[divie23]

Hey Guys,

I've been trying to figure this problem out for several days now but seem no closer to solving it. However, I have carefully observed its effects. Some of the following may sound incredibly tedious so I hope I explain it all nice and simple!

Firstly I'll explain the amp I've built. It's basically a buffered CMoy using OPA637s and a single BUF634 per channel.

In essence the amp when initially switched on produces a buzzing kind of a sound in both channels. However, after about 2 minutes it gradually fades to silence and the amp functions perfectly. Also, the buzz only seems to occur when the Pot is set to a minimum (maximum resistance) and decreases in volume as the Pot is increased, and finally ceases when the resistance of the Pot is zero. My timing of the buzz is when the Pot is left at full resistance.

Removing the buffers from the amp and making it a voltage gain amp only removes this buzzing completely. And likewise removing the gain stage and leaving simply the buffers in also removes the turn-on buzzing completely as does removing either a single Op Amp OR a single buffer while leaving the others intact. Make sense?? I hope so! The same is true when I remove an Op Amp and a buffer from a single channel and feed the signal raw through it but amplified through the other channel.

I suspect it's a power supply issue but I can't see how it can be. I mean I'm using a virtual ground driver with 2 BUF634s, about 400/500 microfarads per rail in capacitance (aluminium electrolytics), a 24V regulated mains supply which is rated at 300mA. The current draw which I measured sometime ago was approx. 60mA.

Can anyone offer me some suggestions as to how to tackle this please?! I mean I've spent close to £150 on this amp and this is really testing my patience to the point where I feel like smashing the thing!

Divie

 

[divie23]

It seems as though something is taking a while to stabilise? Because after the buzzing has ceased I can turn the amp OFF and straight back ON without any of the problems I've mentioned.

 

[kwkarth]

maybe a leaky electrolytic that's not forming properly??

 

[ppl]

You might also check short term stability whitch is determoned by the overall stability of the Amp. If the 637 is operated within the BUF-634 loop use High Bias on the BUF-634. and determon that the Design is normaly stable. If it is then suspect the Past as mentioned.

 

[divie23]

The BUf634s are within the 637s bandwidth loop and are operating in high bandwidth mode. However the Bufs used within the virtual ground driver are operating in low bandwidth mode, is that a problem?

 

[divie23]

Ok, this is what I've tried so far:

Putting the BUFs in the virtual ground circuit in high bandwidth makes no difference.

Removing the electrolytics makes the buzz louder.

Removing the input RCA cables makes the buzz sound regardless of the position of the Pot.

Once again though the buzz fades after say 2/3 minutes.

I've tried absolutely everything and am completely clueless.
Divie

 

[kwkarth]

Replace the electrolytics

 

[Nezer]

I concur with kwkarth. The fact that it goes away after a few minutes would seemingly point to the electrolytics not properly taking a charge. Couple that with the fact that the buzz is louder when the caps are removed and everything points right to those caps.

Replace em and I would bet you $5 to a doughnut it goes away.

 

[Apheared]

No, that's probably not it. If the 'lytics were hosed, then running the amp without em would make it work just fine. You don't need reserve caps when a BUF634 is your rail splitter!

If removing them makes it louder, it's probably ground related... they're acting as a filter when in the circuit. Same for your pot rotation description.

This is a courtesy post; I saw this at our other home and answered there, head over to...
http://headwize.powerpill.org/ubb/sh...num=3&tid=1909

I'll reply to whichever you answer to on either "home"

 

[Nezer]

Apheared, don't use logic, that's just dirty fighting!

That's a good point, check the grounding. But it's strange that it goes away after a few minutes... That makes it sound like it's a cap. Maybe there's another cap that's got an intermitent grounding?

 

[paulb]

"Goes away after a few minutes", without considering the other symptoms, sounds like a heat-related problem. Maybe this is secondary to an original problem like oscillation.
Anything in the circuit getting warm / hot?
Can you put a scope on the output?

 

[divie23]

Just to let you guys I've found the source of the problem! Putting the output buffers into low bandwidth mode eliminates the buzzing altogether.

However, what I found intriguing was that putting one of the channels into high bandwidth caused the buzz not only in that channel but in both. Is that because the two channels are linked through the power supply and through ground??

What I'd really like to know though is if I'm sacrificing precious sound quality by using low bandwidth??

Also I'm going to put in a capacitor from Op Amp output to its -ve input as a bandwidth throttle, however the only caps I can find are the polyestyrene type, are these OK?

Divie

 

[Nezer]

Quote:

Originally posted by divie23 What I'd really like to know though is if I'm sacrificing precious sound quality by using low bandwidth?? Also I'm going to put in a capacitor from Op Amp output to its -ve input as a bandwidth throttle, however the only caps I can find are the polyestyrene type, are these OK? Divie

For question #1, only your ears can tell you this. I doubt you are though.

As for the cap, any cap should work but, again, you *might* find that one brand/type sounds better than another. I would suggest using whatever you can get and then judging whether you should try for different brands/types.

The best part of DIY is the tweaking.

I'm glad you found the source! You probably would have found it much faster with a scope so, if your going to be working a lot with high-speed circuts, try and get a used one on ebay or something.

 

[ppl]

Be carfull about putting a cap around an OPA-637 it is not unity gain stable and so any cap you use will need a resistor in series with it to prevent the gain from reaching unity. or in the 637's case a gain less than 5. the 627 is a better choice for this kind of compensation BTW did you have a resistor between the opa's output and the buffer's input? if not use one 100-220 ohms for your combo is required.