LM833 Oscillation problem

[Mark_H]

Hi everyone,

I build a quite nice metal detector which works most of the time very well but sometimes it just starts to oscillate rendering the device unusable. I think the problem lies in the preamplifier:

Here's some info about the circuit and questions:
Input is about 100mV sine wave 14khz
Goes more easily into oscillation if the ambient temperature is near freezing or below
Oscillations stop if I touch C3
I replaced C3 with 15pf but it didn't help
I cannot induce oscillations on bench, even if i put the whole pcb to freezer for hours
I replaced LM833 with LF353, problem fixed but unacceptable low sensitivity
C3 acts as a proximity sensor. If i bring a screwdriver or even a plastic stick near it the detector reacts. Is this normal?
Whats the point of R9?

Any tips I could try are welcome. I have only basic skills in electronics but I'm eagerly trying to learn more.

Thank you.

 

[E-design]

At what frequency is the self-oscillations during faulty behavior?

 

[Borber]

U1a together with R6, R7, R8 and R9 form a differential amplifier. It subtract voltage on pin2 from voltage on pin1 and amplifies difference voltage by 46.8. Touching C3 means adding some capacitance against ground. You can try adding 33pF between GND and pin1 or pin2 of U1a.

 

[Mark_H]

Thanks for the tips. I will try adding that 33pF there and see what it does. I don't know the oscillation frequency as I am unable to induce the oscillations on bench.

 

[E-design]

The AC analysis shows a potential for oscillation when there is enough input level to get above the 0dB mark.

 

[chuckey]

I would increase C41 and C44 - decoupling capacitors to at least 1MF.
Frank