# Stacking BUF634 buffers



## TheSloth

Wondering if anyone has any experience with stacking these, with any particularly noticeable improvement?

 Thanks.


----------



## rreynol

http://www.tangentsoft.net/audio/pimeta/tweaks.html


----------



## mono

I have a couple of Pimetas, one with single and one with doubled BUF634 going into Senn 120 Ohm cans. I'd call it a "last 2%" kind of benefit. seems to make bass a little tighter (though the two pimetas aren't _exactly_ alike, YMMV).


----------



## TheSloth

I get the impression that the benefit of stacking these buffers goes down as the impedence of the headphone being driven goes up. Is that true?

 I wonder if the HA-5002 would be a more worthwhile upgrade. It does lack the protection in the BUF643, but that doesn't seem to have been a problem under 'normal' circumstances?


----------



## steinchen

correct, stacking buffers seem to help low impedance cans since they need more current

 HA-5002 is not pin compatible with buf634, you'd have to build some kind of adapter. I'd rather go for a discrete buffer or even better another amp.


----------



## Dougigs

I've just built a MINT (Tangent's PCB), and I stacked two BUF634s on each channel (a tricky soldering job with SMD chips, but not as hard as you'd think). 

 But is it worth it? I also built a nearly identical MINT -- both have 18v supplies, and I use them to drive 32 ohm Grado SR125s that my wife and I listen to. One unit was "stock" (no class A bias, no bandwidth enhancement on the buffers, no stacked chips), and we listened carefully to each enhancement in A-B comparisons as I added it to the "decked-out" amp... Here, in order of significance and drama, are the effects of the changes:

 1) Increasing the bandwidth of the BUF634 -- this has a HUGE effect - - much more, in fact, than doubling the number of BUF634s. I added 330 ohm resistors on the chips, which raises the quiescent current from 1.5ma to 7ma... I had been highly skeptical of this, since the stock bandwidth of 30mhz is more than anyone needs for audio. But Tangent pointed out to me that what you're really doing is biasing the amp more deeply into class A, an especially useful add with low-impedance cans, so I tried adding it -- and I'm glad I did. It dramatically increased the smoothness and depth of sound on the Grados. It made it sound much less like a chip amp and more like a high end component... With this change, is compares well to my Sheldon Stokes MOSFET headphone amp. It does make the thing drink more current, though.

 2) Doubling up the BUF634s... This was a much, much more subtle improvement. Probably not really worth it, in fact, given the cost of these buffers -- I did it after raising the bandwith, and it was not a huge jump. It didn't add anything to Charles Mingus's bass or Mstislav Rostropovich's cello, but it did give a bit more wham to the Chemical Brothers at high volume (higher volume than I'd ever use)... Next time I'd just open the buffer bandwidth further rather than adding another one.

 3) Biasing the op amps into class A. Of course I did this using a constant-current diode (1.5ma)... I've done this before with older op amps with great effect, but this timem with the AD8620, it was barely noticable. I think that a) the latest generation of chips are calibrated to a pretty nice operating area anyway, and b) in the low-strain environment of a Jung loop, you're not pushing the op amps too far anyway. I'd still keep doing it, since you're only using a diode that costs 15 pence and adding 1.5ma (or 1.1 ma for you Americans who get your diodes from Mouser) -- hardly a current or money blow and worth it just so you can know you're listening to a pure class A amp. But just try to hear the difference. Really.

 So max out your buffer bandwidth, but think twice before stacking them. I find it nifty to think that I'm listening to a pure class A, low-feedback, parallel-driver high-end audio amplifier with perhaps 80 transistors and 14 constant-current sources in a space the size of a postage stamp. But I do have to live with a quiescent current draw of 25ma.


----------



## Kippei

Does the buffer stacking theory also apply to pimeta as well?


----------



## Brent Hutto

In my reading at Headwize I've seen a preference from some builders for the sound quality of the old-fashioned Intersil HA-5002 buffers compared to the more potent BUF634 type. If you were to build two almost identical amps with 3xOPA627 as the gain stages and either one BUF634 or one HA-5002 per channel, how would you expect the sound to be different? 

 In particular, what if you were driving headphones of the HD595/DT880 ilk where any tendency to exaggerate harsh treble energy in the source material is to be avoided? 

 Alternatively, would the answer be different if the #1 thing to avoid were any hint of bloating or boominess in the upper bass/lower midrange?

 Or are we talking an extremely subtle difference in sound due to buffer choice (compared to other design and execution elements like choice of op-amp or sufficient power and ground delivery to the buffer area of the PCB)?


----------



## Dougigs

When discussing the stacked-buffer MINT, I should have noted this: If you're increasing the buffer's bandwidth (which I highly recommend), you need to cut the value of the bandwidth resistor in half for two buffers -- since you're going to need twice as much current. So if you were going to use a 200 ohm resistor (for 15ma bias or so), you'll need 100 ohm instead. I used a 150-ohmer, and the sound is great -- much firmer (and less boomy) bass, owing I think to the higher damping factor of this operating point. (important when driving low-impedance cans).


----------



## Kippei

Is stacked buffer highly recommended for low Z cans?


----------



## n_maher

IMO stacking buffers on a MINT and maybe even the PIMETA should be done with careful consideration. What is the amps intended use? Portable? Transportable? Battery powered? Wall powered? All of these things can play a role in whether or not stacking is worthwhile and will obviously vary from person to person. For example, as previously mentioned doubling buffers doubles the current output of the amp. You've also just made your amp chew through batteries at nearly twice the prevoius rate. Is the resultant run time acceptable given the increased preformance? There is no clear cut answer there as it is a personal preference thing. For me my Class A PIMETA w/ single buffers (work rig) is just fine.

 So yes there is an audible difference with stacking buffers but just make sure you've realized the penalty that you pay for the increased performance. For some performance is the key, for others it's maximum run time, and for most it's probably somewhere in between.

