EMI/RFI noise reduction when inside a case.  Also, try it with a high quality battery delivering at least 16v - that improves the sound too.
 Thanks for your listening impressions.
 X


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## Slater

Looks really nice.
  
 How do you go about recharging the batteries?
  
 Also, is there a way to get longer run times than 4-5 hours?
  
 If I installed the setup into a different case (ie aluminum enclosure), could I run a 18650 batteries? Is there a boost converter for the battery power (for say [2] 3.7v batteries in series), or would I need to run 5 batteries in series to reach the recommended 18v?


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## xrk971

slaterlovesspam said:


> Looks really nice.
> 
> How do you go about recharging the batteries?
> 
> ...




These are removable 9v batteries that come with a wall charger. Look for EBL 9v rechargeable battery on Amazon. $21 for 4 600mAhr batteries and charger. 

You could use inexpensive replacement 3.7v LiPo batteries and a low cost DC to DC step up converter. Just parallel the 3.7v cells (but charge individually) to get more duration. 

If you put into a larger case it it should work fine. 

Alternatively, a low ripple regulated 19vdc power supply from a laptop (SMPS) followed with an LM317 regulator to drop to 16.5v should work really well for desktop use. Add a 2200uF 10R 2200uF CRC filter after the LM to really smooth it out.


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## bogde

> Alternatively, a low ripple regulated 19vdc power supply from a laptop (SMPS) followed with an LM317 regulator to drop to 16.5v should work really well for desktop use. Add a 2200uF 10R 2200uF CRC filter after the LM to really smooth it out.


 
 that sounds like a great idea, i don't care too much about portability, i'm tied to my desk most of the time anyway.


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## xrk971

Instead of LM317, a single MOSFET based cap multiplier would also be very good and lower noise than LM317. It will have 2-4v drop depending on MOSFET used. That will give you 15v to 17v.


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## Slater

xrk971 said:


> You could use inexpensive replacement 3.7v LiPo batteries and a low cost DC to DC step up converter. Just parallel the 3.7v cells (but charge individually) to get more duration.


 
  
 Great, thanks for the info.
  
 I'm a believer in using quality cells (flashaholic), so I'd use something like Orbtronics or Panasonic NCR18650B 3400mAh cells. If 900mAh batteries are providing 5 hrs, then (2) NCR18650B cells in series would provide 18-19 hours between charges. And I wouldn't have to boost voltage as much vs. running the cells in parallel.
  
 I didn't see where there was a kit available with board & all components, with me doing all of the kit assembly. Didn't you say you were doing all of the assembly?
  
 Thanks,
 - Slater


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## xrk971

slaterlovesspam said:


> Great, thanks for the info.
> 
> I'm a believer in using quality cells (flashaholic), so I'd use something like Orbtronics or Panasonic [COLOR=333333]NCR18650B 3400mAh cells. If 900mAh batteries are providing 5 hrs, then (2) NCR18650B cells in series would provide 18-19 hours between charges. And I wouldn't have to boost voltage as much vs. running the cells in parallel.[/COLOR]
> 
> ...




There is a simple Mouser shopping cart link with all the parts. You can order that and the board. If you want me to order for you as a "kit" I can do that too. Just send me a PM.


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## xrk971

Also, if you want the complete amp but don't want to go through eBay feel free to PM me.


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## stellarelephant

xrk971 said:


> Hi StellarElephant,
> Great to hear that the sound is incredible. I knew it was special the first time I listened to it - that was the little hand etched PCB version.  Now with a real board, upgraded caps, matched FETs, I know that it must sound even better. I am surprised you say that the DAP direct output has blacker blacks because this amp with -126dB noise floor should add nothing noise wise.  Perhaps it is because you have not mounted it inside a metal tin yet?  There is some EMI/RFI noise reduction when inside a case.  Also, try it with a high quality battery delivering at least 16v - that improves the sound too.
> Thanks for your listening impressions.
> X


 
  
 Let me clarify what I meant when I said that my DAP's built-in amp has a "blacker background" than my Pocket Class A.  This amp is NOT noisy at all...quite the opposite.  I was more talking about dynamics and how quickly and completely the sound reverts to quiet "blackness" in the spaces between the notes in the music, if that makes sense. 
  
 This amp continues to impress me every time I listen.  Today I let my wife hear it for the first time.  I played her "Take Me Home" by Pentatonix through the Pocket Class A and then again direct from my DAP's headphone jack.  She usually cannot hear subtle differences in SQ, but in this case she said she was surprised at how large the difference is!  She said the Class A sounded "fuller" and "more three-dimensional".  Made me smile.


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## xrk971

I had some time to make my personal no holds barred (NHB) uber pocket amp last night. 

Using super-matched FETs (matched to 4 significant figures and higher Idss - 7mA bias on JFET vs usual 5mA), and some capacitor tweaks so using Nichicon AK 470uF 16v on output coupler bypassed with Wima 2.2uF 50v MKS and on input 10uF 100v electrolytics bypassed with 100nF 100v MKT. Resistor settings hand adjusted for exactly 50mA bias on MOSFET and harmonic profile with -66dB of H2 and H3 of -30dB below that and nothing else. The board was reflow soldered with great care as well - just look at those clean solder joints!  I will listen for long term and do some AB testing with handy dandy AB box and will report back later. 

















Initial listening tests so far are very promising. Superb bass, crystal clear highs. Not fatiguing and excellent lively and engaging presentation.

Here is the AB switcher box: uses two DPDT signal relays activated by 12v momentary toggle switch. Toggling with level matched signals is almost imperceptible. Kind of eerie how well a mechanical double crossbar switch can work actually.


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## gikigill

Can I please buy that uber pocket amp?


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## Venture Guy

xrk971 said:


> I had some time to make my personal no holds barred (NHB) uber pocket amp last night.


 
 The pics are showing up as filenames, not images. Is that just me?


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## xrk971

I assume it's just you since jkgill can see them. 
Can you see earlier images in thread posted by me?


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## gikigill

Nope, can't see the pics. 

I made an offer sight unseen since I've seen your handiwork before.


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## Venture Guy

xrk971 said:


> I assume it's just you since jkgill can see them.
> Can you see earlier images in thread posted by me?


 
 Yes, I can see your lovely prototype and the astounding THD graph above, but not the pics of your magnum opus


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## xrk971

I will work on getting pics resolved asap.

Ok, I had to downsize images and use jpg. Let's see if this works?







Higher res versions are also available here:

http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-40.html#post5017579


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## Venture Guy

There they are! With the big caps, will the magnum opus power my 25 ohm Audioquest NightOwls? (98dB sensitivity)


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## gikigill

Shut up and take my money!!


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## xrk971

Venture Guy,
 Sure they will power 25ohm cans no problem, what SPL levels do you like to listen at?  98dB (per mW right?) is pretty sensitive so if you tell me your average drive voltage level I can show you the calculated harmonic profile. There is probably a minor tweak in a resistor value or two to customize it to your headphone impedance for max performance. Semi custom headphone amps here.
 Cheers,
 X


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## gikigill

How about a bass boost and a gain switch?


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## Venture Guy

gikigill said:


> How about a bass boost and a gain switch?


 
 Aargh!


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## xrk971

In case anyone is wondering, the output impedance from LTSpice simulations (used extensively to design this amp) is 1.08ohms.  
  
 Simulations also show that the amp can drive 60ohm cans to a max of 5.32v (p-p) before clipping, with associated max THD of 0.43%. This is about 60mW into 60ohms if I did the math right. 
  
 For 250ohm cans, it can drive 10v (p-p) before clipping with a max THD of 0.54%, and this is about 50mW into 250ohms max power.  I think pain threshold on my DT880-250's is about 5.5v so I am not power limited by any means.
  
 Gain is about 3x, so to hit the ear-bleed levels, you will need to use your line-outs, not your iPhone.


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## xrk971

gikigill said:


> How about a bass boost and a gain switch?


 

  
 There is a volume knob you know 
  
 In all seriousness, since this is a zero global feedback design, one can't just change the "gain" by setting resistors for Rf/Ri, etc. as would be done on a conventional amp with large amounts of negative feedback.
  
 So the gain is fixed by the topology at 3x so all we can do is attnuate it - and the way that is done is by the volume pot.  I suppose a switch could be added to attenuate by an additional amount but that would require rework of the PCB and finding room for a switch on the ever so small knob/jack real estate of the mint tin.
  
 I don't have bass boost as it is not needed - superb bass that follows the music as intended.  Bass response is absolutely flat measured -0.25dB at 20Hz, extrapolating to 10Hz I estimate -0.5dB at 10Hz.  Upper end falloff is due to 96khz sampling, the inherent bandwidth is in the 100's of kHz (approaching 1MHz) according to sims.  
  
 I listened to some intense sub-bass tracks and it rocks my head - literally feel the air pressure pulsating behind the cans on my my eardrums (not a freqency one can hear but just feel).


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## gikigill

What headphones did you use?
  
 I,m thinking of a 18v power supply for desktop usage too.


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## xrk971

I use DT880-250 special edition chromes. 

Desktop PSU is a good idea and I can provide that. Here I demonstrated the amp working on 120vac mains with my own PSU filtering to provide clean noise free power. Notice lack of 60Hz hum peak.


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## Slater

gikigill said:


> I,m thinking of a 18v power supply for desktop usage too.


 
  
 That's my intended plan as well.


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## gikigill

xrk971 said:


> I use DT880-250 special edition chromes.
> 
> Desktop PSU is a good idea and I can provide that. Here I demonstrated the amp working on 120vac mains with my own PSU filtering to provide clean noise free power. Notice lack of 60Hz hum peak.


 

 Cant see the picture, I would need a 240V power supply though.


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## xrk971

This is really annoying how my images from DIYA cannot simply be linked with a url.  They all seem to fail, so I have to re-upload the images here.
  
 Ok, here is the FFT from a desktop version of this amp I was working on -  a prototype.  But it shows how clean of a power supply I can make without resorting to heroic efforts like a shunt regulator or even a cap multiplier.  Just using a basic SMPS then cleaning that up.
  

  
 Noise floor went up a bit from -126dB I can get from batteries, but still pretty low. Main thing is lack of any 60Hz bump at the AC line frequency. Hardly any PSU shunt or otherwise can achieve this in practice. Almost any other wall-plug PSU will have some sort of 60Hz and oftentimes 120Hz (or 50Hz and 100Hz outside of USA) bumps or peaks.
  
 240VAC supply is not a problem as the main power source is a universal 110vac/240vac 50Hz/60Hz SMPS.


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## gikigill

xrk971 said:


> This is really annoying how my images from DIYA cannot simply be linked with a url.  They all seem to fail, so I have to re-upload the images here.
> 
> Ok, here is the FFT from a desktop version of this amp I was working on -  a prototype.  But it shows how clean of a power supply I can make without resorting to heroic efforts like a shunt regulator or even a cap multiplier.  Just using a basic SMPS then cleaning that up.
> 
> ...


 
  
 I can also obtain a linear supply though of the same rating.


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## xrk971

Congratulations to gikigill as the new owner of the NHB (No Holds Barred) Convertible Ediition (desktop capable) amp!











Truly a one of a kind amp - and I must admit that I am saddened to see it go to Australia tomorrow. Reassuring to know it is going to a great home though.


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## xrk971

Some kind words by member Funch regarding how this amp sounds here:
  
 http://www.head-fi.org/t/833106/new-diy-altoids-tin-amp-the-class-a-wintergreen-handwarmer-by-xrk971/90#post_13344874


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## gikigill

Can hardly wait, managed to source a power supply locally for desktop usage. 

So when is the bigger brother coming? First dibs on a proper desktop setup.


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## xrk971

gikigill said:


> Can hardly wait, managed to source a power supply locally for desktop usage.
> 
> So when is the bigger brother coming? First dibs on a proper desktop setup.


 
 You mean this?  I am waiting for BabyDontHertzMe to help me with the new layout.  The prototype using a pocket PCB seems to show that it works well though.  93mA bias current should drive lower impedance cans much better.  Harmonic profile is the same.
  

  
 What power supply did you get?  What regulator chip is it using, if any?  A cap multiplier can work very well too - regulation is not needed more ripple removing is the order of the day and aim for 15v to 16.5v supply as 18v is pushing it with the heat generation.


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## Slater

xrk971 said:


> 93mA bias current should drive lower impedance cans much better.  Harmonic profile is the same.


 
  
 What is the optimal impedance range of each of the amps (the original and the big brother)?


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## xrk971

The original pocket Class A was designed to be used with headphones as low as 60ohms. The bigger desktop one can go to 30ohms. Although I have used the original on 16ohm IEM's and earbuds and they work quite well - perhaps with just a bit less bass extension. 250ohm cans have theoretical reach of 8Hz -0.2dB point with 390uF output caps. So 60ohm would be about 32Hz for -0.2dB. -3dB would probably still be in the tens of Hz. 

Now that you have your amp kit, let us know how the build goes.

Testing amps must go on. AB box used to check for QC vs the reference amp (bone stock one) with blue cord.


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## Slater

Great, thanks for the clarification.


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## xrk971

The good stuff just arrived! Elna Silmic II's 4.7uF 25v and 100uF 16v RFS's


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## gikigill

Is that for the desktop amp?


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## xrk971

Yes. I would be making more progress on a real desktop amp PCB but can't seem to find BDHM. He's been offline for several weeks now. Hope he's ok - but he's the layout artist. 

For the true desktop amp I will add a relay to prevent turn on thump. Although a cap multiplier and cap bank will do same thing via controlled slow start (20 Sec to come up to full voltage. )

Also considering a CFP topology with the venerable Toshiba 2SA1837.

It's a tabula rasa at the moment so any ideas or features folks would like to see incorporated please let me know.


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## gikigill

Calling dibs on the desktop amp


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## xrk971

You got it Gill. 

Let's run 125mA bias through CFP topology with BF862 for master and 2SA1837's as slaves and have 20v rail. We will have cap multiplier and low ESR cap bank. Power will be from 24v SMPS. I think we might be able to get 1w into 30ohm cans. 

Of course the full Silmic II or OSCONs and Wima MKS treatment. 

Crossfeed?

RCA inputs and 1/4in TRS jack out.


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## gikigill

Can you bump into a 10w per channel monster for planars?


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## xrk971

Yes, but that's a different amp altogether. You really need 10w? Isn't that a regular speaker powe amp? I have a very nice single ended quasi topology that will give a similar harmonic profile and 10w if that's what you really need. How many ohms are the headphones?


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## gikigill

I want it to power the HE-6, which can literally suck up current. It sounds thin and harsh out of most headphone amps but powered off a speaker amp, it has the best bass I've ever heard. 

A literal wall of bass, you wanna reach out and eat a chunk of that tasty tasty bass.


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## gikigill

Pass Lab amps are very popular with the HE-6 crowd. Low wattage, relatively speaking but high current.


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## xrk971

Ok the website says HE6 needs a 2w amp and 50ohms is very different than 10w. I have some designs using the LU1014D power JFET that might work for you in this case. Let me run some LTSpicesims and see what I can come up with. So a 2w rating means it needs to run at least 100mA bias in order to swing 200mA into 50ohms. The LU1014D has an on impedance of only 3milliohm so can really drive stuff hard. But if you want some headroom we should double that to 200mA bias. That's a pretty hot running headphone amp. With 24v rails it will drop about 12v through the power JFET for 2.4w dissipation.


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## xrk971

Have you tried an ACA Pass amp with your HE-6? I have built a few of those and they are quite nice SE Class A designs - 5w into 8ohms. Not sure how many into 50ohms.


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## gikigill

The 10W per channel Pass Labs amp is really successful with the HE-6. Can't remember the name.


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## gikigill

It's the F2, F2J, Aleph, F6 and F7 that are mostly used for the HE-6. 

I use a vintage Onkyo with a 800w power supply and 110 watts per channel @8 ohms.


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## xrk971

Those are all "big iron" amps. I have now a design in mind that would use the CFP SE Class A topology, but use large MOSFETs similar to the Pass Class A amps to boost the output current. It would have the same harmonic signature of my little amps but can drive about 19v p-p into 8ohms or about 7w into 50ohms. It is a home heater though with about 70w dissipation per channel and requiring two 100VA transformers.  A very serious amp.  All the distortion would be H2 and very little H3 and nothing else.


----------



## gikigill

As long as it's at a reasonable price, I'm in.


----------



## xrk971

Ok, I am designing a new desktop amp. It will be quite something special. It will be a JFET input stage with a classic CFP output stage. Capable of 4w of pure Single Ended Class A power into 50ohm cans. At moderate drive levels of 320mW into 50ohms it has 0.04% THD and a similar harmonic profile as my pocket amp - wonderful sounding in other words. At full 4w drive level of 14v p-p into 50ohms it has 0.9% THD. At 119dB in 83dB/mW HE6's, THD will be the least of your worries 

I will be designing this with Hugh Dean's help (Aspen Amplifiers) and initial simulations in LTSpice are very encouraging. The price to pay for 4w of glorious SE Class A output capability is 75w of dissipation with music playing or not. That's per channel so 150w Like a large old style 75w incandescent lightbulb. 

Here is a preliminary prediction of the FFT at 320mW into 50ohms:


Oh, and btw, you could use this amp to drive your 8ohm room speaker to 10w. Or perhaps you have super inefficient cans that need 10w?


----------



## gikigill

Any estimate of the price?


----------



## xrk971

Let me finish spec'ing it and even start building and testing before I give you an estimate. I don't want to give you bad info. My big concern is the size and weight which makes shipping to Australia a big problem. It's a full size speaker amp for all intents and purposes. I ordered the 200VA toroidal trafo and that alone weighs 6lbs. A lot of iron and copper. The heatsink and case will probably come in around 8 lbs to 10 lbs. you can see where this is going. How handy are you at DIY? One option is I sell you the amp module ready to connect to PSU and case with heatsink that you buy locally.


----------



## gikigill

I'm pretty rubbish at DIY but might be able to plug in different items to get them up and running.


----------



## xrk971

Ok. This is pretty extensive work and not for a beginner since it is a 150w dissipation Class A project. 
I will see what we can do.


----------



## gikigill

Let's hope so, is it similar to the Beta 24.


----------



## xrk971

gikigill said:


> Let's hope so, is it similar to the Beta 24.




Hi Gil,
I hope by now, you would have realized the philosophy that I have to designing amps that sound great is not through the use of circa 45 (I lost count) actives per channel (that's a record high count for diy amps as far as I know) such as used by the Beta 24. Whic uses extensive differential pairs and lots of negative feedback to achieve perhaps immeasurable low levels of distortion. Will it sound better than an amp with maybe 3 actives and no global feedback but designed for natural sounding musicality? Probably not. No disrespect for designer of Beta 24 - just a different philosophy. 

Here is Beta 24







My amp will have 3 actives in audio signal path: JFET, BJT, MOSFET. It will have two ancillary N channel MOSFETs for a Constant Current Sink (CCS) and a capacitance multiplier (Cx). So 5 total but signal will only going through 3 (vs 30-40?) Which amp do you think will have less noise, and transistor junction artifacts imposed on it? It's also much simpler to build and smaller. So no, not similar at all - maybe in current draw? 2.3 amps per channel all the time via 32v single rail.


----------



## gikigill

Bring it on, I got a Beta 22 coming so should be a good comparison


----------



## xrk971

Beta 22 or 24?

You are a certified amp addict! How many do you have?

Not having ever heard the Beta 22/24 - but looking at the topology, I can be quite certain you will think the new Aksa-designed CFP amp (the "Glass Harmony") will sound better. 

But we will see.


----------



## gikigill

Beta 22. 

I'm on about 15-20 amps. 

Maybe more.......


----------



## DBaldock9

@xrk971 - I'm curious about the feasibility of using multiples of the original amp, set up to drive low impedance IEMs or Earbuds, using a single-ended line-level input and providing a balanced output?
  
 Thanks,
 David Baldock


----------



## xrk971

dbaldock9 said:


> @xrk971
> - I'm curious about the feasibility of using multiples of the original amp, set up to drive low impedance IEMs or Earbuds, using a single-ended line-level input and providing a balanced output?
> 
> Thanks,
> David Baldock




Let me look at this again. It may not be possible with current amp but it may be possible with a new variant using 4 FETs. It will have to be DC coupled though. The problem is that the power supplies have to be balanced or else the cans get a bad dose of DC offset or worse, a catastrophic rail imbalance. This is why AC coupled phones are much safer. 

I will mull on it and maybe run a simulation or two to confirm. 

Cheers,
X


----------



## xrk971

Dbaldock9,
Do you already have something that puts or balanced line drive so that I don't have to do the single ended to balanced line conversion? There may be a simple way to do this with two amps tied together as you suggest. Investigating it now on LTSpice. 
Cheers,
X


----------



## DBaldock9

xrk971 said:


> Dbaldock9,
> Do you already have something that puts or balanced line drive so that I don't have to do the single ended to balanced line conversion? There may be a simple way to do this with two amps tied together as you suggest. Investigating it now on LTSpice.
> Cheers,
> X


 
  
 Currently, the only Balanced audio device I've got, is an Onkyo DP-X1 DAP - which has a balanced 2.5mm amplifier output, but the Line Level output is on the 3.5mm single-ended jack.
 I haven't yet investigated balanced line output sources.


----------



## xrk971

If your DAP has balanced output that is the best thing to use as it is a perfect digital inversion for the negative side. I need to read up on 2.5mm balanced output jacks but assume it is the 4 wire connector normally used for smartphone headsets with mic's?

Problem is, I don't have a balanced output DAP or headphone to test my amp if I were to build one.

Here is what I have in mind and conceptually it should work. To implement this could mean one amp board per balanced headphone channel. So practically it could work out well. Just get two amps (specially wired for balanced). What is good here is that it is still AC coupled - which isn't going to ruin the bass or anything. With large caps and 250ohm cans, the low corner will be about 8Hz (-0.2dB) Plenty of bass. But because it is AC coupled - there is no danger of blowing out your cans when one of the batteries in the supply goes kaput before the other one.


