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Audio Op Amps

Posted 26th June 2015 at 02:59 AM by rjm
Updated 1st October 2015 at 07:04 AM by rjm

I'm often asked "which op amp sounds better".

The reply is usually a scowl and muttered "does it look like I care!?" Which is something of a lie... I do care about getting op amps to sound good. It's the phrasing of the question I dislike.

Op amps do not come in "good, better, best" flavors. All it is - and this is pretty obvious I would have thought but apparently not - all this is about is matching an op amp to the job it's going to do; the circuit it's going to be sitting in.

The op amp you'd choose to use as a DAC IV converter is different from the one you'd choose to back a 100k volume potentiometer in a preamp is different from the one you'd choose for an MC phono preamp input stage...

Why do you think there are like a thousands of different op amps to choose from in the first place? It's because there are thousands of combinations of op amp characteristics and properties ... not because companies like Nat Semi want to finely grade op amps by sound quality.

Even ignoring all the industrial, test, and measurement applications and just sticking to audio, there are still dozens of options.

Indeed many op amps are marketed as being "for audio". To the cynics among you: no, that doesn't mean the prices are inflated to market them to gullible audiophiles (well, maybe Muses...). In fact audio op amps are usually on the cheaper end of the price scale. These ICs really are designed for, or at least well-suited to, audio applications. And finally, no, contrary to what you might imagine, it doesn't just mean low distortion. That is just one relevant aspect of the performance, and there are many others.

Since I'm not up to writing a textbook on this, let me just state that probably most important thing is not distortion but the how the op amp copes with the circuit impedances seen at the op amp inputs, which for audio tend to run between hundreds of ohms to tens of kohms.

A good general-purpose audio op amp is designed to be able to handle an unbalanced DC resistance of 100 kohms on one of its inputs, since a fairly typical application is a preamplifier where you have a 100k volume control, a coupling capacitor, then a 100k resistor to ground, followed by a non-inverting op amp gain stage. See the attached Bryston preamp schematic as a generic example. While certain audio applications like IV conversion or phono preamps can use op amps designed for lower impedances, an "audio" op amp should have low distortion and low offsets when dumped into a circuit like the Bryston.

Impedances of audio circuit are on the borderline where bipolar hands off the baton to jFET input types in terms of optimal performance. Both types are workable and both are common actually, though in the particular case of the Bryston, being on the upper range of impedances, FET input would be most likely be a better match from a noise, distortion, and offset perspective.

And I haven't even touched on the loading, drive, PSRR, or gain-bandwidth considerations yet.

And you want to just know which op amp sounds better? Of course you do. I just hope you have a better appreciation after reading this why I don't answer such a question.
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  1. Old Comment
    CHiroshi's Avatar
    Well written, thank you!
    permalink
    Posted 1st July 2015 at 12:26 PM by CHiroshi CHiroshi is offline
 

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