 My 2¢,

 Nate


----------



## Voodoochile

Search for a post by KurtW about buffers. He did a very comprehensive test, with single, double, and quad stacks of Buf634, HA5002, and the sweet old Elantec buffers. Included distortion measurements, everything. A very good thread.

Found the thread here. 

 I recall that most of the benefit comes with doubling. The benefits after that are still there, but not nearly as pronounced. Doubling them makes a notable diff, though.


----------



## Voodoochile

Dougigs: nice post. You mentioned the buffer bandwidth and how it related to the sound. Too often, you just get a pat answer like "I like this", without much supporting dialog as to why.

 In the KurtW link I posted, there is some discussion of gain and the limiting effect on bandwidth it has. There are a lot of options available between adjusting the bandwidth of the front-end, the buffer, the gain, and the bias of each stage as well.

 What are you building, TheSloth?
 (or, what is TheSloth building?)


----------



## TheSloth

I'm not building anything per se. I bought a PIMETA-like amp from Akuan, which has the unique design feature of having double buffered ground channels (sepearing left and right, connecting the headphones via 2 1/8th in mono connectors). Was too curious to pass it up - I'd been looking at an old style aeroplane adapter, and was thinking of the potential benefits of such a dual mono system. The next day this little amp popped up.

 Search for more information. He calls it a '4 channel amp'. I haven't actually heard it yet, but it will be waiting for me when I return to NY on thursday, so I'll post some impressions.


----------



## Blorton

Does anyone know of a way to see if an in-circuit BUF634 is already running in "wide bandwidth" mode? The DAC-1 uses them and I'm chasing down upgrade options.

 Cheers!
 Dan


----------



## Jazper

Quote:


  Originally Posted by *Blorton* 
_Does anyone know of a way to see if an in-circuit BUF634 is already running in "wide bandwidth" mode? The DAC-1 uses them and I'm chasing down upgrade options.

 Cheers!
 Dan_

 

Read the datasheet, check the bandwidth restriction resistor with a multimeter to figure out what it is.. (you could simply check the resistance between the two pins on the chip)

 doesn't get much easier than that


----------



## Sinbios

I think tangent mentioned that distortion goes up again at 200 ohms, and "wide bandwidth mode" means zero resistance.


----------



## tangent

Quote:


  Originally Posted by *Sinbios* 
_I think tangent mentioned that distortion goes up again at 200 ohms, and "wide bandwidth mode" means zero resistance._

 

That's covered in the KurtW article Voodoochile is referring you all to. Find it, read it, love it.


----------



## Dougigs

That whole thread is extremely interesting. His tests confirm what our listening tests indicated: Raising the bandwidth on the BUF634 (i.e., giving it a class A bias) dramatically improves both the level and the quality of distortion, while doubling up the buffers makes only a tiny (and probably inaudible) extra difference. 

 And the bandwidth-bias issue seems to swamp other factors like improved output impedance, damping, power (which by the way is probably not a great improvement in SMD buffers, whose thermal limitations mean that stacking isn't doing all that much -- might be interesting for someone to try the TO220 versions which can deliver the whole 250ma, with heatsinks, and see if stacking *those* has an effect...).

 It's an important lesson for the MINT (and one that Tangent has already made clear... I just have to check things out for myself). I would say it's probably also useful for AC-powered circuits: Yes, you can double up the $12 buffers for a tiny and probably inaudible improvement, but if you really want that money to affect a tangible improvement, probably better to move to a different (discrete) topology....


----------



## Voodoochile

Quote:


  Originally Posted by *Dougigs* 
_That whole thread is extremely interesting._

 

Yeah it is! 
	

	
	
		
		

		
		
	


	



 I think it should be added to the stickies reference. I have referred back to that thread more than a dozen times in the last couple years. It's one of the gems.

 As for the doubling of buffers... what I gather from that thread is that the benefit is notable with doubling, and marginal with quadrupling. Going from two or three buffers (amp total, not per channel!) to four or six is not a big deal, financially. But going from four or six to a eight or twelve, well, the cost outlay versus performance gain starts to slide big time. For a number of those buffers, doubling them makes an easily discernible improvement. Something to consider carefully. It's a small tweak in the scope of an entire DIY amp build.

 I like discrete buffers, but for many uninitiated (or more accurately- folks with limited test equipment and time, and/or limited money for extra parts and parts matching), simply adding another buffer is a quick and significant improvement over the single integrated buffer.


----------



## Dougigs

From my reading, it seems to show now benefit at all to doubling up...

 BUF634 WB, 0.045%, 
 BUF634 WB x 2, 0.037%, 

 The difference between .045 and .037 per cent distortion is statistically and audibly meaningless, and they're both such low figures as to make distortion no longer an issue. I'm willing to believe that quadrupling has even less benefit, but it seems from this that even doubling accomplishes nothing useful, beyond doubling your current consumption. Or am I missing something.


----------



## mono

Quote:


  Originally Posted by *Voodoochile* 
_I like discrete buffers, but for many uninitiated (or more accurately- folks with limited test equipment and time, and/or limited money for extra parts and parts matching), simply adding another buffer is a quick and significant improvement over the single integrated buffer._

 

So how do some of the various discretes stack up against these, and against each other? There seems to be a concensus that more elaborate designs like Jungs are superior, but how much within the context of the linked tests? I'm not necessarily trying to prompt anyone else to do tests for me but subjective perceptions might be useful.

 Then there's the simplier discrete buffers, 2 pairs of transistors. Following is a picture of one Sijosae has used a few times... where does it stand? Significantly better than no buffer? Relatively, how much improvement over no buffer and how much better than it, is a typical Jung Diamond buffer, glassman's buffers, those in a PPA2, or one of the chip buffers?