----------



## DBaldock9

xrk971 said:


> If your DAP has balanced output that is the best thing to use as it is a perfect digital inversion for the negative side. I need to read up on 2.5mm balanced output jacks but assume it is the 4 wire connector normally used for smartphone headsets with mic's?
> 
> Problem is, I don't have a balanced output DAP or headphone to test my amp if I were to build one.
> 
> Here is what I have in mind and conceptually it should work. To implement this could mean one amp board per balanced headphone channel. So practically it could work out well. Just get two amps (specially wired for balanced). What is good here is that it is still AC coupled - which isn't going to ruin the bass or anything. With large caps and 250ohm cans, the low corner will be about 8Hz (-0.2dB) Plenty of bass. But because it is AC coupled - there is no danger of blowing out your cans when one of the batteries in the supply goes kaput before the other one.


 
 The Onkyo DP-X1 has the same balanced output amplifier wiring as the Astell Kern DAPs on the 2.5mm 4-pole jack.
 Tip = Right (-)
 Ring1 = Right (+)
 Ring2 = Left (+)
 Sleeve = Left (-)
  
 I was hoping to get (or assemble) an amp that could take either a balanced or unbalanced line-level input, and have a balanced output.  If that is rather difficult or expensive, I can use a 3.5mm TRS (3-pole) to 2.5mm TRRS (4-pole) adapter, to allow my balanced earphones & earbuds to be driven by single-ended amps.
  
 If buying or assembling one of your standard amps, are there mods necessary so that it can drive earphones & earbuds that range from ~12Ω to 650Ω?
  
 Thanks,
 David Baldock


----------



## xrk971

First, if you want a version of this amp that drives balanced phones - it's not something easily reversible. The internal power supply has to be split - a necessary requirement for balanced (aka circlotron Class A topology). A balanced amp can always drive from an unbalanced source - you won't get the advantages but it plays music as if it were single ended. 

12ohms might be pushing it. It is designed for cans as low as 50ohms. Will still sound decent with 30ohms but bass extension may be lacking at 16ohms. 

I have not tested with 600ohm cans yet but should work very well. You can get 10v peak to peak swing.


----------



## xrk971

dbaldock9 said:


> The Onkyo DP-X1 has the same balanced output amplifier wiring as the Astell Kern DAPs on the 2.5mm 4-pole jack.
> Tip = Right (-)
> Ring1 = Right (+)
> Ring2 = Left (+)
> ...




Hi David,
The initial simulations look very good and it appears that there is a way to do balanced drive with these amps. Gikigill has kindly offered to loan me his balanced drive DAP and a pair of HE400's to test out the concept in reality. In LTSpice sims it looks great. Here are some results showing the FFT for balanced drive into 250ohm cans at 1.4v peak-peak. This corresponds to 0.02%THD with most of it as second harmonic - a very different signature that the predominant H3 of differential input push pull balanced amps. I will keep folks posted on how this goes but looks very promising so far and may be, again, one of the first pocket sized SE Pure Class A balanced headphone amps around. To keep it simple if your DAP has balanced outputs that is best ofherwause I would have to add a single ended to balanced driver circuit and that makes setup bigger and less transparent. We are still trying to keep it to 2 transistors per balanced side for 4 total per channel. Cheers,
X


----------



## DBaldock9

xrk971 said:


> Hi David,
> The initial simulations look very good and it appears that there is a way to do balanced drive with these amps. Gikigill has kindly offered to loan me his balanced drive DAP and a pair of HE400's to test out the concept in reality. In LTSpice sims it looks great. Here are some results showing the FFT for balanced drive into 250ohm cans at 1.4v peak-peak. This corresponds to 0.02%THD with most of it as second harmonic - a very different signature that the predominant H3 of differential input push pull balanced amps. I will keep folks posted on how this goes but looks very promising so far and may be, again, one of the first pocket sized SE Pure Class A balanced headphone amps around. To keep it simple if your DAP has balanced outputs that is best ofherwause I would have to add a single ended to balanced driver circuit and that makes setup bigger and less transparent. We are still trying to keep it to 2 transistors per balanced side for 4 total per channel. Cheers,
> X


 
  
 Thanks for looking into this alternate use of your amplifier circuits.
  
 I suppose the single-ended to balanced driver could be switched in or out of the circuit, depending on whether it's needed - and thus, not affecting the audio if a balanced input is available.


----------



## xrk971

You are welcome, David. Yes, if you don't have a balanced source one needs to be inserted in between the source and the amp to generate the balanced drive. Of course, if it s removed, the amp can function as balanced amp for balanced sources like a DAP. Or even switch back to regular single ended amp for single ended sources. Nothing is irreversible. It's quite adaptable and flexible yet maintaining great performance of pure class A SE topology of the original amp. 

One thing with balanced drive is that it naturally removes the pleasant second harmonic distortion - as you can see the levels of my amp were in the -66 to -72dB H2 but is driven to -80dB and below. 

There are some tricks to restore the naturalness of the slightly higher H2 sound of a SE Class A topology. 

Here's an example of such a profile even in balanced drive mode. 



Note the nice -20dB difference between H3 from H2. It will sound a lot more engaging and less dry - yet still have the dynamics and attack of the high slew rates from balanced drive.


----------



## DBaldock9

xrk971 said:


> You are welcome, David. Yes, if you don't have a balanced source one needs to be inserted in between the source and the amp to generate the balanced drive. Of course, if it s removed, the amp can function as balanced amp for balanced sources like a DAP. Or even switch back to regular single ended amp for single ended sources. Nothing is irreversible. It's quite adaptable and flexible yet maintaining great performance of pure class A SE topology of the original amp.
> 
> One thing with balanced drive is that it naturally removes the pleasant second harmonic distortion - as you can see the levels of my amp were in the -66 to -72dB H2 but is driven to -80dB and below.
> 
> ...


 
  
 Sounds like you could end up with a whole family of circuit modules (power supplies, inputs, outputs), so people can custom assemble whatever sort of amplifier they require (portable, desktop, balanced or unbalanced inputs, balanced or unbalanced outputs).


----------



## xrk971

Yes, modular mix and match is one way. I think some folks may be looking for a nice SE Class A balanced drive desktop unit as well. I have another version that would have even higher resolution using the venerable (but end of life) Toshiba 2SA1837 BJT for the output rather than a MOSFET. It's capable of 1amp bias so I could easily run it at say 200mA and have some nice current for lower impedence cans. 

Of course let's not forgot the big 4w into 50ohm SE class A amp (desktop) that is currently under development. It's singing really nicely right now. I stress tested it yesterday at 4v rmsinto 8ohms sine wave for 4 hrs and it didn't flinch. That's 2 watts. 

Sounds wonderful. Similar harmonic profile of all my SE amps. Just on steroids. 

You pay a price in heat though. 50w per channel no matter what.


----------



## xrk971

A quick update - I indeed tested the amps in SE balanced drive mode and it works very well.  I modified my old Sony MDRv6 to accept balanced drive and used digital inversion to produce a balanced signal 180 deg out of phase.
  
 As expected, running without the phase flip results in no sound.  Running with one side sounds like old (normal SE way).  Flippin phase on one side for balanced drive sounds louder and more dynamic.  The transient peaks are actually kind of uncomfortable.  I estimate the dynamic range went up by +12dB if that is correct for balanced drive.
  
 I am waiting for Gikigill to get me a real balanced drive DAP and phones to test out as this was a quic and dirty proof of concept test.


----------



## DBaldock9

Sounds like a promising result already! 
  
 About the higher output - in the review of the balanced mod of the Chord Mojo, there is a reference to the mod'er reducing the gain by 14dB.
  
 On another note - I haven't seen your amp schematic, so I'm wondering if the 9V batteries are wired in parallel, for more current - or in series, for more voltage?
  
  
 Thanks,
 David Baldock


----------



## bogde

the batteries are wired in series.


----------



## xrk971

Here is schematic from this thread:

http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb.html







Caps for rails is now 2200uF 16v per side with two 0.1uF x7r MLCC bypass pet side.


----------



## xrk971

My eBay listing closed and I can't relist for 30 days since I am a new seller. If anyone is interested in a pre-built amp, just send me a PM.


----------



## SDBiotek

Sorry guys, I'm the one that bought the last one on eBay. Maybe someday when I finally catch up with household projects I'll be able to take a shot at building one.


----------



## xrk971

sdbiotek said:


> Sorry guys, I'm the one that bought the last one on eBay. Maybe someday when I finally catch up with household projects I'll be able to take a shot at building one.




Thanks for the order SDBiotek! Your amp is shipping out today. 

In case anyone wants to get more prebuilt amps you can order one through my new shop on etsy. 

https://www.etsy.com/your/shops/XRKAudio/tools/listings/522023157

I hope it is not against forum rules for me to simply tell people where to go if they want another one since eBay won't let me re-list for 30 days. Kind of crazy policy actually for doing business.


----------



## DBaldock9

xrk971 said:


> Thanks for the order SDBiotek! Your amp is shipping out today.
> 
> In case anyone wants to get more prebuilt amps you can order one through my new shop on etsy.
> 
> ...


 
  
 For shoppers, we only need this much of the link - https://www.etsy.com/shop/XRKAudio  
	

	
	
		
		

		
		
	


	



  
 Are you still offering the "ultra" model, with the matched transistors, and lower Bass response?


----------



## xrk971

dbaldock9 said:


> For shoppers, we only need this much of the link - https://www.etsy.com/shop/XRKAudio  :bigsmile_face:
> 
> Are you still offering the "ultra" model, with the matched transistors, and lower Bass response?




Yes, if you look at the buying options - scroll down to NHB option.

You can also choose your color. For a limited time, I am offering the amp at the promotional pricing hat you can select as an option. This is only for the standard version with 390uF oscon caps. 

Thanks for showing me the proper link - a newbie here.


----------



## stellarelephant

I think it is OK to tell people where they can purchase your amps. I have seen large manufacturers do the same thing in other Head-Fi theads when they roll out new products.


----------



## SDBiotek

Received my amp yesterday, but waiting on some 9V batteries from Amazon. Looking forward to trying it out tonight!


----------



## xrk971

You can put regular 9v batteries in to test out. Should be good for at least 3hrs of use if not more. 

Congrats on your new amp - looking forward to listening impressions. What headphone will you be using?


----------



## xrk971

Just an update folks. My shop now includes bare PCBs, kits for the BOM, pre-soldered SMT PCBs, and hand-matched FETs.
  
https://www.etsy.com/shop/XRKAudio


----------



## DBaldock9

xrk971 said:


> Just an update folks. My shop now includes bare PCBs, kits for the BOM, pre-soldered SMT PCBs, and hand-matched FETs.
> 
> https://www.etsy.com/shop/XRKAudio


 
  
 So, we can buy the parts, including the hand-matched FETs, to build a NHB version of your Amp?


----------



## xrk971

dbaldock9 said:


> So, we can buy the parts, including the hand-matched FETs, to build a NHB version of your Amp?




Yes, almost. I have not added the priced out option for FETs matched to 4 significant figures yet but will add it soon. It will be $30 per set. However, besides the Uber matched FETs and top of the line capacitor combo, perhaps the most important aspect of the NHB option that I sell is the "hand-tuning". This involves measuring the amp's performance and looking at the harmonic distortion profile. Then, fine-tuning the resistor network to achieve the optimal balance of low distortion and musicality. If you have your own measurement rig you can do it yourself and I can give pointers even.


----------



## xrk971

Just a reminder that if your amp with Li-ion batteries doesn't seem to turn on - LED flashes then flickers then off. It's normal:

The Li-ion batteries have a smart self protect circuit that shuts off the battery if it thinks you shorted it out. The in-rush current to charge the large rail caps causes it to shutdown (in the battery). Only turning off resets it. Turn amp on and off quickly several times and the caps should charge a bit each time. Usually no more than two clicks back and forth. But it's perfectly normal with the EBL (and other rechargeable) Li-ion batteries. Alkalines and carbon-zinc cells don't have his problem.


----------



## DBaldock9

xrk971 said:


> Yes, almost. I have not added the priced out option for FETs matched to 4 significant figures yet but will add it soon. It will be $30 per set. However, besides the Uber matched FETs and top of the line capacitor combo, perhaps the most important aspect of the NHB option that I sell is the "hand-tuning". This involves measuring the amp's performance and looking at the harmonic distortion profile. Then, fine-tuning the resistor network to achieve the optimal balance of low distortion and musicality. If you have your own measurement rig you can do it yourself and I can give pointers even.


 
  
 Well then, your personal tuning service is worth the cost of an assembled NHB model.
 Need to get my US Federal Taxes calculated & filed, and I expect my refund will cover the cost of an NHB.


----------



## xrk971

I love spending tax rebates before they come in too. 

If you are serious, I can start assembling an NHB for you. Are you into low impedance headphones?

I can tune it for a higher bias current if so.


----------



## gikigill

Saving up my refund in July for a desktop setup.


----------



## xrk971

gikigill said:


> Saving up my refund in July for a desktop setup.




Are we talking about the 4w into 50ohms SE Class A or the 7w into 50ohms Class AB quasi complimentary MOSFET/BJT hybrid?


----------



## gikigill

7 watts Pure Class A @50ohms would be even better.


----------



## xrk971

gikigill said:


> 7 watts Pure Class A @50ohms would be even better.




That can be done but it's no desktop amp! Probably 150w of dissipation (per channel!) whether music plays or not. A 300w room heater the size of standard 50w into 8ohms speaker Class A amp.


----------



## gikigill

5w should be alright too.


----------



## DBaldock9

xrk971 said:


> I love spending tax rebates before they come in too.
> 
> 
> 
> ...


 
  
 Well, the one pair of actual Headphones that I've got, is the Beyerdynamic DT-831 (250Ω).
  
 But, now that I've built up my collection of Earphones & Earbuds, I don't really listen to the DT-831s very often.
  
 Most of my Earphones are 16Ω or 32Ω; and my Earbuds range from 16Ω to 650Ω (although the ones I listen to most, are 32Ω or higher).
  
 Have you had any feedback from people buying and using this set of batteries with your Amp?  http://www.onlybatteries.com/showitem.asp?ItemID=19514.11&cat1=14&uid=1970


----------



## xrk971

Ok I can aim for 32ohms design then. Looks like you need boosted bias current and big caps. I have not seen those batteries before - look nice at 720mA but is that real or at 3.7v internal before step up to 9v? Also the price is a bit high $114!

The EBL cells I use are cheap $21 for 4 including charger. But plagued by quality problems. I have had to send back 2 sets. I know others who also have. So their ratio of good batteries is about 60%. In fact, it looks line SDBiotek is having problems with his right now shutting down upon turn on.


----------



## DBaldock9

Just found the same set, on Amazon, for $96 - https://smile.amazon.com/Hitech-Four-Lithium-Polymer-Batteries-Charger/dp/B00890HIE4
 And they've got the 2 batteries, plus charger set, for $55 - https://smile.amazon.com/Hitech-Two-Lithium-Polymer-Batteries-Charger/dp/B0088R79RE
  
 On Amazon, there's more info about the battery - and it says the nominal voltage is only 7.4V.


----------



## SDBiotek

My amp is working very well now. The EBL rechargeable 9V batteries turned out to be duds, even though they appeared to charge normally.I still want to get some rechargables for this amp...I will be using it a lot!
  
 So far, Ive only had a chance to try it with my Fostex TH-X00 and Shure SRH 440, using my AK 100 II DAP as the source. The amp has no noise floor with those headphones, and drives both very well. I am amazed by the bit of extra clarity it seems to bring out of the headphones. The Fostex retains is strong, fun midbass signature, but with less of the "bloated" sound. The Shure sounds quite articulate and has a more solid bass foundation.


----------



## xrk971

Great! So glad it's working out for you and you like the sound. What are the inpedances of your Foster and Sure headphones?

Sorry about the EBL's - I have had several duds myself.


----------



## DBaldock9

xrk971 said:


> I love spending tax rebates before they come in too.
> 
> 
> 
> ...


 
  
 OK Taxes filed, and my refund will definitely cover a NHB.
  
 Should I go ahead and place the order on Etsy?


----------



## xrk971

Dbaldock9,
Yes, please do. 
Thanks!


----------



## DBaldock9

xrk971 said:


> Dbaldock9,
> Yes, please do.
> Thanks!


 
  
 Done.
  
 Will optimizing it for 32Ω, cause problems with it being able to drive 250Ω Headphones, or 500Ω - 650Ω Earbuds?
  
  
 Are there alternate Li-Ion batteries, other than standard 9V sized cells, that could be used to power your amp design?
 How low of a ripple voltage would an AC powered linear supply need to have, to avoid audio degradation?
 Would the optimum voltage be the same 9Vdc, as the batteries?


----------



## xrk971

Optimizing for 32ohms is a good all-around deal as it won't affect performance at higher impedences and this amp can actually do well at 32 ohms with big caps and higher bias. 

I will get to work on building it tonight - I have just the right pair of FETs I have been saving for this. Hot running high Idss units that will give a nice high bias current naturally.


----------



## DBaldock9

xrk971 said:


> Optimizing for 32ohms is a good all-around deal as it won't affect performance at higher impedences and this amp can actually do well at 32 ohms with big caps and higher bias.
> 
> I will get to work on building it tonight - I have just the right pair of FETs I have been saving for this. Hot running high Idss units that will give a nice high bias current naturally.


 
  
 Thanks!  Looking forward to seeing & hearing your handiwork.


----------



## SDBiotek

xrk971 said:


> Great! So glad it's working out for you and you like the sound. What are the inpedances of your Foster and Sure headphones?
> 
> 
> The Shure SRH440 is 44 Ohm, and the Fostex is 25 Ohms. Both are pretty sensitive and work ok directly from my computer or portable player, but do not sound as controlled compared with using the pocket amp.


----------



## DBaldock9

dbaldock9 said:


> Done.
> 
> Will optimizing it for 32Ω, cause problems with it being able to drive 250Ω Headphones, or 500Ω - 650Ω Earbuds?
> 
> ...


 
  
 Doing a bit more searching, and I found this site - http://www.batterysupports.com/
 Two of these 11.1V Li-Ion batteries [ http://www.batterysupports.com/108v-nbsp-111v-nbsp-12v-c-7_11.html ] wouldn't fit in an Altoids tin, but the 2600mAh model ($24.50 ea) would have 3x to 4x the run time of a pair of 9V batteries.


----------



## xrk971

Those look interesting. I wonder if they have built in protection circuitry. If you can find a 50mm x 50mm x 20mm thick battery it will fit fine.


----------



## stellarelephant

Hey hey. Lots of talk about batteries, so I'll share my review of the OKCell USB Lipo batteries I'm using in my Pocket Class A. 
http://www.head-fi.org/t/833106/new-diy-altoids-tin-amp-the-class-a-wintergreen-handwarmer-by-xrk971/225#post_13410233


----------



## DBaldock9

xrk971 said:


> Those look interesting. I wonder if they have built in protection circuitry. If you can find a 50mm x 50mm x 20mm thick battery it will fit fine.


 
  
 The description contains -
 ~~~
Built-in IC chip will prevent battery pack from over charge, over discharge and prolongs battery life
Charged by Li-ion 12.6V Charging Voltage Battery Charger
 ~~~
  
 Unfortunately, the 2600mAh model says -  Dimensions: 68 x 56 x 20 mm (Height x Length x Thickness)
And, we need two of them, to get the +11.1V & -11.1V rails.


----------



## DBaldock9

A pair of these will require a fair sized housing, but you'll have +/-12V @ 9800mAh, and easy recharging, at $36 for the pair!  
	

	
	
		
		

		
		
	


	



  
http://www.ebay.com/itm/Pro-DC12V-9800mAh-Rechargeable-Portable-Li-ion-Battery-US-Plug-Battery-Pack-TOP/282330243512


----------



## xrk971

You can try running amp with just 12v rail it may work for high impedance sensitive cans. Putting 12v in series makes 24v and that is too much. Heat dissipation is an issue as those four resistors in R4 are designed to burn off no more than 500mW. It's close to 1500mW at 24v. The MOSFET would require more heat sinking as well. And maybe if you placed the circuit SMT face down on a thick silicone heatsink pad and clamped to a large aluminum heatsink capable of removing 5w it might work. You can't have any large through hole parts on SMT side as it needs to be flat to clamp against heatsink. The other issue is whether or not the BF862 JFET, rated for 20v max can survive - it's got a 1k resistor above it to limit voltage but probably running very close to max. I will need to run a simulation to see where it stands and how t works at 24v.


----------



## DBaldock9

xrk971 said:


> You can try running amp with just 12v rail it may work for high impedance sensitive cans. Putting 12v in series makes 24v and that is too much. Heat dissipation is an issue as those four resistors in R4 are designed to burn off no more than 500mW. It's close to 1500mW at 24v. The MOSFET would require more heat sinking as well. And maybe if you placed the circuit SMT face down on a thick silicone heatsink pad and clamped to a large aluminum heatsink capable of removing 5w it might work. You can't have any large through hole parts on SMT side as it needs to be flat to clamp against heatsink. The other issue is whether or not the BF862 JFET, rated for 20v max can survive - it's got a 1k resistor above it to limit voltage but probably running very close to max. I will need to run a simulation to see where it stands and how t works at 24v.


 
  
 So, is +/-9V rails the ideal power supply for this Amp, or is it using that because of the convenience and size of standard 9V batteries?
  
 If it can operate on a single +18V rail, then there are some 68 x 90 x 20 mm 18V Li-Ion batteries, up to 3400mAh, available here - http://www.batterysupports.com/18v-21v-518650-3400mah-5s-lithium-ion-liion-battery-pack-p-509.html


----------



## xrk971

18v will work fine but space available in Altoids tin is about 50mm x 50mm x 19mm, which happens to fit two 9v cells perfectly.
  
 There are 50mm x 50mm LiPo cells out there. Use them with DC step up can work and that is under development now.


----------



## DBaldock9

Yeah, most of the high value mAh Li-Ion battery "solutions" I've been finding, are too large to fit inside the Altoids tin - and would require an alternate housing.


----------



## SDBiotek

If there could be a variation that allowed for charging via USB, maybe in a larger Hammond case, that would be cool. I'm happy to deal with 9V for now, though. still need to find some time to try the amp with some of my big headphones.


----------



## DBaldock9

sdbiotek said:


> If there could be a variation that allowed for charging via USB, maybe in a larger Hammond case, that would be cool. I'm happy to deal with 9V for now, though. still need to find some time to try the amp with some of my big headphones.


 
  
 Are you talking about an option on his pre-built Amps?  That's the feature he mentions above, as being in development.
  
 Otherwise, you're free to make whatever mods you want, once you've received, or built, an Amp.


