# doobooloo's TPA6120-based balanced headphone amp



## doobooloo

Hello everyone!

 So I am extremely bored over summer, and I was reminded of my old TPA6120-based balanced headamp project. I was excited as I found something to kill time with, but I realized that when I reformatted my computer a while back all my Eagle files and libraries that I had built up had perished.

 But guess what. That just gave me another reason to design the same amp from scratch, just much better this time. 
	

	
	
		
		

		
			





 So after a few days of playing around, here is my design on a 2.5" by 4" board (fits in the smallest Lansing case):

 Everything:





 Just pads and traces:





 Top layer:





 Bottom layer:





 Click here for higher-resolution pictures.

 So what makes this amp exciting? To list a few features that I really wanted when I was designing this amp...

 1. It is a fully balanced amp
 2. It is extremely high-performance (TPA6120)
 3. But, it takes single-ended inputs, allowing the use of numerous (read: simple, cheap, small, proven, good) sources
 4. Outputs are in the form of two 1/4" jacks - one inverted and the other non-inverted - which allows the use of conventional headphones and balanced headphones with the use of a simple adapter cable
 5. It employs the DS1802 digital potentiometer coupled with a rotary pulse switch allowing for easy and precise volume adjustment as well as a higher sound quality, better channel matching, and long-term durability compared to analog pots
 6. All necessary jacks can be mounted onboard - at the same time standard 0.10" pads are provided for external wiring and the use of 3-pin Molex KK connectors if desired
 7. A desired sound signature can be achieved by opamp selection (since it's SMD, no easy opamp rolling, though)

 I used Eagle Light to design this board, so no groundplane fill function is available. Also, board space is limited by the Light version, and the only reasonable board size within these limitations with a widely available case was 2.5" x 4" using Lansing MicroPak cases.

 Anyway... I have a breadboard prototype amp employing a very similar amp circuitry running and it sounds damn good. With a more optimized board design the amp section can only sound better... and this is very exciting.

 The part that I am a bit nervous about is the DS1802 digital pot section which I took a lot of hints from Armond Chen's design and the accompanying Headwize thread. I replaced the LM336Z-2.5 with an LM4040 since the latter has higher accuracy and is available in a smaller surface-mount package. Other than that, I am pretty much following Armond's design for this board.

 Anyway... any comments?


----------



## doobooloo

Oh, forgot to add... The amp requires a bipolar power supply, so something like the Elpac WM71 (+15V/-15V) with a male DIN-5 plug. Different PSU manufacturers, including Elpac, seemed to be using the same DIN-5 standard for dual and triple output power supplies so I thought that would be nice in terms of flexibility.

 Of course, my opamp of choice is the AD8066 which can only take up to +12/-12V... I am probably going to build myself a simple dual-TREAD bipolar supply to go with this unit or make another small matching PSU PCB so that the two cases can neatly be stacked or something...

 Another thing I forgot to add again... All the surface mount resistors and capacitors are in 0603 package.


----------



## ATAT

How are you splitting the single ended signal to balanced? 
 DRV134s? those arn't that great last I checked.. the high performance amp circuit may be for nothing if you're using line drivers as splitters..


----------



## doobooloo

Quote:


  Originally Posted by *ATAT* 
_How are you splitting the single ended signal to balanced? 
 DRV134s? those arn't that great last I checked.. the high performance amp circuit may be for nothing if you're using line drivers as splitters.._

 

Two dual opamps take in signal, acts as buffer into TPA6120 providing low impedance output. One TPA6120 is set at positive gain and the other at negative gain. Gain magnitude should both be equal, of course, just inversed.


----------



## Garbz

True the DRV134 isn't great but it's not a bad rail splitter either. I found it better then discrete solutions when I tried it but it did fail to outperform Transformers.

 My guess looking by the layout, one of the opamps will flip the signal.

 The 2.5V -2.5V option for running the DS1802 works like a treat. The problem I had when integrating it in my balanced amp was that the input signal is often overpowering. I think 10V p-p are some of the balanced output signal swings, so I had to put a 10k resistor in series with the input to bring the level down slightly. A studer cdplayer that becomethemould dropped off here one day did however still clip the input.

 The other thing is what rotary pulse switch are you using? Double check the datasheets to ensure the pulse time of the switch is long enough. I used a quadrature rotary encoder and the decoder for it didn't pulse long enough for the chip to register a click. In the end Daki][er sent up a uC with a new decoder programmed in.


----------



## doobooloo

Quote:


  Originally Posted by *Garbz* 
_True the DRV134 isn't great but it's not a bad rail splitter either. I found it better then discrete solutions when I tried it but it did fail to outperform Transformers.

 My guess looking by the layout, one of the opamps will flip the signal.

 The 2.5V -2.5V option for running the DS1802 works like a treat. The problem I had when integrating it in my balanced amp was that the input signal is often overpowering. I think 10V p-p are some of the balanced output signal swings, so I had to put a 10k resistor in series with the input to bring the level down slightly. A studer cdplayer that becomethemould dropped off here one day did however still clip the input.

 The other thing is what rotary pulse switch are you using? Double check the datasheets to ensure the pulse time of the switch is long enough. I used a quadrature rotary encoder and the decoder for it didn't pulse long enough for the chip to register a click. In the end Daki][er sent up a uC with a new decoder programmed in._

 

Thanks for the comments! The opamps are both configured identically, and it is one of the TPA6120 chips that does the flipping.

 Regarding the input clipping... maybe I should make space for that option on the board. Two resistors to form a voltage divider would be better than a simple resistor though, right?

 The rotary switch was simply taken from the Headwize thread. I have no clue if it will work well or not. I'll just have to try it - but for now that's really the only option I have since I don't want to set up a uC... 
	

	
	
		
		

		
		
	


	




 If the prototype board fails with the rotary switch then I'm going to have to redesign with two pushbutton switches or something... 
	

	
	
		
		

		
		
	


	




 Also... regarding aos's comments on the Headwize thread:

  Quote:


  Originally Posted by *aos* 
_It looks pretty sweet. One thing I'd reccommend is a small inductor or ferrite to isolate digital from analog part of 5V rail to reduce HF noise' conquest of the volume pot._

 

Is this necessary? I think I've heard some arguments both for and against this "dividing" of analog and digital grounds with an inductor or ferrite, so I am not sure what to do. I omitted it in this design but it wouldn't be too hard to add space for it.

 Actually, maybe I can just make space for it so I can play around and simply jumper across it if it's not necessary?


----------



## doobooloo

Also, any suggestions for a PCB house for this small board?


----------



## Jazper

sparkfun.com used to be ok for small boards.. might want to try them

 thoughts:

 1. not fully balanced, fully balanced would have balanced inputs and balanced outputs.

 2. TPA chip works fine on +12/-12v and with balanced config the extra 6v swing in running 15/15 will make very little difference.

 3. Amps in this configuration tend to have the sonic signature of the TPA chip rather than smd opamps, provided the smd opamp is fast enough. Very little difference in sound signature with the input opamp selection. 

 4. Wouldn't it make more sense to just have 4 electrolytics bypassed with 4 polyprop or polyester caps (possibly even 4 silver mica as well if you want to go extreme) to filter the power??

 5. it looks like you have input ground connected to power ground which could cause a ground loop. Suggest maybe using a ground loop breaker or sticking to one or the other.

 6. considering you're happy to mount resistors/caps on the opposite sides of the board you might want to think about mounting the resistors/caps on the tpa right under the pins or under the chip.

 7. nothing stopping you having the input opamps socketable.


----------



## ATAT

dooob. i think what AOS says is a good idea.. putting a ferrite there would help a fair bit with the hf noise (if any).. 

 4pcb is by far one of the nicest boardhouses evar. $33 / 60 square inches.. if you're a college student that is.. its a very nice deal to say the least.


----------



## nikongod

unlike ******* in other sections, i line by line because you roxor.
  Quote:


  Originally Posted by *Jazper* 
_sparkfun.com used to be ok for small boards.. might want to try them

 thoughts:

 1. not fully balanced, fully balanced would have balanced inputs and balanced outputs._

 

find a portable balanced source. this amp looks VERY well sized for portable use.

  Quote:


 2. TPA chip works fine on +12/-12v and with balanced config the extra 6v swing in running 15/15 will make very little difference. 
 

 it is actually closer to 12V of swing because it is balanced. FWIW the swing with a 12V sup-ly would be DOUBLE the swing of a single chip with a +-12V input probably about 40vP-P without getting anywhere near the rails yea, you dont need this.... drop suply to +12V/-12V. alternately, a 2X9V system could be used... you would still have about 32vP-P swing. can anyone say "uberportable"

  Quote:


 3. Amps in this configuration tend to have the sonic signature of the TPA chip rather than smd opamps, provided the smd opamp is fast enough. Very little difference in sound signature with the input opamp selection. 
 

 the tpa chip has the sound signature that is to sound like the source... it is VERY uncolored. i would wory more about the input opamps than the tpa chips.

  Quote:


 4. Wouldn't it make more sense to just have 4 electrolytics bypassed with 4 polyprop or polyester caps (possibly even 4 silver mica as well if you want to go extreme) to filter the power?? 
 

 the chip used is SOO insensative to power suply "quality" that it is really unnecessary.
  Quote:


 5. it looks like you have input ground connected to power ground which could cause a ground loop. Suggest maybe using a ground loop breaker or sticking to one or the other.

 6. considering you're happy to mount resistors/caps on the opposite sides of the board you might want to think about mounting the resistors/caps on the tpa right under the pins or under the chip. 
 

 you may run into space/heat issues from the power-pad solder point.
  Quote:


 7. nothing stopping you having the input opamps socketable. 
 

+1, DO IT.


----------



## doobooloo

Thanks for all your replies! Regarding some comments...

  Quote:


 dooob. i think what AOS says is a good idea.. putting a ferrite there would help a fair bit with the hf noise (if any).. 
 

What impedance and DCR values should I look for when choosing a ferrite for that position? I'm looking at the Vishay-Dale SMD multilayer ferrite beads on Mouser here: http://www.mouser.com/catalog/626/710.pdf and I'm not quite sure which will work best.

  Quote:


 drop suply to +12V/-12V. alternately, a 2X9V system could be used... you would still have about 32vP-P swing. can anyone say "uberportable" 
 

Yup, +12/-12V would be ideal, since many nice opamps will not tolerate higher anyway. 
	

	
	
		
		

		
		
	


	




 2x9V is very interesting but battery life probably will suffer...

  Quote:


 find a portable balanced source. this amp looks VERY well sized for portable use. 
 

Why find a portable balanced source if the amp takes unbalanced? I was thinking of mating this with the Alien DAC or something that's small which is much easier to find unbalanced than balanced... All I care about is the balanced output, really. 
	

	
	
		
		

		
		
	


	




 Anything balanced that's cheap and small is likely to have balancing mechanism that's crappier (using lesser parts) than what I've done on this board anyway.

  Quote:


 considering you're happy to mount resistors/caps on the opposite sides of the board you might want to think about mounting the resistors/caps on the tpa right under the pins or under the chip. 
 

Some SMD resistors and caps (related to power and ground) are mounted under the board. I tried to minimize the use of vias in the signal path so all the resistors in the signal path are on the top layer.

  Quote:


 you may run into space/heat issues from the power-pad solder point. 
 

Heat issues? The TPA that I'm running now doesn't get that hot - with a small "groundplane" attached to the powerpad solder point, will heat be such an issue with what I've done?

  Quote:


 nothing stopping you having the input opamps socketable. 
 

I personally didn't like the DIP socket idea - potential stability problems with using higher speed opamps on adapters and sockets and space issues as well. Also, more of my favorite opamps only come in SOIC anyway so why bother with DIP? 
	

	
	
		
		

		
		
	


	




 Anyway, thank you so much for all your comments so far. I'll try to get a revised version up soon with your feedback applied!


----------



## motherone

With the amount of board space you're dedicating to the caps, any reason you could just use an AC input (either voltage doubler with single AC, or CT Xformer) and just go with an on-board regulated bipolar PSU?


----------



## Garbz

I personally don't like the idea of inductors in HF or switching applications, which is what incidently a lot of people recomend them for. The main problem is implimentation. I may say at this point too that while testing the DS1802 for my amp I ran them from a very basic shunt based on the amp's powersupplies. A simple capacitor on the outside was all the filtering I put in and there is precisely nada difference from going to a separate supply. So that said I wouldn't bother with an inductor.

 If you do pick an inductor you should pick one that minimally reacts with the capacitor beforehand. Putting an Ls, Rs, and Cs in the same circuit is a recepie for ringing. Info on how to impliment this correctly: http://www.hagtech.com/pdf/snubber.pdf, http://www.designers-guide.org/Design/bypassing.pdf.


----------



## Nisbeth

Quote:


  Originally Posted by *doobooloo* 
_I used Eagle Light to design this board, so no groundplane fill function is available._

 

Don't know if this has been covered, but why not? Select the polygon tool, type in the name of you ground net and hit enter and you can draw a ground plane. When you're done, use the ratsnest tool to render it 
	

	
	
		
		

		
		
	


	




 /U.


----------



## 00940

A ferrite will create no problem (at higher frequencies, its inductance is about zero) and they're dirt cheap. Pick a ferrite with enough current capacity and a middle of the road resistance.


----------



## guzzler

Well worth a read:

http://www.tentlabs.com/Info/Article...decoupling.pdf

 Specifically applicable for digital designs, but the principles still remain


----------



## Garbz

another good one for my library thanks.


----------



## 00940

here's an idea.

 The OPA1632 differential opamp can be viewed as two inverting voltage feedback opamps. What about using a TPA6120a2 in its feedback loops ? You get differential or single ended inputs, both section of the TPA6120a2 work now non inverting, you get SUSY (see the X-Gainclone thread on diyaudio), you can fit the 10R resistors inside a feedback loop and reduce their impact.


