Bypassing opamps - hiss reduction?
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Sinbios

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In the past, I've had some hissing problems with low-impedance phones and my MINT amp.

- I'm not sure if it's oscillation, so how would I measure it with my o-scope?

- If it is oscillation, would adding bypass caps to the opamps get rid of the noise?

- What values should I use, and what configuration - rail to rail or rails to ground?

- I've been reading on the issue, and it seems some people recommend both a low capacitance ceramic parallel with a high capacitance film or electrolytic. Is the high capacitance cap necessary? Also, is it plausible to share the high capacitance cap between several opamps?

- Would buffers benefit the same way?
 
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Megaptera

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If you're using anything resembling a typical configuration in the Mint, then oscillation in this case wouldn't be my first guess. Try R8 if you already haven't, and of course check and make sure it's not a hissy source, but I'd put it down to the physical construction (cold joints, etc.) if it's neither of those.

From what I've seen, people tend to bypass large caps with smaller ceramics, but I don't know how common it is to use a large electrolytic as a bypass cap itself. If you were to use bypass caps on the opamps or buf634s in the Mint, you'd want to use, say, 0.1uF ceramics or 10uF tantalums, or smaller, from the V+ and V- pins to ground. If it's oscillation, then this might do the trick -- increasing the bandwith of the buffers can help too sometimes -- but again, be careful that you don't opt for a neat solution before you figure out what the problem is. Without knowing if you've installed R8 or not, I'd say that's where the balance of probability is.
 
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Sinbios

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Quote:

Originally Posted by Megaptera
If you're using anything resembling a typical configuration in the Mint, then oscillation in this case wouldn't be my first guess. Try R8 if you already haven't, and of course check and make sure it's not a hissy source, but I'd put it down to the physical construction (cold joints, etc.) if it's neither of those.

From what I've seen, people tend to bypass large caps with smaller ceramics, but I don't know how common it is to use a large electrolytic as a bypass cap itself. If you were to use bypass caps on the opamps or buf634s in the Mint, you'd want to use, say, 0.1uF ceramics or 10uF tantalums, or smaller, from the V+ and V- pins to ground. If it's oscillation, then this might do the trick -- increasing the bandwith of the buffers can help too sometimes -- but again, be careful that you don't opt for a neat solution before you figure out what the problem is. Without knowing if you've installed R8 or not, I'd say that's where the balance of probability is.



I had R8 (100ohms) installed, but that didn't fix the problem. I pulled those out and use them in an impedance adapter now, and that blocks most of the noise. Since I do have an o-scope, I might as well use it to find out for sure whether it's oscillation.
 
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Add up the current of the amp and buffers (as configured, see the chips' spec sheets). If actual current significantly exceeds that, (or if the chips are getting hot) that would be sign of oscillation.

While I don't think adding bypass caps will help in this regard unless you were running a particularly difficult opamp, I am among those who feel it would have potential to improve sound quality.

You should not get rid of the electrolytics, but it is possible that smaller ones would work as well IF you added some ceramic decoupling. Tantalum 10uF would help but is not a substitute for smaller/faster ~ 0.1uF ceramic. One compromise wtihout adding tantalums might be a slightly larger ceramic, or a pair of them like .047uF and .22uF. A film cap could replace the larger ceramic but ideally a ceramic of .22uF or lower would be used no matter what else you have, and it would be placed as close to opamp supply pins as possible. Even better would be adding two more pairs as close to the buffer pins as possible.

You can do it rail-rail, that will help, but it will also cause fluctuations from one channel to have minor impact on the other. How much on a Mint I don't know. Ideally it would be rail-vgnd for each rail on every chip.

Yes it is plausible to share the high capacitance cap between several chips. On a hold-no-bars design you probably would try for multiple paralleled electrolytics but the whole point of a mint is small size, IMO.
 
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