----------



## SDBiotek

dbaldock9 said:


> Are you talking about an option on his pre-built Amps?  That's the feature he mentions above, as being in development.
> 
> Otherwise, you're free to make whatever mods you want, once you've received, or built, an Amp.




 Yes, I meant pre-built...while I've done some dabbling with a soldering iron, I'd feel a bit better having someone else build one for me. I haven't kept track of all the variations in development, so I'll have to go search the thread when I have some time.


----------



## xrk971

Just to show you that the prototype system under development does indeed still fit inside the mint tin, here is a photo. It uses a 3.7v 2600mAhr LiPo cell that is 50mm x 50mm x 6mm thick.  The cell has built in smart protection circuit for over-charging, over-current, and over-depletion.  A DC-DC step up regulator takes that to any voltage but I set it at 15.5v for a good balance of long life and dynamics. A bank 2200uF 16v caps and resistors form a CRCRC filter to clean up the broadband residual switching noise.  An automatic USB charging circuit handles charging duties.
  
 I am getting 3.5hrs per charge so far - that will also depend on your bias current setting on the amp.  The battery auto-shutdown is also tricky to deal with as the in-rush current to the caps on the amp tends to kick it into self-protect. Right now, use of a series resistor between the battery and the caps limits the current but wastes power as heat.  Similarly, the resistors in the CRC filter waste heat as well. Otherwise this amp should run close to 5.5hrs.


----------



## SDBiotek

Very cool! Looking forward to seeing how it develops.


----------



## DBaldock9

xrk971 said:


> Just to show you that the prototype system under development does indeed still fit inside the mint tin, here is a photo. It uses a 3.7v 2600mAhr LiPo cell that is 50mm x 50mm x 6mm thick.  The cell has built in smart protection circuit for over-charging, over-current, and over-depletion.  A DC-DC step up regulator takes that to any voltage but I set it at 15.5v for a good balance of long life and dynamics. A bank 2200uF 16v caps and resistors form a CRCRC filter to clean up the broadband residual switching noise.  An automatic USB charging circuit handles charging duties.
> 
> I am getting 3.5hrs per charge so far - that will also depend on your bias current setting on the amp.  The battery auto-shutdown is also tricky to deal with as the in-rush current to the caps on the amp tends to kick it into self-protect. Right now, use of a series resistor between the battery and the caps limits the current but wastes power as heat.  Similarly, the resistors in the CRC filter waste heat as well. Otherwise this amp should run close to 5.5hrs.


 
  
 It would require more complexity, but the series resister could be bypassed by a solid state switch or relay, after a time delay.


----------



## xrk971

Would need to flow through resistors to charge caps then bypass the resistor with a relay.

Btw, your new NHB amp should be in your mailbox.


----------



## DBaldock9

xrk971 said:


> Would need to flow through resistors to charge caps then bypass the resistor with a relay.
> 
> Btw, your new NHB amp should be in your mailbox.




There were several boxes in my mailbox today. I'm about to go open the packages now. 


*EDIT:* The NHB Amp is here, safe and sound (the Post Office did a bit of folding and spindling of the envelope, but the Amp is fine).

Now, I need to figure out which jack is the input, and which one is the output...


----------



## DBaldock9

xrk971 said:


> Optimizing for 32ohms is a good all-around deal as it won't affect performance at higher impedences and this amp can actually do well at 32 ohms with big caps and higher bias.
> 
> I will get to work on building it tonight - I have just the right pair of FETs I have been saving for this. Hot running high Idss units that will give a nice high bias current naturally.


 
  
 This one seriously great sounding NHB Amp!
  
 I've got my LZ A4 earphones connected to the output (using my new VE Espresso 2.5mm TRRS -to- MMCX cable), and the _Fixed Line Output_ of my DP-X1 driving the Amp, playing the 24-Bit / 192-KHz FLAC music files from the *Chesky - Ultimate Headphone Demonstration Disc*.  I know some (maybe all) of the tracks were recorded with their B&K Binaural Head, and the sound-stage is pretty amazing to hear with earphones. 
  
 Thanks for creating such a cool (well it's Class A, so warm really... 
	

	
	
		
		

		
			





 ) Amp!


----------



## xrk971

dbaldock9 said:


> This one seriously great sounding NHB Amp!
> 
> I've got my LZ A4 earphones connected to the output (using my new VE Espresso 2.5mm TRRS -to- MMCX cable), and the _Fixed Line Output_ of my DP-X1 driving the Amp, playing the 24-Bit / 192-KHz FLAC music files from the *Chesky - Ultimate Headphone Demonstration Disc*.  I know some (maybe all) of the tracks were recorded with their B&K Binaural Head, and the sound-stage is pretty amazing to hear with earphones.
> 
> ...


 
 You are very welcome!  So glad it got to you safely and you like how it sounds with your earphones. 
	

	
	
		
		

		
		
	


	



  
 What is impedance of your earphones?  That demo Chesky disc wih FLAC files sounds like a nice one to have.


----------



## DBaldock9

xrk971 said:


> You are very welcome!  So glad it got to you safely and you like how it sounds with your earphones.
> 
> 
> 
> ...


 
  
 The LZ A4 earphones are 16Ω, but very sensitive (120dB @ 1mW).
  
 I've switched over to my K's 500Ω earbuds, and they sound great, as well.
  
 Both of these have been Balanced cables, connected to the output of the Amp with an Impact Audio Cables 3.5mm TRS Plug -to- 2.5mm TRRS Jack adapter.
  
 The Chesky Demo Disc is a $25 download from HDTracks - http://www.hdtracks.com/the-ultimate-headphone-demonstration-disc-168033


----------



## xrk971

16ohms to 500ohms. Quite the range. Is the 3.5mm end the one that goes into the earphone? Let me know if you are still interested in running this amp in balanced mode.


----------



## DBaldock9

xrk971 said:


> 16ohms to 500ohms. Quite the range. Is the 3.5mm end the one that goes into the earphone? Let me know if you are still interested in running this amp in balanced mode.


 
  
 This is the adapter that I'm using - https://www.etsy.com/listing/387459214/impact-audio-cables-ultra-mini-adapter
  
 I would be interested in a Balanced Amp - but do you have another matched sets of transistors (which match the set in my NHB model)?


----------



## xrk971

Wow some of those cables cost as much as my amp. For balanced drive, it's important that the positive and negative phases of each channel be matched. But not critical that the left and right channels also match, as that is handled with the volume knob or level setting on each amp. So really all you need is another NHB amp. Although I can get you another one that will be close. It's just not critical. You will need balanced drive cables that terminate in 3.5mm on amp input and output. Also important is ring in is same as ring out and tip in is same as tip out. On the earphone and DAP side, it can be whatever your earphone/DAP uses.


----------



## DBaldock9

xrk971 said:


> Wow some of those cables cost as much as my amp. For balanced drive, it's important that the positive and negative phases of each channel be matched. But not critical that the left and right channels also match, as that is handled with the volume knob or level setting on each amp. So really all you need is another NHB amp. Although I can get you another one that will be close. It's just not critical. You will need balanced drive cables that terminate in 3.5mm on amp input and output. Also important is ring in is same as ring out and tip in is same as tip out. On the earphone and DAP side, it can be whatever your earphone/DAP uses.


 
  
 Would this be a Balanced Input to Balanced Output design, or could it also work as a Single-Ended Input to Balanced Output Amp?


----------



## stellarelephant

xrk971 said:


> Just to show you that the prototype system under development does indeed still fit inside the mint tin, here is a photo. It uses a 3.7v 2600mAhr LiPo cell that is 50mm x 50mm x 6mm thick.  The cell has built in smart protection circuit for over-charging, over-current, and over-depletion.  A DC-DC step up regulator takes that to any voltage but I set it at 15.5v for a good balance of long life and dynamics. A bank 2200uF 16v caps and resistors form a CRCRC filter to clean up the broadband residual switching noise.  An automatic USB charging circuit handles charging duties.
> 
> I am getting 3.5hrs per charge so far - that will also depend on your bias current setting on the amp.  The battery auto-shutdown is also tricky to deal with as the in-rush current to the caps on the amp tends to kick it into self-protect. Right now, use of a series resistor between the battery and the caps limits the current but wastes power as heat.  Similarly, the resistors in the CRC filter waste heat as well. Otherwise this amp should run close to 5.5hrs.


 
 Wow, you did it.  Is it a lot warmer?


----------



## xrk971

The current limit resistor and CRC filter resistors probably burn off an extra watt. The amp is nominally 1.5w so yes it is a bit warmer. But not hot or uncomfortable. If you think about how much heat that is for 3.5hrs it's pretty incredible the energy density in a LiPo battery. Almost like TNT per weight.


----------



## xrk971

dbaldock9 said:


> Would this be a Balanced Input to Balanced Output design, or could it also work as a Single-Ended Input to Balanced Output Amp?




The amp can work either as SE to SE or balanced to balanced but for balanced you need to give it balanced input (left and right channels out of phase by 180deg). But if in balanced connection phones and you give it SE input on one side it plays as if SE. If you give SE inputs to both sides and phones connected as balanced - the signals cancel and you get no sound (very very faint barely audible).


----------



## DBaldock9

xrk971 said:


> The amp can work either as SE to SE or balanced to balanced but for balanced you need to give it balanced input (left and right channels out of phase by 180deg). But if in balanced connection phones and you give it SE input on one side it plays as if SE. If you give SE inputs to both sides and phones connected as balanced - the signals cancel and you get no sound (very very faint barely audible).


 

 Ah, so I would need to come up with an analog SE to Balanced conversion stage, in order to drive balanced earphones using a single-ended input?


----------



## xrk971

dbaldock9 said:


> Ah, so I would need to come up with an analog SE to Balanced conversion stage, in order to drive balanced earphones using a single-ended input?


 
 I was assuming that you have a DAP or DAC that provides balanced outputs already - that is the ideal way to do it as the 180deg phase flip is done at the digital level with a simple sign inversion. Whereas doing it at the amp requires additional analog circuitry to do a SE to Balanced conversion (2 op amps). Linear Technologies makes a nice single IC one that I could add to a special balanced amp I suppose but that's a ways off.  However, you are getting away from the thesis of this amp: simple and pure 2 transistors for SE pure Class A.  Another option is a passive SE to Balanced signal transformer.  They can be very good but, impart their own harmonic distortion profile (usually H3 for transformers).  
  
 Here is such a transformer - note that it has 600ohm impedance so meant to be driven by DAC with 600ohm drive capability:
 http://cinemag.biz/output/PDF/CMOL-3x600T2.pdf
  
 Most DACs that are designed for pro-audio usage have built in balanced XLR outputs - you can use these with cable converted to 3.5mm TRS to drive the amp directly in balanced mode.  I want to get the Focusrite  2i4 for this very reason.  An example is here - look at back panel it has two balanced outputs.  The DAC in this is very good. If you don't need a portable option, it's less than typical $500 balanced out DAPs.
  
 https://www.sweetwater.com/store/detail/Scarlet2i4G2


----------



## DBaldock9

xrk971 said:


> I was assuming that you have a DAP or DAC that provides balanced outputs already - that is the ideal way to do it as the 180deg phase flip is done at the digital level with a simple sign inversion. Whereas doing it at the amp requires additional analog circuitry to do a SE to Balanced conversion (2 op amps). Linear Technologies makes a nice single IC one that I could add to a special balanced amp I suppose but that's a ways off.  However, you are getting away from the thesis of this amp: simple and pure 2 transistors for SE pure Class A.  Another option is a passive SE to Balanced signal transformer.  They can be very good but, impart their own harmonic distortion profile (usually H3 for transformers).
> 
> Here is such a transformer - note that it has 600ohm impedance so meant to be driven by DAC with 600ohm drive capability:
> http://cinemag.biz/output/PDF/CMOL-3x600T2.pdf
> ...


 
  
 Thanks for the info!
  
 The one Balanced output device I've got (currently), is my DP-X1 DAP.
 There are four separate wires, and the (L-) & (R-) cannot be connected to anything which would short them together.
 So, if the Amplifier (-) inputs are at GND, and the GND is common between channels, that won't work for the output of my DAP.


----------



## xrk971

That is an amazing DAP btw, I am jealous. 
  

  
  
 http://www.onkyousa.com/Products/model.php?m=DP-X1&class=Accessory
  
  
 I think you are fine with that DAP. Here is how you would wire it (whether ring or tip for +/- is not important, as long as consistent on output cable to phones):
  
*Amp 1:*
 DAP Output  L+ to Amp1 input Tip
 DAP Output  L- to Amp1 input Ring
 DAP Ground to to Amp1 input Sleeve (GND1 on Amp1)
  
 Amp1 output Tip to Headphone L+
 Amp1 output Ring to Headphone L-
 Amp 1 Sleeve unconnected
  
*Amp 2:*
 DAP Output  R+ to Amp2 input Tip
 DAP Output  R- to Amp2 input Ring
 DAP Ground to to Amp2 input Sleeve (GND2 on Amp2)
  
 Amp2 output Tip to Headphone R+
 Amp2 output Ring to Headphone R-
 Amp2 Sleeve unconnected
  
 Note that L- and R- are never shorted or connected as on separate circuits completely. Monoblocks essentially.
  
 If your DAP is 2.5mm TRRS connector, it looks like all you need is a 4-ring TRRS to a Y-split 3.5mm dual TRS cable?
  
 All amp connectors are 3.5mm TRS
  
 Note that GND1 and GND2 are not in common between Right and Left as they are two separate amps with separate power supplies. You have to be careful not to stack amp cases so they touch directly as case is at amp GND.  Use a silicone insulator sheet, etc.


----------



## Slater

xrk971 said:


> That is an amazing DAP btw, I am jealous.


 
  
 +1.
  
 Onkyo makes a lot of great stuff. I had a number of their old school components back in the day.


----------



## DBaldock9

BTW - What do we listen for, to know when the Amp's batteries need to be replaced (Alkaline), or recharged (Li-Ion)?


----------



## xrk971

Distortion crackling sound with regular batteries. Li-ions simply stop. The self over discharge protection kicks in.


----------



## DBaldock9

I paired the XRKAudio Class A NHB amp with my Cayin N3 and K's 500 (using an Impact Audio 3.5mm TRS Plug -to- 2.5mm TRRS Jack Adapter Cable), for a while at work today.
 Sounds great together!


----------



## xrk971

Nice setup. Looks like you are mostly an earphone type guy rather than headphones? SE Class A amps are ideal for earphones where ultralow self noise is important because the phones are so sensitive that noise / hiss / hum can be very apparent on an amp that is less than silent. 

Did your rechargeables arrive yet?


----------



## DBaldock9

xrk971 said:


> Nice setup. Looks like you are mostly an earphone type guy rather than headphones? SE Class A amps are ideal for earphones where ultralow self noise is important because the phones are so sensitive that noise / hiss / hum can be very apparent on an amp that is less than silent.
> 
> Did your rechargeables arrive yet?


 
  
 The rechargeable batteries are still in-transit, so I picked up a pair of 9V Alkaline batteries this afternoon - just in-case I need a new set, before the others arrive.


----------



## DBaldock9

I was looking around to see what sort of discrete circuitry would be required to convert from single-ended to differential (balanced), and the simplest representation consists of 3 resistors, and 2 NPN transistors.
  
 Some additional components will be needed, if operating from a single +18VDC supply, rather than a split supply - since if Vee = GND, and you GND the Base input on Q2, the circuit won't work.  I guess the additional components would set the bias voltage on the Bases high enough above GND, to flow enough current through the Collector resistors, so that with no Audio input, the voltage at Vout is ~9VDC.
  
 (From: https://en.wikipedia.org/wiki/Differential_amplifier)
  

  
  
 Two of these circuits would take the single-ended (L+ / GND) (R+ / GND) input, and produce a balanced (L+ / L-) (R+ / R-) output - which could feed the input to your XRK Class A amp boards.
  
 Currently, each of your amp circuit boards has a dual 10K potentiometer, to control L / R volume (by attenuating the input to the amp).  So, if you're using two boards, in a balanced configuration, you have to adjust two separate knobs (one for L, one for R) to control volume.
  
 It's a bit tricky trying to determine the best place to connect the potentiometer(s), if the amp can be used as - 
 Single-ended Input to Single-ended Output
 Single-ended Input to Balanced Output
 Balanced Input to Single-ended Output
 Balanced Input to Balanced Output


----------



## xrk971

Let me look into making a discrete balanced drive converter. What you show is a differential input stage which I try to avoid. Most modern amps have differential transistor inputs but they inherently cancel 2nd order distortion. BJTs also have a finite current flow at the base. If you put a pot in front of this you will get scratchy sounds when changing volume. That's because there is DC current through the pot. Notice that the pot action on my amp is noiseless. That's because of the JFET input stage. JFETs have no current flow through the gate. Perhaps there is a way to make the converter from maybe 6 of the same JFETs. 

But if your Onkyo Dap already has balanced output why not just use that? I would just set volume to max on both and have gain set identically in both amps. Volume control would.ve via DAP.


----------



## DBaldock9

xrk971 said:


> Let me look into making a discrete balanced drive converter. What you show is a differential input stage which I try to avoid. Most modern amps have differential transistor inputs but they inherently cancel 2nd order distortion. BJTs also have a finite current flow at the base. If you put a pot in front of this you will get scratchy sounds when changing volume. That's because there is DC current through the pot. Notice that the pot action on my amp is noiseless. That's because of the JFET input stage. JFETs have no current flow through the gate. Perhaps there is a way to make the converter from maybe 6 of the same JFETs.
> 
> But if your Onkyo Dap already has balanced output why not just use that? I would just set volume to max on both and have gain set identically in both amps. Volume control would.ve via DAP.


 
  
 There are some articles on audioXpress, originally published in Audio Electronics by Erno Borbely in 1999, where he shows circuits which use JFETs - http://www.audioxpress.com/article/JFETs-The-New-Frontier-Part-2


----------



## xrk971

Yes, very familiar with the Borbelly articles.


----------



## DBaldock9

If replacing the two 9V series connected batteries, with one higher capacity Li-Ion battery pack, which would be better for the amp? 
   
14.4V @ 3400mAh (68mm x 74mm x 20mm)  Uses a 16.8V charger, and has 11.2V Over-Discharge Protection. 

 18V @ 3400mAh (68mm x 90mm x 20mm)  Uses a 21V charger, and has 14V Over-Discharge Protection.
  
 I realize that neither will fit in the Altoids tin, so I'm looking for an alternative housing.
  
 Thanks!


----------



## xrk971

dbaldock9 said:


> If replacing the two 9V series connected batteries, with one higher capacity Li-Ion battery pack, which would be better for the amp?
> 
> 14.4V @ 3400mAh (68mm x 74mm x 20mm)  Uses a 16.8V charger, and has 11.2V Over-Discharge Protection.
> 
> ...


 
 The 18v one will give you better performance for sure - just make sure your case allows adequate cooling as the MOSFETs will each be disspating about 0.7W each and the resistor arrays 0.6W each or 2.6W total heat generation vs 1.8w total output when running 16v.  You might want to add a 0.47R 1W series resistor between the batteryand the amp to limit the in-rush current that may trip the battery to go into self-protect when turning on.  Try without it first of course.


----------



## xrk971

I finally called eBay and asked them what's the deal with making me wait a month to be able to list another item - got it all straightened out so I am now back on eBay (as well as Etsy).  So if you have a preference for eBay you can go there.
  
 Here is the NHB Edition listing:
  
 http://www.ebay.com/itm/-/222479147739?
  
 Here is the standard one:
  
 http://www.ebay.com/itm/222479138123?
  
 or if you prefer Etsy - options for bare PCB's etc are available there:
  
 https://www.etsy.com/shop/XRKAudio?ref=hdr_shop_menu


----------



## xrk971

dbaldock9 said:


> I was looking around to see what sort of discrete circuitry would be required to convert from single-ended to differential (balanced), and the simplest representation consists of 3 resistors, and 2 NPN transistors.
> 
> Some additional components will be needed, if operating from a single +18VDC supply, rather than a split supply - since if Vee = GND, and you GND the Base input on Q2, the circuit won't work.  I guess the additional components would set the bias voltage on the Bases high enough above GND, to flow enough current through the Collector resistors, so that with no Audio input, the voltage at Vout is ~9VDC.
> 
> ...




I'm looking into this option for a discrete Class A balanced line driver that will convert single ended input into balanced phases for driving the power amp portion in balanced drive mode. It will probably add 6 more JFETs but have the advantage that the signal will only be processed with a SE Class A zero feedback topology.


----------



## DBaldock9

dbaldock9 said:


> Just found the same set, on Amazon, for $96 - https://www.amazon.com/Hitech-Four-Lithium-Polymer-Batteries-Charger/dp/B00890HIE4
> And they've got the 2 batteries, plus charger set, for $55 - https://www.amazon.com/Hitech-Two-Lithium-Polymer-Batteries-Charger/dp/B0088R79RE
> 
> On Amazon, there's more info about the battery - and it says the nominal voltage is only 7.4V.


 

 I can report that the Hitech 9V @ 720mAh Li-Ion batteries *DO NOT* work in the Pocket Class A NHB amp.
 When the knob is turned from Off to On, there's a soft "pop" in the earphones, and the LED flashes on, then fades off.
 It's probably the in-rush current causing a battery protection circuit to activate.
 If the earphones are disconnected, and the power is switched On / Off / On rapidly, the LED lights up brighter, but the amp doesn't stay on.

 So for now, I'll just have to buy a box of Alkaline batteries.


----------



## xrk971

Sorry to hear that Dbaldock9,  I know you have been waiting for those for some time now - I hope you can return them.  You might want to try the EBL 9v Li-ion ones.  The do suffer in-rush the occasional self-protect shutoff but can be made to work reliably.  Mine actually do not suffer any auto shutdown now. It seems the more you use them, the less this happens.
  
 Here is what I use:
  
 https://www.amazon.com/EBL-Self-Discharge-Lithium-ion-Rechargeable-Batteries/dp/B00EQ3U2AA


----------



## DBaldock9

xrk971 said:


> Sorry to hear that Dbaldock9,  I know you have been waiting for those for some time now - I hope you can return them.  You might want to try the EBL 9v Li-ion ones.  The do suffer in-rush the occasional self-protect shutoff but can be made to work reliably.  Mine actually do not suffer any auto shutdown now. It seems the more you use them, the less this happens.
> 
> Here is what I use:
> 
> https://www.amazon.com/EBL-Self-Discharge-Lithium-ion-Rechargeable-Batteries/dp/B00EQ3U2AA


 
  
 I wonder how small of a series resistance could be used to keep the self-protect circuit from tripping?