----------



## doobooloo

First, thanks again for all your replies!

 Over the past few days and all this weekend I've been working on porting this design from Eagle Light to Pad2Pad. Two reasons for this move were:

 1. Pad2Pad is free and has no limitations of Eagle Lite
 2. Instant price quotes and reasonable prices

 Anyway, with this I've lengthened the board by 1" - for a total board size of 2.51" x 5". This still fits the smallest Lansing Micropak cases.

 Here is a preview of what's cooking:







 Click here for a high-resolution picture.

 So... as you can see it is very incomplete with all the wrong part names. I'll get those fixed and post a BOM with all the part values and numbers soon, I'm working on it right now as I move towards completing the board layout first. A few changes from the previous version are:

 1. Isolated analog and digital grounds with a ferrite underneath the DS1802 chip
 2. "Separated" power supplies - a TLE2426 for the volume control, and a TLE2426 followed by a BUF634 for the amp sections. Plenty of room for capacitance both prior and after the rail splitters. This allows for the use of any single-pole power supply, and allows for a more compact standard 2.1mm/2.5mm power jack onboard. Actually, the height restriction of the large DIN-5 bipolar power jack was the big issue (the Micropak case would not have acommodated the jack), with this solution hopefully a better "channel separation" between the two inverting and non-inverting sections will be achieved.

 Why did I not go for some onboard regulation? That way I am more limited with regards to power supply choice - I was thinking of a +/-10V virtual ground with two positive and negative regulators as well but realized that this would limit the possibility of using a battery pack for portable use or other lower voltage power supplies.

 Anyway... I also added a power LED between the output jacks, and am planning to add a 1/8" input jack between the power and RCA jacks. Oh, and I replaced the crappy RCA jacks from before and put in a much nicer dual RCA jack.

 In defense of all that excess capacitor space... I am planning to use a slimline case so the max capacitor height is going to be around 15mm. The diameter is 10mm. Hence, especially with boutique caps, I won't be able to get much capacitance in there without having so many - but if anyone feels strongly about reducing the rail capacitance and putting something more interesting in there (I was thinking onboard USB DAC or just cutting back board size to 4" length)... please let me know!

 Anyway, at this size the board is expected to cost about $8~$9 when I order 20. Not too shabby. 
	

	
	
		
		

		
		
	


	




 Any criticism or suggestions welcome!


----------



## doobooloo

Oh, forgot to add:

 1. The groundplane is now larger for each channel, extending all the way to the rail capacitors.

 2. All SMD components are now 1206 size, before they were 0603. The output resistors for the TPA6120s have also been changed to SMD 1206 resistors.

 3. Changed the voltage reference diode package to the more common TO-92, allowing the use of a greater selection of diodes.

 4. All traces are now 20mils, except for a few short sections (between IC pads, for example) where they are 10mils.

 5. The jacks have been laid out for a 3mm front panel and 1.5mm rear panel. I am planning to use FPE for the panels.


----------



## nikongod

a couple things i want to ask:

 1: why not dump the "multiple" tle 2426's im favor of 1. the virtual ground is buffered seperately for each chip, so why not? the possibility of one of the railsplitters "drifting" at some time is eliminated, and if nothing else the parts count would drop.

 2: any thoughts on the possibility of a switch for "turning off" the "second" tpa chip? this would save a little power when used in single ended mode. probably just a pipe dream though.

 added: 
 any thoughts on thickening the power suply traces where possible? again, it may not be necesary, but it probbaly wont hurt.


----------



## jcx

Quote:


  Originally Posted by *doobooloo* 
_Two dual opamps take in signal, acts as buffer into TPA6120 providing low impedance output. One TPA6120 is set at positive gain and the other at negative gain. Gain magnitude should both be equal, of course, just inversed._

 

Congratulations! you might want to know that you have found all of the ways I can readily imagine to give yourself the worst possible power supply distribution arrangement

 in a balanced amp there is no external driver gnd connection, the +/- drive currents are equal and opposite in one channel (R or L), this strongly suggests putting the +/- polarity amplifiers for one channel in the same dual op amp chip - one chip for R and the other for Left

 since the +/- ps currents can be equal and opposite the least external magnetic field coupling can be had by putting the +/- supply traces as close together as possible – you have done the opposite by putting them as far apart as you can, this gives large loop areas for the rectified drive current (when operating Class B at higher levels) in each supply to spray this nasty nonlinear magnetic field over all of the signal traces in your layout

 By putting both + channels in one dual op amp and both – in the other you actually force the output currents for both channels to complete their loops through your gnd traces to make up the current balance otherwise inherent in balanced drive

 By forcing output current to unnecessarily complete through the gnd you are using many large caps between the +/- rails and ground, putting them in series relative to the traces that should be delivering the drive current, reducing the cap’s effectiveness by half

 By forcing +/- supply currents to complete through a pair of buffered active gnds that aren’t even connected together you have both driver’s currents flowing through your gnd buffers – adding cost, complexity, and potentially distortion

 More comments in this general application are in another tpa layout thread:

http://www6.head-fi.org/forums/showthread.php?t=127473

 Since you expressed a need to avoid boredom you will I hope welcome this opportunity to ripup your layout and start over


----------



## doobooloo

Thanks for your comments, jcx!

 First, regarding power supply line arrangement... what about something along the lines of this...






 And with only one buffer driver and shared ground?

 Also, regarding this comment...

  Quote:


 in a balanced amp there is no external driver gnd connection, the +/- drive currents are equal and opposite in one channel (R or L), this strongly suggests putting the +/- polarity amplifiers for one channel in the same dual op amp chip - one chip for R and the other for Left

 By putting both + channels in one dual op amp and both – in the other you actually force the output currents for both channels to complete their loops through your gnd traces to make up the current balance otherwise inherent in balanced drive 
 

The only way to do this in my design (single ended -> balanced) as far as I can see is to add an additional buffer to provide a low-impedance output to two pairs of dual opamps, one inverting and the other not, and then switch the signals around so that each TPA gets either both +s or both -s as you mention. So, there's a tradeoff - of adding an extra opamp in the signal path, but having the TPAs not "fight" for output current.

 On the flip side, in that new configuration, if I ever want to use the amp in single-ended mode, the two chips would be fighting for current in that configuration instead, right?

  Quote:


 By forcing +/- supply currents to complete through a pair of buffered active gnds that aren’t even connected together you have both driver’s currents flowing through your gnd buffers – adding cost, complexity, and potentially distortion 
 

So, if I use the new layout above using a single buffered active ground for both, this problem is (at least slightly) alleviated?

 Also thanks for the link - I've read through the thread once but I'm not quite sure I get everything you and peranders talk about. 
	

	
	
		
		

		
		
	


	




 I'll have to read it through a few more times and study the board layouts and see what more I can do to improve mine. Meanwhile, please feel free to shoot down whatever I'm doing as much as possible!


----------



## doobooloo

I've also been creating a groundplane (shared between the two "channels") with polygons - rather painstaking job but it's coming together rather nice. Also I'm going to dramatically reduce the number of caps and make the board shorter again to 2.51" x 4".

 I'll post some pics later today when I've cleaned the layout up a bit more.

 Any more suggestions/criticism welcome in the meanwhile...


----------



## skilpad

Hi, you could also consider the following app-notes as supplied with the 
 TPA6120A2 EVM board (slou169.pdf) fromTI.

 Please note their concerns and solutions regarding the ground plane's proximity
 to the input pins (The input is very sensitive to stray capacitance )

 Interestingly enough they opted for a ferrite bead in the supply line's
 330R @ 100MHz.

 Hope you get it working


----------



## jcx

If you want to build something really good it is better to be very specific about its purpose, balanced output can be a good approach to some problems, single ended is OK too but trying to make the same hardware do both will likely compromise the performance. Likewise you say you want to use a dual ps and a single as a secondary option – if you really intend to build a dedicated supply I would go with a dual supply only, have a “real“ ground and not play with active ground buffers at all, this choice makes the 1st option of balanced/single ended operation far easier 

 A slight twist that may let you have some options in one layout is to use stuffing options – 2 different circuits can be built up on the common pcb, but you can only have one choice for each assembled pcb – only practical when most of the circuit functions are common

 But in any approach it really is helpful to use the schematic drawing function and keep circuit and layout in sync and avoid gross errors of miswiring – and of course keep straight what it is you are trying to build as well as communicate with others

 The ps wiring in the previous post is much better if you use balanced load, putting the +/- from one channel in the same dual is still likely to be a little better. But if you really want to have a good layout for single ended load too you have to put gnd plane under the ps traces and get much lower Z gnd out to the output jack – even layout is influenced by operating mode

 I can't imagine having the extra op amps you are using to buffer the digital pot and not using them to make multiloop amps servoing the tpa's - even a simple multiloop with a +2 gain tpa greatly reduces the distortion compared to your use of the buffers to drive the low Z of the inverting tpa input R

 Try reading Walt’s chapter on audio amps

http://www.analog.com/library/analog...h6_final_I.pdf

 pdf page 50 + 6.48 in book, heavy duty current boosted multiloop stuff starts ~ 6.60


----------



## doobooloo

Thanks again for all your interest! 
	

	
	
		
		

		
		
	


	




  Quote:


 If you want to build something really good it is better to be very specific about its purpose 
 

Well - to get one thing straight... I'm not aiming to create the ultimate high performance balanced amp here. I'm trying to create a very small amp capable of driving both single ended and balanced headphones from a single ended source and standard 24V wallwart power supply or battery pack. I wouldn't really mind sacrificing some of the design "optimality" to get the above... Of course, a clean true biploar power supply would be better, but if I can get away with a buffered active ground then that's good enough for me. Running two TPA chips with one doing + and the other - may not be optimal for running balanced phones but if the audible differences are merely subtle compared to both +/- on one chip then I really wouldn't mind.

 Size and convenience are really the key drivers here, I guess.

 The thing is - I have a prototype of this rough design on a perfboard with all through-hole components and the TPA6120 mounted on adapters (gasp!) and in sockets (gasp!!!). Grounding is a horror, it's all over the place. I'll be the first one to say that it's a horrible way to make use of these fine chips, but guess what - they sound great even so. So, I am sure that doing a slightly more optimized PCB layout is only going to improve things and I'd be very happy even if it's not 100% optimal. 
	

	
	
		
		

		
		
	


	




 At the same time - that said, I'd definately like to see the last drop of performance squeezed out of this configuration _*given the design criteria limitations*_.

  Quote:


 But in any approach it really is helpful to use the schematic drawing function and keep circuit and layout in sync and avoid gross errors of miswiring – and of course keep straight what it is you are trying to build as well as communicate with others 
 

I would love to draw a schematic and link it to the board layout but the Pad2Pad program has no such function. 
	

	
	
		
		

		
		
	


	




 I'll at least re-label all the parts once the design is more or less solid. Maybe once I think things are done I'll try to draw the schematic by hand to make sure I've got everything correct.

 Meanwhile then... any good simple free schematic drawing programs out there? 
	

	
	
		
		

		
		
	


	




*Anyway, the newest revision...*

 All layers:





 Top only:





 Bottom only:





 So yeah... I drew that groundplane by hand by stitching numerous polygons... 
	

	
	
		
		

		
		
	


	




 Hopefully it's worth that effort. Groundplane clearance is 20 mils. Since the DS1802 chip groundplane is not the -2.5V ground but the same 0V virtual ground set up by the TLE.

 Some changes:

 1. Obviously, the groundplane. Of course it's not there under the input pins to minimize capacitance as recommended per skilpad and TI.

 2. Power supply. Traces put close together between the two "channels" and using only one active buffer for both. Capacitance post-buffer has been reduced to four caps, but cap diameter has been increased to 12.5mm. The DS1802 uses a single TLE2426 and four 6.3mm caps after the virtual ground. There are four 10mm caps before both virtual grounds and the power jack. I forgot to put a protection diode - I'll do so in a later revision.

 3. I can't fit a 3.5mm jack at the rear anymore! 
	

	
	
		
		

		
		
	


	




 I can if I remove one of the rear 10mm caps but... but...)

 Is there anything I'm still doing horribly wrong? I'm sure the amp will "work" and produce sound fine but given the design limitations above I would like to know if there's anything else I can do to improve performance. Thanks again in advance!


----------



## doobooloo

Quote:


  Originally Posted by *skilpad* 
_Interestingly enough they opted for a ferrite bead in the supply line's
 330R @ 100MHz._

 

I noticed that too - do you think it's a good idea on this board as well?


----------



## skilpad

Maybe,


 You should however place it as close to the DC input connector as possible 
 And it should be the first series component after the DC input proper.
 (Please check total current consumption + peak current of total loads so that 
 the bead don’t saturate, or current starve the circuit)

 A potentially better solution would be to use an external Clamp-on Ferrite tube 
 over the entire DC cable just before it enters the DC input connector
 This way people could use it, or not and experiment with different types, and
 Amount of turns around the ferrite tube.

 Still on the filter subject, one could use a series resistor Say 10 to 50 Ohm in the positive supply to the
 Volume control circuit between the closest big capacitor and the rest of them feeding the TPA’s

 The idea is to isolate the capacitors decoupling the volume control circuit from the current peaks 
 Drawn by the TPA’s

 All of this is of course to extract the very last bit of performance and should be viewed more
 As academic than absolute requirements.


