# A nice(?), simple chip amplifier



## PJPro

I've built a handful of headphone amps during my short time in DIY audio including the TangentSoft CMoy, GS DAK Novo and Beezar SSMH as well as a pair of Chip Amp mono power amps (still under construction) with associated ESP soft starts, AMB Labs E24s and ESP DC offset protection. Now I'd like to build something from the ground up. 

 I'd like to make something simple but a step up from the TS CMoy to drive my RS1s. So, I've been browsing the data sheets to come up with the basis of a design and stumbled across a schematic for a high performance headphone driver in the BUF634 datasheet (fig 5, page 9). I have reproduced this schematic below, adding a pot along the way.






 High Performance Headphone Driver

 Well, I guess that TI/BB must know what they're on about, right? So what do you think of the above schematic?

 I'm assuming that the gain is altered by changing the relative resistance of R2 to R3. I make the gain in the above circuit 20 (R3/R2), although BB report the gain as 21. Ideally, I'd prefer a gain of over 5 but less than 10.

 I'm surprised to see that there isn't a resistor between between the opamp and buffer. The TS CMoy certainly had one after the opamp (R5) to reduce noise....although it was optional. I also notice that the M3 also has one between the opamp and discrete buffer.

 In fact, (to my inexperienced eyes) there would appear to be some similarities between the simple circuit above and the M3 / PPA circuits except that the above uses an IC for the buffer whereas the others use discrete components. Am I right?

 Once the circuit for a single channel is sorted, I intend to use it implement a three channel design. Then I'll consider the power circuit. 

 Any thoughts, pearls of wisdom or recommendations greatly appreciated.


----------



## Slash47

Place a 10k resistor from 6 to 4 on the opamp for class A biasing. OPA627 respond well to that, lowering the distortion without dropping load impedance too much for a nicer sound.

 Also, replace the 5k feedback resistor with 2.5-ish for a gain of 11. Then you can use the OPA637 instead of the 627 which is a nice upgrade.

 Maybe a couple o' Ohms from opamp to buffer, but whatever. The less resistors in the signal path the better, although a small one is insignificant. It's more about elegance.

 Add a really nice PSU and you're done 
	

	
	
		
		

		
		
	


	




 Nothing new though...


----------



## PJPro

Thanks Slash47. Grateful for the reply. I'll include your suggestions. 

 Not trying to change the world, just do something nice and simple.


----------



## Slash47

It's a good design: tried, tested and loved by many. I wasn't hating


----------



## PJPro

Hmmm. Just googled "opamp class a biasing" and Tangent's site came top of the returned results. A quick scan reveals a number of ways to implement the biasing. Perhaps I'll take a read and consider something more complex than a single resistor to bias the opamp.....I sense an increase in complexity coming on!


----------



## Steve Eddy

_Holly came from Miami, F.L.A.
 Hitch-hiked her way across the USA
 Plucked her eyebrows on the way
 Shaved her legs and then he was a she
 She says, Hey babe
 Take a walk on the wild side
 Hey honey
 Take a walk on the wild side_











 se


----------



## Slash47

Please note that you'd really just be doing a PPA in that case. You might as well just do that! Nice and easy, with a ready made PCB.

 I personally don't like the lack of space on some ready made PCB's, though. I like expensive caps when I DIY 
	

	
	
		
		

		
		
	


	




 (bigger and shinier is always better, right?)

 EDIT: Cool design with the transformer, but do you think it'd sound nice? I'm not sure... It'd prefer a good 1uF film cap and incorporate the buffer in a low noise opamp's feedback loop.

 It certainly earns some cool point, though, which is very important.


----------



## PJPro

Yeah, but the simplicity of the BUF634 over the discrete buffer on the PPA has its attractions. And the JFET cascode doesn't look too complex does it?


----------



## PJPro

Quote:


  Originally Posted by *Koyaan I. Sqatsi* /img/forum/go_quote.gif 
_Holly came from Miami, F.L.A.
 Hitch-hiked her way across the USA
 Plucked her eyebrows on the way
 Shaved her legs and then he was a she
 She says, Hey babe
 Take a walk on the wild side
 Hey honey
 Take a walk on the wild side











 se_

 

Radical! Have you actually tried this?


----------



## Slash47

No, it's very easy. I just don't think it sounds better then a large value resistor.

 I might've meant the Pimeta, BTW. I confuse this stuff all the time, sorry.

 If you use the opamp for line driving and want to bias it, you'd have to use ~2.5k for biasing which might kill some dynamics. In that case something else might be nicer, like JFETs.


----------



## Steve Eddy

Quote:


  Originally Posted by *Slash47* /img/forum/go_quote.gif 
_EDIT: Cool design with the transformer, but do you think it'd sound nice?_

 

I've every reason to believe it would sound wonderful, provided certain input criteria were met (i.e. no DC offset, 200 ohm source impedance or less, no more than 2 VRMS peak).

 I've been designing using high quality microphone step-up transformers for voltage gain for years and when used within their limits, I'll take a good quality transformer over any active solution.

  Quote:


 I'm not sure... It'd prefer a good 1uF film cap and incorporate the buffer in a low noise opamp's feedback loop. 
 

Hey, whatever works best for you.

 Just thought I'd toss out something a little more out of the ordinary is all. 

  Quote:


 It certainly earns some cool point, though, which is very important. 
 

Thanks. Though cool points only go so far. Still have to satisfy on other levels. 

 se


----------



## Steve Eddy

Quote:


  Originally Posted by *PJPro* /img/forum/go_quote.gif 
_Radical! Have you actually tried this?_

 

Not that specific transformer with that specific buffer IC, but as I mentioned previously, I've been using this same basic approach for preamps and amps for years.

 Haven't previously done a dedicated headphone amp yet, but this is what I'm in the process of working on toward that end:






 Once I get it fully fleshed out I'll post all the details of it here in the DIY forum if anyone wants to build their own.

 se


----------



## Ant1Xr1st

*PJPro*, try this one 
	

	
	
		
		

		
		
	


	






author site (in russian)


----------



## Rescue Toaster

Quote:


  Originally Posted by *Koyaan I. Sqatsi* /img/forum/go_quote.gif 
_
 I've been designing using high quality microphone step-up transformers for voltage gain for years and when used within their limits, I'll take a good quality transformer over any active solution.

 se_

 

Steve?


----------



## Steve Eddy

Quote:


  Originally Posted by *Rescue Toaster* /img/forum/go_quote.gif 
_Steve?_

 

...Dad? 






 Yeah, it's me.

 se


----------



## Anonanimal

^----- Hahaha!


----------



## FallenAngel

Oh, that's priceless


----------



## cobaltmute

Quote:


  Originally Posted by *Koyaan I. Sqatsi* /img/forum/go_quote.gif 
_




_

 

totally off topic for the thread (and apologies for that), but what do you use to draw these schematics?


----------



## nattonrice

They do look like something from the 40's =P


----------



## Steve Eddy

Quote:


  Originally Posted by *cobaltmute* /img/forum/go_quote.gif 
_totally off topic for the thread (and apologies for that), but what do you use to draw these schematics?_

 

Well, I first started creating the symbols in AutoCAD some 20 years ago or so, and then some years later ported them into a vector graphics program similar to CorelDRAW called Xara (and at the time I bought it many years ago, Xara was owned by Corel). 

