preamp opamp problems

[jnxw2]

I am having some problems with my preamp, i made a simple pramp using AD811 current feedback opamps as output line drivers, it works and sounds great at mid to high volumes, but for some reason at low volumes i got a kind of a "scratching" sound when i changed volume, so I got my DMM out and it turns out that at low volumes there is about a 100mV DC offset at the outputs, but if you turn the volume up about a fifth of the way it suddenly disappears.
Also at low volumes the ICs draw about 45mA when they should only draw about 17-20mA. This problem also disapears at higher volume at the same time as the DC offset disappears.

I can kind of reduce the problem by lowering the resistance at the +In of the opamps but that greatly reduces gain, can anybody think of what the problem might be?

Any input greatly appreciated

cheers

jnxw2

 

[rickpt]

Hi

Put a 1k resistor in series with the non-inverting input as close to the opamp as possible...

I would also remove the 750ohm resistor that connects to ground.

Parasitic oscillation, I had the same problem on my discreet current feedback Preamp...

Cheers

 

[jnxw2]

Hi

Put a 1k resistor in series with the non-inverting input as close to the opamp as possible...

Sorry about the simple question but when you say that, do you mean between the non-inverting input and gnd or between the non-inverting input and the output?


Thanks

jnxw2
PS What is parasitic oscillation caused by?

 

[rickpt]

Hi

Connect the 1k between the wiper of the pot and the non-inverting input.

About parasitic oscillation:
A parasitic oscillation is caused by stray resonant circuits in an amplifier causing the amplifier to act as an oscillator. That’s why they only happen at certain volume settings...
The resistor forms a RC filter whit the input capacitance of the opamp making it slower and less prone to that kind of oscillation.

Cheers

 

[jnxw2]

Ok i tried that but it didnt seem to help very much, but i did some more testing and the DC offset is negligeable as long as i dont connect the interconnect that goes to the power amp. As soon as i connect it, i get about 80-120mV DC and it doesnt seem to matter if the power amp is switched on or not, i get the same result. If i plug the interconnect into the preamp but leave the other end unconnected i get about 35mV DC.


Any suggestions?

thanks

jnxw2

 

[andy_c]

the DC offset is negligeable as long as i dont connect the interconnect that goes to the power amp. As soon as i connect it, i get about 80-120mV DC and it doesnt seem to matter if the power amp is switched on or not, i get the same result.

Maybe the capacitive loading of the cable between preamp and power amp is causing the op-amp to oscillate for certain source resistances. Do you have a resistor in series with the op-amp output (that is, between the op-amp output and the power amp input)? With a resistor in place, any load capacitance will appear in series with this resistor and not affect the stability as much.

 

[jnxw2]

Thanks for the tip , i dont have any suitable resistors to hand but will get some and try it.


cheers

jnxw2

 

[mac]

I agree that a 100 ohm series output resistor should solve your problems. Good luck.