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Are buffers designed to work at low impedances (16-600 ohms) (since opamps aren't)

post #1 of 10
Thread Starter 

After reading this article I was wondering if buffers, with their higher output current, were designed to work with low impedance loads. 

 

The article also mentioned that, when used properly, opamps are mostly transparent and the "cheap" 5532 will more often then not perform just as well if not better than the more expensive options. So, for a headphone amp, can you add buffers to make it able to work with lower impedance loads easier? Or are the buffers (like the BUF634) still designed to work with high impedance loads?

 

Thanks in advance.

post #2 of 10

The definition of a buffer is a device that allows a circuit to drive a lower impedance than it could otherwise.

 

An opamp has a maximum current that it can output. Sometimes this is expressed on the datasheet as Output Short-Circuit current. If you load an opamp with a low-impedance load that would draw in excess of the max output current, the opamp will misbehave. The output current may collapse or the output may clip.

 

So you put in a buffer. This will prevent the overall circuit misbehaving up to its max output current, i.e. with a low impedance load. What happens now if the load is high impedance (draws a small current)? Nothing, or very close to nothing. The buffer may slightly reduce the output voltage, it may slightly increase the distortion, but although more current is available, the load doesn't make use of it, so the buffer is unnecessary, until you want to use a low-impedance set of phones.

 

That's all she wrote (on that subject).

 

The audibility of opamps is a highly contentious subject. This thread http://www.diyaudio.com/forums/everything-else/245455-ultimate-opamp-shootout-where-you-get-decide.html tends to suggest that there are audible differences in blind testing, but the test loads the opamps heavily (~580 ohms), sometimes outside what they are rated for, then uses 2 circuits in series to exaggerate the effect, so the conclusions are of dubious value. It does look, however, as though at least some individuals can detect an NE5532 in these circumstances

 

w

post #3 of 10
Thread Starter 
Quote:
Originally Posted by SeriousListener View Post
 

http://web.telecom.cz/macura/buffer_en.html

thank you for this I'll make sure to study it.

 

Quote:
Originally Posted by wakibaki View Post
 

The definition of a buffer is a device that allows a circuit to drive a lower impedance than it could otherwise.

 

An opamp has a maximum current that it can output. Sometimes this is expressed on the datasheet as Output Short-Circuit current. If you load an opamp with a low-impedance load that would draw in excess of the max output current, the opamp will misbehave. The output current may collapse or the output may clip.

 

So you put in a buffer. This will prevent the overall circuit misbehaving up to its max output current, i.e. with a low impedance load. What happens now if the load is high impedance (draws a small current)? Nothing, or very close to nothing. The buffer may slightly reduce the output voltage, it may slightly increase the distortion, but although more current is available, the load doesn't make use of it, so the buffer is unnecessary, until you want to use a low-impedance set of phones.

 

That's all she wrote (on that subject).

 

The audibility of opamps is a highly contentious subject. This thread http://www.diyaudio.com/forums/everything-else/245455-ultimate-opamp-shootout-where-you-get-decide.html tends to suggest that there are audible differences in blind testing, but the test loads the opamps heavily (~580 ohms), sometimes outside what they are rated for, then uses 2 circuits in series to exaggerate the effect, so the conclusions are of dubious value. It does look, however, as though at least some individuals can detect an NE5532 in these circumstances

 

w

Thanks for the explanation. I believe the first thing I'll do for my cmoy then is an output buffer. 

post #4 of 10
Thread Starter 
Quote:
Originally Posted by SeriousListener View Post
 

I have Headphone Amplifier version and sound is perfect. OPA134.

headphone amplifier version of what? Since you're using single opamps it would probably have ok power handling even without the buffer...

post #5 of 10

there are op amps with higher output current specs:

 

njm/jrc4556 used in the O2 headphone amp

AD8397 in mini3 project

tpa6120/ths6012 TI sells a headphone amp eval board, peranders has a headphone project - used commerically in Benchmark, Eximus, Xonar for headphone output

 

I think the best use is one of the high current DSL driver op amps inside a feeback loop with a great fet input audio op amp

you can get a little more performance by configuring the high current output op amp to provide some V gain too - but you do need to know a bit more about stabilizing feedback loops


Edited by jcx - 2/10/14 at 9:44am
post #6 of 10
Quote:
Originally Posted by jcx View Post
 

you can get a little more performance by configuring the high current output op amp to provide some V gain too - but you do need to know a bit more about stabilizing feedback loops

 

I have been thinking of making something like the O2 with many output chips. Jan Meier made something like what I am thinking of ages ago on headwize, although with LM6172(? I think...) chips. 