----------



## jcx

I still advocate jumpers, by using twisted or star quad magnet wire for the horizontal run to the tpa's power you can have a nearly solid ground plane, the V+, V- currents in a tightly coupled twisted jumper radiate even less than the present pcb trace, when the jumper is taped down to the backside ground plane it is closer than traces running over solid gnd plane too (a few mils of varnish on magnet wire vs 0.062" of FR4)

 I've also put together this LtSpice schematic/sim of the lowest risk multiloop approach that works in your amp - no extra op amps, only a couple of extra resistors in the outer feedback loops






 U2,4 are just your buffers recycled to improve amp performance, there is still high Z at U2 input for the pot (LT1022 is a fet op amp input similar to opa134, ths6012 is the industrial version of TPA6120, the LT1022 model is included in free LtSpice Sw CAD III http://www.linear.com/company/software.jsp )

 some people object to inverting the amplified + gain circuit's output to get the - side drive on "unequal delay/gain" grounds, complete nonsense for audio frequencies with these op amps - the improved quality of the multiloop and convenience of the circuit implementation more than make up for the "error":

 in the sim the +r,-r phase difference is 179.833 degrees => 0.167 degrees error from the ideal 180, this is at 20 KHz !, actual "phase error" across the headphone is 1/2 that value - how much and how reproducable do you think dynamic headphone's phase is at this frequency? (if you've got hair you can't even put the same pair of headphones on twice with this phase accuracy @ 20KHz)

 if you use the 1st inverting amp topology in my diyAudio thread:
http://www.diyaudio.com/forums/showt...806#post512806
 the +r,-r phase diff reads 179.999 in the 20 KHz sim

 its not just Walt pushing multiloops:
http://focus.ti.com/lit/an/sboa002/sboa002.pdf

 [rename the attachment, stripping the .txt to run in LtSpice]


----------



## nikongod

the good thing about just about any multiloop boards is that the multiloop is easy to bypass. 

 single feedback is enough when no feedback is soooooo nice.


----------



## doobooloo

Quote:


  Originally Posted by *nikongod* 
_single feedback is enough when no feedback is soooooo nice._

 

Not quite sure if I got this... I'm interpreting your post as something along the lines of "multiloop is not necessary" (please correct me if I'm off) but what exactly is no feedback?


----------



## jcx

probably saw "multiloop" and assumed unity gain buffers (as is popular here) not reading thread title or looking at any posts to determine that "no feedback" isn't on the table

 some people think that unity gain buffers don't have feedback and that feedback is bad for sound quality, since they also often believe that they can reliably tell us about universally applicable subjective effects of circuit topology without even taking elementary steps to level match and blind their comparisons - if they are even based on listening to any comparable hardware the same day - I think it is best to ignore them


----------



## doobooloo

OK I was going on about this update, but the browser screwed up and I lost everything. So I'm going to make it really short.

 1. No multiloop in this amp. Will implement in a more proper balanced amp project later.

 2. Replaced dual RCA (too tall) with single RCA jacks.

 3. Added 3.5mm jack.

 4. Corrected power supply line polarity.

 5. Added component names.

 6. Added .1" spaced holes at rear for panel mounting of other components or testing purposes.

 6. Created a full BOM.

 Now, pictures...

















 Full size pics available here.

 I'll post the BOM soon after I revise it a bit!

 Also, what should I call this amp?


----------



## rsabo

Quote:


  Originally Posted by *doobooloo* 
_Also, what should I call this amp?_

 

Doobooloo's Balanced Transportable?
 TPA-Inna-Box?


 doobooloo, get this done quickly! there's talk of a potential philly meet this september!


----------



## motherone

DBTPA? Doobooloo's Balanced TPA? Or maybe the MBA: Micro Balanced Amp


----------



## doobooloo

Quote:


  Originally Posted by *motherone* 
_Micro Balanced Amp 
	

	
	
		
		

		
		
	


	


_

 

Haha - Masters in Balanced Amplification.


----------



## ferds

hi doobooloo

 i've been following this thread since the first post. im very interested if there will be a group buy...

 excellent job doobooloo!!


----------



## doobooloo

Quote:


  Originally Posted by *ferds* 
_hi doobooloo

 i've been following this thread since the first post. im very interested if there will be a group buy...

 excellent job doobooloo!!_

 

Well - I'd like to order 20 boards for the first run since it's not too much more expensive than ordering 5. Anyone interested in prototyping (at cost)? 
	

	
	
		
		

		
		
	


	




 If prototyping is a success and people are happy with the results, I will ask Jude if a group buy for boards can take place.

 As a rough estimate, in case a group buy does happen (note the word "in case" here, I can't promise anything at this point of time)... the prices through Pad2Pad with the board as it is now are:

 50 boards: $4.79 each
 100 boards: $3.25 each
 150 boards: $2.72 each
 200 boards: $2.44 each
 250 boards: $2.27 each

 Meanwhile, a tentative BOM has been attached to this post. I sort of splurged on the parts, using expensive 0.1% Vishay resistors and so on... Including EVERYTHING - case, board, custom FPE panels, etc., the total cost comes out to around $130. Using cheaper passives, expect to save another $15 or so. Also, with potential group buy board prices, slash another $5 or so. Oh, these are all without shipping and handling, so potentially another $20 or more may be added for shipping.

 Once the board layout is finalized (I'm going to take a few more days to really double-check and triple-check for errors before I hit the order button) I'm going to work on the front and rear panels using FPE. I'll post pictures and original files here as well for anyone to play around with.

 Meanwhile... any more name suggestions? 
	

	
	
		
		

		
		
	


	




 A few that I like so far:

 dba
 diet
 mba

 [EDIT]

 Let's keep this thread on topic - meaning, no further discussion of group buy YET. I'd like to concentrate my efforts on development and prototyping first, and when that's complete we can discuss group buy possibilities in a separate official GB thread. Meanwhile I'd really appreciate further input and comments/suggestions to make this a better amp!


----------



## doobooloo

Just a minor update to the layout... Realized that the L/R input RCA jacks were switched around (and consequently the 1/8" jack too) so I fixed that.

 Picture:






 Large file here.


----------



## doobooloo

It's 5:50 am and I really need to sleep, but before I do that...

 Here's an initial design attempt in FPE of the front panel layout:






 The 1/4" jack holes have been "countersunk" at 45-degree angle by 0.025" to simulate a chromed rim. The volume knob is a 0.5" diameter kilo knob, and the hole is recessed so that the knob will look fitted. Since the screws are so close to the left 1/4" jack, they have been countersunk for a flat appearance so that it will not interfere with the operation of the headphone plug/jack.

 Total front panel width = case width = 2.75"

 Too tired to work on the rear panel right now, I'm off to bed... 
	

	
	
		
		

		
		
	


	




 [EDIT] Mounting screws are 4-40.


----------



## jcx

You might rethink multiple virtual grounds (Agnd = Amplifier, Sgnd=Signal input/pot ground)

 2 cases:

 they are always at identical potential (only a logical possibility not in fact happening here)

 – then they could have been connected together


 they are different (DC, AC or both)

 –then the difference appears in series with your signal input and is added(subtraced) to the amplified signal because the amplifier feedback network reference grounds are different from the signal input ground

 Your signal and amplifier gnds will be different in both balanced and single ended operation

 with balanced output load little current flows in the amplifier gnd and the Agnd will be a the DC potential set by its ref and buff Vos, as frequency increases the Agnd becomes the C divider determined point between the ps voltages (with their small dynamic V drops from load I) – the ps cap C divider ratio is unlikely to match either the Aref or Sref divider ratio to single digit % levels given Al electro tolerances

 with single ended drive all of the load current flows through the Agnd, with 32 Ohm cans and BUF634 you have to have enough ps-Agnd C to short out the paralleled load Z at 20 Hz (because the BUF634 has more output series Z than the combined headphone 16 Ohm Zload it can’t even hold the Agnd to < ½ of the amplifier output swing for signals in phase at the R,L outputs, like most deep bass frequencies)

 since you’ve even cut back on ps C # from the earlier layout I wonder if you’ve done the low frequency corner math for 32 Ohm cans and single ended output load


 By using a common gnd for signal input and amplifier feedback divider reference you turn some of the problems above that are direct cross coupling/channel isolation issues into power supply rejection issues and your op amps have much better psrr than you have room for big enough ps-gnd C to make the problems small enough to ignore


----------



## doobooloo

Thanks again for your help, jcx! 
	

	
	
		
		

		
		
	


	




 What about using JFETs to isolate rails as with the PPA and M3 to help with some of the problems you mention? Such as...






 Also, the TLE2426s are accurate within microvolts, so even if there is a difference in ground potential between Agnd and Sgnd, it can be safely regarded as negligible, right? Also since neither TLE is expected to draw anywhere near the max current capacity, both should be reliably accurate - well, that's my potentially faulty reasoning, at least.

 Finally, would putting in a capacitance multiplier for the amp section result in any noticeable difference?

 Thanks again!


----------



## doobooloo

Regarding capacitance multiplier...

 My rough initial attempts at the multiplier led to this:






 So, basically...

 TLE -> BUF -> Multiplier (2 caps) -> 2 caps -> amp

 Does this make sense?

 [EDIT]

 Oh, I haven't reconnected the groundplane yet, imagine that the groundplane extends all the way as in previous layouts...


----------



## doobooloo

Also, for the JFET isolation for the amp section... Since it's driving a hefty load (two TPAs plus 2 opamps plus cap multiplier) the current draw may exceed 100mA. So, can I use a pair of J105~J107 JFET N switches that have minimum IDSS of 500, 200, and 100 mA respectively?

 And probably just a PN4393 pair with min IDSS of 5mA would work for the DS1802?


----------



## jcx

A single ground plane is often not optimal, but is usually pretty good and a lot better than poorly implemented alternatives

 For this amp I would recommend a single ground plane with the buf634 and as much ps-gnd C as you can fit ( big ps C compromises bal load operation psrr but is necessary for single ended load)

 A extra pair of R-C sections to filter buffer op amp and digital pot local +/- ps could be helpful to give “clean(er) power” with the tpa’s directly connected to the big ps C and batt in terminals

 How much ps-gnd C? consider both channels same 1 V, 20 Hz signal w/32 Ohm cans, this gives 16 Ohm load which is in parallel with the buf634

 According to ppl, the buf634 output Z ~= 20 Ohms, simplify calcs with optimistic assumption of BUF634_ Zout = 16 Ohms

 Load || Buf634 = 8 Ohms, without any C the “gnd” would move ½ V with 1V signal!

 To reduce gnd AC impedance at 20 Hz requires lots of C,

 Z_C = 1/(2*pi*20*C)

 For Z_C = 1 Ohm, C = 8000 uF 
 (fortunately you get to count the + and – side caps as operating in parallel, as long as the external ps Z is low, << 1 Ohm )


----------



## jcx

I am very unhappy with the ppa team’s misnomer of “fet isolated” power supply – if the fets were used as ccs, which requires several V across them and the constant current was operating into a shunt regulator the term would be OK

 But as seen in too many schematics here the fets are merely operating as poor tolerance, slightly nonlinear resistors


----------



## doobooloo

Quote:


  Originally Posted by *jcx* 
_To reduce gnd AC impedance at 20 Hz requires lots of C_

 

Instead of physically putting more caps in there, would a capacitance multiplier as I implemented above work as well? After all, the "effective" rail capacitance given gain of 100 and using 2x 820uF capacitors in the multiplier will lead to about 164,000uF...?

  Quote:


  Originally Posted by *jcx* 
_I am very unhappy with the ppa team’s misnomer of “fet isolated” power supply – if the fets were used as ccs, which requires several V across them and the constant current was operating into a shunt regulator the term would be OK

 But as seen in too many schematics here the fets are merely operating as poor tolerance, slightly nonlinear resistors_

 

But still better than nothing, right? Are you saying that rather than FETs to "isolate" rails, a high quality resistor would do the job better?


----------



## doobooloo

OK fixed up the groundplane...


----------



## jcx

C multiplier might be useful for the low power filtered supply if your battery or wall wart output Z is lo enough – if the load current from driving100 mA peaks into low Z cans on both channels causes more than a few 100 mV ps droop then the C multiplier pass transistors saturate and you don’t have any gain to multiply your C with any more

 It is unable to help the single ended drive problem – draw out the current path – current must flow in closed loops that complete through your ps, then look at the AC and DC impedance in the loop and see where the voltage drops appear, remember the BUF634 has relatively Hi Zout - and you don’t want output current induced voltages in your signal path

 I think jfet “power supply isolation” is worse than a resistor – if your current demand spikes above the jfet Idss then it turns into a ccs, meaning it prevents the load from drawing any more current by collapsing the supplied voltage – as a fault protector maybe it is useful but it doesn’t do anything for normal operation that a resistor doesn’t and the fet makes clipping worse when the ps could have delivered the dynamic peak current you needed

 Why don’t you try using spice to sim some of these options to improve understanding of how they work – the LtSpice SwCAD III is a really great free spice – you do need other spice source material to learn how to use it though – its own doc is oriented to the advanced user

 for wasting time over the summer simulation ranks up there with layout


----------



## doobooloo

OK - how about this:

 1. Got rid of JFET rail isolation. Added two caps back on the board.

 1.5. Capacitance multiplier is still there, post-BUF.

 2. One TLE2426 for one virtual ground... the TLE drives the DS1802 and the BUF634, the BUF634 then drives the amp portion. Ground planes are therefore separated into two parts coming from the TLE - the one right out of TLE, and the one buffered. Signal ground is connected post TLE and pre BUF.

 Pictures:


----------



## t52

could you also post an image of the current schematics please?


----------



## jcx

I think you could learn some useful things about ps and active ground currents playing with this in LtSpice

 I've attached a buf634 model

 the ths6012 should be the same as the tpa6120 in my previous sim

 both buf and ths models appear to have output current reflected in the power supply pins (not all Spice models do)

 putting together parts from both sims you can model your circuit and "measure" the gnd, ps currents and crosstalk issues, find a good number for your expected battery series impedance and the model gets even more useful

 on your own til late tonite - have fun

 [the .asy file is the symbol for the buf634, since I modded a op amp to make it it is in my op amp lib\sym folder: C:\Program Files\LTC\SwCADIII\lib\sym\Opamps you have to add it to your LtSpice lib folder - some people prefer to keep custom/added parts in a seperate lib folder]


----------



## doobooloo

I apologize to those who have been following the thread... I'm out of town for a few days so I won't be posting any updates until probably next week. Not sure how many people are actually interested in what I'm doing, but thought I'd let this thread know anyway.


