What does a buffer do for the output stage of an audio amplifier?

An op amp is like a gain adjustable buffer but has limited current capability. A buffer circuit provides more current. Often needed with harder to drive head phones or dealing with certain op amps. To go discrete or not depends on the engineering and component selection. I prefer a proven chip design because you take a number of variables out of the equation. If you can hand pick matched transistors and killer capacitors then discrete could sound better.

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Originally Posted by FloydCouncil 

I have been studying the various schematics of op-amp based audio amplifiers and I notice almost all of them include a buffer stage. Why is this so?

A buffer isolates the gain stage from the relatively low-impedance of the load. Think of one stage to provide voltage gain and one to provide current gain.

http://gilmore2.chem.northwestern.edu/library.htm could keep you in reading material for a while

I cite this one a lot http://gilmore2.chem.northwestern.edu/articles/hearing_art.htm - read twice - for protecting yourself and then for Dynamic Peak SPL estimates - which lets you size a amp for a given headphone's impedance, sensitivity

for the HD-650 getting to 120 dB SPL peak requires ~ 12 Vpk, 40 mApk - this is a pretty high amount of dynamic headroom you may never have a clipping event depending on music genre and whether you really ever try to literally reproduce live event SPL peaks

yet with a op amp + buffer it is a pretty easy goal to meet for the HD-650

another overkill approach common in headphone amps is to run the output in Class A bias - avoids a few error mechanisms and at headphone power levels it doesn't cost too much for desktop amps

the current and power to do this with the HD650 are within the limits of some op amps but the easiest way is to use a separate buffer amp giving greater freedom for choosing the input op amp

I strongly advocate putting the buffer inside the op amp Global feedback loop - Walt Jung has articles, explanations, check out his Op Amp Applications book (free online) - particularly the audio line diver circuits - section 6 http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html

like Jung I prefer current feedback op amps for the output "buffer" - the TPA6120 gives an impressive spec as a headphone amp on its own - but I still wrapped a OPA627 around it in my amp (avatar)

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Originally Posted by jcx 

like Jung I prefer current feedback op amps for the output "buffer" - the TPA6120 gives an impressive spec as a headphone amp on its own - but I still wrapped a OPA627 around it in my amp (avatar)

This isn't really related, but what is the giant heatsink for? Did you bias your TPA6120 into class A?

-Kris

yes I did, it is possible to bias parallel op amp outputs "against each other" for push-pull Class A, simple sim examples next page:

http://www.head-fi.org/t/188758/ad8397-class-a#post_2263321

you could do similar with parallel buffer chips

really inefficient is to load the ouput stage to a rail in SE Class A output