----------



## xrk971

Try a 2.2ohms 0.5W resistor, then go lower, double it in parallel for equivalent 1.1ohm - the smaller, the better for battery life. Or go the other way if 2.2ohms doesn't help. If you did not want to suffer more than say, 0.5v drop across resistor and want to allow it to pass 200mA then Ohm's law (V=IR) gives you R=V/I=0.5v/0.2A)=2.5ohms. Which is why I picked 2.2ohms.


----------



## xrk971

Here's an interesting Single to differential driver. It might even work with two JFETs on the output. 



I want to avoid IC opamps but if I used one, a THAT 1646 would probably be the IC of choice. it would be simple and very transparent - ultra low distortion and the SE amps can then do their magic. 

https://cdn.sparkfun.com/datasheets/BreakoutBoards/THAT_1606-1646_Datasheet.pdf


----------



## DBaldock9

xrk971 said:


> Here's an interesting Single to differential driver. It might even work with two JFETs on the output.
> 
> 
> 
> ...




I hope the supply voltages can be scaled, since you're already using 18VDC, and it would be a hassle to also have a 35VDC battery.


----------



## xrk971

I think it can be done with same power supply voltage. Will run a simulation and see what happens.  I will also look at a balanced JFET input long tail pair (LTP) like you suggested above. I think it can be done with just 3 JFETs, two for the balanced pair and one for the constant current source (CCS).  
  
 A general question is, do you guys want a larger singel integrated balanced headphone amp with the single ended to balamced driver built in, or as an add on that allows you to use two separate existing Pocket Amps as a balanced amp?


----------



## DBaldock9

xrk971 said:


> I think it can be done with same power supply voltage. Will run a simulation and see what happens.  I will also look at a balanced JFET input long tail pair (LTP) like you suggested above. I think it can be done with just 3 JFETs, two for the balanced pair and one for the constant current source (CCS).
> 
> A general question is, do you guys want a larger singel integrated balanced headphone amp with the single ended to balamced driver built in, or as an add on that allows you to use two separate existing Pocket Amps as a balanced amp?


 
  
 That second question is the one I was about to ask you - Were you planning to make a single assembly that takes single-ended (or balanced) input, and provides balanced output - or were you thinking of a modular approach, with a single-ended to balanced driver board (with a volume pot) that could be added to a pair of standard amp boards (minus their volume pots)?
  
 Modular means you can stick with the basic design / layout of your standard amp, but a single assembly would probably be smaller.


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## xrk971

I think I am going to make an integrated balanced drive amp.  The modular approach is already within reach of anyone willing to buy two amps and drive with their DAP that has balanced outputs.  Having three units is too unwieldy.  I am going to give the integrated balanced drive more current and thus will dissipate more and will not be portable but a desktop version with about 100mA bias current on one side.
  
 Btw, here are some new NHB amp builds going on right now for *Etsy shop customers*.  They are in the middle of testing and hand-tuning at present.
  
 4.7uF Elna Silmic II inputs with 2.2uF Wima bypass and 1uF Wimas on outpiut.
  

  
 Dual 1000uF OSCON outputs with 1uF Wima bypass:

  
  
 1000uF OSCON and 470uF Nichicon AK + 1uF Wima's on output.


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## DBaldock9

> Originally Posted by *xrk971* /img/forum/go_quote.gif
> 
> ...
> 
> ...


 
  
 Do you remember which components you used on my NHB amp? I haven't disassemble it, to take a look...


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## DBaldock9

xrk971 said:


> I think I am going to make an integrated balanced drive amp.  The modular approach is already within reach of anyone willing to buy two amps and drive with their DAP that has balanced outputs.  Having three units is too unwieldy.  I am going to give the integrated balanced drive more current and thus will dissipate more and will not be portable but a desktop version with about 100mA bias current on one side.
> 
> ...


 
  
 Well, since my Onkyo DP-X1 has a Balanced Amplifier Output, rather than Balanced Line Output, I can drive my balanced earphones / earbuds directly, with up to 150mW /ch @ 32Ω.
 How much output does the NHB version of your Pocket Class A amp have, at 32Ω?
  
 I think it would be cool to have a Balanced Output amp, that I could drive from the Line Output of my Cayin N3, or other "budget" DAP with a Single-ended Line Output.
  
 Take Care,
 David Baldock


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## xrk971

dbaldock9 said:


> Well, since my Onkyo DP-X1 has a Balanced Amplifier Output, rather than Balanced Line Output, I can drive my balanced earphones / earbuds directly, with up to 150mW /ch @ 32Ω.
> How much output does the NHB version of your Pocket Class A amp have, at 32Ω?
> 
> I think it would be cool to have a Balanced Output amp, that I could drive from the Line Output of my Cayin N3, or other "budget" DAP with a Single-ended Line Output.
> ...




Your NHB amp has 1000uF OSCON + 100uF Elna Silmic II's + 1uF Wima film cap on outputs. It had Elna Silmic II 4.7uF + 2.2uF Wima film caps on inputs. The Elna Silmic II 100uF on output was reported by a member on DIYA as the best sounding in his comparison. I added the 1000uF oscon for deep bass extension. You should have the best of all worlds: Silmic for smooth mids and clear highs, Oscon for deep impactful bass, Wima for sparkling highs. 

The Onkyo DP-1X is an impressive piece of equipment and 150mA into 32ohms is a nice amount of power. But keep in mind they are not doing it via pure Class A SE topology. It's very easy to get high power with complementary push pull Class AB amps. So my simulations show max power at 60ohms is 55mW. I have not run a simulation for 32ohms but hink it's about the same or maybe a little more. So probably in range 50mW to 60mW. 

If you ever want to try Class A SE balanced drive - you just need another amp and cables.


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## DBaldock9

I'm looking at the schematic in this post - http://www.head-fi.org/t/837656/new-portable-amp-pocket-class-a-by-xrk971-now-available-as-complete-pcb/75#post_13389240
 - and some of the photos you posted above, and I'm wondering if the etch between _BATT-4_ and the _Power Switch_ is accessible enough to cut, in order to solder a couple of parallel SMT 1/4W or 1/2W Resistors, in series with the power lead, to reduce the in-rush current from the batteries?
  
 Thanks,
 David Baldock


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## xrk971

Before cutting the trace, you might want to try externally mounted axial lead resistors in line with batter leads. Easier to try - re-splice and cover up with shrink tube alter. There appears to be enough room though. You should return those batteries (Amazon right?) and order the EBL's they are proven to work.


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## DBaldock9

I think I'll just buy some of the 4-pin power connectors, to build one harness with a series in-rush current limiting resistor for the 9V batteries, and another with a connection for one of the 18V @ 3400mAh Li-Ion battery packs (it may also need a series resistor).


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## xrk971

New clear finish mint tin cases *now available.*  Here is an NHB in clear.


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## macky112

In my attempt to learn and understand the function of bypass capacitor (C2_2) I stumbled upon this YouTube video



This makes sense to me because my high school physics tells me that caps in parallel form a bigger capacitor with the sum of all their values, so I am still unclear how the smaller 1uF cap would Specifically improve the higher frequencies


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## xrk971 (May 2, 2017)

Presenting the "Silicon Harmony" a pure SE Class A speaker/headphone amp developed in collaboration with Hugh Dean (Aspen Amplifier).  It uses CFP topology with a constant current source (CCS) and has 3 actives in the signal path with a JFET input and a BJT/MOSFET complementary feedback pair (CFP) and zero global feedback.  The sound signature is just like the Class A Pocket Amp but with immense power reserves and dynamics.  As the bias current is 1.5amps and it can drive headphones (or speakers) with impedances as low as 8ohms, it can bring out the bass response form perhaps the most difficult to drive cans around.  Max power will be in the 6w to 10w at 8ohms (TBC).

I had a quick listen last night in mono (pseudo stereo) on 55ohm Status OB-1's and the sound was stunning.  The mid bass slam was unforgettable and the bass extension was head moving. I could feel the headphone frame recoiling from the movement of the diaphragm.

I will reserve final judgement once I have it up and running in true stereo. But from what I have heard so far, this amp is special. It may be he best amp I have heard.

I am still working out details on which power supply to go with: linear toroidal trafo and cap bank to get 34v or SMPS approach that will actually be lower noise (in measurement - as I could not hear any hiss or hum with my ears).  The tests shown here are with linear trafo.


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## gikigill (May 2, 2017)

How much for this one mate ☺️☺️


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## xrk971

Hi Gikigill,
It's still under development, so the PSU, enclosure, etc still has to be spec'd and built. I will have a more accurate estimate once the proto of the completed (cased and ready-to-use) amp is done.  However, as this is perhaps the premier head amp offering from XRKAudio, it will get the appropriate and well-deserved premium and top-of-the-line components and peripherals to go along with it.  It will be a sizable amp that draws lots of power and makes lots of heat.


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## gikigill

Sounds good. 
Big brash and power hungry.


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## xrk971 (May 2, 2017)

Well brash might not be the best word to describe the sound.  It's power hungry that's for sure.  The sound is smooth and liquid like in texture but capable of tsunami like bass slam and dynamics with seemingly unending reserves into the depths of sub-audio frequencies.  Very musical, engaging, natural and never sterile.  It has huge power but at the same time, plays quiet passages with zero self noise.


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## stellarelephant

Images not loading for me.  



xrk971 said:


> I could feel the headphone frame recoiling from the movement of the diaphragm.



Holy cow.  That's some bass.


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## Zyder

Sound like a monster and i am interested to get 1!


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## xrk971

Great to see some interest in the Silicon Harmony.  I will certainly keep you in mind as I get closer to a final amp. 

It's quite impressive what a big low impedance power mosfet designed for 12amps continuous and 48amps peak current can do to drive little. HP voice coils (or planar HP zig zags).


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## gikigill

Lets see what it does against the mighty HE-6. I got a second one coming so it will be interesting.


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## xrk971

Measurement FFT of Silicon Harmony driving a very heavy 8ohm load:


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## xrk971

I've been listening to the Silicon Harmony on my 55ohm OB-1's for past week.  I have to say that this may be just the best sounding headphone amp I have in my collection.  Certainly the most powerful and articulate bass ever. The sensation of experiencing Newton's 2nd Law with the headphone frame holding the driver recoiling in the opposite direction on a kickdrum is a big mind blowing.  It has to be felt to be believed.


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## xrk971 (May 17, 2017)

I just built another daily listener, this time with 470uF 16v Nichicon AK's + Elna Silmic II 100uF 16v + RVF 47uF 25v SMT output coupling caps, no film bypass.  On inputs I am using usual 2.2uF Wima film, but adding no-name 10uF 100v rated electrolytics for deep bass and low distortion due to the high voltage rating.  Using a white LED this time plus an antique "Made in Britain' Altoids tin.

I had actually gone a week without a Pocket Class A amp (all stock either sold or given away as presents) and was using my Fiio A5.  Having built this one up and listening to it - boy what a difference - it's like the music is alive once again.  There is a definite improvement in sound quality.


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## DBaldock9

What's the Brand & P/N of the Volume Pot, with the "D" shaft?


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## xrk971

It's an Alps 10k pot with integrated power switch.  

Mouser part number 

RK0971221Z05


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## DBaldock9

Thanks, I just received an order from Digi-Key, which had some Bournes pots, with the knurled/ridged shaft. I also got a couple of conductive plastic pots with "D" shafts, but they're larger than the Alps & Bournes pots.


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## xrk971

I just upgraded to some 10uF 35v Elna Silmic II's on the input caps. 

Sweet sounding!


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## xrk971 (May 30, 2017)

Who says a SE Class A pre-amp/desktop amp needs to be big?

Here is the Pocket Class A outfitted with an internal 1.2MHz DC step up and RCRCRC filter (0.47R / 2200uF 25v) and 5.5mm jack for a 12v SMPS input. It is serving as the main pre-amp for my 100w Class AB power amp system.  Here it is next to an XMOS U8/BB PCM5102A DAC. Besides providing volume control, it gives the system a wonderful SE Class A sound and has ability to drive 10v p-p so enough to get any amp to go into clipping.

I have the DC step adjusted to provide 17.0v after the RCRCRC filter.

Makes the stereo amp and speakers sound lively and engaging.  The power amp, a Vzaudio VHEX+ and is a DIY dual mosfet output stage amp running from +/-53v PSU with very low HD levels.


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## xrk971

The review of this amp by Headfonics.com just came out:

*http://headfonics.com/2017/06/the-class-a-nhb-pocket-amp-by-xrk-audio/*


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## gikigill

For once I unequivocally agree with an audio review. The musicality is something that's very hard to find.


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## xrk971

Thanks Gikigill.  I am glad you agree


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## gikigill

II have been so busy listening, I'm not even ready to send it back for the modifications I proposed

One of these days, I'll send it.....


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## xrk971

I know how that goes. Take your time but I do think you will benefit tremendously if the caps are upgraded to 1000uF Oscons to give you more bass extension.


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## gikigill

Its clean, crisp, musical and very soothing to listen to. The background blackness rivals the mighty Beta 22 and that's saying something although it falls short of the body and muscle in the sound overall compared to the Beta 22 but then again, there aren't too many amps that can keep up with a Balanced B22. The NHB tries it's best but it's a little small compared to the Beta 22. Both were compared with the TH900 as it doesn't need a lot of power.

Now imagine a grown up version of the NHB with the Beta22 specs and it would be something.

Xkr971 has a real shot at making a once in a lifetime item here. A grown up NHB could be a worthy successor to the Beta 22.

4 Board balanced with an outboard power supply would be good.


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## xrk971 (Jun 3, 2017)

Gikigill,
Do you think the big amp would need a single ended to balanced drive converter?
Four SE Class A amp boards each burning 30watts is a full size chassis. Perhaps maybe even 50w dissipation ea board.  Depending on what you want to drive.  The SE Class A topology sounds glorious but comes at a price of thermal inefficiency. It will be able to do about 1w into 50ohms at reasonable distortion levels. For HE-6, I would go with singleton input stage Class AB quasi-complementary hybrid MOSFET-BJT output stage for delivering  up to 10w into 50ohms with maybe 15w dissipation per board. 

The Silicon Harmony may just be the ticket qnty 4 to provide balanced drive (ignore puny heatsink it is mounted on for testing purposes - way too small):






The SE Class A four board balanced amp would need a case like this:


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## gikigill

How about a separate board summing up to both balanced and single ended. 

A Pic of the Beta22. Not mine but similar to mine.


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## gikigill

At the risk of sounding like a philistine, how about low rpm fan running at say 800-1200rpm.

Krell uses them in their monster amps so maybe that might be something that can be used to facilitate heat dissipation. Just a fixed fan running at 800 or so rpm quietly moving air out of the sides.


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## gikigill

Specs for the Beta22. 

www.amb.org/audio/beta22/


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## xrk971 (Jun 3, 2017)

I use speed reduced fans all the time to keep heatsinks reasonable in size.   But for products aimed at high end low noise - some people sneer at such things even though they are very quiet.  Simple 100ohm resistor in series with 12v fan really slows it down to silent. There is a risk that fan can malfunction. In which case an auto shutoff due to heat is needed.

I was reading the description of the Beta22 and it says it is fully Class A and capable of 18w into 8ohms or 50w into 8ohms balanced.  How can Class A be achieved with single pinner local heatsinks rated for maybe 3w ea? A Class A amp capable of 50w into 8ohms would normally have heatsinks the size of a 5U Dissipante case.  Huge in other words. 

Maybe its Class A for first 2watts and goes into Class AB after that?


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## gikigill

Its pure Class A, just checked the specs again and outputs 18w at the headphone jack.


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## xrk971 (Jun 4, 2017)

I understand that is what the spec says but I don't believe it is possible. Because to make 18W Class A means you need to dissipate about 43w of power continuously.  There is no way a locally mounted little heatsink on a TO220 MOSFET can dissipate heat like a fin on large case, which is what is supposed to be used to keep temps below 55C. Or the Beta22 runs with temps that are much hotter thereby risking premature failure. How hot do the little heatsinks get? Can you touch them for more than 10seconds?

Here is a 25w Class A amp (F5). Notice size of heat sinks and this is for just 2 MOSFETs per channel.






Compared to this for 18w (or 50w in balanced - which means it needs to dissipate twice as much as above Amp):





In reading the detailed technical description of the Beta22 it says this about the power dissipation:

"

For headphone amp application, I = quiescent current of the output stage, typically 160mA. For speaker amp application, I = output current. To determine this current, we use the following formula (assuming a load impedance of 8Ω): 

I = √(Po / 8) 

Where Po is the output power of the amplifier. For ±30V rails, this would be 18W, and for ±24V rails, this would be 12W. Note that this gives the _continuous_ current assuming that the amplifier is playing a sine wave at maximum output all the time. When playing real music the average current is a lot less, you can safely use 30% (which corresponds to about -10dB) of the calculated current, as long as the amplifier is only to be used in a home audio application."
So they are saying the amp dissipates more heat when playing sine waves than when playing music. This is clearly Class AB behavior as Class AB amps run much cooler and only get hot when delivering a lot of power.  Also, the quiescent current is only 160mA.

True Class A amps dissipate the same amount of heat no matter what is playing and in fact, the heat dissipated by the heatsinks is *less* when the speakers (or phones) are producing a lot of power.  That is basically the definition of Class A.   They burn hot no matter what.  In addition, quiescent current on Class A amps like the Silicon Harmony or F5 is about 1250mA (not 160mA).  Max output current swing can be no more than 2-3x the quiescent current. So if you have cans that need current - a Class A amp with 1.25amps can swing 2.5amps easily.

160mA quiescent current x 60v (both rails) = 0.16amps x 60v = 10w dissipation in heatsinks. With 4 locally mounted heatsinks this is 2.5w ea.

So, I am not saying that Beta22 is not Class A, but probably runs Class A within the 160mA bias range. For 30v rails, that is about 3W max in Class A before switching to Class AB. Note that being a fully symmetric circuit it will reduce all H2 (and even orders) and emphasize H3 and H5 distortion.

If you like H3 and H5, H7, ...


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## xrk971

This latest bad boy NHB is shipping out to Poland tomorrow.  It's got dual 1000uF OSCONs and 10uF Silmics on the output and Silmics on the input.  Uber hand tuned and measures 0.02%THD driving 270ohm load at 700mV with almost all of it H2.  The huge output caps are so that it can drive the user's favorite 13.8ohm 115dB sensitive Andromeda's.  Sim in LTSpice shows THD is only 0.05% at listening level of 106dB. Frequency response is flat to 15Hz (-0.5dB point).

So ultra-low impedance is not a problem if you have very high sensitivity.


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## Carmantom

xrk971 said:


> This latest bad boy NHB is shipping out to Poland tomorrow.  It's got dual 1000uF OSCONs and 10uF Silmics on the output and Silmics on the input.  Uber hand tuned and measures 0.02%THD driving 270ohm load at 700mV with almost all of it H2.  The huge output caps are so that it can drive the user's favorite 13.8ohm 115dB sensitive Andromeda's.  Sim in LTSpice shows THD is only 0.05% at listening level of 106dB. Frequency response is flat to 15Hz (-0.5dB point).
> 
> So ultra-low impedance is not a problem if you have very high sensitivity.


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## Carmantom (Jun 8, 2017)

This is my short review of my new " Pocket Class A Headphone Amplifier / Desktop Amplifier NHB Edition "  Made custom by xrk971  Let me tell you I had a great experience with this purchase. This is one of those rare purchases where you get some of the man who built the product as well as this wonderful and magical amp.

Let me start by saying my review is only of my listening enjoyment and how I hear it. I have absolutely no idea of any technical aspects or any parts included with this build. He is welcomed to post any of the specs if he likes.

xrk971 asked for my listening preferences and gear I was using. It is Fiio X5 gen 3 DAP and Audeze LCD 2 headphones. Although many reviewers indicate the Fiio X5 gen 3 to be plenty strong to drive the Audeze LCD 2s, I did not. It seemed to me to be so much more there that wasn't being revealed.  One day recently while trolling on Head Fi I came across what looked like a little jewel if 1/2 of what was being said about it was true.

I decided to try it and boy am I glad I did.  xrk971
kept me up to speed continuously.  He custom built it to my hardware and listening specs and he sure delivered. All in all from time I ordered, he built, shipped and delivered it was only about 5 days.

Short and sweet, these are my impressions.  As always I try to remember to listen clean and mastered recordings. In this case I listened to the DSD recording of Christian Mc Brides " Live at Village Vanguard "  This would be a sure workout for the bass.  I'm a fan of " Steve Hoffman " masterings. I listened to some of his " Dreamboat Annie  "  Pee Wee Russell and his clarinet. And others.

Please be aware, this amp is very unforgiving in its reproduction of poor music source. This is clear evidence of not trying to color but to convey music as artist intended.

The sound stage was huge and wide.  Most folks know LCD2S are well know for bass. The base this amp gave my setup was nothing less than articulate, good transit response,  natural and detailed. I am a big believer in " It's not how low it goes. It's how well it goes low " .  The speed of Mc Brides fingers up and down his bass is accurate. Who would ever believe this box could deliver so much bass.  Next up instrument imaging and placement very distinct in height, width and depth. However vocals more up front in my face than I prefer. This may be unfair since I did not listen to very many recordings yet.

I could not believe the musicality this magical amp would provide me with. Here's how it was for me laying in my recliner listening Lincoln Mayorga and Distinguished Gentlemen on the Sheffield lable. " Limehouse Blues " toes are tapping relentlessly, shoulders rolling in and around and head rhythmically swaying side to side.

I found mids to be airy not congested. The highs are accurate and warm with some sparkle. Not as much sparkle as I prefer.

All in all this is a gem of an amp and great value at that. I have had amps costing much more not deliver nearly as much.

The man, the amp and the music are all delivered in this amp.


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## xrk971

Hi Carmantom,
Thank you for the detailed listening impressions. So glad the amp worked out well and was able to drive your LCD-2's! Your review and impressions are not unlike others that I have heard from others regarding the musicality, bass articulation, to the sound stage and imaging. One really important technical detail about this amp is that it uses only two transistors! There is something to be said for having the audio signal pass through as few as possible to keep the noise low and the purity of the signal clean. If you have very sensitive IEMs, you will find that the self noise of this amp is truly the blackest of blacks - inaudible. 
Cheers,
xrk971


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## alpha421

Damn it! Waiting for the batteries to charge to test out the amp (low gain version, purchased from Funch).