----------



## doobooloo

Looks like not too many people are interested in this project after all. 
	

	
	
		
		

		
		
	


	




 Anyway, I just got back from my trip and am kind of tired... I'm wondering, the buffered ground solution seems to work just fine in other amps, and the performance impact when used in balanced mode is going to be minimal so I'm just going to do a simple final check to make sure there aren't any stupid errors on the board and finally press the order button. I'm not an EE by training (was that obvious?) and it looks like my "have nothing better to do" days are going to be over very soon. So, I'm really feeling the pressure to at least get the first batch of boards done and have myself at least one completed amp before summer's over.

 I'll post the final board layout and if I have time, a schematic in the next day or so. It won't be too different from the last layout though. Anyone feeling experimental and interested in prototyping please PM me!


----------



## jcx

Independent of everything else I’ve mentioned it is not the best idea to have an alternate analog input thread past the power jack, why don’t you swap the upper input and the pwr jack?

 I‘m sorry my comments seem to have just confused you – power and active ground still far from what I consider minimally correct even within your constraints

 If I had to kick out this board in only a few hrs I would lose the ”C multiplier” (which isn’t even a C-multiplier without additional impedance between the supply and the Q base/C junction – are they even connected to the supply at all?)

 Tle and buf split the supply and produce a Single Ground for the whole circuit with One gnd plane

 All of the C you can fit divided equally between +,- rails and the gnd plane, hopefully > 5000 uF


----------



## mirlo

I'm certainly interested, but for me to really follow along, I need to be able to read a recent schematic ...

 -- mirlo

  Quote:


  Originally Posted by *doobooloo* 
_Looks like not too many people are interested in this project after all. 
	

	
	
		
		

		
		
	


	




 Anyway, I just got back from my trip and am kind of tired... I'm wondering, the buffered ground solution seems to work just fine in other amps, and the performance impact when used in balanced mode is going to be minimal so I'm just going to do a simple final check to make sure there aren't any stupid errors on the board and finally press the order button. I'm not an EE by training (was that obvious?) and it looks like my "have nothing better to do" days are going to be over very soon. So, I'm really feeling the pressure to at least get the first batch of boards done and have myself at least one completed amp before summer's over.

 I'll post the final board layout and if I have time, a schematic in the next day or so. It won't be too different from the last layout though. Anyone feeling experimental and interested in prototyping please PM me! 
	

	
	
		
		

		
		
	


	


_


----------



## MisterX

I can only imagine what a PITA it will be to hand solder those resistors when the pads are end to end like that.


----------



## Voodoochile

Quote:


  Originally Posted by *doobooloo* 
_Looks like not too many people are interested in this project after all. 
	

	
	
		
		

		
		
	


	


_

 

It's interesting to me 
	

	
	
		
		

		
		
	


	




.
 I'm just watching it unfold.


----------



## doobooloo

So here is another update reflecting the changes suggested by jcx:






 The groundplane (single, throughout entire board) will be filled in both the top and bottom layers, they were omitted here for clarity.

 So, there are two 10mm caps before the TLE and BUF, and twelve afterwards. Using 16V Panasonic FM capacitors with a height limit of 16mm, I can use 680uF per cap so 8160uF of active ground rail capacitance. Would this be sufficient?


----------



## jcx

Quote:


  Originally Posted by *doobooloo* 
_So here is another update reflecting the changes suggested by jcx:







 The groundplane (single, throughout entire board) will be filled in both the top and bottom layers, they were omitted here for clarity.

 So, there are two 10mm caps before the TLE and BUF, and twelve afterwards. Using 16V Panasonic FM capacitors with a height limit of 16mm, I can use 680uF per cap so 8160uF of active ground rail capacitance. Would this be sufficient?_

 

I'm fairly confident that this approach should yield a reasonably good performance amplifier 

 the remaining question for me would be whether all power supplies and switching arrangements would be happy with the ps C - with low impedance batteries charging currents could reach 10s of A - use a big switch, worry if the batteries might have internal overcurrent fuses - some sort of inrush current protection would be a good idea for a commercial product

 with a reg supply like steps the reg will (safely) current limit during turn-on for a fraction of a second, some types of regulators may not like the large C load on their output

 it is possible some unreg wallwart supplies might lose their diodes due to the inrush current

 these questions would have to be answered in a commercial product - for a hobby project you can just see what works and avoid what doesn't


----------



## doobooloo

Quote:


  Originally Posted by *MisterX* 
_I can only imagine what a PITA it will be to hand solder those resistors when the pads are end to end like that. 
	

	
	
		
		

		
		
	


	


_

 

Well the SMD resistors are 1206 size so they should be relatively easy to handle. Also, given that the spacing between resistors is going to be greater than the pin spacing on the SO20 parts, it should really be OK since hand soldering an SO part pin by pin isn't difficult at all as long as one has a sharp iron tip. 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *jcx* 
_I'm fairly confident that this approach should yield a reasonably good performance amplifier_

 

Yay! 
	

	
	
		
		

		
		
	


	




 Thanks for your help along the way. I really appreciate it!

  Quote:


  Originally Posted by *jcx* 
_the remaining question for me would be whether all power supplies and switching arrangements would be happy with the ps C - with low impedance batteries charging currents could reach 10s of A - use a big switch, worry if the batteries might have internal overcurrent fuses - some sort of inrush current protection would be a good idea for a commercial product

 with a reg supply like steps the reg will (safely) current limit during turn-on for a fraction of a second, some types of regulators may not like the large C load on their output

 it is possible some unreg wallwart supplies might lose their diodes due to the inrush current

 these questions would have to be answered in a commercial product - for a hobby project you can just see what works and avoid what doesn't_

 

I'm kind of worried about that as well - I've had several PSUs blow due to high rail capacitance in previous PPA builds... Oh well. As you say I'll have to see what works and what doesn't.

  Quote:


  Originally Posted by *Voodoochile* 
_It's interesting to me .
 I'm just watching it unfold._

 

Thanks! Well it looks like the project is going to wrap up soon...


----------



## jcx

since you seem comfortable slicing up gnd planes, a minor optimization would be to add a slot in the still uitary gnd plane to reduce output stage/ps cap gnd current from affecting some of the low level input signal

 starting at the rt edge cut a narrow slot in the gnd plane (top and botom layers) at the top edge of the uppermost signal input jack, follow its outline left and down, extend the slot downwards to just avoid the ouline of the lower rt ps cap and move left horizontally to near the digital pot chip and just stop the slot - essentially create a tongue of gnd under the input traces that is partially seperated from the ps C gnd

 the slot just stops, leaving a single gnd plane but keeps some of the ps cap gnd/output return current from adding a tiny voltage signal to the input jack gnds


----------



## doobooloo

Quote:


  Originally Posted by *jcx* 
_since you seem comfortable slicing up gnd planes, a minor optimization would be to add a slot in the still uitary gnd plane to reduce output stage/ps cap gnd current from affecting some of the low level input signal_

 

Did that, and I placed the order last night for 20 boards. Came out to $185.24 including shipping for 12-day turnaround, not too bad. 
	

	
	
		
		

		
		
	


	




 Anyone want to join me in this prototype phase? Please PM me if you're interested!

 I tried to upload some photos of the final layout but FTP somehow kept spitting out errors. But there's nothing fundamentally different from the last layout other than the input ground is somewhat isolated as per jcx's suggestion and there is a real groundplane both on top and bottom.

 Also, I noticed in another thread that LMH6321 may be a better replacement for the BUF634. Not only does it have higher current output capacity, but the datasheet shows its output impedance as around 5 ohms (increases slightly with higher sinking current), much lower than the "optimistic assumption" of 16-ohm for the BUF634. And plus it's actually possible to purchase them from Digi-Key, although they only have around 50 in stock so who knows when these will be out of stock as well. 
	

	
	
		
		

		
		
	


	




 Anyway, I'm heading up to NYC again to take care of business, hopefully I'll have some time up there to finish up the front panels and order all the parts so that in a week or two's time I can actually put this damn thing together finally. 
	

	
	
		
		

		
		
	


	




 Thanks for all your help and encouragement so far everyone!


----------



## nikongod

with this many pads for caps, if there is a problem of tooo much capacitance, we can always leve some positions empty, or use caps that are less "volumetrically" efficent. ex a BG 100uf that is the same case size as a 200uf elna.


----------



## doobooloo

Quote:


  Originally Posted by *nikongod* 
_with this many pads for caps, if there is a problem of tooo much capacitance, we can always leve some positions empty, or use caps that are less "volumetrically" efficent. ex a BG 100uf that is the same case size as a 200uf elna._

 

Yup. These are also 10mm dia caps and there's a ~15mm (IIRC, could be lower) height limit imposed by the case, so it will not be very hard to get a "reasonable" rail capacitance (if that's what one desires) with more boutique low density caps.


----------



## motherone

Doobooloo,

 I've been following your progress, and this project definitely looks promising. I may want one of your proto boards, but I'll have to wait until next month. Too many projects in the queue right now!


----------



## doobooloo

OK FTP finally works. Pics of what I ordered:

 All:





 Top:





 Bottom:





 Full size pictures here:
http://www.seas.upenn.edu/~doo/Head-Fi/TPA6120/v1final/


----------



## doobooloo

Updated front and rear panels so far:











 Decided to keep it simple and clean, after all...


----------



## doobooloo

Oh forgot to mention...

 Although the images are rather big, the amp is really small...

 Panel dimensions are only 2.75" x 1.124".


----------



## doobooloo

Here's an updated final BOM for the board. 
	

	
	
		
		

		
		
	


	




 [edit] Oops, I forgot to revise a cap description, uploaded an updated version. The part numbers were all correct in the previous version, just the part description was a bit outdated. Sorry!


----------



## doobooloo

Woohoo! The boards have shipped and are expected to arrive later today (8/2)!!!

 I'll take some photos of the boards later when I get a chance.


----------



## mikemacwillie

Still an option to get in on the proto phase? I could go for a couple boards..


----------



## Voodoochile

Quote:


  Originally Posted by *doobooloo* 
_Decided to keep it simple and clean, after all... 
	

	
	
		
		

		
		
	


	


_

 

Simple and clean never goes out of style!


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_Still an option to get in on the proto phase? I could go for a couple boards.. 
	

	
	
		
		

		
		
	


	


_

 

The boards arrived yesterday, all 20 and they look absolutely beautiful... 
	

	
	
		
		

		
		
	


	




 Only a few have been claimed so far so if anyone wants in, as I said before, just PM me with the number of boards you want and I'll get them out to you as soon as I can. It cost me $185.24 for the entire set, so that comes to $9.26 per board without shipping or PayPal fees.

 Currently I'm moving back and forth between home in Philly and NYC (I'm in NYC as I type this now, just came up today...) so I can't take pics of the boards now but they do look and feel very nicely done.

 Also, since I have accumulated a massive surplus of 10mm diameter 150uF (IIRC...) 20V Sanyo OS-CON low-impedance electrolytics during the past few years, I'll throw in fouteen pieces per board that can be used for all the C1~C3 positions FOR FREE. Sort of a "thank you" for prototyping this project, I guess. 
	

	
	
		
		

		
		
	


	




 Note though that since these caps are rated at 20V it may be a better idea to use higher rated caps for the two C1 positions (35V recommended) but otherwise (18V or lower power supply) this should work.

 I'll probably post an identical FS post later but I would like these boards to go to those who have been following this thread first... 
	

	
	
		
		

		
		
	


	




 [size=xx-small]Moderators - if this is not in line with Head-Fi rules, please feel free to delete this post or modify as necessary. Note however that this is only an invitation for prototyping and is not a group-buy (I only have 20 boards on my hands right now, with no planned additional order to take place in the near future). It is also not an offer for commercial sale either, as there will be no financial gain realized. Please let me know if there are any issues. Thanks![/size]


----------



## doobooloo

Oh, also if any prototyper's interested, I can order the necessary parts too and ship them over along with the board(s). Case and front panel I don't think I'll have time to order though... but I'll definately post the front panel express files and maybe another person can organize a mini organized purchase among the prototypers or something?


----------



## motherone

Dangit Doobooloo.. I'm trying really hard to resist your project. I already want a TPA based headphone amp, but I just have too much going on.

 I hope you can get a few folks to build some amps off those prototype boards. It looks like a great project!


----------



## doobooloo

OK... I'm flying out for a few months this week so I can't really do a parts buy. However, there are 10 boards left and I'll be shipping a batch (US only for now, please!) out on Monday so if anyone else is interested please let me know!


----------



## n_maher

Doo,

 Have you managed to build one off the proto boards to make sure that the layout works? I'm getting ready to place a parts order and it'd be great to know if there's confirmation that it's a go. 

 Nate


----------



## mikemacwillie

I just recieved my boards today.. I will start building soon.


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_Doo,

 Have you managed to build one off the proto boards to make sure that the layout works? I'm getting ready to place a parts order and it'd be great to know if there's confirmation that it's a go. 

 Nate_

 

Unfortunately I am out of the country right now and won't be returning until early November. 
	

	
	
		
		

		
			





 I barely had enough time to receive the boards, pack them, and get a few shipped out. I haven't even placed a parts order yet myself.

 So yeah, you may have to do a bit of troubleshooting, but I did double and triple check the layout on the screen before pressing the order button, so hopefully everything's all right.

 Please let us know how it goes. If anything doesn't go as expected I'll do my best to help out.