----------



## Voodoochile

Quote:


  Originally Posted by *Dougigs* 
_From my reading, it seems to show now benefit at all to doubling up..._

 

No, I'm missing something. Like the title to this thread: BUF634. 
	

	
	
		
		

		
		
	


	



 I was thinking of the Elantec buffers, which are what I was using at the time that thread appeared. The 2001 and 2002s roughly halved their distortion when doubled.

 The distortion bit is fairly academic anyway, as the principal benefit is the increased current handling. I just got all nostalgic. If you have cans that demand a lot of current, then doubling the buffers current capacity can make a significant impact of the performance, regardless of the distortion figure.

 mono: I'd love to see those tests run again, with the HA3-5002 vs glassman vs the little sijosae buffer. I really enjoy technical comparisons like that, even though what sounds good does not neccesarily correspond to the best figures.


----------



## mono

I just built the pictured Sijosae buffer on a DIP socket and swapped it into a Pimeta's Right Channel buffer position. Left channel still has 2 x stacked BUF634 in *wide* bandwidth mode (100 Ohm R11).

 Initial impression- Sijosae buffer seems stable at lower voltage, though I'm hitting the floor for the L/R opamp too so it's hard to tell how much. It seems to remove a veil compared to BUF634, even with a BUF634 still in Gnd buffer position, and sounds good... good enough that I may build a few more of these (tediously, I am not a miniturization expert like Sijosae).

 With the Sijosae buffer built on a DIP socket, plugged into a dip socket soldered to the Pimeta PCB too, it looks like it could be a really tight fit in a Serpac H65 or similar shallow case... but I wasn't thinking about that when building it, if I deliberated over it I might be able to make it 1-2 millimeters shorter, as the one I made would sit "about" 12mm above the PCB (as a point of reference, if it didnt' have the empty socket on the PCB too, about 16mm with that socket).

 My initial impression of sound quality and possibility that it might be stable at lower voltage is piquing my interest... and it doesn't hurt that, *IF* they do end up sounding decent, the total cost for buffering 3 channels could be under $4. I can't yet claim it's a viable replacement for BUF634 though, not having but one channel swapped in- This is just a first impression but it might sound better than BUF634. I've a backlog of stuff I need to do this weekend so I may not get a whole Sijosae-buffered Pimeta running as soon as I'd like.


----------



## Voodoochile

That's great, man! Sounds very cool.
 Got a digicam??? 
	

	
	
		
		

		
		
	


	




 Love to see some pics.


----------



## Blorton

Quote:


  Originally Posted by *Jazper* 
_Read the datasheet, check the bandwidth restriction resistor with a multimeter to figure out what it is.. (you could simply check the resistance between the two pins on the chip)

 doesn't get much easier than that 
	

	
	
		
		

		
		
	


	


_

 


 Thanks! I just measured, and sure enough, the DAC-1's BUF634's are already in WB mode.

 Dan


----------



## amb

Sijosae's buffer will work fine in a Pimeta (or other amps where it's driven from an opamp), but it will not work universally where BUF634s are used because its input impedance is very low, and could be a difficult load for the preceding stage. For example, it should not be used in a Millett Hybrid. Also, its PSRR is probably much lower than that of the BUF634 due to the fact that the output transistors are biased with resistors going directly to the supply rails. Just fyi.


----------



## nikongod

there may be room for an input resistor if you move the transitor near "in" to the other side of the v-pin, and move the 2k-ohm resistor (and of course the v- line).... im not sure what an input resistor would do to the circuit though.

 it is doable, laying the circuit slightly diferently than sijose did it... i dunno if its worthwhile though.


----------



## amb

Quote:


  Originally Posted by *nikongod* 
_there may be room for an input resistor if you move the transitor near "in" to the other side of the v-pin, and move the 2k-ohm resistor (and of course the v- line).... im not sure what an input resistor would do to the circuit though.

 it is doable, laying the circuit slightly diferently than sijose did it... i dunno if its worthwhile though._

 

Adding a series input resistor to the Sijosae buffer (of high enough value to make the input impedance high enough for general BUF634 replacement use) will cause severe voltage loss such that it is no longer near unity-gain. As it stands the input impedance of this buffer is less than 1KΩ. If you add a 10KΩ series resistor, it will introduce a 20dB+ loss. Compare this to the input impedance of the BUF634 of 8MΩ in wideband mode or 80MΩ in low-current mode.


----------



## mono

So is there a lesser value that would at least provide some benefit in the Sijosae buffer, would 1K series input resistor seem a reasonable choice? I was just about to build 1 (or 3) with 1K.


----------



## amb

Quote:


  Originally Posted by *mono* 
_So is there a lesser value that would at least provide some benefit in the Sijosae buffer, would 1K series input resistor seem a reasonable choice? I was just about to build 1 (or 3) with 1K._

 

If you're planning to use it in a Pimeta, then you don't need to add an input resistor. The opamps should have no trouble trouble driving a 1KΩ load.

 Adding a 1KΩ series resistor will only raise the input impedance of the buffer to 2KΩ, which doesn't help much in other circuits (i.e., Millett Hybrid, etc.) that require higher input impedances. Also, even that 1KΩ resistor will introduce ~6dB of loss.

 A better way to increase the input impedance of that circuit is not to add a series resistor, but to replace those two 2KΩ resistors going to the rails with current sources (either CRDs or JFETs), similar to PPL's LISA II amp. This will not only dramatically increase the input impedance of the buffer, but will also improve the PSRR, and make the biasing current mostly invariant to supply voltage. If you go this route you need to match the CRDs (or the Idss of the JFETs).


----------



## mono

I'd noticed on amps like PPA2, There is often this series resistance. Tangent wrote about this as implemented in PPA2 as:

  Quote:


 This resistor isolates the input capacitance of the buffer stage from the output stage of the op-amp. Without this resistor, you're likely to get electrical ringing, or even instability. 
 