 I still use the same process and programs today.

 se


----------



## Steve Eddy

Quote:


  Originally Posted by *nattonrice* /img/forum/go_quote.gif 
_They do look like something from the 40's =P_

 

Being something of an anachronist, I was influenced by schematic drawings from the 30's and 40's when I created the symbols that I use. 
	

	
	
		
		

		
		
	


	




 se


----------



## IPodPJ

Is that really your father?


----------



## Steve Eddy

Quote:


  Originally Posted by *IPodPJ* /img/forum/go_quote.gif 
_Is that really your father? 
	

	
	
		
		

		
		
	


	


_

 

Naah. Just me being a smartass. 
	

	
	
		
		

		
		
	


	




 se


----------



## cobaltmute

Quote:


  Originally Posted by *Koyaan I. Sqatsi* /img/forum/go_quote.gif 
_Well, I first started creating the symbols in AutoCAD some 20 years ago or so, and then some years later ported them into a vector graphics program similar to CorelDRAW called Xara (and at the time I bought it many years ago, Xara was owned by Corel). 

 I still use the same process and programs today.

 se_

 

That explains why they look like art. I do like the look.


----------



## 00940

Quote:


  Originally Posted by *Koyaan I. Sqatsi* /img/forum/go_quote.gif 
_Holly came from Miami, F.L.A.
 Hitch-hiked her way across the USA
 Plucked her eyebrows on the way
 Shaved her legs and then he was a she
 She says, Hey babe
 Take a walk on the wild side
 Hey honey
 Take a walk on the wild side











 se_

 

Open loop diamond buffer (that's what the BUF634 is) ? You'll have to deal with DC offset.


----------



## diditmyself

With OPA627 you can't go (much) wrong. This could be a fine sounding amp.

 Some suggestions:
 1. I think the gain is far to high. You'll have problem adjusting the volume and the amp will be noisy.

 2. If your source doesn't have DC on it's output (it's probably already capacitor coupled) you dont need the C1 cap. I think an input resistor will do more good.

 3. If you're using a 50k pot, the R1 resistor should be something like 470k to 1M.

 4. A resistor between opamp and buffer might increase the stability. Use something like 47R to 100R.

 5. How the return currents are handled is as important as L/R channels. 3-channel topology, IC-regulators, large and good capacitors - the choice is yours, but make it good. I know what I like but I'm not going to start a war in this thread.

 6. Why not use a PIMETA v2 board? Room for fancy capacitors? With a DC-coupled 3 ch amp, who cares about capacitors!


----------



## Steve Eddy

Quote:


  Originally Posted by *cobaltmute* /img/forum/go_quote.gif 
_That explains why they look like art. I do like the look._

 

Thanks. 

 se


----------



## Steve Eddy

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_Open loop diamond buffer (that's what the BUF634 is) ? You'll have to deal with DC offset._

 

True enough. It was a bit too flippant of me to tack on the BUF-634 label like that. I was only intending to get the basic idea across rather than produce a buildable schematic.

 I've edited the image to make it more generic.

 se


----------



## diditmyself

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_Open loop diamond buffer (that's what the BUF634 is) ? You'll have to deal with DC offset._

 

No problems. Just add a resistor or CCS to the input to balance the bias currents.


----------



## PJPro

Quote:


  Originally Posted by *Slash47* /img/forum/go_quote.gif 
_.......I personally don't like the lack of space on some ready made PCB's....._

 

Yeah, I know where you're coming from. I'd prefer a bit more space sometimes.


----------



## PJPro

Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_1. I think the gain is far to high. You'll have problem adjusting the volume and the amp will be noisy._

 

OK. I'll tweak the values.
  Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_2. If your source doesn't have DC on it's output (it's probably already capacitor coupled) you dont need the C1 cap. I think an input resistor will do more good._

 

Need to return to this later methinks.
  Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_3. If you're using a 50k pot, the R1 resistor should be something like 470k to 1M._

 

OK. Thanks.
  Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_4. A resistor between opamp and buffer might increase the stability. Use something like 47R to 100R._

 

I'll place it on the schematic as optional.
  Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_5. How the return currents are handled is as important as L/R channels. 3-channel topology, IC-regulators, large and good capacitors - the choice is yours, but make it good. I know what I like but I'm not going to start a war in this thread._

 

Agreed. I'm intending 3 channel. Thought it best to get a L/R basically right first.
  Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_6. Why not use a PIMETA v2 board? Room for fancy capacitors? With a DC-coupled 3 ch amp, who cares about capacitors!_

 

I didn't realise that the Pimeta used an IC buffer. Having looked, the Pv1 actually uses the BUF634! However, I'd still like to press on as one of my intending learning objectives from this exercise will/may be the production of a PCB (I've never done one before).


----------



## diditmyself

I understand your urge to make your own board. Best of luck with it!


----------



## TzeYang

Try them on a perfboard layout first.

 PCB is not something you want to risk unless you're very sure about what you're doing.

 The cost of fabricating PCB is not worth the risk. Unless you're loaded, that's another thing lol.


----------



## cobaltmute

Doing a PCB is definitely a case of measure twice, order once. 
	

	
	
		
		

		
			





 amb noted something that I have found very useful: Print you PCB layount on paper at 100% scale and make sure everything fits. It will save you from making coasters.

 BatchPCB can make it cost effective to run a single board or two.


----------



## PJPro

Thanks for the advice guys. 

 Yes, I intended to do a protoboard version first to see if it's worth going to the next stage. Printing out the design sounds like a good idea which I hadn't considered.....but then again I'm not going to minimise the layout (like the professionals). But I'll do the print out anyway.


----------



## cobaltmute

Print it out anyways - it helps ensure that the footprint is correct. I speak from experience where I made a whole panel of coasters due the fact that the software had the wrong footprint in it. 

 And leave it a week between when you decide on your "final" layout and before you send out the job. A week away from the PCB and you may see something you didn't the first time.


----------



## PJPro

Well, I've been having a bit if a read and have introduced the spirit of changes to the schematic as recommended. 

 It now looks like a TangentSoft CMoy with a buffer plugged on the end, with Class A biasing and a cap (33pF) in the feedback loop as used by the M3. TBH, I could arrived almost directly at the design by reading the Headwise Design Paper. Still, it's kept me busy.

 I'll post the schematic shortly.


----------



## PJPro

Hmm. Here's the latest...






 Ground channel next....I expect very similar to the L/R channel.


----------



## cobaltmute

You want decoupling caps on the op amps and the buffer. 0.1uF on all supplies and a 0.1uf across the pins as a starting point.

 Someone with more experience than me may comment on this one, but I think you want R5 after R3, other wise R5 and R3 form a resistor divider.


----------



## PJPro

Well, I was certainly going to place decoupling caps on the power supplies to the opamps (as per the datasheet) but wasn't going to do this for the buffers. Should I? 

 R5 is optional and serves to reduce noise going into the buffer. Tangent notes that the BUF634 has 200 ohms of series resistance on its input, so R5 isn't really needed. I wanted the option to bung it in, just in case. However, I think you are right, I'll move it to the right of the JFET cascode.