----------



## mikemacwillie

Just need a few more parts then it'll be ready to power up.


 Sorry for the crappy photo.

http://vortexsoundservices.com/P1011345.JPG

 I have a few suggestions that I'll post in the morning.


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_Just need a few more parts then it'll be ready to power up.


 Sorry for the crappy photo.

http://vortexsoundservices.com/P1011345.JPG

 I have a few suggestions that I'll post in the morning._

 

Woohoo! Looking good so far! I hope it all goes smooth once you finish populating the board.

 Please keep us updated and do post your suggestions! (Small hole sizes?)


----------



## mikemacwillie

The power jack was a tight fit, I had to cut a bit off the width of the rear pin. The silkscreen letters of U2 are also not there because of a via. Not that big of a deal. the last one is more of a personal preference.. But using the same part designator for more than one part makes it kinda annoying when assembling. It's harder to make sure you've put all the parts on.


 I had OPA551 on hand instad of BUF634, so I jumpered pin 6 and 2. Connecting pin 6 and 2 on the pcb allows another option for builders, and doesn't affect the buf634.


 I just need the pulse switch, and refrence diode then it will be time to power up. 


 The layout looks excelent, and the PCB's are of very high quality.


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_The power jack was a tight fit, I had to cut a bit off the width of the rear pin._

 

Aiyarrrr... 
	

	
	
		
		

		
		
	


	




 Thanks for pointing that out. I thought I used different hole sizes for the front and rear pins, but it looks like I used the hole size for the side pin (thinner at 2.5mm vs. 3mm) for all three. I did two or three iterations of the power jack, must have forgotten to change the hole sizes in the later iterations. I apologize for that mistake.

 Anyway, there IS a version of the same power switch by CUI with thinner pcb mount pins - the PJ-102 series (instead of the 202 series) - which have the same footprint but with 1mm pins instead of 2.5~3mm (and not kinked as well). The current hole diameter on the board is 2.8mm so it's going to be a very loose fit but one can choose this option if one's not very comfortable with shaving metal off the power connectors before soldering.

  Quote:


 The silkscreen letters of U2 are also not there because of a via. 
 

Yup, that was the first thing I noticed when I unpacked the boards, and hit myself in the head. Luckily there's only one U2 and it's not too hard to figure out which is U2... and it's not visible once the chips are on anyway. 
	

	
	
		
		

		
		
	


	




 I'll definately fix this for the next revision, though.

  Quote:


 the last one is more of a personal preference.. But using the same part designator for more than one part makes it kinda annoying when assembling. It's harder to make sure you've put all the parts on. 
 

I was debating on this issue... The reason why I did what I did was going with one number for each part would have made for many double-digit part numbers and it would have been a bit more difficult making the silkscreen pretty. 
	

	
	
		
		

		
		
	


	




 But I do agree that it would have been easier that way, so I'll see if I can still manage to fit all the part numbers like that without making a mess.

  Quote:


 I had OPA551 on hand instad of BUF634, so I jumpered pin 6 and 2. Connecting pin 6 and 2 on the pcb allows another option for builders, and doesn't affect the buf634. 
 

That's a good idea for the next revision. Thanks for the input!

  Quote:


 I just need the pulse switch, and refrence diode then it will be time to power up. 
 

Nice! Please keep us updated! And thanks again for prototyping and sharing your results!


----------



## doobooloo

Also, regarding the DC jack... found this on Mouser:

http://www.kycon.com/Pub_Eng_Draw/KLDX-0202-xHT.pdf

 Part numbers 806-KLDX-0202-AHT or BHT (2.0mm and 2.5mm inner pin, respectively)

 The rear pin is slightly thinner at 2.8mm x 0.4mm (compared to 3.00mm)- so with a bit of force this may just fit without having to cut the pin.

 Aah... stupid mistake, hitting myself in the head over and over again...


----------



## mikemacwillie

I didn't have to cut much of the pin at all, so not that big of a deal. Probably could have forced it in. My mouser order with the pulse switch and the ref. diode shipped today.. Should be here fairly soon!


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_I didn't have to cut much of the pin at all, so not that big of a deal. Probably could have forced it in._

 

Well when I get to building mine in November, I'll try all three mentioned jacks including the original suggested part and report my results here.


----------



## doobooloo

Also - have you gotten the Lansing case and/or the front panels yet? If not, are you planning to get them in the near future?

 I'll post my FPD files for the appropriate Lansing case here if anyone's interested. I've pretty much finalized them.


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_Also - have you gotten the Lansing case and/or the front panels yet? If not, are you planning to get them in the near future?_

 

I believe that at least one of the amps that I'm building will be a in a larger case 
	

	
	
		
		

		
		
	


	




 so no rush on posting the panels for me Doo. I hope to start ordering parts this week for the project though 
	

	
	
		
		

		
		
	


	




.

 Nate


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_I believe that at least one of the amps that I'm building will be a in a larger case_

 

There are no mounting holes on the board - how do you plan to secure the board in a larger enclosure? I came up with a few alternatives myself but would like to know what others are thinking.


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_There are no mounting holes on the board - how do you plan to secure the board in a larger enclosure? I came up with a few alternatives myself but would like to know what others are thinking. 
	

	
	
		
		

		
		
	


	


_

 

Based on a quick look at the boards it would appear that there is plenty of room at the corners to create mounting holes, especially since I do not plan on board mounting any of the rear panel components. That's where I'm starting anyway, we'll see how it goes during actual construction.

 I'll take plenty of pictures. 
	

	
	
		
		

		
		
	


	




 Nate


----------



## Thaddy

**sniff sniff**

 Smells like the curse is strong within this thread...


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_Based on a quick look at the boards it would appear that there is plenty of room at the corners to create mounting holes, especially since I do not plan on board mounting any of the rear panel components. That's where I'm starting anyway, we'll see how it goes during actual construction.

 I'll take plenty of pictures. 
	

	
	
		
		

		
		
	


	




 Nate_

 

I should have just added some pads for standard 0.1" spacing Molex KK connectors over the current space for all the I/O and power/volume connectors. I actually HAD them in place at a certain point of time and got rid of them. Should have just left them in there...

 Holes - I guess I can put them over or overlapping with the component footprint since it's going to be either or... 
	

	
	
		
		

		
		
	


	




 Didn't think of this before.

 Anyway - good luck with prototyping everyone... hopefully it works and sounds great on the first try!


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_Anyway - good luck with prototyping everyone... hopefully it works and sounds great on the first try!_

 

Parts ordered! DigiKey/Mouser are definitely out of some of the "stock" parts, here's to hoping that I picked the correct alternates! With a little luck I'll start building this weekend.

 Nate


----------



## mikemacwillie

My order for the couple of parts I was missing is due to be delivered today according to fedex tracking.. Can't wait!


----------



## mikemacwillie

It works!!!


----------



## Thaddy

Pictures and impressions ASAP!


----------



## mikemacwillie

I will try to get some decent pictures very soon.. my camera is crap!


----------



## n_maher

Quote:


  Originally Posted by *Thaddy* 
_Pictures and impressions ASAP!_

 

What are you so anxious about??? 
	

	
	
		
		

		
		
	


	




 Mike, 

 That's very cool! Any hints or tips you care to pass along to the rest of us? Looking forward to seeing the pics and of course hearing what you think of the balanced drive. 

 Nate


----------



## mikemacwillie

I don't really have many tips, I just assembled it, and it worked first time!





 I've been listening for a little while now with my grado's. It is easily capable of driving them to insane levels.

 One thing I've noticed is that it's very easy to clip the input with a strong signal. If I connect it right to my CD player, it makes the nastiest clicking noise. If I attenuate the signal from the CD player, or use my MP3 player, it's not a problem. The horrible clicking noise seems to lessen when increasing the volume, must be something to do with the digital pot.


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_I don't really have many tips, I just assembled it, and it worked first time!





 I've been listening for a little while now with my grado's. It is easily capable of driving them to insane levels.

 One thing I've noticed is that it's very easy to clip the input with a strong signal. If I connect it right to my CD player, it makes the nastiest clicking noise. If I attenuate the signal from the CD player, or use my MP3 player, it's not a problem. The horrible clicking noise seems to lessen when increasing the volume, must be something to do with the digital pot._

 

Awesome to know that it works! 
	

	
	
		
		

		
		
	


	




 What was your specific configuration (gain, opamps, etc.?)

 Regarding the clipping - as you pointed out it's an inherent problem with the DS1802 digital pot. Currently it is running at +/- 2.5V - and if the source signal goes beyond that it clips and distorts nastily.

 What you can do is have a voltage divider cable (or adapter) for your high signal source - not very convenient, but I don't think there's a simple solution to this given the chip's limitations. I still thought the better sound quality and channel matching of the chip compared to small analog pots was worth the tradeoff (and it's way cooler to begin with). 
	

	
	
		
		

		
		
	


	




 One thing several people voiced concerns about was if the Alpha rotary switch would actually work well with the DS1802. Any problems so far? (i.e. when rotating fast or slow, does volume go up and down proportionately, etc.?)

 Anyway, for more info on the digital pot and it's clipping characteristics check out Pedja Rogic's page on the digital pot: http://www.pedjarogic.com/spot/index.html

 Congrats on the successful build - keep the impressions coming please!


----------



## doobooloo

Oh, also - are you using your grados in a balanced configuration? If not, are there any sonic differences between the regular and inverted channels?

 And if you're using them balanced - how is it different from single ended? (I hope it's better!)


----------



## MASantos

This thread is useless without pictures!!!


----------



## phergus_25

im really wanting to know what peole think of this project, Iv been following it. so we need pics and impressions!
 -greg


----------



## doobooloo

Quote:


  Originally Posted by *phergus_25* 
_im really wanting to know what peole think of this project, Iv been following it. so we need pics and impressions!
 -greg_

 

well my very biased opinion of this project, as the initiator, is that it's a wonderful and highly sexy project. 
	

	
	
		
		

		
			





 seriously though, this should sound better than my breadboard prototype and that already sounded ridiculously good. so yeah, hopefully the current beta testers will find that the tpa6120 chip is a very tasty chip indeed, particularly when driven in balanced mode as in this case.


----------



## xaudiox

Hi mikemacwillie,

 Any Pics?


----------



## palchiu

Quote:


  Originally Posted by *xaudiox* 
_Hi mikemacwillie,

 Any Pics?_

 


 Not only Pics, we should ask "Any PCBs?"


----------



## phergus_25

im sure that this has been gone over, and pardon my ignorane, but how this will not allow for a truely balanced system will it, just the chs of the amp completely seprate?


----------



## doobooloo

Quote:


  Originally Posted by *palchiu* 
_Not only Pics, we should ask "Any PCBs?" 
	

	
	
		
		

		
		
	


	


_

 

If you mean any more PCBs left from the initial buy, yes there are a few remaining. But I'm out of the country and will be returning early November so I can't get anything shipped out until then. Shoot me a PM around the first week of November if anyone's interested.

  Quote:


 im sure that this has been gone over, and pardon my ignorane, but how this will not allow for a truely balanced system will it, just the chs of the amp completely seprate? 
 

Not quite sure what you're asking - but let me try to answer anyway.

 The amp takes in regular single-ended inputs and produces "balanced" outputs.

 The amp does this by having two separate amplification "channels" - one inverting and the other non-inverting. Very simple opamp inversion, nothing fancy.

 Since we have one inverting and one non-inverting channels, it is not possible to connect a balanced source and have balanced outputs.


----------



## palchiu

Quote:


  Originally Posted by *doobooloo* 
_If you mean any more PCBs left from the initial buy, yes there are a few remaining. But I'm out of the country and will be returning early November so I can't get anything shipped out until then. Shoot me a PM around the first week of November if anyone's interested.
_

 

That's great! Please save 1 pcb for me!

 Thanks doobooloo!!!

 Now, I need to start collect these parts. And wait the pcb arrive.


----------



## Mod_Evil

Can anybody send to me the schematic of these amplifiers?

 I'm thinking to make a balanced amp with OPA2227.

 Thanks,
 Felipe


----------



## doobooloo

Quote:


  Originally Posted by *palchiu* 
_That's great! Please save 1 pcb for me!

 Thanks doobooloo!!!

 Now, I need to start collect these parts. And wait the pcb arrive._

 

Sure. Just remember to send me a PM early November so that I can remember when I return. 
	

	
	
		
		

		
		
	


	




  Quote:


 Can anybody send to me the schematic of these amplifiers?

 I'm thinking to make a balanced amp with OPA2227.

 Thanks,
 Felipe 
 

Sorry, I didn't draw out a schematic when I did the layout. The layout was my schematic. 
	

	
	
		
		

		
		
	


	




 All I did was combine three elements into one amp:

 1. The buffered virtual ground circuit - TLE2426 followed by a BUF634 or equivalent. Check out Tangent's page here for more info and other variations.

 2. The digital pot section. check out Armond's page here and Pedja Rogic's page here for more info.

 3. The amp section. Nothing fancy here, no multiloop or anything - just very basic inverting/non-inverting configurations for the TPA6120s, buffered by non-inverting opamps for low-impedance output into the TPA6120s.

 Hope this helps...


----------



## mikemacwillie

Quote:


  Originally Posted by *doobooloo* 
_Awesome to know that it works! What was your specific configuration (gain, opamps, etc.?)_

 

My gain resistors are as in the BOM, and opamps are opa2132 for now (what I have on hand)



  Quote:


  Originally Posted by *doobooloo* 
_Regarding the clipping - as you pointed out it's an inherent problem with the DS1802 digital pot. Currently it is running at +/- 2.5V - and if the source signal goes beyond that it clips and distorts nastily._

 

Yeah, it's definitely the digital pot clipping, not that big of a deal really! Just need o okeep an eye on input signal level. 