I swapped in a few different opamps and *observed* no audible instability but would this ringing or high frequency be effected by omission of the resistor in a (for the purposes of this post, stock Pimeta)?


----------



## sijosae

This is my lower input impedance version.
 I add only two components. (LM317LZ & LM337LZ)
 In my previous test ( http://www.headphoneamp.co.kr/bbs/zb...sijosae&no=211 ), "LM317+resistor". CCS showed best result.

 I did not test yet.
 How about this buffer?


----------



## amb

Quote:


  Originally Posted by *mono* 
_I'd noticed on amps like PPA2, There is often this series resistance._

 

Yes, a small series resistance is a good thing to eliminate the chance of parasitic oscillation, about 100Ω should be enough.


----------



## mono

Quote:


  Originally Posted by *sijosae* 
_This is my lower input impedance version.
 I add only two components. (LM317LZ & LM337LZ)
 In my previous test ( http://www.headphoneamp.co.kr/bbs/zb...sijosae&no=211 ), "LM317+resistor". CCS showed best result.

 I did not test yet.
 How about this buffer?_

 

Thank you Sijosae.

 Easy for you, but I run out of space on my DIP sockets. 
	

	
	
		
		

		
			




 I was hoping to keep the size of the buffer very small if it isn't too great a penalty but I don't know how bad it would be.

 How important is a true constant current instead of one varying with supply voltage? I see your improved buffer has ~ 6-12mA CCS. Is it going to be a significantly worse sound to have the 2K resistor with expected operation somewhere in the 6-24V range?


----------



## sijosae

mono,
 My buffer has 3-6 mA CCS.

 I think, 
 in your application (OP-Amp + Output Buffer), there is almost no difference between this new (w/ CCS) version and original (w/o CCS) version.
 I recommend you to use original buffer.


----------



## amb

Maintaining absolutely constant current over a wide range of supply voltages is not the most important factor here. I don't know how well the LM317L/337L performs as far as noise, input capacitance, etc., compared to simple JFETs, but I tend to think that these should be higher priority.


----------



## steinchen

*LM317*
 I tried LM137 as ccs for the push-pull buffers for the Millet and found them way too noisy, I'd recommend a jfet cascode or even a lone jfet. The noise floor introduced by the lm317 was louder than low volume passages of the music.

*100 Ohms input resistor*
 I tried it on my PPA v2 and on my Millet and found it way too low. Bass weakens and highs become bright and sharp. I'd recommend at least 470 ohms.


----------



## mono

more


----------



## mono

Quote:


  Originally Posted by *amb* 
_Maintaining absolutely constant current over a wide range of supply voltages is not the most important factor here. I don't know how well the LM317L/337L performs as far as noise, input capacitance, etc., compared to simple JFETs, but I tend to think that these should be higher priority._

 

I appreciate what you're saying and given another mounting method it could be considered. It was enough of a PITA to just add an input resistor on the DIP (DIP16 cut down to be a DIP12) socket. To implement it I feel it would be better to use a proper PCB... and I may eventually come up with a pattern to etch but one thing at a time.

 Do you feel the noise issue more significant would be that getting through the supply or amp-induced? So far the supply has been a common LM317 based linear, maybe even a sloppy one at that as it wasn't made with amp use in mind, but listening, it seems pretty quiet.

 What JFETs would be a decent match here? IIRC I have a few spare 2N5486 but only somewhere between 0 and 1 LM317L/something/TO-92 and no inclination to get any if/with a JFET being a better choice. So far I haven't been able to determine that noise is a problem. Do you think it'll need a capacitor after the JFET? I did put a cap on the socket in the two already finished but it's a very tight fit, only .1uF. 

 Could probably substitute 1uF ceramic, and that's about all before the buffer starts growing beyond the bounds of the space allotted on a Pimeta. The Gnd channel buffer is already a tight fit with a 10mm cap behind it, I had to slant the cap but the socket might be filed down a little more.


----------



## amb

Quote:


  Originally Posted by *mono* 
_Do you feel the noise issue more significant would be that getting through the supply or amp-induced? So far the supply has been a common LM317 based linear, maybe even a sloppy one at that as it wasn't made with amp use in mind, but listening, it seems pretty quiet._

 

When an LM317 is used to regulate the rails to power an amp with reasonable PSRR, then noise isn't an issue. However when it's called upon to serve as a current source to bias the output stage of the amp, then it could be too noisy. I haven't actually tested this myself but steinchen had tried this in a similar circuit and reported noise issues.

  Quote:


 What JFETs would be a decent match here? 
 

2N5486 is fine.

  Quote:


 Do you think it'll need a capacitor after the JFET? 
 

Connected across the base pins of the output transistors? While there is a theoretical benefit, for simplicity you can skip the cap.


----------



## sijosae

I can't understand how LM317 induced noise affects the sound in my buffer.
 I think, noise hardly contaminate signal path.

 I used LM317 in my previous amp. ( http://www.headphoneamp.co.kr/bbs/zb...sijosae&no=226 )
 It is dead quiet.

 Attached circuits are LM317-CCS based buffer idea.


----------



## Mod_Evil

Can I double the Sijosae buffer? I will make a Sijosae buffer to compare to the BUF634 and 2xBUF634 in my oscilioscope.


----------



## amb

Quote:


  Originally Posted by *Mod_Evil* 
_Can I double the Sijosae buffer? I will make a Sijosae buffer to compare to the BUF634 and 2xBUF634 in my oscilioscope. 
	

	
	
		
		

		
		
	


	


_

 

Not like how you would stack BUF634s. The equivalent of that would be to change to higher powered output transistors and increasing the bias. This particular Sijosae buffer doesn't have adjustable bias so you would have to change the circuit and add to its complexity.