----------



## TzeYang

why not just build a PPA V1/PIMETA V1?


----------



## cobaltmute

From the BUF634 datasheet:

  Quote:


 Power supplies should be bypassed with capacitors connected close to the device pins. Capacitor values as low as 0.1uF will assure stable operation in most applications, but high output current and fast output slewing can demand large current transients from the power supplies. Solid tantalum 10uF capacitors are recommended.


----------



## PJPro

Quote:


  Originally Posted by *TzeYang* /img/forum/go_quote.gif 
_





 why not just build a PPA V1/PIMETA V1?_

 

Well, it's true the schematic is more or less the same but I'd like to have a go at producing my PCB. I thought an opamp based design would arrive at the best sounding, simple design. But I wanted something more than a CMoy variant, hence the buffer.

 The guidance given here, plus the other posts I've read (here, Headwise & diyaudio) have lead me to produce a PIMENTA / M3 (with an IC buffer) clone. Perhaps it is not surprising given the influence these designs have made on the diy community. And I can take some comfort in the fact that the heritage of the above design is good and should produce a decent amp if I can get everything else right (gulp).


----------



## PJPro

Quote:


  Originally Posted by *cobaltmute* /img/forum/go_quote.gif 
_From the BUF634 datasheet:_

 

Noted. Thanks.


----------



## PJPro

OK. Power supplies. Should I go for a single supply design and use a TLE rail splitter or just go for dual supply?


----------



## wap32

I'm working on a similar design but using a MOSFET buffer and I have a question about C2.

 If I'm correct, it works as a bandwidth limiter to avoid stability issues.
 I've also seen it placed on the output of the buffer instead, as such (circuit b):







 Are there any differences with such placements?

 More importantly, how do I pick the correct value for C2, what resistance does it see?
 Also, since the ground channel will be in a unity gain configuration, its value must change, no?


----------



## diditmyself

A single supply is a lot easier to build. I vote for TLE2426, but I'm biased because of bad luck with LM317/337 as ground. There are other options like using two resistors and a high quality opamp instead of TLE2426 or you could use shunt regulators.


----------



## PJPro

Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_A single supply is a lot easier to build. I vote for TLE2426, but I'm biased because of bad luck with LM317/337 as ground. There are other options like using two resistors and a high quality opamp instead of TLE2426 or you could use shunt regulators._

 

If I go the single route, I'll do something like this (not the battery)....





 From Tangentsoft


----------



## diditmyself

Quote:


  Originally Posted by *wap32* /img/forum/go_quote.gif 
_I'm working on a similar design but using a MOSFET buffer and I have a question about C2.

 If I'm correct, it works as a bandwidth limiter to avoid stability issues.
 I've also seen it placed on the output of the buffer instead, as such (circuit b):






 Are there any differences with such placements?

 More importantly, how do I pick the correct value for C2, what resistance does it see?
 Also, since the ground channel will be in a unity gain configuration, its value must change, no?_

 

The parts in the schematic aren't numbered. Do you mean the 20 pF compensation cap? In that case it depends on what opamp you use and what gain/feedback resistors you use. Some times it's needed and sometimes it's not. Often it is better to use it locally - from opamp output to inverting input. I'd suggest you either use "known-to-work-values" for a particular opamp or you learn LTSpice. Sometimes you can find recommended values in the opamp data sheet. The best way is to use a signal generator and an oscilloscope.

 The cap on the output in the 1st schematic is a coupling cap (high pass filter) to block out DC. This buffer will have lots of it otherwise. When thrown in the loop of an opamp this DC will be corrected (if the opamp has low DC offset).


----------



## diditmyself

Quote:


  Originally Posted by *PJPro* /img/forum/go_quote.gif 
_If I go the single route, I'll do something like this (not the battery)....





 From Tangentsoft_

 

Yes, it's a good, reliable, tweakable and easy to build ground. I think it could be wise to use smallish caps from rails to ground to handle higher frequencies. The TLE2426's output impedance rises steeply at higher frequencies. Look at how amb does it in eg M3 and Mini3. If amb does it, you should do it. He really knows his stuff.


----------



## wap32

Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_The parts in the schematic aren't numbered. Do you mean the 20 pF compensation cap?_

 

I was talking about C2 in PJPro's latest schematic and the 20pF cap in the schematic I posted.

  Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_In that case it depends on what opamp you use and what gain/feedback resistors you use. Some times it's needed and sometimes it's not. Often it is better to use it locally - from opamp output to inverting input. I'd suggest you either use "known-to-work-values" for a particular opamp or you learn LTSpice. Sometimes you can find recommended values in the opamp data sheet. The best way is to use a signal generator and an oscilloscope.

 The cap on the output in the 1:st schematic is a coupling cap (high pass filter) to block out DC._

 

But even "known-to-work-values" for an opamp would be tied to a particular gain/feedback setting right?

 I guess I'll be doing some simulations.


----------



## diditmyself

Quote:


  Originally Posted by *wap32* /img/forum/go_quote.gif 
_I was talking about C2 in PJPro's latest schematic and the 20pF cap in the schematic I posted.



 But even "known-to-work-values" for an opamp would be tied to a particular gain/feedback setting right?

 I guess I'll be doing some simulations._

 

Yes, but sometimes you can find the right resistors/comp cap values in the data sheet or you can find it in posts here, at Headwize or at Diyaudio if you're lucky.

 LTSpice is free and it usually predicts the behaviour quite well, most of the time spot on, better than it's rumour. Good luck with it, and I hope you build yourself a nice amp. Like always, if you build it please post your impressions and pictures here in the DIY forum.


----------



## PJPro

I plucked the C2 cap from Amb's M3 design. I thought I meantioned that above? Have a look at his parts selection pages for the M3. The value is tweaked depending on the opamp used. 15pF is the value recommended for the OPA637. I think the opamp rolling option was 33pF.


----------



## diditmyself

Quote:


  Originally Posted by *PJPro* /img/forum/go_quote.gif 
_I plucked the C2 cap from Amb's M3 design. I thought I meantioned that above? Have a look at his parts selection pages for the M3. The value is tweaked depending on the opamp used. 15pF is the value recommended for the OPA637. I think the opamp rolling option was 33pF._

 

From simulation there's almost no need for a cap if 1k/10k is choosen as feedback resistors. In that case 1 pF seems to be enough. In M3 the opamp drives MOSFETs and your amp uses a bipolar input buffer so you can't extrapolate the M3 values to your amp.

 If you look in the data sheet you can see an example: 500R/1k feedback resistors and a 6 pF feedback cap. It also simulates perfect in LTSpice.


----------



## diditmyself

This is what a square looks like with 1k/10k with a 15 pF cap. It's not what you want.


----------



## PJPro

Yes. Not good. 

 I downloaded TINA-TI today and am in the process of adding the schematic. Is LTSpice better?


----------



## diditmyself

I've never learnt TINA-TI. I gave it up early. I only know the basic stuff with LTSpice, but I find it inevitable now, and like I said it almost always predict the circuit spot on. Some slight adjustments have to be done if working with discrete circuits because of varying Hfe, Idss, Vgs etc. It's not very intuitive, and there aren't any good manuals except for the basic stuff. I think I've learnt the most at Diyaudio.com. I have an oscilloscope, but if you don't a simulator is a must.