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## xrk971

If you are in a bind and want to hear it, plain old 9v from drugstore works. Just cheap carbon cells from dollar store works for a few hrs while you wait.  Alkalines work nicely too.

When using Li-ion 9v's or Lipo 9v's note that they have a smart charger chip inside that sometimes cuts off power when first turned on as the in rush current to charge the two large 2200uF caps is seen as a short circuit.  Click power on off several times until LED stays lit brightly.

The gain is all the same so not sure by what you mean by "low gain version" from Funch.  Maybe lower value capacitors?


----------



## Carmantom

xrk971 said:


> Hi Carmantom,
> Thank you for the detailed listening impressions. So glad the amp worked out well and was able to drive your LCD-2's! Your review and impressions are not unlike others that I have heard from others regarding the musicality, bass articulation, to the sound stage and imaging. One really important technical detail about this amp is that it uses only two transistors! There is something to be said for having the audio signal pass through as few as possible to keep the noise low and the purity of the signal clean. If you have very sensitive IEMs, you will find that the self noise of this amp is truly the blackest of blacks - inaudible.
> Cheers,
> xrk971


Absolutely true about the black.  Sorry I failed to mention how quiet it is.  I am truly impressed with this amp.  I also made the changes you asked for.


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## alpha421

xrk971 said:


> If you are in a bind and want to hear it, plain old 9v from drugstore works. Just cheap carbon cells from dollar store works for a few hrs while you wait.  Alkalines work nicely too.
> 
> When using Li-ion 9v's or Lipo 9v's note that they have a smart charger chip inside that sometimes cuts off power when first turned on as the in rush current to charge the two large 2200uF caps is seen as a short circuit.  Click power on off several times until LED stays lit brightly.
> 
> The gain is all the same so not sure by what you mean by "low gain version" from Funch.  Maybe lower value capacitors?



Possibly.  Funch was selling a high gain and this low gain.  Yes, he relayed me the info of turning on/off the pot quickly a few times with the Li-ion 9Vs'


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## xrk971 (Jun 9, 2017)

alpha421 said:


> Possibly. Funch was selling a high gain and this low gain.



I have competition from Funch it seems 

Let me know if you are interested in a hand-tuned NHB model. The tuning uses a resistor network that is different than run of the mill standard amps and makes a difference.  Anyone can roll caps and match FETs, but the tuning is key to the upgraded sound.

If you read the review by Michael on Headfonics.com - he noticed a substantial improvement with the NHB.

http://headfonics.com/tag/xrk-audio/


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## alpha421

Temping.  Other than the loud initial pop, this is an outstanding sound.  Is the pop inherent to the design? I've own desktop amps that produced a loud pop.  I made a dummy resistor plug to absorb the pop before I connect the head/earphones.

Kudos to your design.  I listened with my Grado and although it's been a while since I had a Melos amp, the dynamics and rich tone had me with a flashback.


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## xrk971 (Jun 10, 2017)

Hi Alpha421,
Turn on pop is something I avoid by turning on before inserting phones. It can be avoided electronically with a delay timer and relay circuit. A lot of added complexity and cost.  Moreover, it takes up valuable real estate that is used for larger caps. I have a version with turn on pop suppression - it works wonderfully but with 2 extra TO92 MOSFETs, a TO92 7812 regulator, two small caps, a resistor, added 4mA current Drain, a gold/iridium contact low current relay, so the board space is limited to a single output capacitor due to space limitations (so max is 470uF). In my opinion, not worth the added complexity and loss of real estate.

What caps are you using now?


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## alpha421

I tried waiting for a minute or two before inserting my headphones, and still got a pop.  No biggie as the dummy plug works like a charm.  The caps installed on the board, assuming, they are the set on top of the board facing up are brown colored Elna (about the size of a tic tac).


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## xrk971 (Jun 10, 2017)

There shouldn't be any pop after 2 seconds. Strange that you get it after such a long time. Basically it is the output cap charging up.

It sounds like Funch may have opted to not install any dummy load resistors on the board. There is provision for two and I usually put 270R there. If you open board you will see two labeled RL near the output jack. Install two 0805 270R there. Or at least 680R or 1k if you are afraid of it pulling down impedance any lower. That will discharge the caps on power up.  

The tic tac Elna is on the SMT (visible side) is just a 10uF bypass. The big one are hidden underneath.


----------



## DBaldock9

DBaldock9 said:


> I can report that the Hitech 9V @ 720mAh Li-Ion batteries *DO NOT* work in the Pocket Class A NHB amp.
> When the knob is turned from Off to On, there's a soft "pop" in the earphones, and the LED flashes on, then fades off.
> It's probably the in-rush current causing a battery protection circuit to activate.
> If the earphones are disconnected, and the power is switched On / Off / On rapidly, the LED lights up brighter, but the amp doesn't stay on.
> ...



Just for fun today, I attempted once again to see if I could get the Hitech 9V @ 720mAh Li-Ion batteries to work in my XRK NHB Amp:

First - With the earbuds disconnected, I tried turning the power switch On / Off / On rapidly - many, many times - but I'm still seeing getting the same result - the LED lights up brighter, but the amp doesn't stay on.

Second - I tried leaving the power switch On, and unplugging / re-plugging one of the batteries - and the amp came on and stayed on!
So, then I plugged in my TY Hi-Z HP-650 earbuds, and I'm listening to my Roku 4 Net Streamer, playing ThistleRadio through the XRK NHB amp, running on the Hitech 9V @ 720mAh Li-Ion batteries.  

The analog audio is being decoded from the Roku HDMI output, using one of these - http://www.ebay.com/itm/HDMI-Dolby-...Digital-Audio-Converter-Decoder-/230840436904 - but I'm changing the setup of my audio system, by adding a 4x4 Matrix SPDIF (Toslink) switch, with inputs from my PC, Roku 4, VCR/DVD, and a stand-alone ADC; and outputs going to my Micro iDSD (feeding my powered monitor speakers & sub-woofer), a Bluetooth transmitter, and a FiiO D3 DAC (to connect to amps, like the XRK NHB, Walnut v2, and Zishan Z1).

Since the amp powered On when the battery was unplugged / re-plugged, I'm wondering if there might be a inrush current carrying issue with the power switch part of the Volume Pot?


----------



## xrk971

The little power switch on the Alps pot is not designed to carry huge continuous current but a turn on transient shouldn't be a problem. It's rated for 10,000 cycles and 3amps at 16v. Proof tested at 300vac for 1min. 

The part specs are here:
http://www.alps.com/prod/info/E/HTML/Potentiometer/RotaryPotentiometers/RK097/RK0971221Z05.html


----------



## DBaldock9

xrk971 said:


> The little power switch on the Alps pot is not designed to carry huge continuous current but a turn on transient shouldn't be a problem. It's rated for 10,000 cycles and 3amps at 16v. Proof tested at 300vac for 1min.
> 
> The part specs are here:
> http://www.alps.com/prod/info/E/HTML/Potentiometer/RotaryPotentiometers/RK097/RK0971221Z05.html



I had read the specs, and was a bit concerned about using one switch to control power for a pair of amps, and the THAT 1646 circuitry, as I'm assembling a balanced output amp model.
But for single amp operation - I can't figure out what's causing these batteries to work fine when one is unplugged / re-plugged vs. leaving the batteries connected, and switching the power On with the switch?


----------



## DBaldock9

I wish I had noted the exact time that I got the batteries to work with the amp this afternoon, but they just stopped - after 4 or 5 hours of operation, driving my TY Hi-Z HP-650 earbuds.
The way these particular batteries "run down", is to cause the amp to make what to me sounds like someone hitting an Irish Bodhran drum (a percussive thumping sound, almost in-time to the music), for about 10-15 seconds - and then silence.


----------



## xrk971

My amps simply stop making sound suddenly. No warning and it happens because the internal autoshutoff circuitry on the batteries kick in to prevent over depletion. This is with EBL batteries.  

Why yours work when one is in first and then add the second one could be that the current in series when suddenly turned on is too much. But half as much when one is in? But batteries are in series so no current flows with one disconnected.  So very strange.


----------



## xrk971

Another NHB amp tuned for LCD-2 just shipped to a member in Singapore.






Sounds great and measures superbly well:


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## xrk971 (Jun 17, 2017)

Silicon Harmony is getting closer to being a real amp.  It's in a nice Class A case now - heatsinks are a cool 50 deg C.  Running of a 19.5v SMPS brick which is then stepped up to 30v. Wonderful sounding amp. The most powerful bass I have felt. 

Capable of about 6w of pure SE Class A power into 8ohms.


----------



## xrk971

I connected Silicon Harmony to my Focsurite DAC (capable of 192kHz at 32bit), and played a high res 176k FLAC file of Spyro Gyra's Dreams of Toledo. Wow. The better the source, the better it sounds. Here is the FFT profile of this amp into 33ohms. Predominantly H2 distortion at -66dB and H3 at -90dB. Not much else. There is a bit of odd noise sneaking in from one of the DC step ups which may be going bad as I can hear a slight whining noise from it. Will change those out shortly for some new ones with a proper heatsink.



 

FFT for 33ohm load:


 

I love the dynamic range of this amp. From the blackest blackest blacks (no noise when no music playing even at full volume - cannot tell it is turned on), the how hard it hits you when the headphone frame recoils on a kick drum.


----------



## gikigill

EWhat's the output at 50ohms?


----------



## xrk971 (Jun 18, 2017)

You mean power at 50ohms?  I am listening with 55ohm headphones but they are 96dB and it can get painfully loud if not careful with volume knob.  Let me rig up a 51ohm power resistor and measure the max output.

Edit:  Ok, just measured the max power into 51 ohm 2w power resistor. With O-scope attached to show unclipped, max is 17.1\7v peak-peak or 6.28vac as shown on Fluke.  That's about 800mW into 51 ohms or 5w into 8ohms - per design spec.  Clipping limit could be increased but requires running amp at higher voltage and case heatsink should not be pressed for more than circa 32w/channel as this current configuration at 29v Vcc is giving 55 deg C heatsink temps.  The bias current is currently at 1.1amps and can be adjusted as high as 1.5amps to increase power, but at the expense of higher heat output.

If you think 800mW into 51ohms is not much, well it's pure SE Class A power derived from burning 32w in the heatsinks.  2.5% thermal efficiency is very Class A manly-man power


----------



## gikigill

Sounds tempting!


----------



## xrk971

I have had it on and running three days straight to burn in and make sure all is well from reliability stand point. I am adding local heatsinks to the power supplies to further enhance their reliability. The PSU auto detects for 110/220 vac so good for using anywhere.


----------



## xrk971

gikigill said:


> Sounds tempting!



Yes, the Silicon Harmony is the Class A temptress from the Audio Underworld!


----------



## Lucas_ob

your AMP REALLY cute


----------



## xrk971

Lucas_ob said:


> your AMP REALLY cute



Which one, the little one or the big one?


----------



## xrk971

Silicon harmony with internal SMPS and neater wiring.


----------



## xrk971

Sounds wonderful with good full range speakers too.  Playing in the speakerlab: on the left is an XKi (Xrk's Karlson ab-Initio 6th order bandpass) with Tang Band W5-2143 and on right is tapered TL with Fostex FF105WK.  Both are circa 89dB sensitive drivers with cabinets that give them bass extension down to about 55Hz,





Also note that you can plug in headphones in addition to listening to your 8ohm speakers simultaneosuly. It can handle it.


----------



## xrk971

Silicon Harmony finally makes it out of the lab and into the house.  Here it is paired with a Focusrite 2i4 2gen as a DAC.  Currently listening to high res FLACs with Status OB-1's.  Superb sounding combo.


----------



## xrk971 (Jul 1, 2017)

Single Ended (SE) pure Class A topology single to balanced converters that now enable two Pocket Class A amps to power balanced drive headphones.  Converters use premium Wima film and Elna Silmic II caps. Capable of driving 10v peak to peak from 48v single rail. Super low distortion for a very transparent insertion in audio chain, yet retains sonic character of the Pocket Class A amp.


----------



## DBaldock9

xrk971 said:


> Single Ended (SE) pure Class A topology single to balanced converters that now enable two Pocket Class A amps to power balanced drive headphones.  Converters use premium Wima film and Elna Silmic II caps. Capable of driving 10v peak to peak from 48v single rail. Super low distortion for a very transparent insertion in audio chain, yet retains sonic character of the Pocket Class A amp.



I'm very interested in this particular project!


----------



## xrk971 (Jul 1, 2017)

Hi Dbaldock9,
I thought you would like this.  I'm going to modify my OB-1's tonight to be balanced drive. Then have a test listen in stereo.  From the sims and oscope traces it should sound fantastic.  Another Hugh Dean (Aspen  Amplifier) design and collaboration. I was originally just going to make an all inclusive desktop amp with this converter and two pocket amps all on one large PCB. But it seems there may be interest in a standalone converter.  Four 9v batteries will give about 32v to 35v rails which should be more than enough for most portable uses.  It won't fit in an Altoids tin though. Bias current through the two BJT's is less than 7mA so 4 x 9v (600mAhr) batteries should run for a long time for 14mA combined - circa 40hrs.


----------



## DBaldock9

xrk971 said:


> Sounds wonderful with good full range speakers too.  Playing in the speakerlab: on the left is an XKi (Xrk's Karlson ab-Initio 6th order bandpass) with Tang Band W5-2143 and on right is tapered TL with Fostex FF105WK.  Both are circa 89dB sensitive drivers with cabinets that give them bass extension down to about 55Hz,
> 
> 
> 
> Also note that you can plug in headphones in addition to listening to your 8ohm speakers simultaneosuly. It can handle it.



Back in the '90s, one of the companies I worked for, sent a group of us Techs to a training class taught by Sy Levine [ http://theconversation.com/profiles/sy-levine-126969 ].
During the break between sessions, the subject of loudspeakers came up - and he mentioned having some original Karlson enclosures.
I don't remember now, whether he said that he had known John Karlson, or had just bought the speakers because of the unique design.


----------



## xrk971

Karlson's are unique speakers. They are very efficient and have a clean dynamic sound. Wide dispersion via acoustic lens.  I happen to be a Karlson fan and user. I have even developed a computational model of the Karlson and it's variants in Akabak (acoustic modeling and simulation software package). 

Here are my threads in DIYA on the Karlson and variations (Karlsonator, Kazba, and XKi):

http://www.diyaudio.com/forums/full-range/237948-speaker-kicks-butt-large-spaces.html

http://www.diyaudio.com/forums/full-range/239338-mini-karlsonator-0-53x-dual-tc9fds.html

http://www.diyaudio.com/forums/full-range/271011-rockin-kazba-dipole-k-aperture-z-baffle-dipole.html

http://www.diyaudio.com/forums/full-range/268524-xki-xs-ab-initio-karlson-6th-order-bandpass.html


----------



## DBaldock9

xrk971 said:


> Hi Dbaldock9,
> I thought you would like this.  I'm going to modify my OB-1's tonight to be balanced drive. Then have a test listen in stereo.  From the sims and oscope traces it should sound fantastic.  Another Hugh Dean (Aspen  Amplifier) design and collaboration. I was originally just going to make an all inclusive desktop amp with this converter and two pocket amps all on one large PCB. But it seems there may be interest in a standalone converter.  Four 9v batteries will give about 32v to 35v rails which should be more than enough for most portable uses.  It won't fit in an Altoids tin though. Bias current through the two BJT's is less than 7mA so 4 x 9v (600mAhr) batteries should run for a long time for 14mA combined - circa 40hrs.



Is this circuit, which operates on twice the supply voltage of the NHB Class A amplifier - just an input / driver to connect to a pair of NHB boards, or is it a whole stand-alone Balanced Amp?
Also, is it one additional circuit card per pair of NHB Amps, or is it one new card for each amp?

If this new circuit is meant to drive a pair of NHB Amps, how many 9V batteries are going to be needed?
Is it 4 for the driver, and then 2-each for the amps?

I'm still awaiting the 18V @ 3400mAh Li-Ion Battery Pack & Charger that I ordered from SuPower Battery (there was a mix-up by the freight forwarder here in the USA, and they misdirected 3 packages).
If I had known that the input circuit would need twice the voltage, I would have ordered a pair of the battery packs.


----------



## xrk971

I am wiring up a 4 x 9v battery pack now and just tested it.  Works well to power them.  You need two converter boards for stereo. And two NHB amps.  Basically one converter board and one NHB amp per headphone can.  This just provides the NHB with balanced (180 deg out of phase pair of signals). The left and right of one NHB drive the +ve and -be phase of the headphone diaphragm in opposition. 

I only came up with this circuit in past week and had no idea I would need 35v to 48v to drive it.  Four 9v batteries is the cleanest way to do it. Very low noise and 40hrs is great life.


----------



## xrk971 (Jul 1, 2017)

First sound from Pocket NHB's and balanced drive converter. 




It was a pain but I rewired my OB-1'a for balanced drive using two 3.5mm stereo jacks (taken by cutting a 3.5mm male-male cable in half) and then connecting that to the 4 wires from the OB-1 (luckily it is a 4 wire headphone and not 3 wire). I learned how to solder Litz wire for the first time. 
	

	
	
		
		

		
		
	


	




So I have the single to balanced converters running of a stack of 4 rechargeable 9v EBL batteries for a Vcc of about 34v. Max output swing is 25% of that minus 1v or 7.5v p-p. More than enough to hit max SPL after 12dB gain on the power amps.

Source is the DAC from Focsrite Solo 2G playing high res FLACs. Power amps are NHB's running about 67mA bias with the works. Red amp is Right channel and Green amp is Left channel.

First impressions... WOW!!! It's a completely different ball game. The sheer dynamics, depth, crescendos, all kill a non balanced drive amp/phone combo. It appears to be much louder - probably 12dB? Volume knob is used sparingly.

First track played was Nils Lofgren's "Keith Don't Go" a song know well. The attack on the acoustic guitar is incredible.

Next up are some well tested tracks and all are amazing sounding. It sounds like a full blown desktop amp even though they are pocket amps.

Very very happy with the result. This is going to get made into a PCB for sure and probably is the one thing one can get to really change your listening sound quality to a whole 'nother level.


----------



## DBaldock9

xrk971 said:


> First sound from Pocket NHB's and balanced drive converter.
> 
> It was a pain but I rewired my OB-1'a for balanced drive using two 3.5mm stereo jacks (taken by cutting a 3.5mm male-male cable in half) and then connecting that to the 4 wires from the OB-1 (luckily it is a 4 wire headphone and not 3 wire). I learned how to solder Litz wire for the first time.
> 
> ...



So, with the NHB amps running from their own pairs of 9V batteries - you're using a total of 8x 9V batteries?
Currently, I've only got 6x of the 9V batteries, and two dual battery chargers (1 at home, 1 at work).
How many batteries & chargers do you have?


----------



## xrk971

I have a lot of batteries and chargers.  Probably 16 and 3 chargers.  I have them on hand because some customers can't get their own in their country.  This was just a test and by no means represents how this would be run in its ultimtate configuration.  The phase converter is a relatively low current draw and there are plenty of IC's made for generating 48v phantom power via charge pumps.  But really I think balanced drive is ultimately best realized with a new PCB that has everything on it: 2 phase converters and 2 amps, plus the necessary power supply system so that a LiPo battery can be used. But for now, it takes 8 batteries to power.


----------



## xrk971

Some more notes...

1. After a little more listening and tests I am finding that the interconnects between the phase splitter driver board and the power amps is critical. Use of 3.5mm stereo jacks is prone to noise as jiggling it can result in changes in background noise and a lot of popping if a disconnect occurs. Balanced drive is very very sensitive - essentially increases sensitivity of the headphones and makes them very resolving of low level detail. So I think this may not be practical to do with removable interconnects between balanced driver and power amp stages. 

2. Because the output of the phase splitter is already capacitively coupled, there is no need to add more caps on the input of the power stage. Or vice versa, one can removed the caps on the phase splitter output stage.

3. If carefully matched FETs such as that used in the NHB are employed, and the bias of both +ve and -ve channels on each amp is carefully matched, then one can forgo output caps altogether as the voltage on each side will balance out for net zero current flow through the voice coil. Alas, about 7v DC will be sitting on each side of the voice coil but net DC offset of order 0mV to 5mV it can work. However, an uncomfortable proposition it will be should one side be inadvertently become unbalanced due to interconnect jostle or failure. But there will probably be increased resolution and dynamics possible in a DC coupled output configuration. It may be Ok to test and experiment with full knowledge one could destroy a set of cans with one false move. Hmm... I think I will leave the output coupler caps in place 
	

	
	
		
		

		
		
	


	




I think this will be best implemented as a new fresh *integrated* PCB with all circuits on one board with small tight traces, full use of SMT components for the input phase splitter BJT's (vs TO92 through holes that I am now using). A delay turn on relay and possibly DC detect and protect output cut-off can also be added. This is all a lot of work though... but the resulting desktop amp can be very glorious sounding.

I may even switch to a CFP BJT topology for the output stage which will give even more resolution and better dynamics.
Attached Thumbnails


----------



## xrk971

Presenting a prototype PCB of the HyQu amp - a Class AB hybrid quasi complimentary amp with the ability to drive low impedance cans with authority while providing the harmonic distortion profile that resembles a low distortion SE Class A amp.  Here is the first sound test of the amp with a single rail 24v power supply.  Very dynamic, powerful, transparent sound with super resolution.


----------



## xrk971 (Jul 22, 2017)

Now in stereo. Wow!


----------



## DBaldock9

xrk971 said:


> Now in stereo. Wow!



Is the green inductor part of the power supply voltage step-up circuit?


----------



## xrk971

The inductor is in parallel to a power resistor and is part of the Thiele network to ensure unconditional stability into any load.  This is standard practice for Class AB amps with some global feedback (although very small as feedback amps go).


----------



## jonmbarlow

Ordered the uber NHB with rechargeable batteries.....really looking forward to the sound that this little device is reportedly capable of


----------



## xrk971 (Jul 23, 2017)

Measurements of the HyQu are very nice.  1.41vrms into 270ohms shows 0.003%THD with 28ppm H2 and about 9ppm H3 and monotonically descending higher orders. Flat -130dB noise floor (this is running of AC mains).  Sound is exceptionally detailed and resolving.  Data shown for 24v Vcc and 95mA bias current.