  Quote:


  Originally Posted by *doobooloo* 
_What you can do is have a voltage divider cable (or adapter) for your high signal source - not very convenient, but I don't think there's a simple solution to this given the chip's limitations. I still thought the better sound quality and channel matching of the chip compared to small analog pots was worth the tradeoff (and it's way cooler to begin with)._

 

I agree it's worth the tradeoff. 

  Quote:


  Originally Posted by *doobooloo* 
_One thing several people voiced concerns about was if the Alpha rotary switch would actually work well with the DS1802. Any problems so far? (i.e. when rotating fast or slow, does volume go up and down proportionately, etc.?)_

 

It works just as it should 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *doobooloo* 
_Anyway, for more info on the digital pot and it's clipping characteristics check out Pedja Rogic's page on the digital pot: http://www.pedjarogic.com/spot/index.html

 Congrats on the successful build - keep the impressions coming please!_

 

Thanks!

  Quote:


  Originally Posted by *doobooloo* 
_Oh, also - are you using your grados in a balanced configuration? If not, are there any sonic differences between the regular and inverted channels?

 And if you're using them balanced - how is it different from single ended? (I hope it's better!)_

 

My grados aren't wired for a balanced configuration.. Yet. The will be soon.
 I can't seem to find a sonic difference between either of the channels. 


 I used the os-con caps you sent me for the C2 and C3 positions, and I used Nichicon UPW 220uf 35v that I had for the C1 positions. I also used OPA551 instead of buf634.. placing a jumper between pins 6 and 2.


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_My gain resistors are as in the BOM, and opamps are opa2132 for now (what I have on hand)_

 

IIRC the default gain should be 4 - 2 during the opamp buffer stage and 2 during the tpa6120 amp stage. Pretty standard value, not too high but not too low either. Should work pretty well with high impedance phones too...

 Will you be trying another build with different opamps? I'm thinking of trying several high-speed opamps such as AD8066, THS4062, THS4012, etc. - hopefully I'll albe able to try more traditional audio opamps such as the AD8620 as well.

  Quote:


 My grados aren't wired for a balanced configuration.. Yet. The will be soon.
 I can't seem to find a sonic difference between either of the channels. 
 

What Grados are you using now? I had a pair of RS-1s before running balanced and they sounded absolutely fantastic.

  Quote:


 I used the os-con caps you sent me for the C2 and C3 positions, and I used Nichicon UPW 220uf 35v that I had for the C1 positions. I also used OPA551 instead of buf634.. placing a jumper between pins 6 and 2. 
 

Nice, glad to hear that the os-cons are working out well. I still have more than 1000 (yes, one thousand) pieces of them boxed in original containers from Sanyo... never used a single one myself yet. 
	

	
	
		
		

		
		
	


	




 I don't know what I was thinking when I got so many before, that I'll somehow use them all for DIY... 
	

	
	
		
		

		
		
	


	




 At least they were pretty cheap.


----------



## mikemacwillie

Quote:


  Originally Posted by *doobooloo* 
_
 Will you be trying another build with different opamps? I'm thinking of trying several high-speed opamps such as AD8066, THS4062, THS4012, etc. - hopefully I'll albe able to try more traditional audio opamps such as the AD8620 as well._

 

I will be trying some high speed opamps as well. the THS look interesting to try out 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *doobooloo* 
_What Grados are you using now? I had a pair of RS-1s before running balanced and they sounded absolutely fantastic._

 

Just a pair of SR125's.. 

  Quote:


  Originally Posted by *doobooloo* 
_Nice, glad to hear that the os-cons are working out well. I still have more than 1000 (yes, one thousand) pieces of them boxed in original containers from Sanyo... never used a single one myself yet. 
	

	
	
		
		

		
		
	


	




 I don't know what I was thinking when I got so many before, that I'll somehow use them all for DIY... 
	

	
	
		
		

		
		
	


	




 At least they were pretty cheap._

 

Perhaps when you get back I could purchase some off you?


----------



## phergus_25

dibs on a PCB.


----------



## n_maher

Quick question for Doo (or anyone else that knows) - could you confirm that C2 and C3 only see half the rail voltage? It sure looks that way to me but given my luck (and lack of skills) verification would be nice. 
	

	
	
		
		

		
		
	


	




 Thanks,

 Nate

 P.s. Solder starts to fly tomorrow.


----------



## mikemacwillie

They only see half rail voltage. C1 will see full rail voltage, Which is why I didn't use the os-con's that doo gave me for C1. I used some 220uf 35v nichicon UPW caps I had on hand.


----------



## doobooloo

Quote:


 Quick question for Doo (or anyone else that knows) - could you confirm that C2 and C3 only see half the rail voltage? It sure looks that way to me but given my luck (and lack of skills) verification would be nice. 
 

Yup, only half rail. 
	

	
	
		
		

		
		
	


	




 Just out of curiosity - Are you going to be using the supplied os-cons or other caps?

  Quote:


 P.s. Solder starts to fly tomorrow 
 

Whoa. I wish my solder would fly someday. 
	

	
	
		
		

		
		
	


	




 Wait, that sounds marginally dangerous.


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_I will be trying some high speed opamps as well. the THS look interesting to try out 
	

	
	
		
		

		
			



_

 

Yup, I've never tried the THS's myself, they definately do look very interesting.

  Quote:


 Perhaps when you get back I could purchase some off you? 
	

	
	
		
		

		
		
	


	



 

Sure - I've only got a thousand or so, so hopefully I can meet your demand.


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_Yup, only half rail. 
	

	
	
		
		

		
		
	


	




 Just out of curiosity - Are you going to be using the supplied os-cons or other caps?_

 

One one I'll use the oscons, on the other I'll use Panny FMs. I can't imagine this is going to make any kind of difference but figured I'd do it anyway. Also, the only surface mount opamps that I could find at digikey were the 8620s so that's what I'll be using. The amp is going to be used for Senns so I think that'll work just fine.

  Quote:


 Whoa. I wish my solder would fly someday. 
	

	
	
		
		

		
		
	


	



 

What can I say, I'm special 
	

	
	
		
		

		
		
	


	




.


----------



## Thaddy

Ohh yay I like reading updates


----------



## t52

regarding the buf634/opa551 choice:
 wouldn't it be better to use the opa551 or any hight current opamp than a buffer like the buf634? the buf634's output impedance is said to be around 10~20ohms, whereas an opamps output impedance in unity gain config should be much lower, right? in this case one could also replace the tle rails splitter with a resistor devider and a cap to ground for noise suppression, because the opamp has a very high input impedance...


 and now something completely different: how do you guys solder the TPA6120 chip, especially the thermal pad?


----------



## t52

oh yeah, and yet another thing: could someone post pics of this sexy beast?


----------



## n_maher

Quote:


  Originally Posted by *t52* 
_and now something completely different: how do you guys solder the TPA6120 chip, especially the thermal pad?_

 

I'm also interested in hearing about this, I didn't see it mentioned anywhere earlier in the thread. Knowing that Doo had a breadboard prototype going I'm wondering if it's even required?


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_I'm also interested in hearing about this, I didn't see it mentioned anywhere earlier in the thread. Knowing that Doo had a breadboard prototype going I'm wondering if it's even required?_

 

The solder pads are on the most recent board, but to be honest, during normal operations the chip doesn't get very hot even without the pad soldered. Perhaps around 60 degrees celsius?

 To get the PowerPad down on this board, you'll need a pretty powerful iron to keep the solder flowing in that ~ 3mm x 3mm area while the heat is dissipated throughout the groundplane.

 And please - those who have completed the boards - post some pics for us! 
	

	
	
		
		

		
		
	


	




 I know Mike posted a nearly completed pic earlier but we need more!


----------



## n_maher

Ok, another question mostly as a result of me wanting to be careful since fixing SMD stuff is the devil.

 Can anyone tell me why R3 and R6 are different values (49.9ohms vs. 499ohms)? From what I can see on the board they look to be in the same position relative to each channel. My thought is that on each channel one should be R3 and one should be R6, not two R3's on one channel and two R6's on the other. Either that or perhaps they are all supposed to be 499ohms.

 Clarification would be stellar.

 Nate


----------



## mikemacwillie

To oslder the powerpad.. I put lots of flux on the powerpad on the bottom of the chip, and the spot on the pcb before soldering down the TPA chip. Then using a large chisel chip I heated the area on the bottom of the board with all the vias. The vias will conduct the ehat to the top side, and the flux to the chip until you get some solder in there.. Then just slowly add solder to the board and it will flow through the many vias 
	

	
	
		
		

		
		
	


	




 I clipped all the legs off one TPA chip to see if the power pad was soldered.. and it was. 



 I will try to get more pictures, but my camera isn't any good! Perhaps natural lighting will help.


----------



## mikemacwillie




----------



## n_maher

Made good progress today, as much as I can without the digi chip (on order) and without the answer to my above question regarding R3 and R6.


----------



## JohnH

Wow, that's some nice clean soldering Mike and Nate.

 Did you guys do that freehand with a fine tip soldering iron and some fine solder?

 It sure takes a steady hand.

 I nearly trashed a sound card that I was modifying with a fine tip soldering iron using the thick Cardas solder.


----------



## mikemacwillie

Yes, all done freehand with a hakko station. I'm experienced though.. Done lots of SMT rework for a day job. 


 Looking good so far Nathan! Reguarding your questions about the resistors.. One of the TPA chips is in an inverting circuit, and the other in a non inverting circuit. You need different resistor values to get the same gain.


----------



## Thaddy

mikemacwillie, are those XLR connectors? They look quite a bit like 1/4th outputs.


----------



## mikemacwillie

They are the 1/4" connectors the board was designed for.


----------



## n_maher

Quote:


  Originally Posted by *mikemacwillie* 
_Yes, all done freehand with a hakko station. I'm experienced though.. Done lots of SMT rework for a day job._

 

Like Mike I'm just soldering freehand. I use a very small tip and relatively thin solder. It just takes practice (I've build 10 or so MINTs) and patience, laying off coffee doesn't hurt either 
	

	
	
		
		

		
			





.

  Quote:


 Looking good so far Nathan! Reguarding your questions about the resistors.. One of the TPA chips is in an inverting circuit, and the other in a non inverting circuit. You need different resistor values to get the same gain. 
 

Thanks for the compliment and thanks for the help, I wondered if it wasn't something like that but I'd rather be safe than sorry since everything up to this point seems to have gone well. The volume control chip should be in this week so with any luck I'll have a working amp next weekend. I've got a 24V TREAD ready to go and also have a STEPS ready to get built in case I decide to go a bit crazy 
	

	
	
		
		

		
		
	


	




.


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_Ok, another question mostly as a result of me wanting to be careful since fixing SMD stuff is the devil.

 Can anyone tell me why R3 and R6 are different values (49.9ohms vs. 499ohms)? From what I can see on the board they look to be in the same position relative to each channel. My thought is that on each channel one should be R3 and one should be R6, not two R3's on one channel and two R6's on the other. Either that or perhaps they are all supposed to be 499ohms.

 Clarification would be stellar.

 Nate_

 

The R3s and R6s are in the same position but wired differently. R3 is the noninverting R_S and R6 is the inverting R_I. The values are correct.


----------



## doobooloo

Quote:


  Originally Posted by *Thaddy* 
_mikemacwillie, are those XLR connectors? They look quite a bit like 1/4th outputs._

 

Indeed they're 1/4" outputs - so that regular headphones can be plugged in for normal use, and for balanced, an adaptor can be made.

 Plus, for the small case no XLR jack would fit anyway.


----------



## doobooloo

Great photos, Mike and Nate!

 I have one question for Mike who's got all the jacks mounted already... When the jacks are board mounted, they're supposed to be "flush" with the board - meaning everything that's not supposed to protrude through holes when cased should not be protruding at all. From the photos it looks like the jacks are sticking out a bit more than they should - and this may cause some problems when casing them up in the originally intended case.

 And, unfortunately I did not make the board slightly shorter to allow for mfg tolerances - which I really should have. 
	

	
	
		
		

		
		
	


	




 From your judgment, do you think it's going to be a big problem?


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_Plus, for the small case no XLR jack would fit anyway. 
	

	
	
		
		

		
		
	


	


_

 

Speaking of XLR jacks can anyone shed any light on the industry standard for wiring XLR's? I've got 3-pin jacks and plugs (I'm making my own balanced cable) and while I'm sure I could figure out how to wire the amp to make it work it'd be great to know that both the cable and amp would be compatible with other pieces of gear.


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_Great photos, Mike and Nate!_

 

I should update it since I've added R3, R6 and the volume pot now. 
	

	
	
		
		

		
		
	


	




 Really just waiting on U5 now! Let me know if there's anything in particular that you'd like a photo of as things progress.


----------



## doobooloo

Here they are... sorry it took so long to post.

 One thing to be careful with the front panels, if one decides to use the Lansing MicroPak case that fits the board perfectly, is that the screw holes are very close to the left 1/4" jack and the volume pot. This means that the screws should be countersunk properly so that the front panel is flat, without any protruding screw heads.

 I looked through McMaster's extensive screw catalog to see which one would work, and the best one I found was the 4-40 thread flat machine screw 82-degree cut screws such as this:






 This particular one, #91771A108, is stainless steel, 3/8" length, and is $2.80 per pack of 100.

 All others with flat heads (socket cap screws, etc.) would not really work because the head would be slightly too big.

 The screw mount holes on the FPE file is designed for the one pictured above (or different lengths). Note though, that the FPE file is currently black but there are only natural (stainless steel) colored screws of this type at McMaster - so you should change the color to natural before you order so that it doesn't look so weird when put together.

 Also you can elect to remove the edge countersinking that I applied for some of the holes to give them a nice chrome beveled edge.