----------



## diditmyself

It's getting late in Sweden, I have to go to bed now.


----------



## Mr.Unknown

Heed canamp seem to be a simple amp 
	

	
	
		
		

		
		
	


	




. I'm going to build AD815 headphone amp and mod heed can to use AD815 in input stage. I will using MJE15030 in output and shunt regulator in power supply 
	

	
	
		
		

		
		
	


	




. 
 AD815 is very good chip, another diyer in my country have finished AD815 pre, It sound very good( I prefer 815 than 275, 627).

 P/s: what's BJT good to replace BD139-140???


----------



## diditmyself

Quote:


  Originally Posted by *Mr.Unknown* /img/forum/go_quote.gif 
_Heed canamp seem to be a simple amp 
	

	
	
		
		

		
		
	


	




. I'm going to build AD815 headphone amp and mod heed can to use AD815 in input stage. I will using MJE15030 in output and shunt regulator in power supply 
	

	
	
		
		

		
		
	


	




. 
 AD815 is very good chip, another diyer in my country have finished AD815 pre, It sound very good( I prefer 815 than 275, 627).

 P/s: what's BJT good to replace BD139-140???_

 

AD815 isn't the opamp of choice for an input amp because of the high bias currents, but they can be dealt with and it's done in the carlosfm preamp. If you use AD815, I can't see why you should use output buffers.

 Regarding BD139/140, it depends on what you're going to use them for.


----------



## PJPro

OK. I've tweaked the circuit a little and performed some initial analysis with TINA-TI. Find this below...






 One Channel with Analysis - 100K Square Wave

 With C2 at 75pF, the leading edge of the wave was distinctly curved, as predicted/shown by diditmyself. 8.2pF provides the squarest wave.

 So. What do you think? Does that wave look pretty good or is there more to do?


----------



## cobaltmute

I'd say that looks good enough for a sim - remember reality can be a different beast.

 I don't know if you'd need a load sharing resistor before the buffers (like the PPAv1)?


----------



## diditmyself

That looks like the square wave you want. You're a fast learner. Isn't simulating fantastic? Noobs like us can design amps!


----------



## PJPro

Quote:


  Originally Posted by *cobaltmute* /img/forum/go_quote.gif 
_I'd say that looks good enough for a sim - remember reality can be a different beast.

 I don't know if you'd need a load sharing resistor before the buffers (like the PPAv1)?_

 

Having read Tangent's comments in his parts selection pages for the PPAV1, I think you're right. Actually, I'm a bit surprised by the PPAV1. I thought it used a discrete buffer....obviously that was a change introduced for V2.


----------



## diditmyself

Do you already have the parts? If you're planning on stacking BUF634 why not go for a better one instead like LME49600? One LME is cheaper than two BUFs. I'm afraid I haven't heard any of them but people report that LMH6321 sounds better than BUF634 and LME49600 should be an improvement over LMH6321. I have experience from LMH6321, high current opamps and discrete buffers. The advantage of discretes is that you can run them at the bias you want. I prefer class A to class AB. It's warmer, softer and more natural to my ears. Some seem to prefer class AB.


----------



## PJPro

Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_That looks like the square wave you want. You're a fast learner. Isn't simulating fantastic? Noobs like us can design amps!_

 

Yeah it's great playing with the software....although I did get a little annoyed at first when I couldn't get anything to work. I took a look at LTSpice as well but felt that TINA-TI was the best option for me to get up to speed quickly.

 As far as the design of amps go, I'm definately standing on the shoulders of giants. Without the work by Tangent, AMB, CMoy and the posters on Headfi, on Headwise and on diyaudio I doubt whether I could have come up with the design myself......although the BUF634 datasheet provided a pretty good start point. 

 What the sim software does allow is for someone like me to tweak designs, combine ideas and come up with something that shouldn't be too shabby. However, there's a long way to go and plenty of opportunities along the way for me to make a mess of it. With a lot of luck and a bit of help it might turn out alright.


----------



## PJPro

Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_Do you already have the parts? If you're planning on stacking BUF634 why not go for a better one instead like LME49600? One LME is cheaper than two BUFs. I'm afraid I haven't heard any of them but people report that LMH6321 sounds better than BUF634 and LME49600 should be an improvement over LMH6321. I have experience from LMH6321, high current opamps and discrete buffers. The advantage of discretes is that you can run them at the bias you want. I prefer class A to class AB. It's warmer, softer and more natural to my ears. Some seem to prefer class AB._

 

No, I haven't got the parts yet. I've taken a look over at National and couldn't spot a model for the LME49600. Do you have one? Also, I'm not sure I'm keen on the package.....I wanted DIP8


----------



## cobaltmute

I've got a Pimeta V2 with the LMH6321 as the buffers and I prefer my V1 with discrete buffers (the Monofied Sijosae buffers). I don't know what it is but there is some slightly wrong sounding to the LMH6321s to my ears.


----------



## MisterX

Quote:


 Having read Tangent's comments in his parts selection pages for the PPAV1, I think you're right 
 

You're looking in the wrong place. 
	

	
	
		
		

		
		
	


	





 Check figure 1 of the datasheet 

http://focus.ti.com/lit/ds/symlink/buf634.pdf

 Or consult the Pimeta parts selection guide. 

Part Selection Guide

  Quote:


 R7

 We reserved this part number in the PIMETA for those familiar with the META42 or PPA. In the PIMETA, there is no series resistor between the op-amp and the buffer since the BUF634 has 200 ohms of series resistance on its input already. Hence, there is no need for an “R7”.


----------



## wap32

Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_That looks like the square wave you want. You're a fast learner. Isn't simulating fantastic? Noobs like us can design amps!_

 

Speaking of noobs... 
	

	
	
		
		

		
		
	


	




 I've also been doing some simulations to adjust the compensation cap value, but I haven't been looking at square waves, I just plotted the frequency response.
 Right now, with 33pF, the cutoff frequency (gain-3dB) is around 350kHz and it's just enough to keep the gain equal on all audible frequencies (up to 20kHz).

 Should I be aiming for something else?

 EDIT: I think I answered my own question, or better, AMB did. On the M3 page the caps values are all aimed in order to provide a 1~2Mhz cutoff frequency, so I need to lower the value of my cap.


----------



## PJPro

Quote:


  Originally Posted by *MisterX* /img/forum/go_quote.gif 
_You're looking in the wrong place. 
	

	
	
		
		

		
		
	


	





 Check figure 1 of the datasheet 

http://focus.ti.com/lit/ds/symlink/buf634.pdf

 Or consult the Pimeta parts selection guide. 

Part Selection Guide_

 

Hmmm. I was looking at R11, R12, R13 & R14 on the PPAV1 and the associated comments....

  Quote:


 R11, R12, R13, R14

 These resistors balance the current from the op-amp among the buffers, and reduce electrical ringing in the amp. R11 goes with BUF1, R12 with BUF2, etc.

 There is some wiggle room on the value of this resistor, but for most purposes 1 KΩ is a good value.