----------



## DBaldock9

xrk971 said:


> The inductor is in parallel to a power resistor and is part of the Thiele network to ensure unconditional stability into any load.  This is standard practice for Class AB amps with some global feedback (although very small as feedback amps go).



Is the power resistor in series with the output?
What is the output resistance of this amp model?


----------



## xrk971 (Jul 23, 2017)

This amp has a topology similar to its big brother - a 50w/ch into 8ohms Class AB Quasi Complementary amp shown here.  The headphone amp is different in that all the transistors are different along with DC setpoints, and it uses a single rail supply with capacitive output for headphone safety.

The output Thiele network consists of a 10ohm power resistor in parallel with a 1.5uH inductor and those are in series with the output. They serve to suppreas an oscillation should an instability arise if the outputs are connected to a nasty reactive load.

I will have to run some simulations to see what the output impedance is. I suspect very low as it can drive a 16ohm load no problem.


----------



## xrk971

I just ran the LTSpice simulation to calculate the output impedance of the HyQu and got Zout=0.183ohms. Pretty low so ability to drive low impedance phones ois definitely there.


----------



## xrk971

New review of the Pocket Class A amp on AudioPrimate. 

https://audioprimate.blog/2017/07/24/pocket-class-a-diy-headphone-amp-by-xrk971/


----------



## DBaldock9

The 18V 3400mAh Li-Ion Battery Pack that I ordered from China, isn't going to be delivered (due to new shipping restrictions for Li-Ion batteries coming into the USA) - the vendor refunded my purchase price.

So, now I'm looking at Li-Ion Battery Packs from vendors in the USA.  
I've found an 18.5V, 2600mAh, 48Wh pack, that can supply 7A peak current, and 5.2A of continuous current.
It has a Low Voltage Cutoff of 12.5V.

What is the series voltage of your two EBL 9V Li-ion batteries at the time that a Pocket Class A NHB Amp shuts off?


----------



## xrk971

That's too bad. You could do the trick I suggested which is to use a DC-DC step up converter to go from 3.6v to 18v.  I never actually measured the voltage at switch off. It follows guidelines set accepted standards for LiPo batteries to prevent damage.  If we assume it is a 2s protection circuit, the cutoff is programmed at 2.5v or 5v per battery. So two in series would be 10v cutoff.  Fully charged they are 8.4v ea or 16.8v. So you get 16.4v to 10v drain.

Example of 2s self protect circuit (look at specs):
https://www.amazon.com/Tenergy-Protection-Circuit-Battery-Cut-off/dp/B00LV9Q69I


----------



## xrk971 (Aug 1, 2017)

Some new tweaks after long listening sessions and much experimentation to get the tuning just right, I think I have finally struck Yukon Gold. The HyQu amp now has the harmonic distortion profile of a SE Class A amp, but the power reserves of a discrete Class AB amp. It is small, compact, and powerful.  Driving 270ohm load to 1,41vrms it achieves remarkably low distortion levels of 0.007%THD with -82dB H2 and H3 at -12dB below that, and nothing else.  The noise floor is flat.  So you say 270ohms is no big deal?  Ok, for 51ohm load - similar to typical demanding planar magnetic offerings, it can drive 22.6v peak-peak or 8.0v rms for 1250mW with 0.06%THD and a harmonic profile that still resemble a nice SE Class A amp. Intermodulation distortion is also excellent at only 0.05% for 1kHz and 10kHz.  Stereo crosstalk is only -82dB without resorting to independent PSU's.  For 51ohm load at 5.05vrms (500mW power), the amp remains in fully Class A operation with very low distortion.  This is a level that is generally threshold of pain on most 96dB or more sensitive headphones. 

So how does it sound?  Magnificent! Superb clarity, bass authority and articulation - even with 16ohm headphones no problem.  Beautiful airy highs and wonderful glass smooth mids.  The best sound stage and spatialization I have heard without a doubt.  The classic sound signature of a little warmth and body as if it were a SE Class amp.  The bass is unbelievable.


----------



## DBaldock9

xrk971 said:


> Some new tweaks after long listening sessions and much experimentation to get the tuning just right, I think I have finally struck Yukon Gold. The HyQu amp now has the harmonic distortion profile of a SE Class A amp, but the power reserves of a discrete Class AB amp. It is small, compact, and powerful.  Driving 270ohm load to 1,41vrms it achieves remarkably low distortion levels of 0.007%THD with -82dB H2 and H3 at -12dB below that, and nothing else.  The noise floor is flat.  So you say 270ohms is no big deal?  Ok, for 51ohm load - similar to typical demanding planar magnetic offerings, it can drive 22.6v peak-peak or 8.0v rms for 1250mW with 0.06%THD and a harmonic profile that still resemble a nice SE Class A amp. Intermodulation distortion is also excellent at only 0.05% for 1kHz and 10kHz.  Stereo crosstalk is only -82dB without resorting to independent PSU's.  For 51ohm load at 5.05vrms (500mW power), the amp remains in fully Class A operation with very low distortion.  This is a level that is generally threshold of pain on most 96dB or more sensitive headphones.
> 
> So how does it sound?  Magnificent! Superb clarity, bass authority and articulation - even with 16ohm headphones no problem.  Beautiful airy highs and wonderful glass smooth mids.  The best sound stage and spatialization I have heard without a doubt.  The classic sound signature of a little warmth and body as if it were a SE Class amp.  The bass is unbelievable.



Can you describe the 4 or 5 assemblies that we're seeing in the photo?
Looks like two, single-channel amplifiers in the foreground.
Is the item on the left, a DC-DC power converter?  What is input voltage?  How much current supply is needed?  What voltage(s) & current requirements do the amplifiers have?
Is the board, that's full of capacitors, part of the power supply?
What is that single large capacitor, that's in the center, behind the amp boards?

Is there an estimate for what an unbalanced, or a balanced, version of this amp would cost?

Thanks!


----------



## gikigill

That looks pretty good, if it can be done for 500, it would be great.


----------



## xrk971 (Aug 2, 2017)

The amp uses a small 19v SMPS to power a DC to DC step up converter, this feeds a MOSFET capacitance multiplier (underneath the big cap in between the two amp modules which has a large IRFP240 bolted to the heatsink), this then feeds the CRCRC filter which has qnty 6x470uF 50v low ESR caps in parallel for providing a low impedance path for deep bass transients. The clean and plentiful 28v power from these 3 components then feeds the amp modules.  The bias is about 110mA quiescent so it burns about 3w per channel. Since this is Class AB, it stays Class A below 110mA and digs into Class B when the huge transients or excessive volume is desired. It will be Class A up to 5.05vrms 14.24v peak-peak. That's good enough for 500mW into 50ohm cans.  The amp can supply up to 1.25watts rms sustained power with excellent low distortion that sounds and measures like a SE Class A amp.  The function of the cap multiplier (besides reducing any AC mains ripple by -50dB) is to provide a smooth graceful turn on and turn off ramp to eliminate any possibility of thump/pop. DC protection for headphones is inherent in the cap-coupled output stage.  I am using modest caps right now and it sounds great. Final version will use several ultra-low ESR caps in a array for even better bass articulation. 

This is strictly a proof of concept prototype - and despite being a prototype that is hand made and in separate parts-pieces, sounds fantastic and measures exceedingly well.  In fact, it's the best measuring amp I have built, ever.

Production version will be a single integrated PCB for both channels, cap array, CRCRC filter, and capacitance multiplier. The DC step up will will be separate but that gives flexibility for even a large toroidal trafo linear regulated supply to feed all this.

The final PCB, cost of components, and how much effort it takes to make it fit into the casework etc really determines the final cost and pricing. I think given then amount of componentry, the large size of the board and final case - this is very preliminary, but I am guessing a $700 to $1000 price.  Looking at the performance and listening to the sound - this is a bargain. I predict it will beat just about any other amp out there, regardless of price.


----------



## xrk971

For anyone in DC area going to the Schiit Storm meet in Gaithersburg this Saturday, I will bring Pocket Amps, Silicon Harmony, and HyQu along for demos. 

https://www.head-fi.org/threads/dc-...-august-12-2017-gaithersburg-maryland.855135/

It's free but you need to register.  Hope to see you there.


----------



## xrk971 (Aug 13, 2017)

Above: a table full of Schiit 

The Head-Fi organized SchiitStorm in the DC area was a great event. Lots of very high end cool gear. I feel like more than 10 people brought their Focal Utopia headphones, and several members brought theirs over to try with the Silicon Harmony.  So now I know what those sound like on the Silicon Harmony. Sublime. HiFiMan and Audeze were also there in force. I got to listen to a lot of SE Class A tube amps. Also heard some very nice DACs. A great environment to check out what stuff sounds like and everyone is so nice and easy going with you testing their multi thousand dollar headphones or amps. 

Lots of feedback on my amps and by far, everyone agrees that Silicon Harmony is still king when compared to HyQu despite HyQu having an edge in power.  I was really impressed by the sound that some of the $1000+ headphones sound like. And they all sound very good on the SH, and the PCA was also very well received. The HyQu was also well liked but in the end, could not hold up to the SH when people listened with resolving headphones like the Utopia.

Here are some photos of the event, which Schiit sponsored by providing the venue (a very nice clubhouse in a beautiful neighborhood in Gaithersburg, Maryland). I listened to several of the top Schiit tube amps and they sounded very nice. They ran very hot - the case felt about as hot as my SH (about 55 Celcius). I enjoyed all the amps. There was a young DIY'er there (sprigs) who had some impressive sounding P2P SE Class fleawatt tube amps. Overall it was very nice.

Here was a really nice sounding tube amp - the Zana Deux:




A bunch of cool DIY amps:




The XRKAudio table:




It was a nice way to spend a Saturday afternoon. Btw, for those of you who saw something you liked from my table, feel free to PM me or visit my shop on Etsy. 

A big thanks to Pjabber and others from Head-Fi for organizing this.


----------



## xrk971

There is a 10% off Labor Day sale going on now for the PCA's in my shop.


----------



## daid1

How do you see this amp with a shozy alien gold?


----------



## xrk971

This amp is perfectly suited for the most resolving and highest quality digital sources you can find as it will give the device the best possible sound.  Users have paired it with their ultimate DAPs and DACs including Yggdrasil and found it works even better. The noise floor is so low and use of only two transistors in signal path ensure the ultimate in signal transparency and realism.


----------



## jonmbarlow (Aug 24, 2017)

I purchased this amp recently, after a lengthy hold-up at customs I finally got my hands on this amp and have spent some time with it so here are my impressions......

Firstly I must say that the ordering process is either done via etsy or eBay, I ordered via etsy and found it a simple process....shortly after my order was placed the maker of the amp xrk971 got in touch to enquirer about the headphones that I would be using with the amp, my favourite kinds of music and my preferred sound signature....never before have I had such a personalised experience purchasing such an item and it raised my expectancy that this amp would deliver the sound signature I was looking for.

I purchased rechargeable 9v batteries and a charger to go with the amp, I went for the blue version, the amp arrived very well packaged and after a quick charge of the batteries it was ready to go.....

I told xrk971 that I like a bass presence in my music and this amp delivers the bass in a way that I'm not particularly accustomed to.....I use Uapp via my Shanling m2s and s8 and have always had to have the bass dialled up near to the max to get the sound I desire.....With this amp the bass was immediately distorted and I had to turn it right down to the neutral level, my instincts told me this was a bad thing but how wrong was I, you see this amp is custom made to suit my musical tastes so this amp is delivering what I consider the perfect amount of bass without having to alter the frequencies hardly at all....volume wise I shifted the gain up a little and this amp has got my fidelio x2's singing like I have never heard before (and I own a mojo) also my Lz a4's have gone up a level which I didn't think possible.

If anybody is considering purchasing one of these amps or anything else from xrk971 I cannot recommend it highly enough.


----------



## stellarelephant

This amp indeed delivers incredible bass like nothing else I have heard.  I also love the fine details it brings out.  I have been considering the M2s as my next DAP...good to hear you like the combo.


----------



## xrk971

Thanks for your review Jonmbarlow.  Glad that the amp is living up to your expectations.  It seems to be able to drive a wide variety of headphones well.


----------



## BucketInABucket

Just a question. Are those 9V batteries wired in series or parallel?


----------



## xrk971 (Oct 6, 2017)

In series to provide about 16v to 18v full rail voltage for single ended operation.

Btw, Campfire Audio Andromeda has been reported by about 5 users as working superbly with this amp.  I assume Lyra would be excellent match as well.


----------



## BucketInABucket

I'd assume an 18V lithium battery would do the trick too, right?

Ooo, I have a Lyra 2, I'm seriously considering buying one of these amps and pairing it with a Nuansa P1.


----------



## xrk971

If you can get the 18v lithium ion battery to fit in the tin. Otherwise works in a bigger case too. What I have done is use a 3.7v 3600mAHr LiPo cell and a DC step up converter. The followed by a small capacitance multiplier to reduce noise and allow a soft start mechanism. If you are handy and into DIY'ing it is all described in the threads on DIYAudio. 

Else. Just use a pair of EBL 9v rechargeable Lii-ion cells (4 for $21 including wall charger). Those last 4-5hrs per charge.

Just google EBL 9v Amazon.


----------



## xrk971

Thanks for the likes Dbaldock!

Have you gotten anywhere in your quest to make a balanced drive SE Pocket Class A amp?

I have been working on a new design for a desktop amp that will amaze you guys. Hugh Dean of Aspen PLC, Melbourne is doing the layout as we speak. We will offer it as a product as a collaboration from both our shops. It's a single ended push pull amp. Or SEPP - sort of the holy grail in amps.  Uses SE topology, but has the oomph and snap of a push pull.  Of course pure Class A and can drive up to 1.2w into 8ohms.  Has very low distortion (lower than usual SE Class A) and what it has is dominant 2nd order and little 3rd 

It will come in a nice aluminum case with analog VU meters. XLR and 1/4in TRS jacks. 

Stay tuned.


----------



## DBaldock9

xrk971 said:


> Thanks for the likes Dbaldock!
> 
> Have you gotten anywhere in your quest to make a balanced drive SE Pocket Class A amp?
> 
> ...



Check your PM...


----------



## Lost Cosmonaut (Apr 6, 2018)

,


----------



## xrk971

The HD650's are 300ohm and high sensitivity. Basically the PERFECT headphone to pair with this amp. The amp was designed and optimied around a 270ohm impedance so 300ohms is perfect.


----------



## Lost Cosmonaut (May 23, 2018)

.


----------



## DBaldock9

Lost Cosmonaut said:


> Great! Having an HD650, one tends to get relegated to desktop amps. I love my HD650, but I need to move.  And from what I read, this amp competes with desktops.



I've used my Pocket Class A amp with Beyerdynamic DT-831 Headphones (250Ω), and it's a good sounding combo.


----------



## xrk971 (Oct 14, 2017)

There was a list from a while back from all the people that have tried this amp and reported good results.  needs to be updated, but here it is again:

- AKG K240 (Bogde)
- HD600 with Apollo 8 Blackface (John H. personal communication to xrk971)
- LCD-2 (70ohm) (Morde)
- DT990 pro (Bogde)
- DT880-250 special ed. Chrome with Focusrite Solo 2G (xrk971)
- DT770-250 (pcgab)
- MA900 (Bogde)
- MDRV6 with Focusrite Solo 2G (xrk971)
- HD800 (modified) with Yggdrasil (Funch)
- HD800 (Bogde)
- HD598-50 (StellarElephant)
- HD598c (Bogde)

I have had a lot (at least 6) that have paired this amp with Campfire Audio Andromeda (16ohms but very high sensitivity) and love the results.

- Campfire Audio Andromeda (many people)
- KZ ZS5 (4 way dynamic and bal) (xrk971  - this is a superb sounding combo, one of the best)
- Status Audio OB-1 (xrk971 and several others)
- DT831 (dbaldock)

I have even listened to the Focal Utopia with this amp at the last Head-Fi Schiitstorm meet in Gaithersburg, MD and they sound awesome together.  Also, LCD-X, HiFiman Edition X, Sony MDR-Z1R.  So to add to the list:

- Focal Utopia 
- LCD-X 
- HiFiman Edition X
- Sony MDR-ZR1


----------



## Lost Cosmonaut (Apr 6, 2018)

.


----------



## xrk971

Thanks for the order!  Let's customize for you now.


----------



## Mboom

For the DIY kit, What kind of soldering iron do you recommend to use ? Ive only soldered a few things year's ago.


----------



## xrk971 (Oct 15, 2017)

Nothing fancy, but I personally like a moderately small chisel tip like this:







Watch some YouTube videos on how to do it. Basically solder one end first while holding it with a good tweeter.  Then solder other end. 
Solder paste from syringe works well too - use hot plate and cook on medium heat for about 2 minutes until all paste liquifies and forms neat fillets.

A basic kit like this with variable temp and good collection of tips will work just fine. 

GHB Soldering Iron Kit Micro Soldering Kit 110V Adjustable Temperature Electric Iron Sets for Various Repairing Usage https://www.amazon.com/dp/B01IZDG08E/ref=cm_sw_r_cp_api_fHa5zbZV0JYSY


----------



## xrk971

Lostcosmonaut,
Thanks so much for the order!


----------



## Lost Cosmonaut (Apr 6, 2018)

.


----------



## DBaldock9

Lost Cosmonaut said:


> You're welcome -
> 
> I wish you success with your amps!



...especially the Balanced Desktop model...


----------



## Lost Cosmonaut (Apr 6, 2018)

.


----------



## xrk971 (Nov 20, 2017)

I just built a new one today and found that Nichicon KA 2200uF 16v audio grade caps (big fat blue ones) fit perfectly. I am bypassing on the other side with a Elna Silmic II 10uF 35v.   Now this combo has the most output capacitance of any I have built as a pocket amp.  Sure to please the deep bass, low impedance IEM crowd. Sounds very nice.  Measures wonderfully too, here is 1000mVpp into 270ohms.


----------



## xrk971

I built two just like this yesterday. I wish I could have kept one for myself but alas, one went out to a customer in Colorado. The other is on its way to New Zealand - you guessed it, to member Brooko for a review on Head-Fi.


----------



## DBaldock9

Which batteries are you using with that new amp?


----------



## xrk971

Those are just Tenergy brand Alkalines that I ship out as standard with every amp I sell. It allows the customer to listen and enjoy immediately while waiting for rechargeables to arrive, and allows verification that all still works after shipping.  Alkalines are nice in that you never have that auto-shutdown problem on startup due to in-rush which requires clicking on-off a few times.


----------



## xrk971

My new commuter rig: Cayin N3, NHB Pocket Amp, and KZ ZS5v1 quad driver IEM's.  Superb sounding combo.

Thanks to @Lost Cosmonaut for the recommendation of the N3 - very happy with it!


----------



## krunchcrispy

Just wanted to say that I got some EBL1200 mAh rechargeables to go with my ARKAudio NHB amp -- they are slightly bigger than the 600, and so fiddly to fit, but they will fit.   And they last a good long time, too.


----------



## xrk971

Can you please give a link for the rechargeable 1200mAhr batteries?  The only one I could find are EBL 9v 1200mAhr Li non-rechargeable batteries. These have a 10yr shelf life.


----------



## DBaldock9

When trying to find a single high-output 18V Li-Po battery pack (to substitute for the dual 9V batteries), all that seems to come up in searches, are the handheld drill batteries.
I did find that one Chinese company that sells 18V Li-Po batteries, but they can't ship them into the USA any longer.


----------



## xrk971 (Nov 23, 2017)

The way to do it is a DC step up from 3.7v and the cap multiplier for soft start then CRCRC for filter before going to amp.  This method produces a very low noise flat noise floor FFT. Plus 3.7v batteries in huge 10,000mAhr or more can be found.


----------



## DBaldock9

Are there already assembled 3.7V -> 18V step-up circuits available, or is it something I would have to design and assemble on a breadboard?


----------



## xrk971 (Nov 23, 2017)

It’s all been worked out - RaptorLightning even has Gerbers for you to order the tiny cap multipliers from OSH.  The DC step up is a $1 board from AliExpress/eBay. 

http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-118.html

Gerbers here:
http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-118.html#post5168688

MT3608 DC step up:
http://s.aliexpress.com/7NRRvIjE


----------



## DBaldock9

xrk971 said:


> It’s all been worked out - RaptorLightning even has Gerbers for you to order the tiny cap multipliers from OSH.  The DC step up is a $1 board from AliExpress/eBay.
> 
> http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-118.html
> 
> ...



So the MT3608 DC step-up circuit, with its 2A rating, is enough for a NHB Amp, driving high & low impedance earphones & headphones?


----------



## xrk971

DBaldock9 said:


> So the MT3608 DC step-up circuit, with its 2A rating, is enough for a NHB Amp, driving high & low impedance earphones & headphones?



Yes, you can see it powering amps in the DIYA thread.  You need the cap multiplier circuit or it won’t work.  That circuit provides a soft start so that batteries and DC step up do not shutdown.


----------



## DBaldock9

xrk971 said:


> Yes, you can see it powering amps in the DIYA thread.  You need the cap multiplier circuit or it won’t work.  That circuit provides a soft start so that batteries and DC step up do not shutdown.



Is the cap multiplier circuit something that can be purchased, or does it have to be assembled from scratch?


----------



## stellarelephant

Right now the only way to get the CapMx is to have your own PCBs printed using the files provided by RaptorLightning and then build it yourself.


----------



## xrk971

I might be able to make you a cap Mx Dbaldock9.  Send me a PM if interested.


----------



## krunchcrispy

xrk971 said:


> Can you please give a link for the rechargeable 1200mAhr batteries?  The only one I could find are EBL 9v 1200mAhr Li non-rechargeable batteries. These have a 10yr shelf life.


HEy good call -- I checked, and they are NOT rechargeable, I thought they were. . by bad!


----------



## xrk971

I just got word that Brooko received my NHB safely in the mail. He will listen and provide a review in the near future. Stay tuned!


