[TzeYang]

When it hits 70-72%, you can hear a certain buzz, and once it goes over 72% there is no more buzz and after you go much higher around 85% you get the normal hiss due to high gain.

Amplifier is AD825 feeding a diamond buffer, properly compensated, three channel, gain of 4, Alps RK27 properly grounded.

I don't think there's anything wrong with my work but I'm speculating that the pot contact is bad and when it slightly lifts the higher input impedance is seen(ground resistor) and thus it picks up external interference and leads to the buzzing.


Anyone had similar experience? Does it concern the pot? I don't have an extra RK27 with me so I cant do a comparison.

Thanks in advance.

 

[Pars]

This is in both channels? I would think that would be odd for both to do this at the same point. I take it you do have a resistor to ground in parallel with the wiper of ~10x pot value?

 

[wolf18t]

Amplifier oscillations? You could verify this if you have a scope. If not you can try to add a Zobel network at the amp output.

 

[guzzler]

Definitely oscillation. My PPAv1 with AD797s used to do it. Either change opamp, or reconsider your bypass scheme...

 

[TzeYang]

Quote:

Originally Posted by Pars /img/forum/go_quote.gif This is in both channels? I would think that would be odd for both to do this at the same point. I take it you do have a resistor to ground in parallel with the wiper of ~10x pot value?

yes, i have that resistor. Any explanation on that?

Quote:

Originally Posted by guzzler /img/forum/go_quote.gif Definitely oscillation. My PPAv1 with AD797s used to do it. Either change opamp, or reconsider your bypass scheme...

Awww schnap. I don't have a scope!!!!




thanks all.

 

[Pars]

That resistor is there (or so I've been told) in case the wiper loses contact with the resistive element. I guess it can cause a big pop or something?

You could try looking at the current draw and see if it spikes or rises when you hit 70-72% or so. If so, that would indicate oscillation. Scope of course would be best.

 

[TzeYang]

Will do that thanks.

You know the funniest thing is that all my perfboard builds with "inferior" layout (due restrictions) never gave me any issues and yet PCBs I etched using proper layout would give me problems like these lol.

EDIT: So assuming it really is oscillation, why does it only oscillate at that position? It's merely a voltage divider and with no signal playing I don't see why it's going to affect the OPAMP. Plus it works perfectly fine at max or min.

 

[audiowize]

How do you have your stoppers configured? Rearrange them so they are as close as possible to the actual device. You might also try temporarily removing that second resisotr across the pot.

 

[diditmyself]

Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif EDIT: So assuming it really is oscillation, why does it only oscillate at that position? It's merely a voltage divider and with no signal playing I don't see why it's going to affect the OPAMP. Plus it works perfectly fine at max or min.

Are you sure it's not a ground issue? If not...

The pot impedance and it's setting do affect speed and stability, and the circuit has to be compensated for the extremes - max/min or 50 %. It's easier to get a perfect compensation without a pot.

The first two simulated examples below is an uncompensated JFET input amp, 10k pot, 1k input resistor. With appropriate input resistor (and/or cap from input to ground), feedback resistors and comp cap(s) you can get the amp to behave nicely at all volume settings.

max volume

50 % volume

with 100R imput resistor and 10pF comp cap

max

50%