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_Speaking of XLR jacks can anyone shed any light on the industry standard for wiring XLR's? I've got 3-pin jacks and plugs (I'm making my own balanced cable) and while I'm sure I could figure out how to wire the amp to make it work it'd be great to know that both the cable and amp would be compatible with other pieces of gear._

 

Here is the industry standard for 3-pin XLR:






 So, for the headphone end, the ground pin would not be connected.

 A conversion cable from dual 3pin XLR to 1/4" would then have pin 3 of both channels terminate to the ground pin of the 1/4" and pin 2 of each channel be the hot channel of each channel.

 For this particular amp, since the two 1/4" outputs are not separated as L/R channels but +/- channels, a conversion cable to dual 3-pin would be criss-crossed - where each 1/4" plug feeds into both XLR plugs.

 Does this make sense? Sorry, I just woke up and my muscles and brain are still sore...


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_I should update it since I've added R3, R6 and the volume pot now. 
	

	
	
		
		

		
		
	


	




 Really just waiting on U5 now! Let me know if there's anything in particular that you'd like a photo of as things progress._

 

Yes - could you please see if you have any jack protrusion tolerance issues on your board as I mentioned in the post below? Maybe a few close-up photos of the jacks, with all the washers and stuff removed, from underneath the board (so that it's easy to see if the jacks are "out of line" would be great.

 Thanks!


----------



## doobooloo

Quote:


  Originally Posted by *t52* 
_regarding the buf634/opa551 choice:
 wouldn't it be better to use the opa551 or any hight current opamp than a buffer like the buf634? the buf634's output impedance is said to be around 10~20ohms, whereas an opamps output impedance in unity gain config should be much lower, right? in this case one could also replace the tle rails splitter with a resistor devider and a cap to ground for noise suppression, because the opamp has a very high input impedance..._

 

I thought of using high-current unity gain stable opamps but really it came down to my laziness in not wanting to solder down more SMD resistors. 
	

	
	
		
		

		
		
	


	




 Yes I'm aware of BUF634's high output impedance - but there is a drop-in replacement buffer from National (as seen in the BOM and earlier in this thread) - the LMH6321MR - which has much lower output impedance, at least from a quick look at the datasheet.

 I am actually going to use the LMH6321 in my builds, not the BUF634 so I'll see how that works out and report on my findings.


----------



## doobooloo

Forgot to attach earlier...


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_For this particular amp, since the two 1/4" outputs are not separated as L/R channels but +/- channels, a conversion cable to dual 3-pin would be criss-crossed - where each 1/4" plug feeds into both XLR plugs.

 Does this make sense? Sorry, I just woke up and my muscles and brain are still sore... 
	

	
	
		
		

		
		
	


	


_

 

Definitely going to have to think on this one some more since I will, for at least one of the amps, just be wiring XLR jacks to the board. If I understand what you wrote above correctly there is no true left or right output on this amp, correct? If so, when one was making an adapter cable for the headphones which connections would be L+ and L-? Sound's to me like the crisscrossing will occur internally for me but some clarification on this would be great and a diagram would be super!


----------



## mikemacwillie

Quote:


  Originally Posted by *doobooloo* 
_I thought of using high-current unity gain stable opamps but really it came down to my laziness in not wanting to solder down more SMD resistors. 
	

	
	
		
		

		
		
	


	




 Yes I'm aware of BUF634's high output impedance - but there is a drop-in replacement buffer from National (as seen in the BOM and earlier in this thread) - the LMH6321MR - which has much lower output impedance, at least from a quick look at the datasheet.

 I am actually going to use the LMH6321 in my builds, not the BUF634 so I'll see how that works out and report on my findings. 
	

	
	
		
		

		
		
	


	


_

 

The change to the high current unity gain stable opamp dows not need any resistors. It just needs a jumper from pin 6 to 2 to close the loop.. That's what the short blue 30ga wire is over my opa551 
	

	
	
		
		

		
		
	


	





  Quote:


  Originally Posted by *doobooloo* 
_I have one question for Mike who's got all the jacks mounted already... When the jacks are board mounted, they're supposed to be "flush" with the board - meaning everything that's not supposed to protrude through holes when cased should not be protruding at all. From the photos it looks like the jacks are sticking out a bit more than they should - and this may cause some problems when casing them up in the originally intended case.

 And, unfortunately I did not make the board slightly shorter to allow for mfg tolerances - which I really should have. 
	

	
	
		
		

		
		
	


	




 From your judgment, do you think it's going to be a big problem?_

 

The threaded part does not stick past the edge of the board Just the smooth part. It does stick past about 1/8" though.. (almost exactly the width of a 3.5mm headphone plug..) Not sure if that'll be a problem with the FPE panels or not.. but it should be fine with a normal panel. Also, the holes for the jack aren't a tight fit at all, there is a fair bit of movement available. I installed them towards the front of the holes rather than the back.. So there is a little adjustment available there.


----------



## doobooloo

Quote:


  Originally Posted by *mikemacwillie* 
_The change to the high current unity gain stable opamp dows not need any resistors. It just needs a jumper from pin 6 to 2 to close the loop.. That's what the short blue 30ga wire is over my opa551 
	

	
	
		
		

		
		
	


	







 The threaded part does not stick past the edge of the board Just the smooth part. It does stick past about 1/8" though.. (almost exactly the width of a 3.5mm headphone plug..) Not sure if that'll be a problem with the FPE panels or not.. but it should be fine with a normal panel. Also, the holes for the jack aren't a tight fit at all, there is a fair bit of movement available. I installed them towards the front of the holes rather than the back.. So there is a little adjustment available there. 




_

 

Thanks for the pic. I'm more worried about the 1/8" jack (the black plastic sticking out from the PCB is visible, not good...) and the volume rotary switch body pressing against the front panel. Also - regarding the little notches on the PCB edge for the RCA jacks... did the plastic "feet" fit in well or did you just snip them off?

 Also - regarding the usage of OPA551 and others - I should definately add a jumper option on board for the next revision. Thanks for the suggestion - I was thinking of a more complete feedback loop, but I guess just jumpering the pins work too.


----------



## n_maher

Doo, 

 Could you point out how to figure out which is the left and right for both the + and - connections? I want to start trying to air wire the panel components so that I'm good-to-go when U5 shows up later this week. I'm sure trial and error would work eventually but there has to be an easier way 
	

	
	
		
		

		
		
	


	




.

 Thanks,

 Nate


----------



## mikemacwillie

Quote:


  Originally Posted by *doobooloo* 
_Thanks for the pic. I'm more worried about the 1/8" jack (the black plastic sticking out from the PCB is visible, not good...) and the volume rotary switch body pressing against the front panel. Also - regarding the little notches on the PCB edge for the RCA jacks... did the plastic "feet" fit in well or did you just snip them off?

 Also - regarding the usage of OPA551 and others - I should definately add a jumper option on board for the next revision. Thanks for the suggestion - I was thinking of a more complete feedback loop, but I guess just jumpering the pins work too. 
	

	
	
		
		

		
			



_

 


 The 1/8" jack doesn't actually stick past the edge of the board, It's perfectly flush. It's just the slight angle the board was on in the pic.


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_Doo, 

 Could you point out how to figure out which is the left and right for both the + and - connections? I want to start trying to air wire the panel components so that I'm good-to-go when U5 shows up later this week. I'm sure trial and error would work eventually but there has to be an easier way 
	

	
	
		
		

		
		
	


	




.

 Thanks,

 Nate_

 

The far-out hole is obviously ground; between the remaining two, the one closest to the volume control is LEFT.

 So, looking at the board with the output jacks on top, left is on the left and right is on the right. 
	

	
	
		
		

		
		
	


	




 Hope this helps!

  Quote:


  Originally Posted by *mikemacwillie* 
_The 1/8" jack doesn't actually stick past the edge of the board, It's perfectly flush. It's just the slight angle the board was on in the pic._

 

Perfect! 
	

	
	
		
		

		
		
	


	




 Thanks for the confirmation!


----------



## n_maher

Ok, I've got everything soldered down - found one loose pin on the "+" channel and fixed that but now I'm nervous and wondering how to check DC offset on the balanced output. When I read each + or - with reference to the output ground (measured at the headphone jack's third pad) I get zip, zero, no mV on either channel. However, when I measure the output of the + channel with reference to the input ground it's whole different story. So, anyone care to shed some light on this subject before I go plugging in my big-dollar headphones? I'm itching to try it but not really up for the smell of burning drivers...

 Nate


----------



## mikemacwillie

Refrenced to output ground is where you're going to want to measure. I'm not sure why it would be different with the input ground as the refrence, unless it's decoupled. I'll have to have a look at mine when I've got it infront of me.


----------



## n_maher

Well, things are definitely not happy in Nate-town. The amp powers up, lights the LED but does not play music. It actually makes a sort of start up hum and then makes a repeated clicking noise that I have NO explanation for. The only thing that stands out as odd is that U2 gets very hot, very fast. In my amp that part is the LMH6321MR and I can't see anything wrong with the rest of the layout 
	

	
	
		
		

		
		
	


	




. Any help or suggestions would be greatly appreciated.

 [edit]FWIW it is stable without headphones plugged in (i.e. U2 doesn't get hot) but the minute I plug in headphones things go downhill VERY fast.


----------



## mikemacwillie

U2 is the ground buffer.. So, either you are putting large loads on the ground channel or you have a short somewhere to ground. Do you have any pictures of the completed bard for us to look over?


----------



## n_maher

No pictures tonight, unfortunately. Tomorrow night when I have more time I'll take high-res shots of the top and bottom of the board. I did find one problem already though, I had my "test" headphones wired incorrectly. This was causing a short to ground that explains the hot U2. The bad thing is that even with that issue corrected the amp still doesn't work. I can now hear audio through it but it is very badly distorted and the opamps started to get warmer than I'd like. Ugh, the fact that this is all SMD and impossible to repair if something is cooked is not sitting well with me 
	

	
	
		
		

		
		
	


	




.


----------



## mikemacwillie

Not iumposible to repair SMD at all. 
	

	
	
		
		

		
		
	


	





 What happens when you turn the volume switch?


----------



## n_maher

Quote:


  Originally Posted by *mikemacwillie* 
_Not iumposible to repair SMD at all. 
	

	
	
		
		

		
		
	


	


_

 

I know, I was just being all "my project doesn't work depressed" 
	

	
	
		
		

		
		
	


	




.

  Quote:


 What happens when you turn the volume switch? 
 

The one time I left it on long enough to try it had no effect on playback whatsoever. So something very strange is going on. I'll get the pictures up tonight and hopefully someone can spot whatever it is.


----------



## n_maher

Ok, here are the pictures - click for even bigger versions:


----------



## mikemacwillie

I see a couple of questionable solder joints.. but that might not be the problem. 

 Pin 19 on the digital pot specially.

 Try fluxing and reflowing the TPA chips and the digital pot.. see if it helps. Also do the same to the resistors around the digital pot. It would appear your problem lies there, as there's no signal to either channel.

 If you feed a sine wave into the amp, you can follow the signal through the circuit and see where the problem lies with a DMM. Also, measure the voltages at the supply pins of the chips.


----------



## n_maher

Reflowing can't hurt, I'll give that a shot some time this weekend. Thanks for taking a look at it and I'll certainly report back the results.


----------



## doobooloo

Sorry to hear about the troubles. 
	

	
	
		
		

		
		
	


	




 Could it also be that the buffer chip is permanently damaged from the shorted output?

 Anyway, I hope you get it up and running soon! 
	

	
	
		
		

		
		
	


	




 Please keep us updated with the results.


----------



## MisterX

Quote:


 Could it also be that the buffer chip is permanently damaged from the shorted output? 
 

That would definitely explain why "the opamps started to get warmer than I'd like"


----------



## n_maher

Right now, after reflowing the joints it's still not happy. I checked even the ones that sort of looked sketchy with the continuity function on my meter and they all come back ok so at this point I think my soldering is ok. 

 Marshall, do you think that it'd be worth changing the buffer chip (U2)? I'm all for giving it a shot but is there anything passively that I could check on it to see if it's roached? Should I check the TLE as well?

 As always, any help greatly appreciated.


----------



## mikemacwillie

Have you tried measuring your voltages at the supply pins of the TPA chips, and the DS1082 chip? What do you get?


----------



## MisterX

I would prolly change the TLE fist just because it is the easier of the two....and then if you still have problems change that buffer. 
 As to weather it's really gonna help or not: 
 Good question.


----------



## n_maher

Quote:


  Originally Posted by *mikemacwillie* 
_Have you tried measuring your voltages at the supply pins of the TPA chips, and the DS1082 chip? What do you get?_

 

When I say that things heat up quickly I mean that after 2 to 3 seconds the left chip in particular gets hot to the touch. There simply is not time to take measurements without worrying about something getting cooked unfortunately. The right channel and the DS1082 do not seem to get warm at all during that brief startup. Right now I can't decide if it's better to just start working on the second board that I have or use those parts to try and fix the first. If there was any testing that I could do on the board without the fear that I could be damaging other parts it would be an easier decision.


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_When I say that things heat up quickly I mean that after 2 to 3 seconds the left chip in particular gets hot to the touch. There simply is not time to take measurements without worrying about something getting cooked unfortunately. The right channel and the DS1082 do not seem to get warm at all during that brief startup. Right now I can't decide if it's better to just start working on the second board that I have or use those parts to try and fix the first. If there was any testing that I could do on the board without the fear that I could be damaging other parts it would be an easier decision._

 

Well, no harm in trying to fix the old board, right? 
	

	
	
		
		

		
			





 First - what do you mean by your "left" channel? There IS no separate left channel on the board - only inverting and non-inverting channels. I'm assuming you're talking about one of those?

 It's also a bit odd that only one channel starts getting hot - do both the TPA6120 and your opamp get hot or just the opamp (and not the TPA)?