----------



## Dougigs

Well, here's another thing about stacked buffers: They cook through batteries at an awesome clip! Never mind the quiescent current (35ma, which sounds modest); under a 36ohm load, with a gain of 3, I'm getting about 5 hours out of 2 series-connected 9v alkalines... that's a lot of current!

 Tangent (or other veteran of such circuits): Do these things sound ok with a 9v (i.e. parallel-connected) supply? I know the chips are all spec'd to work at these voltages, and my Grados certainly don't need much of a swing, but are we getting a bit too close to the noise floor at this voltage? Does sound quality suffer? It not, then I'm taking the low-voltage route.


----------



## Sinbios

Quote:


  Originally Posted by *Dougigs* 
_Well, here's another thing about stacked buffers: They cook through batteries at an awesome clip! Never mind the quiescent current (35ma, which sounds modest); under a 36ohm load, with a gain of 3, I'm getting about 5 hours out of 2 series-connected 9v alkalines... that's a lot of current!

 Tangent (or other veteran of such circuits): Do these things sound ok with a 9v (i.e. parallel-connected) supply? I know the chips are all spec'd to work at these voltages, and my Grados certainly don't need much of a swing, but are we getting a bit too close to the noise floor at this voltage? Does sound quality suffer? It not, then I'm taking the low-voltage route._

 

It depends on your opamp's voltage requirements. Also, I seem to recall that putting batteries in parallel is a bad idea, but I can't seem to remember why - something about unequal charge.


----------



## TheSloth

I'm glad my little question has sparked a lively thread. I stacked my buffers, but accidentally soldered one of them the wrong way round, and can't seperate them. So my experiment is grounded until I get some more buffers, or a rather better desoldering tool.

 For my purposes, I wasn't considering current draw, as the electronics are at 24v and run off AC only. Speaking of which, does anyone know of a good quality 24v supply that can be had a reasonable cost?


----------



## rreynol

Quote:


  Originally Posted by *TheSloth* 
_For my purposes, I wasn't considering current draw, as the electronics are at 24v and run off AC only._

 

Comment Removed.

 Edit: Ah, I see. Wall powered vs Battery powered.


----------



## mono

Quote:


  Originally Posted by *TheSloth* 
_I'm glad my little question has sparked a lively thread. I stacked my buffers, but accidentally soldered one of them the wrong way round, and can't seperate them. So my experiment is grounded until I get some more buffers, or a rather better desoldering tool.

 For my purposes, I wasn't considering current draw, as the electronics are at 24v and run off AC only. Speaking of which, does anyone know of a good quality 24v supply that can be had a reasonable cost?_

 

IMO, one of the most cost effective supplies is a Tread behind a 24V wall-wart, or put the wart behind an AC filter module if necessary.

 I successfully separated a pair of stacked buffers by removing excess solder first, by sliding some (added flux on) desoldering wick down the pin(s) then sliding the tip of a small/thin X-Acto knife blade between the reheated pins (working quickly but letting chip cool some between each pin). 

 The knife blade was slid from top towards botton of pins and it "might've" helped to do it that direction as it resulted in what little solder remained to be pushed towards the larger gap at the bottom of the pins (with top buffer pins bend outwards slightly). Then with them separated another pass with the desoldering wick removed almost all of what little solder remained. It was easier than I thought it would be.


----------



## mono

Quote:


  Originally Posted by *amb* 
_Not like how you would stack BUF634s. The equivalent of that would be to change to higher powered output transistors and increasing the bias._

 

These BC327 / 337 I'm using are 800mA, is that a significant limitation? I'm wondering if there are other audible benefits if one never exceeds 800mA peak output, or if paralleled transistors would be better or worse than one higher current transistor?

 Is 800mA even useful, any benefit over the some of the lower current surface-mount transistors (excluding larger outline types like a D-Pak) ? I suppose I'm wondering, all other things equal, where is the point of diminishing return in current capability, or best compromise for portable vs stationary/AC-powered amp?

 Is increasing BIAS in itself a benefit or just *necessary* with higher-current capable transistors?

 Touch-testing them, the BUF634 stack was getting hotter than this discrete during fairly high volume operation. I have the beginnnings of more buffers, started, may try to socket-pin both legs of the input resistor on the next one if I can manage it.

 Ultimately it's easy to see the limitation of a DIP12 socket, that a larger outline PCB would help. Then again, on some amps like a Pimeta the possibility of having a browndog with separate L/R opamps plus the issue of having one buffer on either side of L/R (so the discrete buffer can't just extend as far past the boundary of the socket and still be identical buffer modules), there is still a fair limit on the size of such a buffer board instead of DIP-socket-built. I'll try to measure that available space on a Pimeta and append it here. 

 Edit: Gnd Buffer already too long on a Pimeta, intrudes on space for 1 10mm dia. electrolytic and output signal pins by about 1mm... socket could be filed down enough to squeeze it in against a 10mm electrolytic if necessary. Buffer could be about 33% (ie- one set of .1" space holes) wider extending to the left of the socket. No space on right side of socket if space is to be kept for a Browndog adapter for L/R channel opamps.


----------



## tangent

Quote:


  Originally Posted by *Dougigs* 
_Do these things sound ok with a 9v supply?_

 

I've taken BUF634s down to 5-6V, as I recall. Naturally, it depends on the headphones whether you will avoid clipping at these extreme levels. And keep in mind, that "9V" supply will drop in voltage as the battery discharges. Depending on the type of 9V you have, 6V might be well below the minimum useful voltage, or there might still be useful amounts of power left in it at that voltage.

  Quote:


 are we getting a bit too close to the noise floor at this voltage? 
 

What does noise floor have to do with it?

 As far as I can tell, only clipping is an issue here.

  Quote:


 Does sound quality suffer? 
 

A little, yes. Nothing's free.