----------



## xrk971 (Dec 2, 2017)

I have a new public-domain DIY preamp designed by Hugh Dean of Aspen Amplifier that was inspired by the Lender topology.  It is capable of 40vpp drive into 7kohm load with low distortion.  The distortion it has is all SE Class A in character (mostly H2, a little H3 and nothing else.)  It has 0.002%THD driving 20v peak-to-peak into a 7kohm load (more than enough to drive any power amplifier).  The dynamic range and headroom are enormous.  Gain is adjustable from 12dB to 22dB with change of feedback resistor,

More info here:
http://www.diyaudio.com/forums/analog-line-level/314563-aksas-lender-preamp-40vpp-output.html

There is a group buy for some nice PCBs designed by a pro (JPS64).  A second round GB may be possible if there are more interested parties:
http://www.diyaudio.com/forums/group-buys/315521-aksas-lender-preamp-40vpp-ouput-gb.html

Briefly, here is the very elegant circuit by Hugh Dean, using only 5 actives (1 is a CCS):






Here was my SMT prototype (there are nice SMT versions of all the TH components in the above schematic, and they actually work even better):





And here is measured FFT for 20vpp into 7kohm load with 0.002% THD, note the absolutely clean flat silent noise floor at -130dB:





Here is the proposed finished PCB with two core amp module daughter boards and input selector, cap multiplier based PSU and onboard CLC and CRCRC filter:





Amp core module daughterboards are available in SMT, TH, or metal core SMT:














This preamp was designed to be able to drive power amps  with unity gain output stages (Pass F4 for example) or a simple single MOSFET common collector output type SE Class A amp.  But I have found that it makes any amplifer sound better by giving it the proper driving headroom and clean low noise floor that it needs.  PSU will be external SMPS walwart from 12v to as high as 35v.  A 19v laptop SMPS is perfect.

Oh, and if you have no need to drive 20vpp into an amp, here is what it can do driving typical Line-Level inputs at 4vpp - 0.0011% THD, and this is at 22dB gain.  If you only need 12dB gain, the distortion will be much lower, about 0.0007%THD 0 that's 7ppm!:





Finally, what does this have to do with headphones?  Well, I use it to drive my Silicon Harmony SE Class A and the sound is glorious. I have also used it to drive a unity gain SE Class A, the MoFo amp as a headphone amp.  Finally, with a simple swap out of 3 resistors to the above circuit (to increase bias current from 12mA to 90mA), this preamp becomes a bonafide low distortion SE Class A headphone amp!

Here is 1vpp into 50ohms at 90mA bias current and achieves 0.0056% THD:  This low level of THD is hardly ever seen in a SE Class A headphone amp:





Sounds very very nice.


----------



## xrk971 (Dec 3, 2017)

I had some time to test the Aksa Lender Preamp driving the single-transistor MoFo amp.  I found a 67mH 0.47R choke from a dead microwave oven.





Using that as a reactive current sink (double the output swing for free). Here is data for 2.83vrms into 8ohms.





Here are various distortion components for 2.83vrms into 8ohms:





Here is max output voltage before clipping, 26.8vpp (from 19v Vcc - extra power free as I said). That’s 11Wrms into 8ohms. Normally a 19v Vcc and MOSFET out limits max swing to 14vpp.





And here is a 10vpp square wave:





If this amp we’re used to drive say 50ohm headphones, distortion would go down due to lower output current.

Anyhow, this is a good demo of what is capable with this pre.

For those not able to make a DIY version of this amp, a fully built XRKaudio version called the Silicon Ephiphany will be available soon.


----------



## xrk971 (Jan 3, 2018)

Happy New Year folks!

I have some cool new tricks for this Pocket Amp.  I just wired up a full-fledged rechargeable LiPo battery solution last night using a DC step up boost regulator that maintains constant 18.0vdc Vcc for constant sound quality (THD doesn’t rise as batteries deplete). The boost goes to a MOSFET cap multiplier that smooths the output of the DC boost, but more importantly, it forces a gradual ramp up of voltage over several seconds. This has the benefit of zero turn on/off pop.  The cap multiplier feeds a CRCRC filter to clean up any residual noise. So the system is absolutely silent even with 106dB IEMs plugged in.  The measured noise floor is also just as low as a plain battery. That is flat down to -130dB so very quiet as you have come to expect from this amp.  I have also optimized the resistor network to provide better performance for low impedance and high impedance sources. Lower THD and lower H3 relative to H2.

So here is he new amp configuration. Can be charged from USB even when playing. LED changes from red to blue when charged as usual.

Testing it now on my commute on the metro. Works very well with my Cayin N3 and 5ohm ZS5 gen1.

Here is FFT for 1.0vpp into 270ohms:





Here is FFT for 1.0vpp into 50ohms (note profile is about the same with just greater THD):


----------



## DBaldock9

I haven't been keeping up with the thread over on DIY Audio, so I was wondering if anyone is working on a compact circuit board layout, that includes all those different power supply sections?


----------



## xrk971 (Jan 3, 2018)

Yes, I am planning at some point on making a new one with charging and battery capability. Not sure about the schedule though.


----------



## Slater

xrk971 said:


> Happy New Year folks!
> 
> I have some cool new tricks for this Pocket Amp.  I just wired up a full-fledged rechargeable LiPo battery solution last night using a DC step up boost regulator that maintains constant 18.0vdc Vcc for constant sound quality (THD doesn’t rise as batteries deplete). The boost goes to a MOSFET cap multiplier that smooths the output of the DC boost, but more importantly, it forces a gradual ramp up of voltage over several seconds. This has the benefit of zero turn on/off pop.  The cap multiplier feeds a CRCRC filter to clean up any residual noise. So the system is absolutely silent even with 106dB IEMs plugged in.  The measured noise floor is also just as low as a plain battery. That is flat down to -130dB so very quiet as you have come to expect from this amp.  I have also optimized the resistor network to provide better performance for low impedance and high impedance sources. Lower THD and lower H3 relative to H2.
> 
> ...



What is the battery capacity/runtime?


----------



## xrk971

Slater said:


> What is the battery capacity/runtime?



Using a 3000mAhr 3.7v LiPo cell and boosting to 16v should give about 4hrs. I need to test duration of run.


----------



## xrk971

In addition to a LiPo and DC step up and Cap Mx, would having BT 4.0 or a built in DAC be more of a desired feature?


----------



## BucketInABucket

xrk971 said:


> In addition to a LiPo and DC step up and Cap Mx, would having BT 4.0 or a built in DAC be more of a desired feature?


Not for me, it's a dedicated amp and should stay one imo.


----------



## DBaldock9

xrk971 said:


> In addition to a LiPo and DC step up and Cap Mx, would having BT 4.0 or a built in DAC be more of a desired feature?



If you're designing the circuit board, considering future upgrades, maybe you could add a header, for a "Digital Audio" daughter-board?


----------



## xrk971 (Jan 12, 2018)

I’m designing a new desktop preamp/headphone amp that will have a DAC and BT. But a header for future expansion is certainly a good feature.  I currently use headers for core amplifier daughter boards on the group but Aksa Lender Preamp.  It lets one swap boards for different gain, bias, even output devices. Daughter boards are inexpensive and allow ultimate flexibility of main amp.

More info here:
http://www.diyaudio.com/forums/group-buys/315521-aksas-lender-preamp-40vpp-ouput-gb.html

Here is one of the daughter boards:





This is the sort of distortion this amp achieves for 20vpp:





The new Pocket amp will be Class A of course but with topology similar to Aksa Lender for lower distortion. Will have LiPo, DC step up, cap multiplier, BT, and am working on adding a DAC even. Won’t really have room for expansion cards - as it will be pretty compact and still pocket sized with custom CNC case.


----------



## xrk971

Hi Folks,
I just wanted to let you know that member Brooko just reviewed this amp. It’s here:

https://www.head-fi.org/showcase/xrk-audio-nhb-portable-class-a.22873/reviews#review-19723

Thank you, Brooko for a thoughtful and well written review. You hit all the key features of his amp nicely and I agree that the one that keeps me listening to it is the sound quality. It is magical. 

Cheers,
X


----------



## DBaldock9

Will you be offering a USB rechargeable Li-Po battery pack, for the Class A NHB Amplifier?


----------



## xrk971

There is a DIY solution you can follow just like I did using a small cap Mx board, a DC boost board, and a LiPo pack.  It requires cutting a trace on the PCB and some wiring.  
The details can be found here:
http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-144.html











Looks like this laid out flat:





And like this all packaged up:





If there is enough demand, maybe I can make an integrated board that has the DC step up, cap Mx, CRCRC, and battery holder all on one board. But that still requires surgery to the main PCB and I don't know how many people are willing to do that.

I am working on an all new design with a CNC case and integrated LiPo USB power charge system though  - but that is kind of down the road.


----------



## Slater

DBaldock9 said:


> Will you be offering a USB rechargeable Li-Po battery pack, for the Class A NHB Amplifier?



Or better yet, let this accept a USB battery power pack (all output 5v 1A, and they even have them with 5v 2.4A nowdays). You can buy the power packs in almost infinite capacities and case configurations, from single 18650 "lipstick" tube style (usually around 1500-2000mAh), all the way to 20,000mAh (and even larger). That way, it's up to the user to decide what amplifier runtime they want. 

You'd just need to have a female microusb port on the amp as the power input (as well as the built-in boost and power filtering circuitry obviously).


----------



## MyPants

Welp, I'm extremely interested in one of these, but I'm torn on whether or not to take the NHB plunge. Decisions decisions...


----------



## xrk971

MyPants said:


> Welp, I'm extremely interested in one of these, but I'm torn on whether or not to take the NHB plunge. Decisions decisions...



You can be certain that the amp will sound excellent and is sure to please.  Everyone who has listened to one or reviewed it has loved it. Even the last review by Brooko, he liked it so much he bought it despite the fact that I gave him a free sample to keep.  

What cans or IEMs are you using? I can custom tune the amp for best performance with your headphone or earphone.


----------



## MyPants

xrk971 said:


> What cans or IEMs are you using? I can custom tune the amp for best performance with your headphone or earphone.



I just read Brooko's review before my post, and I tend to trust his impressions, so I'm confident that either version will indeed satisfy. I'd be using the amp primarily with the HE400i, Fostex T20RP MK3, and LZ A4 iems.


----------



## xrk971

The LZ A4 will be an excellent match with high sensitivity and 16ohms.  The Fostex can work but sensitivity is on low side so may not get as loud as you want. Given that it is 93dB and 50ohms which is about the borderline of what can be driven by this amp. The HE400i being 32ohms and 91dB is too low of sensitivity and low impedance and will probably not sound it’s best. I have not personally listened but will guess that it’s going to be a struggle.  I use 55ohm and 96dB OB-1’s and that works great. 91dB and 250ohms would be fine for example.

Anyhow, that’s just a limitation of the amp in not being a high current amp.


----------



## MyPants

xrk971 said:


> The LZ A4 will be an excellent match with high sensitivity and 16ohms.  The Fostex can work but sensitivity is on low side so may not get as loud as you want. Given that it is 93dB and 50ohms which is about the borderline of what can be driven by this amp. The HE400i being 32ohms and 91dB is too low of sensitivity and low impedance and will probably not sound it’s best. I have not personally listened but will guess that it’s going to be a struggle.  I use 55ohm and 96dB OB-1’s and that works great. 91dB and 250ohms would be fine for example.
> 
> Anyhow, that’s just a limitation of the amp in not being a high current amp.



Hmm, two questions then;

1. Considering that my current mobile amp is a 9v cMoyBB kit from JDS that I put together, would the NHB version of your amp perform at least as well or better than the cMoy with my sub-optimally paired planar cans?

and

2. I also have a 600ohm set of DT880s, and the _extremely_ sensitive NuForce Primo8. Any better luck with these two as far as pairing goes?


----------



## xrk971

The NHB absolutely can blow a 9v CMOY out of the water. No question - I can use a newly developed tuning that can drive lower impedance cans better to give you usability with the HE400i. 

The DT880-600 and NuForce Primo are ideal matches and will work superbly without issues.  As this amp has an 18v supply rail, it can drive high impedance cans very well. Made for them, in fact.  The high sensitivity of the NuForce and 16ohms will work extremely well.


----------



## MyPants

xrk971 said:


> The NHB absolutely can blow a 9v CMOY out of the water. No question - I can use a newly developed tuning that can drive lower impedance cans better to give you usability with the HE400i.



I think you've sold me, let's do this! Would you prefer I run through etsy, ebay, or would you rather me send you a PM and just invoice it through PayPal?


----------



## xrk971

There’s no question that the NHB can blow the 9v CMOY out of the water performance wise and sound quality wise.  A discrete power MOSFET output stage with 18v rail will be superior in power and sound quality compared to a CMOY opamp at 9v. 

I can even apply a new tuning to it to allow operation with lower impedance cans with lower distortion. 

The DT880-600 is a perfect match. This amp was made to drive high impedance loads as it has 18v supply rail.  You will love the sound. 

The NuForce IEM is 114dB and 16ohms. Perfect match and wil sound absolutely gorgeous with the NHB. Such high sensitivity requires a low noise amp and the NHB has zero added hiss. Even with 114dB phones if you remove the source you will not hear any self noise. It is silent to -130dB or better. 

I am confident that it will exceed your expectations for the DT880-600 and the NuForce Primo 8.


----------



## xrk971

Please buy from Etsy. Thanks!


----------



## DBaldock9

xrk971 said:


> There is a DIY solution you can follow just like I did using a small cap Mx board, a DC boost board, and a LiPo pack.  It requires cutting a trace on the PCB and some wiring.
> The details can be found here:
> http://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-144.html
> 
> ...



Do the USB Battery Charger circuit boards come with a spec, for the range of battery sizes (mAH) that they work with?
What mods would be required, to power 2x PCA NHB amps, if I wanted to attempt a Balanced configuration, using a 7,000 - 10,000 mAH battery pack?


----------



## xrk971

You will have to check the specs on the USB power management unit.  You will need two separate step up and cap multipliers for the balanced system with two PCA boards each having its own power rail.  You can tie both DC steps up from same battery though.  Draw a schematic block diagram and I will check to see if it makes sense.


----------



## xrk971

I'm happy to report that MyPants' new amp, now fitted with the low-impedance resistor network mod, with an 80mA bias current, and 2200uF Nichicon AK output caps, is happily driving a brand new pair of HE400i's on my head!  Listening to Daft Punk's Lose Yourself to Dance.  The bass is nice, articulate, and no bass collapse or distortion as reported by some who tried to use a standard NHB with HiFiMan planars.  Although, ideally, the NHB should be paired with higher impedance and higher sensitivity phones than 93dB and 32ohms.  In this case, with these mods, it works very well.  Here is S/N040 with the special mods.  The mods are detailed over in the *DIYA thread here*.


----------



## DBaldock9

xrk971 said:


> You will have to check the specs on the USB power management unit.  You will need two separate step up and cap multipliers for the balanced system with two PCA boards each having its own power rail.  You can tie both DC steps up from same battery though.  Draw a schematic block diagram and I will check to see if it makes sense.



This LiPo charge controller, on AliExpress, is rated for up to 1000mA of current -
https://www.aliexpress.com/item/MCP...-Controller-Board-Module-3-7/32829229126.html
.




.




.


----------



## xrk971 (Jan 27, 2018)

That looks nice, if assuming you run 80mA at 18v per channel so that is 160mA.  But 18v/3.7v is 4.86x so 780mA total at 3.7v. So it is correctly sized.  Good find.


----------



## DBaldock9

xrk971 said:


> You will have to check the specs on the USB power management unit.  You will need two separate step up and cap multipliers for the balanced system with two PCA boards each having its own power rail.  You can tie both DC steps up from same battery though.  Draw a schematic block diagram and I will check to see if it makes sense.



Just out of curiosity, if in a Balanced configuration, where both PCA boards need +18VDC, at ~100mA, why couldn't they be connected to one of these XL6009 4A Output 5V-35V DC-DC Step Up Voltage Boosters, with a separate CMX for each amp?
https://www.aliexpress.com/item/New...Boost-Power-Supply-Converter/32358896984.html


----------



## xrk971

Those are audibly noisy. Literally make noise you can hear with your ears.  

Use the white metal core boards rated for 10amps and 50v. They are very quiet. 

https://m.ebay.com/itm/DC-DC-boost-converter-Constant-Current-Mobile-Power-supply-250W-10A-LED-Driver-/181940997254?


----------



## DBaldock9

xrk971 said:


> Those are audibly noisy. Literally make noise you can hear with your ears.
> 
> Use the white metal core boards rated for 10amps and 50v. They are very quiet.
> 
> https://m.ebay.com/itm/DC-DC-boost-converter-Constant-Current-Mobile-Power-supply-250W-10A-LED-Driver-/181940997254?



How is that unit able to output +18V, when the spec says, "_Input voltage: DC8.5V-48V_" - and the output of the Li-Po batteries are rated at 3.7V?


----------



## xrk971

Sorry, for Lipo use the MT3608 DC step up to get 25v and another one to go higher for the phase splitter.


----------



## MyPants

S/N:040 arrived this evening, and I can safely say that this is a special piece of equipment. Haven't had much time with it yet, but it's currently driving T20RP MkIIIs better than anything else I own. I'll report back with more detailed thoughts on this beauty in a few days once I've had a chance to rotate through a few sets of cans, but I'm extremely pleased and impressed with the NHB at first listen. Bravo xrk.


----------



## xrk971

Glad you like them, MyPants. Let us know how it goes with the rest of the cans. The HE400i’s are challenging but I think they were working admirably well considering how low sensitivity they are. I am curious about how well your 600ohm cans sound.


----------



## kukkurovaca

MyPants said:


> Haven't had much time with it yet, but it's currently driving T20RP MkIIIs better than anything else I own



Oooh. Which other amps have you used with the T20RP's?


----------



## MyPants

xrk971 said:


> Glad you like them, MyPants. Let us know how it goes with the rest of the cans. The HE400i’s are challenging but I think they were working admirably well considering how low sensitivity they are. I am curious about how well your 600ohm cans sound.



I too am curious to see how it performs with the 600ohm DT880s. Tomorrow I have a bunch of clerical work to do at my desk, so I'll have a chance to demo it with the HE400i and the DT880. Tonight I gave it a little over an hour between the T20RP and the thinksound On2 which also arrived today, and the NHB paired wonderfully with both. I'll probably run the first set of batteries out tomorrow, so I'll have to wait till saturday or sunday to test all the IEMs. I don't wanna swap the batteries out until my ESD wrist strap comes in saturday evening.



kukkurovaca said:


> Oooh. Which other amps have you used with the T20RP's?



My other solid state amps are a cMoy BB kit from JDS Labs and an o2 that I built, and I also have a Little Dot MkII OTL tube amp. The cMoy drives them fine volume-wise and does the highs/mids reasonable justice but gets a little loose in the bass. The o2 actually does a really good job, but simply isn't as musical or engaging as the NHB. The NHB also seems to deliver better overall bass response (tighter/deeper/slam-ier). The Little Dot has heaps of voltage and current on tap, but equal heaps of output impedance... This is fine and even somewhat preferable with the 600ohm Beyers, but makes the T20RP sound really incoherent and uncontrolled (relatively speaking of course).


----------



## xrk971

Just make sure you discharge your fingertips on the tin itself and you will be fine to change batteries. Just don’t touch the circuit board directly. Have fun.


----------



## MyPants

Can confirm the NHB does a beautiful job of driving TH-X00 Purplehearts. Kindof a ludicrous headphone to truly take "portable", but the easy chair sure is far from my desktop rig...


----------



## xrk971 (Apr 14, 2018)

Here is the latest custom amp for a HeadFier - based on the Pocket Class A amp core but with removable 9v battery compartments and built in DC-DC step up for regulated 18.0v rail and Cap Multiplier to clean up the wall power followed by 30,000uF of CRCRC filtering. Anyhow, it is running a hot 75mA bias current and uses 1600uF of output coupling capacitance. Drives most headphones with power. Even HE400i’s work.  The transition from wall power to battery or vice versa is seamless. The sound is really clean with 18v rail power. The ability to pick it up off the desk and take it out to the pool, patio, park, etc is a real bonus.


----------



## xrk971 (Apr 14, 2018)

The long awaited desktop amp with built in cap multiplier and CRCRC and ability to run up to 125mA bias current is almost here. Layout by BabyDontHertzMe and JPS64 on DIYA.

Double plane traces with a thousand vias to connect both sides for high current. Central star grounding with separate ground planes for left-right and PSU.  

A cap-roller’s delight with lots of space and holes for many different sizes and BIG caps. 

It will be a very fun amp to have.


----------



## DBaldock9

Both of these are Single-Ended?


----------



## xrk971

Yes same topology as title amp.


----------



## DBaldock9

@xrk971 - Is there any possibility that you could offer an add-on circuit board to fit in the "other half" of the Altoids tin (leaving space for a 3.7V Li-Ion battery), containing the 5V USB to 3.7V charger, cap multiplier, and CRCRC - for us DIYers, who aren't afraid to cut the trace that's required, and solder the 3 or 4 wires to the Amp circuit board - so we can run the PCA from USB power, and not have to constantly swap 9V batteries?


----------



## Slater

DBaldock9 said:


> @xrk971 - Is there any possibility that you could offer an add-on circuit board to fit in the "other half" of the Altoids tin (leaving space for a 3.7V Li-Ion battery), containing the 5V USB to 3.7V charger, cap multiplier, and CRCRC - for us DIYers, who aren't afraid to cut the trace that's required, and solder the 3 or 4 wires to the Amp circuit board - so we can run the PCA from USB power, and not have to constantly swap 9V batteries?



I second this.


----------



## xrk971

Let me think about it - I have other developments coming along that might make this not needed. But seems like a nice aftermarket upgrade board for the Gen 1 amp.


----------



## xrk971 (May 12, 2018)

New Desktop Class A (DCA) amp comes to life.  Fired up the first time no issues. Built in cap multiplier and CRCRC.  Running 22v rail  and 150mA bias current.






Here is FFT for 1.0vpp into 270ohm load with the low THD optimization:





If you like it more tubey sounding with H2 higher than H3 by about 8x, this optimization looks good:


----------



## gikigill

Price for the Desktop Class A?

Can you please provide a figure of wattage @50ohms.


----------



## xrk971 (May 12, 2018)

Right now the DCA is just a PCB.  If you want a custom build in a case, PM me or contact me on my Etsy shop and we can work it out. I just did a test to get max power for you.  With a 51ohm load resistor in parallel with on board 270ohm load, we get 42ohms net. I boosted Vcc to 25v and connected the Cayin N3 as source playing a high res 1kHz FLAC file. I maxed out the volume on the N3 (100%) and still could not get the DCA to clip.  Here is the test with the DMM showing 3.34vrms and O-scope trace showing unclipped behavior.  3.34vrms into 42ohms is 265mW.  So I guess you are looking at some HiFiman ro Audeze planars at 50ohms?  Assuming meager 84dB sensitivity, 265mW is about same as 256, which conveniently is 8 doublings of power from 1mW. So we know 8 doublings x 3dB = 24dB.  So the 84dB cans can play 84dB+24dB=108dB SPL with this amp.  That's pretty loud.