 Optional? You may be able to get away with jumpering these, but the amp will not perform as well as it should if you do. 
 

Given the 200R on the input of the BUF634, should I look to place a 800R resistor in front or is it really not needed?


----------



## Ant1Xr1st

*PJPro*, its not a good idea to parallel buf634 try the discrete Diamond Buffer - Design
  Quote:


 should I look to place a 800R resistor in front 
 

no, you just need 100-200ohm on input of any opamp or buffer to prevent self oscillating


----------



## PJPro

Quote:


  Originally Posted by *Ant1Xr1st* /img/forum/go_quote.gif 
_*PJPro*, its not a good idea to parallel buf634_

 

Why's that?

  Quote:


  Originally Posted by *Ant1Xr1st* /img/forum/go_quote.gif 
_try the discrete Diamond Buffer - Design_

 

Yeah. I've seen that. Thanks. But I was hoping to stick with the simplicity of IC.

  Quote:


  Originally Posted by *Ant1Xr1st* /img/forum/go_quote.gif 
_no, you just need 100-200ohm on input of any opamp or buffer to prevent self oscillating_

 

Hmm. OK. Thanks.


----------



## Ant1Xr1st

Quote:


 Why's that? 
 

Two buf634 cant be 100% indentical, so they will produce a bit more distortion, also you will might need to add approx 10ohm to the output of each buffer 
	

	
	
		
		

		
		
	


	



 I have compared both of them, diamond buffer has a bit more soft and realistic sound
 What are purpuse of using 2 buffers? more class A bias?
 If you want a simplicity, is's possible to make a diamond buffer just with 4 transistors.


----------



## PJPro

I've not made any progress with the modelling tonight. I've been having trouble getting the rail splitter to work properly. Not only that, I wanted to add a non TI component (LM317) and can't work out how to add it to the tool.

 Can anyone give me some tips which will allow me to do this?


----------



## PJPro

OK. I'll take that as a no then. I'd have thought that lots of people on the forum would be using TINA-TI and would have encountered (and overcome) this problem. Seems I am wrong.


----------



## cobaltmute

BTW, the LM317 is a TI part as well.

 AFAIK, Tina-TI is a spice simulator - you should get be able to add the spice model to it.


----------



## PJPro

Yes, thanks cobaltmute. However, TI don't seem to have a model for it and TINA-TI seems to come with a very limited set of models. 

 I've tried plugging a LM317 spice macro behind a symbol for the TLE2425 (which is in TINA-TI). It will compile and I've linked up the pins with model script but I'm getting some crazy results which suggests all is not well.

 TBH, I'm getting a little hacked off with TINA-TI. And LTSpice might be unlimited but the front end is completely counter intuitive (for my simple mind).


----------



## PJPro

Hmmm. I'm just gonna have to learn LTSpice.


----------



## diditmyself

Sorry I can't help you with TINA-TI. I think you'll get more support from LTSpice users, and I think you're going to need it. Like you said there's nothing intuitive about it, a real technocrat product.


----------



## PJPro

Quote:


  Originally Posted by *diditmyself* /img/forum/go_quote.gif 
_......you'll get more support from LTSpice users, and I think you're going to need it......._

 

Hmmm. That sounds worrying. 

 However, I have managed my first limited success with LT Spice. I've placed a 3rd party model into the library of the LM317, created a circuit using the LM317 and ran a DC simulation to arrive at expected results i.e. the correct voltage coming out of the LM317 given the adjustment applied. 
	

	
	
		
		

		
			





 Just need to do the same with the OPA637, TLE rail splitter and BUF634


----------



## diditmyself

Like I said, you seem like a very fast learner. You have more patience with this stuff than I do. The more here that knows LTSpcice the better.


----------



## PJPro

No as fast as I'd like. I've got the same problem with LTSpice that I hit with TINA-TI. For some reason neither will accurately model the function of the rail splitter......or, perhaps more likely, I don't understand how the TLE works!


----------



## 00940

With rail splitters, you have to be very careful about where you place the ground in the simulation. 

 A very good thread about LTspice: Things you should know about LTSpice - diyAudio

 Btw, I've learned first with LTspice. I then tried Tina-TI but really prefers LTspice.

 Edit: here's an amp with virtual ground in LTspice. The true ground is linked to the voltage source and all "grounds" to a trace named VG.


----------



## jcx

a problem with macromodels - Boyle op amp macromodel in particular is the use of Spice node 0 internally for voltage gain reference

 sometimes this simplification is OK, other times it misses important performance characterisics

 also no Spice allows floating nodes, everything has to have a dc path to node 0 - although it can be GigaOhms


----------



## PJPro

Thanks for your comments guys but it's a little more fundamental than that. The rail splitter isn't splitting the rails or arriving at 0V on the output.

 I'm feeding it a single supply eg 0, 20VDC. I'm expecting +/-10VDC on the rails and 0VDC on the output but I'm getting 20VDC on the positive, 0VDC on the negative and 10VDC on the ouput. This is the same behaviour I witnessed on TINA-TI....which sort of suggests I'm doing something wrong, although I am using the same (TI) macro in both.


----------



## cobaltmute

I can't say for LTSpice or Tina-TI, but in Multisim, it depends on where you attach the "ground".

 If you attach it to the negative end of the power supply, you set that as zero potential and the middle of the rail splitter is 10V




 If you attach it to the middle of the rail splitter, you get a +- 10V





 I might guess and could be wrong that the other SPICE programs work the same. Your rail-splitter is doing its job - it is just what point are you measuring 0 from.


----------



## MisterX

Quote:


 I'm getting 20VDC on the positive, 0VDC on the negative and 10VDC on the ouput. 
 

That is what you should be "getting". 

  Quote:


  Originally Posted by *PJPro* /img/forum/go_quote.gif 
_ This is the same behaviour I witnessed on TINA-TI....which sort of suggests I'm doing something wrong, although I am using the same (TI) macro in both._

 

This is what you are doing wrong----> 

  Quote:


 I'm expecting +/-10VDC on the rails and 0VDC on the output


----------



## wap32

The TLE is a "rail splitter" in the sense that it splits the rail-to-rail voltage in half, in your case 20V.

 So you would have the split voltage, 10V, as the output, not 0.
 By using that output as your virtual ground you will effectively have the equivalent of a +/-10V supply (20V-10V-0V), with the exception that the virtual ground won't be able to source/sink much current using the TLE alone.

 Someone please correct me if I'm wrong.


----------



## PJPro

Quote:


  Originally Posted by *wap32* /img/forum/go_quote.gif 
_The TLE is a "rail splitter" in the sense that it splits the rail-to-rail voltage in half, in your case 20V.

 So you would have the split voltage, 10V, as the output, not 0.
 By using that output as your virtual ground you will effectively have the equivalent of a +/-10V supply (20V-10V-0V), with the exception that the virtual ground won't be able to source/sink much current using the TLE alone.

 Someone please correct me if I'm wrong._

 

Well, as far as I can tell, this is the approach used by the Pimeta, PPA and M3. In fact I've just noticed that AMB states, with regard to the M3, that a dual supply is unecessary as the supply is split into two rails with the virtual ground being at one half the supplied voltage (in my case 10VDC).