  Quote:


  Originally Posted by *Sinbios* 
_I seem to recall that putting batteries in parallel is a bad idea, but I can't seem to remember why_

 

Paralleling alkalines and carbon chemistry batteries is fine. Paralleling rechargeables, though, is a definite no-no. The difference is that rechargeables have lower series impedance, so the slightest voltage imbalance between the two mean high currents between the batteries. Also, rechargeables have a pathology called cell reversal, where the voltage of a cell can actually be flipped negative. Once it does that, damage happens quickly. Cell reversal is much less likely to happen in a series-connected pack. (Can still happen, but only if you mistreat the pack.)


----------



## steinchen

Quote:


  Originally Posted by *sijosae* 
_I can't understand how LM317 induced noise affects the sound in my buffer.
 I think, noise hardly contaminate signal path.

 I used LM317 in my previous amp. ( http://www.headphoneamp.co.kr/bbs/zb...sijosae&no=226 )
 It is dead quiet.

 Attached circuits are LM317-CCS based buffer idea._

 

I altered some parts in the similar circuit amb mentioned and the noise is gone, so I consider lm317 to be an option for a ccs depending on the circuit it is used in. Nevertheless, lm317 is much more noisy than a lone jfet or a jfet cascode.


----------



## amb

Quote:


  Originally Posted by *mono* 
_These BC327 / 337 I'm using are 800mA, is that a significant limitation? I'm wondering if there are other audible benefits if one never exceeds 800mA peak output, or if paralleled transistors would be better or worse than one higher current transistor?_

 

800mA is the maximum rated current of the transistor and realistically you shouldn't operate the circuit near there. More often than not, it's the power dissipation of the transistor that becomes the limiting factor. Suppose if the amp is capable of delivering 12V peak into 32 ohms, then the peak power dissipation on the each output transistor at that output level would be over 1W peak, which has exceeded the 625mW limit of the BC327/337. Under normal usage, people aren't really going to be playing a continuous tone into a low impedance headphone at that sort of output level, but we like to design amps that cannot be damaged even under maximum output test conditions. This is especially important for designs without built-in current limiting circuitry (such as most of the discrete buffers we are familiar with here). A bit of overengineering is sometimes worn as a badge of honor... think the M³ with its 45W, 17A MOSFETs, or the dynahi with four parallel sets of 1A, 10W power BJTs.

  Quote:


 Is increasing BIAS in itself a benefit or just *necessary* with higher-current capable transistors? 
 

It's not "necessary", but increasing the bias makes the output stage operate closer to class A (if you go far enough, it becomes pure class A). The point is to prevent the output transistors from cutting off as it swings output into its load, thus avoiding crossover distortion. The price you pay is increased quiescent current draw and heat dissipation.


----------



## mono

My 2N5486 all have IDSS about 15mA in a tight ~ 1mA spread. Is that about right for (single JFET) biasing for an AC/DC powered amp? I'd considered the figures Sijosae showed previously, 3-6mA, more optimal for battery power? 

 I hope I'm getting the JFET implementation right, is the following picture correct? The other thing I was wondering about is whether adding a resistor would help more in spot A, to limit current if desirable, or B, to isolate the JFET's capacitance and limit current "some" (but still variable based on supply voltage) (or is it even important to consider JFET capacitance in this application)?


----------



## Dougigs

If you're trying to fit this onto a DIP-8 header, why don't you use CRD diodes for a constant current source?

 CRD datasheets say you can parallel them to set higher currents... so you could go above 15ma. If you use the SMD versions you could fit them nicely.

 They don't work quite as well as 2 FETs in cascade, but they work as well as 1... this seems like an ideal application.


----------



## mono

Quote:


  Originally Posted by *Dougigs* 
_If you're trying to fit this onto a DIP-8 header, why don't you use CRD diodes for a constant current source?

 CRD datasheets say you can parallel them to set higher currents... so you could go above 15ma. If you use the SMD versions you could fit them nicely.

 They don't work quite as well as 2 FETs in cascade, but they work as well as 1... this seems like an ideal application._

 

Well I'm soliciting opinions, what do you think is the best CCS rate for an AC/DC, and for a battery powered amp? Certainly 6 x 15 = 90mA just for buffer biasing alone is way above what most want running off of a battery. While the sound seems good, I also liked the idea of it running at lower voltage, the channel using the discrete buffer was definitely stable lower than the BUF634 channel was. I suppose that varies per taste though, I seldom want headphones really loud.

 Previously it was mentioned that an precise CCS wasn't really necessary, is there something I've missed? One of the issues I'd supposed was noise rejection, does a CRD do better, worse, same? 

 I wasn't using a DIP8 header though, it was a DIP12 (actually a DIP16 with the bottom end sawed off). A pair of holes overhangs the DIP8 buffer socket on the amp PCB, and there's where one of the clearance issues comes in but it's barely manageable.

 I am contemplating creating a PCB pattern that would provide slightly more space than the DIP12, as the DIP route seemed the most expedient towards a first impression of how this buffer sounded and I may be able to fit everything on above schematic on a DIP12 but I much prefer etching a batch of PCBs instead of wrapping tiny wires around DIP pins... and while I'm at it, even on a cramped Pimeta it seems like there's room for .1" wider board (than a DIP socket), OR if possible, to shorten the PCB some so it's not hitting one of the electrolytic caps.

 If the CRD is every bit the functional equal to the JFET though, it's something to consider. Is it's capacitance an issue (or is it on the JFET) as would be implemented here?

 It does seem almost prohibitively expensive to start paralleling CRDs though. I took at look at Mouser's datasheet (actually the price-list) for the CRD Tangent recommened for the Mint, and it's $2.35 for one 4.3mA CRD. Paralleling them, that's $28.20 just for biasing all 3 buffers.