Here is O-scope screen shot:





So there you have it.  This is running a hot 180mA bias - but heatsinks are doing fine.

I am listening to it with my HE-400i's and at max volume on the Cayin N3, I cannot open the volume on the amp to max without it being too loud to be comfortable.  So I think this amp can drive inefficient planars just fine. Sounds very, very nice. Unbelievably dynamic with no sign of distortion or compression artifacts on bass peaks.

Just realized that I did the max power test with the volume pot not in max postion, so there is probably some more power left.  Maybe 3dB more or at least 500mW or 111dB on the 84dB headphones.

Edit:  quick re-test shows 4.78vrms and that is 544mW into 42ohms. I was close...


----------



## gikigill

How about a sensitivity of -73db@50hms.

I am thinking of commissioning a desktop full Class A amp for planars.


----------



## xrk971

74dB sensitive would not be so good at 98dB max.


----------



## xrk971 (May 13, 2018)

For 74dB planars I think I would recommend a higher bias current even. As is, the max SPL is 10dB less than 111dB or 101dB.  Doubling bias to 250mA would be possibly 2w of output for 6dB more to 105dB.


----------



## xrk971

Gikigill,
I think what you might need is an Aksa Lender HPA.  It is SE Class A and has a constant current source to keep distortion levels lower for the higher current swings plus bit more voltage swing capability.  It should be able to do.

New thread for DCA:

http://www.diyaudio.com/forums/headphone-systems/322638-xrk971-desktop-class-dca-headphone-amp.html


----------



## gikigill

Looking into the AKSA now. A 70V amp with switchable gain would be pretty much end game.


----------



## xrk971 (May 14, 2018)

Hi Gikigill,
For 70v rail, that’s 35v x 125mA bias or 4.4w dissipation on the BJT and CCS each.  Will have to look at using a bigger BJT than 2SA1837 (2w max). A bigger BJT is not hard to find. The DN2540 CCS can handle 15w so that’s not a problem.  This would be an interesting amp though - heatsinks will need to be larger. Possibly a heat spreader to a larger main panel on case wall. 17w total dissipation is no joke but it will Rock your cans that’s for sure. Rough estimates for power assuming 50% swing p-p of rail voltage for SE Class A operation.  So 35vpp is same as 12.4vrms.  That’s 3wrms into 50ohm load.
X


----------



## xrk971 (May 18, 2018)

Hi Gikigill,
I just ran a simulation of the ALPHA 20 Class A amp, but with revised resistors to use +/-25v supply and 140mA bias for 2.25wrms into 50ohms with a beautiful harmonic profile and 0.02%THD.  That’s 30vpp into 50ohms. So the boards already in production and this could be made quickly. The dissipation is 6w/ch not bad for 2.25w SE Class A output.  Here is the predicted harmonic profile at full 30vpp swing into 50ohms.  Is 2.25wrms enough for your 74dB planars?
X


----------



## stellarelephant

The thought of 2.25W pure Class A power into my Fostex planars is making me drool...


----------



## MyPants

stellarelephant said:


> The thought of 2.25W pure Class A power into my Fostex planars is making me drool...


As well it should. Planars of all stripes really seem to just gobble up power. I will say that my T20RPs sound a little shouty with the A100 but that's a damn sight more than 2.25w.


----------



## skwoodwiva

xrk971 said:


> Sure, it's easy.  The graph shows the amplitude of the peaks of the harmonic components.  This is using the de-facto standard of 1kHz excitation signal so we look for second harmonic (H2) at 2kHz and third harmonic (H3) at 3kHz and so forth.  If you look at the amplitude difference between H2 and H3, H3 is about -20dB lower. 20dB is same as factor of 10x. Natural sounding noises or sounds tend to have predominant H2 and are pleasing.  Odd harmonics like H3, H5, H7 tend to be unpleasant. Also, higher order harmonics, odd or even are noted by our subconscious as unatural. The theory is that our brains have evolved to be acutely tuned to odd order harmonics as un-natural sounds equivalent to a high alert for signs of danger.That is the psycho-acoustics behind that anyhow.


Sirs, I am greatly impressed. Have you determined the maximum rail volts it can handle?
As is on ebay?


----------



## xrk971

The Alpha amp can handle even up to 100v rail to rail - it just requires consideration to cooling.  But since large TO247 MOSFETs are used, a reasonable 150mA bias current at 50v is still only. 7.5w per mosfet or 15w per channel.


----------



## gikigill

To answer your question, 8w/ch@50ohms would be enough.


----------



## xrk971

That’s 57vpp so 70v rails would be enough.


----------



## gikigill

Sounds good. Now if it could be done for $350-400, it would be perfect


----------



## xrk971

Pure SE Class A with 8watts rms into 50ohms custom amp for $400?


----------



## gikigill

$425?


----------



## DBaldock9

gikigill said:


> $425?



  Probably more like $1425 ... or $2425


----------



## xrk971

DBaldock9 said:


> Probably more like $1425 ... or $2425



You are pretty good with pricing!


----------



## DBaldock9

I've got a couple of questions about the naming convention on the schematics (specifically the DCA) on your web store.
A few of the capacitors have labels with letter suffixes:
C101
C101A - C101D
C102
C102A - C102D
C111
C111A - C111B
C112
C112A - C112B
C113
C113A
C114
C114A

Are the ones with letters optional, or do all of the capacitors need to be installed?
The Values given for some of those capacitors isn't listed in uF.
For example - what size / values should I try to buy for these?
C101A C22/10 C22.5B10
C101B C27/17 C27.5B17

Thanks,
David Baldock


----------



## xrk971

Those designations refer to all the optional pad sizes for different caps.  This board was made for the DIY cap roller who likes to try different caps. They come in different lead spacings.  

The main thing to know is that the input cap should be at least 4.7uF film or electrolytic. If electrolytic put positive facing the JFET. 

The output cap should be at least 2200uF electrolytic. You can bypass with a film cap of several sizes.  

So to get a working amp, I would suggest a 4.7uF MKP axial or radial film cap with lead spacing fitting one of those spaces.  

On output, use a 2200uF radial electrolytic. Add a 2.2uF MKP or MKS cap and you are good.  Sometimes no film bypass needed. Some people think that sounds better actually.


----------



## xrk971

I think you are overthinking this. You might want to just get a board and start building. Nothing is locked in concrete and you can swap parts to your hearts content as the copper is 2oz thick and held down with lots of vias on 2mm board.  It’s very durable and can withstand desoldering.  Build one board and listen to it. If you like it, build another and make your balanced drive amp then. It’s an easy amp to make and get working and the sound is fantastic. You know the sound - like the Pocket Class A but lots more power. 

Juat start building and trying it out and if you need stuff, Mouser is in Texas and ships you stuff the next day or two.  That will answer many more questions than trying to get it all figured out on paper first.


----------



## xrk971 (Aug 8, 2018)

Hi David,
You might want to start participating in the GB thread - that’s where the community asks questions and share answers.  That way, everyone benefits from the same questions you ask.  There’s a lot of good info in the thread - like matching JFETs or what DC-DC converter to use, or what caps work well, etc. the question you just asked about optional caps was asked in post 22 for example.

http://www.diyaudio.com/forums/head...op-class-dca-headphone-amp-3.html#post5435639


Here is main thread:
http://www.diyaudio.com/forums/headphone-systems/322638-xrk971-desktop-class-dca-headphone-amp.html


----------



## DBaldock9

Thanks!

I see that your web store lists "Matched FETs for DCA" - are those still available?


----------



## xrk971

Yes, I have matched FETs for DCA.  However my basement just got flooded so finding things may take me a few days.


----------



## DBaldock9

xrk971 said:


> Yes, I have matched FETs for DCA.  However my basement just got flooded so finding things may take me a few days.


----------



## MyPants

crap is getting hardcore in here. I like it.


----------



## xrk971

Mypants,
Sorry, don't understand your last post is referring to?


----------



## DBaldock9

Working on the logistics of assembling an integrated Amp (Balanced & Un-Balanced) / Selector Switch unit for headphones.
It will have Balanced & Un-Balanced inputs for line level, and headphone level signals, and be able to switch them through to DCA Amps, and/or  Balanced & Un-Balanced output jacks.
There will have to be some _smarts_ to the relay control switching, to keep from sending a Balanced signal to an Un-Balanced load.
Here's a block diagram of what needs to be inside the chassis - 
.

 
.


----------



## xrk971

Dbaldock9,

I think I have a much simpler passive solution for you.  It's detailed over on *DIYA thread for DCA,* but I will repeat here:

You might be able to do this via passive "switching" on the input/output TRS or XLR jacks.

Here is one approach to try... when using balanced, require a 4 pin XLR input jack to be used and 4 pin XLR output (to phones to be used) and wire accordingly.

When desiring to use unbalanced input and unbalanced output, use 3.5mm or 1/4in TRS input jack with right and left connected to only the +ve side of respective left DCA, and right DCA board. Correspondingly, the DCA board output on +ve side goes to right/left of TRS output jack. So basically, the negative (-ve) side of each DCA board is left unused when in unbalanced mode. 

See attached schematic. Easy peasy... You could in fact power both balanced and unbalanced headphones simultaneously as amp has plenty of power to spare assuming impedance is not too low on either can. Since everything is cap-coupled, there is no fear of DC imbalance blowing something out.

Cheers,
X


----------



## Slater

brentothedan said:


> .



I agree!


----------



## DBaldock9

brentothedan said:


> .





Slater said:


> I agree!



Looks like they've made 4 posts in the _Headphones For Sale_ thread, and then a "*.*" in a bunch of other threads.
Wonder what that's all about?


----------



## MyPants

xrk971 said:


> Mypants,
> Sorry, don't understand your last post is referring to?


Oh I just mean y'all are getting deep into some DIY stuff for some seriously meaty amp designs and it's fun to watch.


----------



## MyPants

DBaldock9 said:


> Looks like they've made 4 posts in the _Headphones For Sale_ thread, and then a "*.*" in a bunch of other threads.
> Wonder what that's all about?


Probably trying to hit the minimum post count required to use the marketplace.


----------



## xrk971

That’s a pretty lame way to bump a thread (worse than people who actually say “bump”). At least add some new info.

I am thinking of offering my Pocket Amp, reconfigured as a balanced input / output amp as a transportable desktop amp. Contact me if you are interested - it will be on a custom ordered basis. It will look similar to this and use the same case. Will have dual 9v batteries in slide out casette trays and also external DC wallwart capability.


----------



## SDBiotek

xrk971 said:


> That’s a pretty lame way to bump a thread (worse than people who actually say “bump”). At least add some new info.
> 
> I am thinking of offering my Pocket Amp, reconfigured as a balanced input / output amp as a transportable desktop amp. Contact me if you are interested - it will be on a custom ordered basis. It will look similar to this and use the same case. Will have dual 9v batteries in slide out casette trays and also external DC wallwart capability.


Does it only have balanced input?


----------



## xrk971

No, it will have optional single ended input (which only gives single ended output.). Will follow diagram posted above.


----------



## MyPants

Hey @xrk971 , what happened to the pocket class a kits on your etsy? I was just recommending the kit to a buddy who wants to build one, but it looks like they're not available anymore.


----------



## xrk971

Simply a matter of parts running out of stock - I can order more of there is interest. I will put it back as soon as parts arrive in a week. Or he can preorder.


----------



## DBaldock9

I've begun ordering some of the parts I'll need to assemble the 6 DCA channels I need, for a Balanced (2 boards, 4 channels) and a Single-Ended (1 board, 2 channels) Amp.
This is in addition to the power supply I found on eBay, and the circuit boards and FETs that I ordered from you previously.
Just purchased a set of the 1% 3W 270Ω resistors, from an eBay vendor.


----------



## xrk971

I was wondering when you were going to build the balanced DCA’s.  Do you have 6 boards?


----------



## xrk971 (Apr 24, 2019)

Here’s a new spin on this amp. Using an ultra low 117uA bias current in the JFET (via new DC setpoint resistors), the sound is sweeter (lower third harmonic relative to second) and lower overall THD, and more dynamic through higher gain. It seems that the input signal had to control a much lower bias current and this seems to give the sound an immediacy and naturalness that’s close to LIVE sound. It’s an experiment that is turning out superb. It’s what I am listening to at present as my main portable amp.

https://www.diyaudio.com/forums/gro...-pocket-class-headamp-gb-189.html#post5750411

Revised schematic:





This is the predicted FFT for 1.4Vpp into 55ohm headphones (THD is only 0.05%):






Test amp with new resistors and only 117uA of bias current through the input JFET:


----------



## DBaldock9

xrk971 said:


> I was wondering when you were going to build the balanced DCA’s.  Do you have 6 boards?



No, just the three boards, and FETs, that I ordered from you previously.
1 board to drive the Left Balanced Channel.
1 board to drive the Right Balanced Channel.
1 board to drive the Left & Right Single-Ended Channels.


----------



## xrk971

New Balanced Drive 2 Transistor SE Class A Headphone Amp with beautiful harmonic profile and ability to drive 50ohm cans.

https://www.diyaudio.com/forums/hea...rive-2-transistor-se-class-headphone-amp.html













Measured FFT for 2Vpp itno 49ohms:


----------



## xrk971

Here’s the new Muscle Amp - uses eight OPA1622 in parallel. Can drive speakers even down to 4ohms. About 10w into 8ohms. Tested T 47ohms and listening with 50ohm HE400i. Fantastic dynamics. Zero noise or background. Powered with 25va 15v trafo through a cap multiplier for low noise power source. 

Here is closeup if the OPA1622’s:
 

Test setup with 47ohm dummy load and 2ppm 1kHz reference source. Focusrite 2i4 interface. 
 

FFT for 2Vpp into 47ohm load (0.0008%THD):


----------



## xrk971 (Nov 18, 2019)

Hi Folks, I haven’t been here for a while but thought you all might be interested in a new project brewing over on DIYA.  It’s going to be a new PCA Mk II.  This time fully assembled and tested and ready to run (RTR) all SMT design.  You just need to install in your own mint tin or 3D printed case.  I will provide STL file.

Main features are still SE Class A amp but this time, using hybrid JFET input and BJT output in a complimentary feedback pair (CFP) topology that had better dynamics and lower distortion.   Built in 4100mAhr LiPo with battery management and LED fuel gauge, DC-DC step up for stable 18v output, and MOSFET cap multiplier for smooth slow startup to prevent turn on thump. USB charging or 24v wallplug wallwart use. Same compact form factor but improved sound and longer lasting LiPo battery. Still in development stage. Will be a group buy in order to get manufacturing cost for a RTR amp down.

https://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-post5974736.html


----------



## DBaldock9

xrk971 said:


> Hi Folks, I haven’t been here for a while but thought you all might be interested in a new project brewing over on DIYA.  It’s going to be a new PCA Mk II.  This time fully assembled and tested and ready to run (RTR) all SMT design.  You just need to install in your own mint tin or 3D printed case.  I will provide STL file.
> 
> Main features are still SE Class A amp but this time, using hybrid JFET input and BJT output in a complimentary feedback pair (CFP) topology that had better dynamics and lower distortion.   Built in 4100mAhr LiPo with battery management and LED fuel gauge, DC-DC step up for stable 18v output, and MOSFET cap multiplier for smooth slow startup to prevent turn on thump. USB charging or 24v wallplug wallwart use. Same compact form factor but improved sound and longer lasting LiPo battery. Still in development stage. Will be a group buy in order to get manufacturing cost for a RTR amp down.
> 
> https://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-post5974736.html



I would like one.


----------



## kukkurovaca

xrk971 said:


> You just need to install in your own mint tin or 3D printed case.



I'm not that handy with DIY stuff -- would I just be able to to have the case printed by a 3d printing service and then screw it together?


----------



## xrk971 (Nov 18, 2019)

Dbaldock,
Cool!  Please respond in the DIYA thread to show interest.


----------



## xrk971 (Nov 18, 2019)

I am looking at option to provide the 3D printed case for a nominal cost circa $30 - depending on how many people interested. There are many onlOne 3D printing services that will print for you if you provide a file.  We may be able to leverage more units to bring costs down.


----------



## xrk971

3D printed case version with 5000 mAhr battery.


----------



## xrk971 (Nov 26, 2019)

New thread on DIYA to discuss technical aspects of the new HPA with battery management system.  Amp will be called the Hakuin HPA. 

Block diagram of LiPo and wall-plug power management system:





https://www.diyaudio.com/forums/headphone-systems/345851-hakuin-se-class-hp-amp.html


----------



## M3NTAL

Big fan of how the Pocket Class A sounded with my IEM's and looking forward to the Hakuin. Any chance to keep us updated over here also? I'm not a DIY guy and not a member of that forum. It makes it hard to follow that thread.  Thank you!


----------



## xrk971 (Feb 28, 2020)

Testing of the Hakuin is going well. I have had a lot of help from member Itsallinmyhead on DIYA.  He built several prototypes and found an issue with the battery fuel gauge meter. Fortunately, the rest of the PSU works well: the smart charger, the DC-DC step up, the ultra low noise LDO regulator, and the solid state relay (SSR) anti turn on thump delay works as well.  Now for the sound, I have built that myself and initial tests have revealed it to be a superb sounding amp. Much more highly resolved than almost any other amp I have heard.  Despite being only 2 transistors- the sound is superb. Great bass dynamics well beyond its size and power rating.

We are about to implement the fix to the fuel gauge system (LEDs).

Here are some photos of the prototype being tested with an external PSU.  Itsallinmyhead built the PSU and I’ll share those photos later.

if you look at the photos, you will notice some multi legged IC’s.  Those are not opamps but dual low RDson MOSFETs used as SSR’s and their 4 pin opto-coupled controllers.  Audio signal is still opamp free.


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## xrk971

Here is a re-post if member Itsallinmyhead’s listening impressions in case you can’t access DIYA.

Link to DIYA review



> *Scuse Me While I Kiss The Sky*
> I had the opportunity to A/B the PCA and Hakuin for a few hours while I was waiing for the Hakuin’s battery to drain down, so I could check the “fuel gauge” circuit.
> 
> *Background*
> ...


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## DBaldock9

I'm interested in how the Hakuin sounds, when driven by a Radsone ES100 or a FiiO BTR5.


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## xrk971

I am using an SMSL Sanskrit 10 which has a nice AK4490 DAC - one of the better ones out their IMO. Perhaps similar in signature to the Radstone's AK4375A.  Sources included mix of FLAC and WAV files from either Jriver or Amazon HD streaming.  You definitely can hear the difference with the Hakuin.


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## gikigill

xrk971 said:


> I am using an SMSL Sanskrit 10 which has a nice AK4490 DAC - one of the better ones out their IMO. Perhaps similar in signature to the Radstone's AK4375A.  Sources included mix of FLAC and WAV files from either Jriver or Amazon HD streaming.  You definitely can hear the difference with the Hakuin.



Long time between drinks Viet, looking forward to the Hakuin now. The original Low and Hi Impedance PCA are still going strong.


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## xrk971

Hi Gill,
Yes it has been ages and 30 amps later. The Hakuin is a really nice amp. I still haven’t had time to finished the pocket Hakuin with LiPo and DCDC and charging system through. Just not enough interest from folks. I have a module version of it for the Yarra as a preamp and that works very well too.  
Cheers,
xrk971


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## gikigill

Do you have any working prototypes for sale. I am definitely interested in the Hakuin.


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## SDBiotek

I'd be interested in a finished pocket Hakuin myself.


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## xrk971

Maybe I should have asked over here if folks were interested. Although this thread has been here for a while and nothing until today.  I don’t have any pocket Hakuin prototypes fully working yet. The amp part works but the charge indicator LED fuel gauge still needs work.


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## gikigill

Happy to wait and can you give me a price please as I'm happy to put in a deposit of 20%.


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## gikigill

Is the LED gauge necessary if a blinking light will do?


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## M3NTAL

I'd be interested in a 120v wall powered version. A dedicated amp for hi-end balanced armature users.  We are all stuck using an iFi adapter or reducing digital volume in the source.


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## xrk971

A wall powered Hakuin?  That’s actually available as a daughterboard module on the Yarra preamp/headphone amp main board.  

https://www.diyaudio.com/forums/group-buys/334457-yarra-preamplifier-hpa-melbourne-db-buy-124.html

It just needs a 24v single rail supply. Provide inputs and outputs and you can make it work as a plank amp too.

btw, in case you don’t know what anYarra is - interchangeable amp cores. The Hakuin would be a small daughterboard that sits on the main board.
Hakuin cores on a plank amp:



 Yarra preamp:


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## iFi audio

M3NTAL said:


> We are all stuck using an iFi adapter



iEMatch saved many setups. It ain't that bad


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## pure5152 (Jun 29, 2022)

I've been an avid lover of my PCA for a couple years now and just discovered the Hakuin!  It looks freaking fantastic.  I saw the last update posted was on the DIYA forums with an updated picture of the proto.  Given it's been half a year, any new updates about it? :O  I'm very interested in it given performance seems to be a step up over the PCA.


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## Peter19500

xrk971 said:


> Here’s a new spin on this amp. Using an ultra low 117uA bias current in the JFET (via new DC setpoint resistors), the sound is sweeter (lower third harmonic relative to second) and lower overall THD, and more dynamic through higher gain. It seems that the input signal had to control a much lower bias current and this seems to give the sound an immediacy and naturalness that’s close to LIVE sound. It’s an experiment that is turning out superb. It’s what I am listening to at present as my main portable amp.
> 
> https://www.diyaudio.com/forums/gro...-pocket-class-headamp-gb-189.html#post5750411
> 
> ...


I bought the circuit board and transistors from you on Etsy.  It works very well.  Could I make this modification?  I only use these headphones: Gjallarhorn GH50 Jm.  
Driver Graphene-coated multilayered composite

 Driver Unit 50 mm

 Frequency Response 
10-50000 Hz

 Sensitivities  116 dB

 Impedance  33 Ohm

 Maximum Input Power 250mW

  Thank you for your response.


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