----------



## jcx

you are free to put the little gnd triangle on any (one) node in a circuit and it will (except in sims with the flawed macromodel issue) behave exactly the same - all "V" readings in a real circuit are V difference readings - and you can choose your reference in any way that aids understanding

 a convenient location for the Spice node 0 gnd symbol in active supply split circuits is the output of the supply splitter

 in LtSpice you can make difference measurements explicit by holding down the button when selecting a circuit node for display and draging the probe to any node you wish to use as reference - the probe symbol will change from red to black and select the reference node when you release the button - the trace in the waveform viewer will have a label like: V(n003,n007) indicating it is an explicit difference; the reference is not displayed when it is referenced to Spice node 0 = gnd


----------



## PJPro

Finally started getting some waves coming out of LTSPice......with a slight backward step in that it's only a CMoy with rail splitter. But useful, nonetheless, to allow me to get my head around the tool.


----------



## PJPro

Have returned to this project after a month of inactivity

 Made quite a bit of progress. I've placed the whole circuit into LTSpice and have tweaked the values of the components based on the "quality" of the square wave coming out of the simulation.

*Question. * Has anyone successfully simulated an active ground design eg M3? For some reason LTSPice complains about a singular network when I try to link the left and ground channels through a 32R load, representing a headphone. It identifies issues with the buffers on the left and ground as the problem.


----------



## PJPro

OK. I have modelled a single, complete channel of an M3 using LT Spice. (I have ignored the complications around the active ground). Has anyone tried this? 

 I've sent a square wave through the circuit using the following values PULSE(-10m 10m 0 0 0 .0000002 .0000004) where the parameters are PULSE(V1 V2 Tdelay Trise Tfall Ton Tperiod). The simulation runs for .0000008s. 

 The results are far from what I was expecting. The square wave has gone, to be replaced by something looking more like a sine wave.

 Is it reasonable to expect the M3 to be able to cope with a wave as fast as the one specified above? If not, can you suggest more reasonable parameters? Or does this simply demonstrate the limitations of modelling?


----------



## PJPro

I am still progressing this project and am documenting the design process here.
   
  I stumbled across an application note for the BUF634 on the Texas Instruments website, written by Burr-Brown. It's called _"Combining an amplifier with the BUF634"_. You can find the application note here.
   
  The application note held some tweaks which I've incorporated into my design. However, it seems that they have made a mistake. The polarised caps on the opamp rails (fig 2) seem to be the wrong way round. I would have expected the negative leg of the cap to be attached to the negative rail and vice-versa for the positive rail.
   
  Can someone confirm this for me please?


----------



## PJPro

Incidentally, if my design thread is cobblers I'd be grateful for corrections


----------



## MisterX

They're right.
   V- has more negative potential then "ground" so the positive lead connects to "ground".
  Quote: 





> The polarised caps on the opamp rails (fig 2) seem to be the wrong way round. I would have expected the negative leg of the cap to be attached to the negative rail and vice-versa for the positive rail.


----------



## PJPro

Thanks for your reply MisterX. 
   
  I got a bit confused there as your statement mirrored mine...so how could they be right? 
   
  Refering back to the original application note, they do indeed have the negative leg of the cap attached to the negative rail, as I would expect.  I must have misread the schematic.  
	

	
	
		
		

		
		
	


	



   
  I'll update my design thread.


----------



## PJPro

This is the design so far. 
   

   
  It's getting more complex than I would have liked and is likely to cost more than I expected.  Anyone spot any silly mistakes? 
   
  I think the design has a little way to go yet.  Clearly, I still need to consider the power circuit and I may have to look at ways at reducing the cost of the components.
   
  Any comments gratefully received.


----------



## Pageygeeza

I'm getting really interested by this project.  I've wanted to design my own, but admittedly I don't think my heart is in it.  
   
  How are you going to set your amp with this design, will you be using three of the exact same circuits for L/R and ground?


----------



## Leny

I’ve not read all of the thread, so here is a quick comment based upon the scheme in post #99.
   
  R1 acts with R2 as a voltage divider to cut the signal by 50%. Why cut the signal in an amplifier? It would seem reasonable to ditch R1 and reduce the gain of the amp to compensate. i.e. reduce gain from 3.7 to 1.85. (OPA627 is stable at low gain). (Or maybe R1, R2 simply represents the half-way point of a 50K pot?).
   
  Why R3? It’s another (yet tiny this time) voltage divider. Or perhaps you intend to use it in combination with a small capacitor across R4 to limit input bandwidth?
   
  I know external biasing of the output of the op-amp can work. However, if you want to get ultra-picky about it consider that according to the Burr-Brown datasheet the negative half of the output has slightly better Power Supply Rejection, hence biasing the op-amp to the positive rail, and hence using the negative output transistor, should bring a theoretical (though small) improvement. (In reality it might not be noticeable). If you do decide to try it then just slide the CCS upwards... top of R11 to positive rail, bottom of R10 to OPA627 output.
   
  I don't know about your CCS values, but you might want to try varying the CCS current a tad (via R10) to see if it helps.
   
  Reference:
http://focus.ti.com/lit/ds/symlink/opa627.pdf
   
  Good luck with it,
  L.


----------



## Leny

I just realised that with R3 you are attempting to balance input impedances to the + and - inputs of the op-amp.
   
  That is necessary mainly with BJT input op-amps that pull input current. i.e. equal current through equal impedances leads to equal voltages; they cancel and hence there is no DC output offset. This is a DC function, not AC. You have blocked DC to R3 via C1 so it has no effect. Also, the 560K would be a party pooper if you were really keen to balance impedances.
   
  (Note for future, especially if you ever choose a BJT input op-amp; R4 provides input AC load *and *DC bias path, which is why choosing a value is a compromise).
   
  However, here you actually have a FET input op-amp with tiny, tiny input current requirements. As such balancing the impedances is less of an issue.
   
  Conclusion: delete R3 !


----------



## Ant1Xr1st

Put 1-4.7ohm after each BUF634
  Put 22-100om between OPA627 and BUF634
  C4/C2 are not needed - use big electrolytic 470-2200u


----------



## Avro_Arrow

It looks like you are trying to build a PIMETA V1
  You should read through the article for design tips.


----------



## Avro_Arrow

FET input amps need to have balanced input impedances too, but for a different reason...it is because of the capacitance
  of the FETs. R3 should equal the impedance seen by the - input, including any multi-loop resistors. Articles by Walt Jung
  explain all this in more detail.
  
  Quote: 





leny said:


> I just realised that with R3 you are attempting to balance input impedances to the + and - inputs of the op-amp.
> 
> That is necessary mainly with BJT input op-amps that pull input current. i.e. equal current through equal impedances leads to equal voltages; they cancel and hence there is no DC output offset. This is a DC function, not AC. You have blocked DC to R3 via C1 so it has no effect. Also, the 560K would be a party pooper if you were really keen to balance impedances.
> 
> ...


----------



## PJPro

Quote: 





pageygeeza said:


> I'm getting really interested by this project.  I've wanted to design my own, but admittedly I don't think my heart is in it.
> 
> How are you going to set your amp with this design, will you be using three of the exact same circuits for L/R and ground?