 I think 2-3 seconds is enough time quickly check if the rail splitter is working or not. Then you can decide what to try replacing/fixing. Given the previous ground short situation, my doubts go to the ground buffer chip - but I'm not sure if the symptoms correlate well with this possibility. Perhaps an expert can help here?

 Good luck - please let us know how it's going.


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* 
_Well, no harm in trying to fix the old board, right? 
	

	
	
		
		

		
		
	


	


_

 

Only further damage to my wallet. 
	

	
	
		
		

		
		
	


	



  Quote:


 First - what do you mean by your "left" channel? There IS no separate left channel on the board - only inverting and non-inverting channels. I'm assuming you're talking about one of those? 
 

It's the "+" channel that gets hot and as far as I've been able to tell it's the TPA that's getting hot, but I can't say that I've really checked the opamp. I'll try checking the TLE tonight and see if I can get any meaningful measurements done in the limited window of time that I feel comfortable leaving the amp on.


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* 
_Only further damage to my wallet. 
	

	
	
		
		

		
		
	


	



 It's the "+" channel that gets hot and as far as I've been able to tell it's the TPA that's getting hot, but I can't say that I've really checked the opamp. I'll try checking the TLE tonight and see if I can get any meaningful measurements done in the limited window of time that I feel comfortable leaving the amp on._

 

Regarding the TLE measurement - have the TLE's outputs hooked up to the voltmeter before you turn it on, so that the instant you turn the amp on you get a reading of the output voltage. There should be very little load on the TLE's outputs if everything's OK so the values are not likely to drift unless the buffer somehow shorts the TLE's outputs.

 Also - if the inverting channel seems to be OK, have you tried doing something similar as above except that you have a pair of single-ended headphones connected to the working channel to see if in the first two or three seconds after the amp is turned on, music plays through properly only with the inverting channel? (In other words, don't test in balanced mode quite yet.)

 Of course, it'd be best to check for DC offsets before you do this. But IIRC you already did this and it was fine at the outputs, correct?


----------



## doobooloo

Also, anyone else doing builds yet? I sent out boards to quite a few people...

 And - anyone planning to do a complete caseup as originally intended (Lansing MicroPak D + FPE panels)?


----------



## TomasA

Hi,

 I am new on this board and I just signed up because of this thread. 

 I would be very interested in your subjective evaluations of this deisgn. I myself have looked at making a headphone amplifier around this same IC.

 Doobooloo, could you supply a PDF-version of your schematic?

 Tomas


----------



## n_maher

Well, when I woke up this morning I wasn't really planning on working on this project. I've had a hard time getting psyched for it since my first board went very, very wrong (my own fault) but I knew I had to get back into the SMD frame of mind so I set out to build a USB DAC. Opened the first parts package and there went that idea, DigiKey had shipped me the wrong part 
	

	
	
		
		

		
		
	


	




 . Since I was all geared up to solder I figured what the heck, I'll give the second TPA a chance. 4 hrs later and #2 is working perfectly! 
	

	
	
		
		

		
		
	


	




 I used a magnifying glass for the whole soldering procedure and it worked beautifully. After finishing this amp I'm sure my first attempt would have worked had I not wired the test headphones incorrectly and shorted the heck out of the output. I'm not sure that board can be saved and really have no desire to find out right now. I will say that balanced Koss KSC75s sound pretty good, more later on how things sound when I get going with my HD650s. I think I'm going to have to make some line attenuators for this amp though as even with the Ipod lineout it was clipping like crazy. 

 Thanks again for putting this all together Doo, it's a cool project for sure.

 Nate


----------



## n_maher

I take it back, this amp hates me 
	

	
	
		
		

		
		
	


	







. I went back after a couple of hours and decided to listen some more, which I guess was a bad idea? The amp now emits a constant low level hum and has a rather unhealthy 3.5V of DC offset on each output. What?!?! The power supply appears to be working fine, I checked some of the voltages on the caps and they are within .1V of each other so I don't think that is it and I've studied this amp under high manification and there are no shorts or bridges. I am completely frustrated...


----------



## doobooloo

I'm sorry to hear that the amp is not working out well for you.

 I'm quite busy at work these days (at work now) so I can't really actively help but if you can't get it working by November I'll send you another board free of charge when I return in November. Just PM me then to remind me.

 Hope it works out... good luck! Anyone else attempt this amp yet?


----------



## n_maher

What is so frustrating is that this last build worked fine for about an hour, then started having issues after being powered down for a few hours. I wiggled a few wires (more specifically the power supply wires) which got it working again but it then abruptly went bad after being powered down again. The only thing that I can describe about it is that it seems almost like a grounding issue as when I hold my hand even close the board the sound of the buzzing changes and if I touch any of the chips it sends massive feedback (loud buzzing, clipping sounds) through the output, especially if I touch the digital volume control chip. The audio signal is still coming through but with almost no bass very thin sounding. I'm also suspicious of the 8620s, I don't know what would be so different about them but that's the only part not directly off the BOM. 

 Again, any thoughts by other builders would be greatly appreciated. The other good thing about this second build is that it does not misbehave so badly that I can't power it up for a few minutes and take measurements while it's somewhat ok. I can also take some measurements like current draw or whatever anyone thinks might be helpful. I may try the amp powered by 2 9V batteries to make sure it's not a power supply issue with the TREAD that I'm using.

 Ugh.


----------



## n_maher

Bump...help...anyone...


----------



## doobooloo

bump... help/updates anyone?


----------



## TimmyMac

What's the word on this? I'm interested...


----------



## ferencvaros

are there any boards for sale ?


----------



## ferencvaros

is still any chance to get board for this project ?


----------



## doobooloo

after over 2 years of hiatus, i am finally going to build this amp! sorry, people have asked me for boards, if this worked out, etc... and i never had a chance to get back to anyone as i was pretty much off head-fi. but i'm sort of back (trying to re-assemble a headphone arsenal and listening whenever i have time) and i really want to build this thing since i actually never even got around to ordering parts for it in the first place...

 anyway, long story short, i'm trying to revive this thing. and i have boards. if anyone wants to join me in this adventure, let me know... 
	

	
	
		
		

		
			





 ah it's good to be back! not sure how i can get all the pic links up, maybe it'll be easier to start a new thread later on.

 i'd really appreciate any help/suggestions through this process!


----------



## doobooloo

Just ordered enough parts for 4+ boards! 
	

	
	
		
		

		
		
	


	




 I've already found a few points that I can make improvements on the board, so I'm going to test one set and order a revised set of boards before I make my final build.

 To be continued...!


----------



## doobooloo

oops double post!


----------



## CD44hi

Man, I always enjoyed checking out your builds. The total symmetry of it, including the alignment of the resistor labels, was outstanding. 

 BTW, the link on your sig probably got screwed after the head-fi crash.


----------



## n_maher

Ugh, after two years I finally threw away the 2 failed builds I had.


----------



## doobooloo

Quote:


  Originally Posted by *CD44hi* /img/forum/go_quote.gif 
_Man, I always enjoyed checking out your builds. The total symmetry of it, including the alignment of the resistor labels, was outstanding. 

 BTW, the link on your sig probably got screwed after the head-fi crash._

 

Thanks for your kind words! Those were fun times when I had access to the laser cutter at school. Too bad now I have to pay for everything! 
	

	
	
		
		

		
		
	


	




  Quote:


 Ugh, after two years I finally threw away the 2 failed builds I had. 
 

How's this - I'll send you two more original boards for you to play with, and if my test proves successful and I order revised boards I'll send you two new boards as well. You in?


----------



## n_maher

Quote:


  Originally Posted by *doobooloo* /img/forum/go_quote.gif 
_How's this - I'll send you two more original boards for you to play with, and if my test proves successful and I order revised boards I'll send you two new boards as well. You in? 
	

	
	
		
		

		
		
	


	


_

 

You meanie! I'd say I'll skip further experimentation with the original boards since clearly we don't get along but if you want help prototyping revised boards I'd be up for the experiment.


----------



## doobooloo

Quote:


  Originally Posted by *n_maher* /img/forum/go_quote.gif 
_You meanie! I'd say I'll skip further experimentation with the original boards since clearly we don't get along but if you want help prototyping revised boards I'd be up for the experiment. 
	

	
	
		
		

		
		
	


	


_

 

Great! Once I test the first batch of boards and work out all the little kinks, I'll send u 2 boards of the new revision. Although, given that the 1st version works and I don't make a huge stupid mistake on the 2nd revision, I don't really plan on making a third revision. 
	

	
	
		
		

		
		
	


	




 Hopefully the next batch will be less of a prototyping and more of a straightforward build.


----------



## doobooloo

Just went through the thread again. Lots of fond memories of staying up all night trying to figure this out! 
	

	
	
		
		

		
		
	


	




 Anyway. For some reason I had thought that there was no successful build yet, but mikemacwillie was able to get his up and running just fine! Which makes me super excited, since I was worried I was going to have to do some layout tweaking, but looks like most of it's going to be little details (like the smaller-than-needed hole size for the DC jack, making it a little more flush with the back panel so I can use 1/1.5mm panels instead of 3mm, switching the order of L and R around for the RCA jacks to comply with widely accepted standards, etc.)...

 Woohoo! Parts arrive Tuesday, which means I am going to work on the build the following Saturday. Can't wait...!


----------



## error401

Kinda interested in this doobooloo, I referenced your thread extensively when I tried to design a TPA6120A-based amp myself. I semi-failed; I felt the DC offset was unacceptable and couldn't really find a simple solution, but the amp does work and sounds pretty good.

 I think I've got a fair few TPA6120's left though, so if you're going ahead with this project it might be worth it to build one or two of them. Especially if they actually work right 
	

	
	
		
		

		
		
	


	




 Any chance you could repost the schematic (or am I missing it somewhere in the 20 pages?)?


----------



## doobooloo

Quote:


  Originally Posted by *error401* /img/forum/go_quote.gif 
_Kinda interested in this doobooloo, I referenced your thread extensively when I tried to design a TPA6120A-based amp myself. I semi-failed; I felt the DC offset was unacceptable and couldn't really find a simple solution, but the amp does work and sounds pretty good.

 I think I've got a fair few TPA6120's left though, so if you're going ahead with this project it might be worth it to build one or two of them. Especially if they actually work right 
	

	
	
		
		

		
		
	


	




 Any chance you could repost the schematic (or am I missing it somewhere in the 20 pages?)?_

 

I never had a proper schematic... just pieces of ideas here and there and the schematic was basically laid directly onto the pcb. I still have the images of the top, bottom, and combined layers on my computer so I'll try to post them again soon but you'll need to reverse engineer those to arrive at a schematic.

 If anything, since it's been over two years since I worked on this thing, I probably have to do the same as well to produce a schematic. 
	

	
	
		
		

		
		
	


	




 But it is very simple on the amp part of things - pretty much follows exactly the schematic and recommendations in the TPA datasheet.


----------



## gyrodec

I have been thinking of a TPA6120 amp for a week or so now since the new Claro sound card thread started which has this chip on the card to driectly drive headphones. I then found per's boards based on the chip, but they are all too big for my plan. This thing might just suit me perectly.

 If anybody builds more than they need, or has a prototype left over, I would like to make an offer on it if the price is reasonable and they turn out good - which of course they will


----------



## doobooloo

i still have boards! 
	

	
	
		
		

		
		
	


	




 pm me if you need one (or a few)! check somewhere in this thread for the full details of how much the boards cost (i forget), i'll still throw in the free oscon capacitors too.


----------



## gyrodec

Thanks doobooloo, but as my DIY skills are, shall we say, under-developed, I was hoping to snag a spare completed board, if any ever surface. Either way I'm watching with interest and good luck to all who build her- smash bottle of champagne over bow of monitor and salute


----------



## doobooloo

Ladies and gentlemen, it works, and marvelously so.

 Amazingly fast and punchy sound with AD8066 (DEEP bass!), smoother lusher sound with beautifully textured midrange with AD8620... Even with an Elpac wallwart, I can't hear much background noise on my Grado HP-2 or Ety ER-4S.

 Anyway, the best thing about this is the digital pot... Works like a charm, coming from the 24-position stepped attenuator of the Headamp GS-1, it's such a relief to have what seems to be infinitely more steps in between! Actually, since the DS1802 has 65 positions, it's really more like two more steps for each step, but regardless, it's a real improvement in being able to get the volume just right and still knowing I've got a form of perfectly matched stepped attenuator working for me!

 I was too busy listening so I haven't had much time to take pics last night but will do so and post them probably in a new thread since this one is littered with broken links from the start.

 Also, n_maher - I think I know your issue, the 2.5v voltage reference pins for the digital pot was incorrect. So, you have to mount it slightly differently, hopefully the pics will show this. This is the only issue from the electronic perspective, other issues are minor such as slightly larger than necessary holes for the 1/4" jacks and the 3.5mm mini jack.

 Anyway, more to come soon, but it works and it's beautiful. 
	

	
	
		
		

		
		
	


	




 Of course I'm a little biased but I think it's true!


----------



## n_maher

Glad to hear that it's up and running well!


----------



## H22

doobooloo, i just found this thread, and man is it long! I haven't gotten past the first 10 pages yet, but i love seeing how your amp has progressed. I have been looking for another fairly simple project (just completed a mini3) and this one looks like a winner. Are kits available yet? if not can i get a PCB and parts list. I apologise if this is already covered elsewhere in the thread, but i want to read the entire thread from the beginning and that will take me awhile
	

	
	
		
		

		
		
	


	



 Thanks and keep up the good work. 
 Joe

 ***Edit , Ok, i am stupid. I am new to this forum, and the structure is a bit different than i am used to. I misunderstood the page layout vs. posts and thought there were 198 pages! not enough coffee this morning i guess. Doopooloo i will pm you for details thanks. *****


----------