----------



## steinchen

Quote:


  Originally Posted by *mono* 
_My 2N5486 all have IDSS about 15mA in a tight ~ 1mA spread. Is that about right for (single JFET) biasing for an AC/DC powered amp? I'd considered the figures Sijosae showed previously, 3-6mA, more optimal for battery power? 

 I hope I'm getting the JFET implementation right, is the following picture correct? The other thing I was wondering about is whether adding a resistor would help more in spot A, to limit current if desirable, or B, to isolate the JFET's capacitance and limit current "some" (but still variable based on supply voltage) (or is it even important to consider JFET capacitance in this application)?




_

 

you'll need a resistor in spot A when using 2n5486 to limit the current
 15mA is a lot, less is better, maybe you try a 2n5484

 did you take a look at the link to the Lisa-II schematic amb posted ? this topology has quite some advantages


----------



## mono

Quote:


  Originally Posted by *steinchen* 
_you'll need a resistor in spot A when using 2n5486 to limit the current
 15mA is a lot, less is better, maybe you try a 2n5484

 did you take a look at the link to the Lisa-II schematic amb posted ? this topology has quite some advantages_

 

Well the CCS amount was one of the things I was wondering about, at what point is it a diminishing return? 2N5484 starts out low at 1mA minimum IDSS, is that enough? Or, should it be viewed as something better optimized depending on whether it's an AC/DC or battery powered amp?

 I saw the Lisa-II, and it seems to just replace one of the transistor pairs with a pair of diodes (ignoring the different ground channel). Could you expand on what you wrote about "quite some advantages"? I don't see what they are except that the diodes are cheaper and smaller, which are two desirable advantages but I don't understand the significance they'll have in buffer operation.


----------



## Dougigs

CRDs aren't usually that expensive -- Rapid Electronics in the UK is carrying 5.6 ma units, which have a lower impedance than the lower-current ones, for 55 pence (a buck) apiece for quantities of less than 25... order # 47-2610 (they're happy with small orders too).

 Or if that's too steep, they're carring 3.5ma surface-mount CRDS, which are really tiny, for 13p each in 10s.


----------



## amb

A CRD is in fact a JFET inside, configured as a two-pin CCS and manufactured to flow a specific current. They are pricey but convenient.

 3 to 5mA is enough for this purpose. The point here is that you want to have enough current to drive the base of the output transistors so that they can deliver the max amount of current you desire without being starved, plus some more for reserve. Let's say you want your output transistors to be able to deliver 300mA, and assuming the Hfe of the transistor is 200, then you will need a minimum of 1.5mA of base drive current for the transistor. Add a bit more for some headroom (e.g., to provide enough charge capability to overcome the transistors' junction capacitance) and that would be a good target CCS current.

 As for transistors vs. diodes for biasing, note that in the Sijosae circuit, the B-C junction of the two bias transistors are short-circuited, so it is in effect using only the B-E diode junction, and is basically equivalent to the two diodes found in the LISA II. The Sijosae scheme provides slightly better matching of the Vbe characteristics between the bias "diodes" and their corresponding output transistor. It gives a finer tolerance of the bias point since this circuit's output transistors have no emitter resistors and the biasing is right at the edge of class B to class AB boundary.


----------



## steinchen

Quote:


  Originally Posted by *mono* 
_Well the CCS amount was one of the things I was wondering about, at what point is it a diminishing return? 2N5484 starts out low at 1mA minimum IDSS, is that enough? Or, should it be viewed as something better optimized depending on whether it's an AC/DC or battery powered amp?_

 

1mA would not be enough, but it is unlikely that you get a jfet with idss at it's rated minimum. 3mA or 4mA are fine, to be sure you'd have to hand select the jfets

  Quote:


  Originally Posted by *mono* 
_I saw the Lisa-II, and it seems to just replace one of the transistor pairs with a pair of diodes (ignoring the different ground channel). Could you expand on what you wrote about "quite some advantages"? I don't see what they are except that the diodes are cheaper and smaller, which are two desirable advantages but I don't understand the significance they'll have in buffer operation._

 

in the first line higher input impedance, though it is no requirement for your application (Pimeta) but would make the buffer more universal. In addition to that cheaper and smaller as you said.


 I'm a little confused about your aim. Do you want to improve sound, reduce quiscient current for longer battery life, optimize the buffer for low voltage operation, ... ?


----------



## mono

Quote:


  Originally Posted by *steinchen* 
_1mA would not be enough, but it is unlikely that you get a jfet with idss at it's rated minimum. 3mA or 4mA are fine, to be sure you'd have to hand select the jfets_

 

Another thought I'd had was, what if the (2n5486) JFET isolated the entire buffer? If it were in series on each power rail for both transistors then (where Sijosae had the 2K) there were resistors still. It goes back to a question I'd had about where the noise is coming from, if/when there's a noise issue.


  Quote:


 in the first line higher input impedance, though it is no requirement for your application (Pimeta) but would make the buffer more universal. In addition to that cheaper and smaller as you said. 
 

It seems while Pimeta has more flexibility in the buffer, it also has less space for it. A universal buffer would be great, but, the devil is in the details. Amps like Millet Hybrid have a lot more space around the socket, yes? You could probably fit a Jung buffer onto a Millet. So one goal is a discrete buffer that fits in Pimeta or other tightly laid-out amps, those that have (at least a) potential to be portable, near pocket-sized. It can expand in size till it exceeds a DIP8 socket a little but not much.

 As it stands, cost per 3 buffers is roughly $5 and IMO, that's OK.


  Quote:


 I'm a little confused about your aim. Do you want to improve sound, reduce quiscient current for longer battery life, optimize the buffer for low voltage operation, ... ? 
 

I'm a little confused about my aim too. 
	

	
	
		
		

		
			





 Was reading the thread and had an impulse to try to squeeze a discrete onto a Pimeta. When I started reading the thread I had no idea I'd be doing it. 
	