 

 Well, I was hoping to produce a single board which could be used for however many channels you wanted eg 2 for passive grounding, 3 for active grounding and 4 for balanced.  That was the aim.....but it would mean a lot of extra wiring e.g. for the pot.


----------



## PJPro

Quote: 





leny said:


> R1 acts with R2 as a voltage divider to cut the signal by 50%. Why cut the signal in an amplifier? It would seem reasonable to ditch R1 and reduce the gain of the amp to compensate. i.e. reduce gain from 3.7 to 1.85. (OPA627 is stable at low gain). (Or maybe R1, R2 simply represents the half-way point of a 50K pot?).


   
  Yes, it's the latter. I'm using LTSPice to produce the schematic and and do a bit of modelling as well. I know that there's better ways of representing a pot in LTSPice but fortunately, my efforts to join the Yahoo forum have failed so far. I sent off my email request but have had no reply.
   


leny said:


> Why R3? It’s another (yet tiny this time) voltage divider. Or perhaps you intend to use it in combination with a small capacitor across R4 to limit input bandwidth?


   
  It's to balance the input impedences. This resistor features in schematics crafted by a number of designs I've canvassed including ppl, tangent and amb. Its value was selected to reflect R5.
   


leny said:


> I know external biasing of the output of the op-amp can work. However, if you want to get ultra-picky about it consider that according to the Burr-Brown datasheet the negative half of the output has slightly better Power Supply Rejection, hence biasing the op-amp to the positive rail, and hence using the negative output transistor, should bring a theoretical (though small) improvement. (In reality it might not be noticeable). If you do decide to try it then just slide the CCS upwards... top of R11 to positive rail, bottom of R10 to OPA627 output.


   
  My research has lead me to believe that the NPN transistors in the opamp are usually better behaved, hence my use of the negative rail.
   


leny said:


> I don't know about your CCS values, but you might want to try varying the CCS current a tad (via R10) to see if it helps.


   
  I am intending to use a trimpot in place of R10. Again, this is a simplification due to my limited experience with LTSpice.
   
  Many thanks for your comments Leny.


----------



## PJPro

Quote: 





ant1xr1st said:


> Put 1-4.7ohm after each BUF634
> Put 22-100om between OPA627 and BUF634
> C4/C2 are not needed - use big electrolytic 470-2200u


 

 From reading the various datasheets/application notes I don't think it's necessary to place resistors after or before the BUF634.  However, I'm happy to be educated.
   
  C2/C4.  Agree.  But isn't 2200u a bit excessive?  I was thinking more along the lines of 100-200u.


----------



## PJPro

Quote: 





avro_arrow said:


> It looks like you are trying to build a PIMETA V1
> You should read through the article for design tips.


 

 Thanks Avro_Arrow.


----------



## Ant1Xr1st

Quote: 





> Agree.  But isn't 2200u a bit excessive?  I was thinking more along the lines of 100-200u.


 
  100u is ok too. but is is a minimum possible value. do you want to make this amplifier xtremly portable? higher capacity will be better anyway.


----------



## Leny

If you are aiming for AC impedance balance you still have some work to do.
   
  If I were to build it I would be inclined to try the CCS on both the negative and the positive rails in turn (in the real / non-sim circuit) and measure the response, even if just for curiosity!
    
  Anyway, good luck with it PJPro.


----------



## jcx

Quote: 





pjpro said:


> ...It's getting more complex than I would have liked and is likely to cost more than I expected.  ...I may have to look at ways at reducing the cost of the components.
> 
> Any comments gratefully received.


 

 there are several, more recent  "replacements" for opa627 if you can use surface mount parts - all are cheaper
   
  for the output buffer function LT1210 with 1 A output is cheaper than 3 buf634; ad8397, tpa6120 duals are cheaper than one buf634 and can do 2 channels if 3-400 mA ea is enough - or parallel the 2 in a chip for each channel
   
  an active supply splitter doesn't require equal "audiophile quality" as the R/L channels - as long as you don't use "3-channel" - or, if you build your own supply, dual V with CT gnd won't add much cost
   
   
  you can't balance the input impedance with fixed R if you use a volume pot - your source R seen at the op amp input changes with setting


----------



## PJPro

Thanks for your comments jcx
   


jcx said:


> there are several, more recent  "replacements" for opa627 if you can use surface mount parts - all are cheaper


   
  I'd like to stick to DIP8 and was considering the AD843. 
   


jcx said:


> for the output buffer function LT1210 with 1 A output is cheaper than 3 buf634; ad8397, tpa6120 duals are cheaper than one buf634 and can do 2 channels if 3-400 mA ea is enough - or parallel the 2 in a chip for each channel


   
  Do they sound any good? I haven't come across too many designs using the LT1210.
   
  PPL clearly rates the HA3-5002 very highly, esp with the AD8610. Presumably Tangent supports this view...hence the PPA v1. So the HA3-5002 is a possible candidate replacement for the BUF634.
   
  The other alternative is the LME49600.....although many appear to see this as a rebadged BUF634.
   
  I have read that the BUF634T is the better sounding variant of the BUF634, presumably due to it maintaining its performance by keeping cooler.  I had intended to go for this TO-220 version, perhaps supplemented with a small heat sink.   That said, I have no idea how hot these get.  They can't get that hot as it would rule out buffer stacking.
   


jcx said:


> an active supply splitter doesn't require equal "audiophile quality" as the R/L channels - as long as you don't use "3-channel" - or, if you build your own supply, dual V with CT gnd won't add much cost


   
  Not sure I fully understand your comments......but I want to use a wallwart to power the amp and was intending to use a rail splitter.


----------



## jcx

Having earned a living for 30 years designing electronics and having followed audio amplifier electronic design evolution in JAES articles by Cherry, Otala, Cordell… and the long standing debates on audio “quality” and its assessment, I don’t put much faith in the local attributions of “sound” qualities to op amps or ”special” electronic parts - I look at their electrical properties and role in the particular circuit
   
  Multiloop composite amplifiers do let you choose input and output op amps separately for better electrical performance in each position
   
  Unity gain chip buffers are easy for beginners to wrap in feedback loops with good chances of success – but you can choose op amps today with higher current outputs at similar speeds to the often 20-30yr old buffer chips that supposedly “have good sound” – with enough current, speed, and heavy enough bias to reduce crossover distortion modern high output current op amps for the DSL market give equivalent or better electrical performance compared to the “traditional” buffer chips and an additional degree of freedom in design with the choice of local feedback gain
   
  The OPA627 was special for its day in having dielectrically isolated input jfets when most of that generation of IC processes had parasitic diode junctions between transistors and the substrate (= one of the supply V, commonly V-)
  For the past 10 yrs there have been many new dielectric isolation or other improved isolation techniques used in IC fabrication – it is now possible for more manufacturers to duplicate that special feature of the OPA627
   
  many other transistor parameters are better or better controlled in newer processes – although the desire for low power consumption and small die size do seem to have hurt input noise, particularly 1/f noise corner in these processes – fortunately in full size headphone amplifiers its not too important a parameter
   
  and its possible that the US$ billions market for A/DSL chips requiring low distortion with the complex multi-tone modem signals of 100 KHz to 1 MHz driving 25/50 Ohm loads has caused analog IC development teams to learn and apply new low distortion design techniques
   
  so I’m willing to work with new chips only issued in smt because they are objectively better in most parameters than the 20-30yr old “audiophile classics”
   
