Strange whistling half way through Gain pot...?

[jwhmca]

This is a simple DOA - transformer in/out NFB Gain...

I get this strange whistling/ noisy thing about have way through the turn of my gain pot. It goes away the higher or lower I turn it... seems to be some particular resistance that causes some kinda feedback or oscillations in the op-amp?

Schematic included... http://i907.photobucket.com/albums/ac273/jwhmca/700MICPREC.jpg

Any thoughts?

 

[JohnRoberts]

Sometime a pot can have a dead spot where the wiper goes open circuit, but from the schematic, an open wiper would be the same as min gain, so that shouldn't cause instability.

Is there any capacitance associated with pot wiring?

JR

 

[jwhmca]

I have the pot on the front panel through about 5 inches of twisted wire.

 

[jwhmca]

It sounds the same if I touch the Gain control wires... Am I missing something elementary about Gain design...? Where is the Cold hearted, brutal slashing of PRR when you need him...

 

[JohnRoberts]

I'll try to be cold and brutal if that helps...

Reversing the order of the pot and 120 ohm resistor will isolate any stray capacitance from the opamp - input, which will help if you are getting oscillation.

If it is just picking up noise, shielded cable, grounded pot shell etc, should help.

If you are touching bare wires and hearing noise, stop doing that.

JR

 

[PRR]

> Where is the Cold hearted, brutal slashing of PRR when you need him...

"about 5 inches of twisted wire" is destabilizing the opamp. There should be one roll-off in the NFB loop. The opamp has roll-off. R12 against 5 inches of twisted wire (capacitance) is another roll-off.

As JR says, put the 120 ohm right AT the opamp then run your long wires from there. But the fact you have howl at R10=~~1K says that may not be enough.

Move the opamp and pot closer together.

Look what ELSE is in this area. (Altho in this one-amp plan, I don't see a major trouble source.)

Untwist the wires. (But the long wire on opamp's naked "-" input is still asking for trouble.)

Increase C2 to 220pFd or 330pFd.

 

[jwhmca]

Thanks JR and PRR, I really appreciate the input. I will try those things in the morning.

One other thing I would like to ask about:

The HF seems lacking... I don't have any numbers yet, just what my ear is telling me. This basic design seemed to have More High freq response until I added all those resistors and caps into the op-amp input C9 C10 C11 r19 etc... before I just had C1 and C8. Anyway I think I know enough do know that those components are bleeding off the HF... How do I know to what degree...? Or, am I right?

I put them in when I copied the Original Yamaha implementation of the NE-80100...