	
	
		
		

		
		
	


	



 It definitely sounds different, whether it's better or not could be a personal preference regardless of whether it measures as accurate as (one or more) BUF634. 

 Given the choice I would optimize towards sound quality and low voltage operation, so if possible the current use would be seen in the context of >= 700mAH cell battery packs, for example AAA, the current then isn't so important as it would be to someone trying to use a ~200mAH 9V. A target I don't expect to hit, but will aim for, is a floor of 6V from 6 cells nearly drained, BUT I don't want to compromise sound at higher voltage to get there. I do realize that the opamp and gain will dominate at some point, then there's finding that point...

 So essentially, looking for best sound out of discrete given available space, including height (else I'd move up past TO-92. It need not be smallest possible, only small enough, and need not be cheapest possible, though I think some things like a CRD vs a JFET, tend towards making a JFET worthwhile since it currently seems possible to fit a JFET for the purpose, unless something else added would be important enough to usurp some of the space.


----------



## mono

Quote:


  Originally Posted by *amb* 
_... 3 to 5mA is enough for this purpose. The point here is that you want to have enough current to drive the base of the output transistors so that they can deliver the max amount of current you desire without being starved, plus some more for reserve. Let's say you want your output transistors to be able to deliver 300mA, and assuming the Hfe of the transistor is 200, then you will need a minimum of 1.5mA of base drive current for the transistor. Add a bit more for some headroom (e.g., to provide enough charge capability to overcome the transistors' junction capacitance) and that would be a good target CCS current._

 

Thank you AMB, this makes it clear. It presents another question though, if I'm biasing for the current potential of each, is there an argument to be made for higher bias on the ground channel buffer than the L & R ?


----------



## amb

Quote:


  Originally Posted by *mono* 
_Thank you AMB, this makes it clear. It presents another question though, if I'm biasing for the current potential of each, is there an argument to be made for higher bias on the ground channel buffer than the L & R ?_

 

Theoretically, yes. In my example above, though, I've allowed for plenty of headroom so it is not necessary.


----------



## mono

Got a bit more done on discretes, knocked out a few boards and built one, may go with it.



 







more


----------



## sijosae

mono,
 It is REALLY IMPRESSIVE work.


----------



## TheSloth

Please excuse my ingnorance, but what exactly is the operational differnce between these discrete buffers and the BUF634? I have mine stacked now, and did notice an improvement in control and dynamics, however was wondering if a discrete buffer would take things even further?


----------



## TheSloth

I'm now going to show just how ignorant I am by asking about biasing buffers. Just as you can bias opamps into class A, is that something you might do with 634 buffers as well?


----------



## mono

Quote:


  Originally Posted by *TheSloth* 
_Please excuse my ingnorance, but what exactly is the operational differnce between these discrete buffers and the BUF634? I have mine stacked now, and did notice an improvement in control and dynamics, however was wondering if a discrete buffer would take things even further?_

 

I now have 3 of the pictured buffer in L/R/G channels on a Pimeta that formerly had 2X stacked BUF634 in Wide BW mode. I would describe the difference as transparency, that I now hear more differences between different opamps and source more, not the coloration of the BB buffers. It gains the ability to increase volume more too, though I never really had a complaint in that department with the BUF634, it just became more obvious that they were limiting when same amp is used with different buffers in it. To a certain extent that makes it more dynamic at median volume levels too.


----------



## amb

Quote:


  Originally Posted by *TheSloth* 
_I'm now going to show just how ignorant I am by asking about biasing buffers. Just as you can bias opamps into class A, is that something you might do with 634 buffers as well?_

 

Yes in theory, but in reality you wouldn't want to do that. See my comments about cmoys and class A in your CRDs thread.


----------



## sijosae

I finished BUF634-pin compatible diamond buffer module.


----------



## mono

Quote:


  Originally Posted by *sijosae* 
_I finished BUF634-pin compatible diamond buffer module._

 






 Another beautiful Sijosae creation, but you didn't tell us how it sounds?


----------



## tangent

Quote:


  Originally Posted by *mono* 
_Another beautiful Sijosae creation, but you didn't tell us how it sounds?_

 

He's said in the past that all op-amps sound the same to him, so he's not the right person to ask.

 You can build this buffer with just Radio Shack parts. (Substitute 2N parts for the BC's, of course.)


----------



## peranders

When you compare things bear in mind that BUF634 uses GHz transistors, highly matched and a much more advanced circuit solution.


----------



## flecom

Quote:


  Originally Posted by *tangent* 
_He's said in the past that all op-amps sound the same to him, so he's not the right person to ask.

 You can build this buffer with just Radio Shack parts. (Substitute 2N parts for the BC's, of course.)_

 

would the generic rs 2n transistors sound different you think?

 sometimes you get decent stuff in those assortments so i guess its hit or miss?


----------



## tangent

Quote:


  Originally Posted by *flecom* 
_would the generic rs 2n transistors sound different you think?_

 

Sure, transistor choice affects sound. I was just pointing out that if you want to try this, you don't have to go to heroic measures to dig up those European transistors. Lots of different transistors will work here.


----------



## mono

A followup note on the discrete buffers I made-

 I was unfortunate in happening upon a marginal/defective 1/4" to 1/8" phone jack adapter. Had my headphones on but unfortunately wasn't playing anything at the moment so no observation of amp cutting out... then I heard a "pop" sound. One of the buffers was fried. I was aware of the loss of output protection going from BUF634 to discrete, but it bears mention- Discrete buffers require more care in use, particularly that your headphones are not unplugged while the amp is turned on (or of course, that you not use a defective phone jack adapter but that goes without saying).

 I had shown a picture of a testbed Pimeta with a discrete buffer plugged in. Since then, I've added a pair of 4.3 Ohm 1W resistors (that value, because that is what I had ATM) to the Pimeta PCB positions R8L & R8R.


----------