 

Anyways, what you posted here intrigues me, could you point me to info on how to stabilize feedback loops? 


Edited by nikongod - 2/10/14 at 11:22am
post #7 of 10
Thread Starter 
Quote:
Originally Posted by jcx View Post
 

there are op amps with higher output current specs:

 

njm/jrc4556 used in the O2 headphone amp

AD8397 in mini3 project

tpa6120/ths6012 TI sells a headphone amp eval board, peranders has a headphone project - used commerically in Benchmark, Eximus, Xonar for headphone output

 

I think the best use is one of the high current DSL driver op amps inside a feeback loop with a great fet input audio op amp

you can get a little more performance by configuring the high current output op amp to provide some V gain too - but you do need to know a bit more about stabilizing feedback loops


I've been thinking of using the OPA2132 with an output buffer (either a discrete one, a BUF634, or an opamp <----about this....I CANNOT afford to put a bunch of $7 chips in this thing (plan to sell them and can't have that much in it) I know the BUF634 is good...but out of these options (discrete, buf, or opamp) which one will perform well while being not $7 a chip? (i.e. 4ish and below )) would this be a good idea? or should I use a high output current chip with the buffer?

post #8 of 10

I really don't see the point of going cheap on the op amps - the volume pot, power supply, case work all likely add lots more than using decent op amps

 

 

unity buffers were probably popular because little thought about stability was needed back when the op amp was likely to have <10 MHz GBW - now you may have to think some, add extra local feedback parts when using a 20-50 MHz GBW op amp to wrap a 30-40 MHz buffer chip - even if nominally OK, you could see problems with added phase shift from cable C load

 

Walt shows composite audio amps ~pp60+ http://www.analog.com/library/analogdialogue/archives/39-05/Web_Ch6_final_I.pdf

 

Quote:

Originally Posted by jcx View Post

I don't always know where to start - some might pick up on the hints, search terms given above, others may need more feedback theory that I can't really teach in a forum post

 

and people can also get offended if I err on either side estimating their present level

 

I like "Classical" Bode frequency response asymptote plots, others may benefit from different formalisms

 

search with key words like: “op amp” “stabilize” it will give hits for free, online papers by Feucht, Green, Graeme, Mancini, manufacturer’s app notes – its not difficult to find a number of useful sources - the principles are similar if you want audio power amp specific understanding read Cherry, Self, Cordell (the later 2 have up to date books)

http://www.analogzone.com/acqt0131.pdf long series, I don’t use Tina otherwise in a quick scan I see lots to like in the presentation

 

http://www.ti.com/lit/an/sboa015/sboa015.pdf - Graeme also has some helpful books which expand on his articles http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=jerald+graeme

 

 

if you put pads for the local input op amp "lead" feedback C between its output and the inverting input you can find a value that will make any "unity gain stable" input op amp work with the 5532 output buffer - you just don't get full advantage of the higher loop gain of the 4562 op amp at audio frequencies

 

the load of 16x 5532 inputs on the input op amp's output may give some peaking from the 5532 input C reacting with the "inductive" impedance of the 4562 output, a small series R of 10-50 Ohms and/or a "Zobel" RC to gnd can damp the potential resonance - again calculating RC time constants, knowing how they affect loop stability is invaluable

post #9 of 10
Thread Starter 
Quote:
Originally Posted by jcx View Post

I really don't see the point of going cheap on the op amps - the volume pot, power supply, case work all likely add lots more than using decent op amps

 

 

unity buffers were probably popular because little thought about stability was needed back when the op amp was likely to have <10 MHz GBW - now you may have to think some, add extra local feedback parts when using a 20-50 MHz GBW op amp to wrap a 30-40 MHz buffer chip - even if nominally OK, you could see problems with added phase shift from cable C load

 

Walt shows composite audio amps ~pp60+ http://www.analog.com/library/analogdialogue/archives/39-05/Web_Ch6_final_I.pdf

 

I print my cases (<$1 per) volume pot is the alps (will not go lower and it saves money on a switch) psu is as cheap as can go. I'm not worried about the main opamp (going to use a nice opa2132/4) I'm worried about the output buffers (offerring bass boost in some models (and my personal one will have such a feature) and it tends to distort a bit at high levels (I think more current will fix this) (distorts without bass too anyway :P) so I'm going to need an output buffer) I was just wondering if there is a cheaper alternative to the BUF634....as it costs over $7 per chip

post #10 of 10
Quote:
Originally Posted by jcx View Post
 

Why didn't it let me quote your text? Weird. 

 

Thank you very much for that information. 

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