What value pot for this pre-amp?

[misterfixit]

Greetings,

I'm working on a pre-amp that uses TL74 op amps. I don't have the pot that connects to J4 of the attached schematic. Any guesses on the type or value?

Assuming a standard pinout of:

gnd -> pot pin 1 (j4 pin 2)
out -> pot pin 2 (j4 pin 3)
in -> pot pin 3 (j4 pin 1)

I have tried several pots of various values without good results.

Thanks!

 

[MarcelvdG]

It looks like it needs a rheostat (variable resistor) rather than a potmeter. Could it be that the ground terminal is only meant for shielding?

 

[misterfixit]

Ah, that is a good possibility! I'll do some more testing with that in mind.

 

[rayma]

Only a two terminal part can work in that position.

 

[misterfixit]

What do you think, a resistive or inductive part?

 

[rayma]

That part must be a resistor for an audio amplifier stage.
For other purposes, it depends.

Can you post the complete schematic of your preamp?
Usually the metal pot bushing/body is grounded directly to the chassis, not to the audio circuit ground.
Is the actual pot intended to be on a different pcb?

 

[misterfixit]

I was thinking about that immediately after I posted, with regard to how a speaker crossover functions. The reactance from the inductor would cut out higher frequencies. Duh.

So, we're just looking at the gain stage of an op amp. Modify the resistor value and you get a different gain level.

Unfortunately I can't post the full schema, but I believe I have what I need now. Sometimes I get to fixated on problems and tunnel vision sets in.

Thanks for the clarity, folks.

👍