  AD8610, ADA4627, OPA827 more recent IC process jfet input op amps appear to be positioned/speced as OPA627 replacements and would be my choices for front end op amps – if you wanted to avoid (some of) the expense and didn’t “need” the slightly lower input noise of the 627
   
  TPA6120/THS6012 CFA DSL driver op amp seems to be more than adequate (ie 3 ppm distortion into 32 Ohms @200mWrms) for most dynamic headphone loads within standard op amp Vsupply limits and is fast enough that stability inside another feedback loop isn’t too big of a problem – the AD8397 may be better at lower supply V but needs local feedback gain >= 2, slowing it and possibly requiring another local feedback cap around the input op amp for loop stability
   
  for my view of the "3-channel" issue: http://www.head-fi.org/forum/thread/457695/impedance-induction-active-ground-three-separate-questions#post_6190248


----------



## PJPro

Thanks for your reply jcx.
   
  I intend to use a DIP8 socket, so could switch to use a SOIC package using an adapter in the future.
   
  I'd like to go with the separate buffer bacause, as you say, they are easy for beginners to produce something which sounds good.
   
  As far as the 3-channel issue, you could well be right.  However,  I've never made a three channel amp, so I thought I'd give it a whirl and see what it sounds like.


----------



## PJPro

Finally managed to get into the LTSpice forum on yahoo and posed some questions around modelling my circuit and I've had a couple of replies.  While I don't doubt the expertise of these guys I am concerned that some of their suggested values for modelling the circuit are a little conservative.
   
  For the Transient analysis, I am using a square wave.  The suggested time for this to be on is 499.9us, with a period of 1ms.  Does this seem too long?


----------



## PJPro

I may have found the answer for this one for myself 
	

	
	
		
		

		
			




   
  I noticed on amb's site that he provides oscilloscope analysis for his amps.  When using square waves, he uses a 1 kHz, 10kHz and 100kHz square wave.  So, if my maths is right (and it often isn't)....
   

 *  kHz  * *  Time on (uS)  * *  Period (uS)  *   1   500   1000   10   50   100   100   5   10
   
  I think it wouldn't hurt to conduct a further test at 20kHz (the upper limit of non-audiophile humans).


----------



## jcx

square waves aren't really informative "audio" test signals - usually the "information" in the rise time, overshoot and ringing response is more useful to estimate amplifier stability - and has limits there too
   
  the square wave high input signal slew rate can cause transient distortions that would never be seen with bandwidth limited audio sources - the vast majority of studio mics are 2nd order low pass <,~=20KHz - a very few can do 40-50 KHz
   
  audio amplifiers for headphones have little excuse for poor audio frequency performance - but you can check with a .AC analysis in spice - but beware that it is a "hack" - no actual "simulation" is going on so impossible, unstable circuits can give "reasonable looking" .AC outputs - only use it after the circuit shows signs of "really working" in .TRAN analysis - and then don't trust the results
   
  looking at the distortion spectrum of audio frequency sine waves is the usual starting place (with LTspice you have to disable waveform data compression, either in the toolbox or with a .options plotwinsize=0 spice directive added to each schematic)
   
  another step up is using multiple sines of different frequency and looking for intermodualtion distortion products, SMTE IMD is a standard frequency, amplitude ratio test that can show some additional distortions that single sine test won't - although "thermal modualtion" distortion that low + high frequency tests are best for aren't simulated in Spice device modeling so won't be seen until you test the real hardware
   
  I also use twin tone signals to look at clipping recovery performance at different overdrive level, time scales
   
  none of these really tell you too much given the approximations/inaccuracies of most freely provided spice device models but could show up a hopeless circuit's logical flaws


----------



## amb

Quote:


jcx said:


> square waves aren't really informative "audio" test signals


   
  Eh... I beg to differ.  An ideal square wave is a superposition of an infinite number of sine waves, each one contributing its amplitude and phase.  When all the sine waves are without amplitude, time and waveform distortions, you will have good square waves.  As such, square waves are _very_ telling of an amplifier's ability to amplify signals accurately, including its ability to handle fast-changing transients.  If there is a single test waveform that could repeatably convey the most information about the performance an amplifier (or, for that matter, any audio system component), it would be the square wave!
   

  Quote:


> Originally Posted by *PJPro* /img/forum/go_quote.gif
> 
> I noticed on amb's site that he provides oscilloscope analysis for his amps.  When using square waves, he uses a 1 kHz, 10kHz and 100kHz square wave.  So, if my maths is right (and it often isn't)....
> 
> ...


   
  There is no additional information to be had by adding a 20KHz square wave.  The edges of the square waves are the same in all cases, the 1KHz, 10KHz and 100KHz oscillograms merely provide you with different horizontal time scales so you can see macroscopic and microscopic views of the waveform.


----------



## PJPro

Quote: 





amb said:


> There is no additional information to be had by adding a 20KHz square wave. The edges of the square waves are the same in all cases, the 1KHz, 10KHz and 100KHz oscillograms merely provide you with different horizontal time scales so you can see macroscopic and microscopic views of the waveform.


 
 I've read elsewhere that a 5kHz wave is also recommended....especially for tube amplifiers.  Is there a reason for this?


----------



## amb

Quote: 





pjpro said:


> I've read elsewhere that a 5kHz wave is also recommended....especially for tube amplifiers.  Is there a reason for this


   
  Same comment applies, I don't know why there is any need for 5KHz when you have all the others already.  In fact, if you're operating a real oscilloscope, you could use just one frequency (say, 1KHz), and turn the horizontal time scale knob to "zoom" in and out of the waveform to see everything you want to see.


----------



## PJPro

Quote: 





pjpro said:


> PPL clearly rates the HA3-5002 very highly, esp with the AD8610. Presumably Tangent supports this view...hence the PPA v1. So the HA3-5002 is a possible candidate replacement for the BUF634.
> 
> The other alternative is the LME49600.....although many appear to see this as a rebadged BUF634.
> 
> I have read that the BUF634T is the better sounding variant of the BUF634, presumably due to it maintaining its performance by keeping cooler. I had intended to go for this TO-220 version, perhaps supplemented with a small heat sink. That said, I have no idea how hot these get. They can't get that hot as it would rule out buffer stacking.


 
   
  I need to make a decision on the buffer to use.  I've done a lot of reading (perhaps too much) but seem unable to arrive at a decision.
   
  If I go for the HA3-5002, this amp will, in terms of the schematic, be essentially a clone of the PPA V1 (Tangent doesn't sell these PCBs anymore?).  As I've already mentioned above, PPL and Tangent state that these buffers have the edge over the BUF634 (even when stacked).
   
  Does anyone else have experience of these two buffers?   Which would you go for?


----------



## PJPro

OK.  So I think I'll go for the BUF634T.  This seems to perform better than the DIP8 package and allows the use of a heaksink.


----------

