"squeeling" noise in mic preamp

[abbey road d enfer]

Case is not but the screen pin is. I can try to ground the case also.

Check continuity of the ground pin with the case; sometimes the ground pin is connected to an internal electroastaic screen only.

Regarding the That 1646, i didn't actually know that.

It's in the datasheet...

What are my options in that case? I could use a lower pot, 5k for example..

You may not notice a problem, because it just ruins the CMRR, which is not immediately noticeable if you have a relatively clean environment. Even with a 5k pot, CMRR would be just about 20dB.
If you want the 1646 to work as it should, you should precede it with a buffer.

 

[aviel]

I am feeding it with a single unbalanced signal, so CMRR here will help only if the same noise is present in my ground also.. A buffer can be a good idea anyway.. Just a TL071 or rather another jfet?

 

[abbey road d enfer]

I am feeding it with a single unbalanced signal, so CMRR here will help only if the same noise is present in my ground also..

I don't get it. Do you mean "feed it to an unbalanced input"?
Do you know a connection to an unbalanced input can be improved by driving it with a balanced source, provided the wiring is also "balanced" (2-wire+shield)?

A buffer can be a good idea anyway.. Just a TL071 or rather another jfet?

Almost any decent audio opamp is suitable, since there is no issue with S/N and the 1646 presents a moderate load, that a TL071 can easily handle.

 

[aviel]

What i mean is that my gain stages are not outputting differential signals, so that 1646 is getting a monolithic input (the inverted input is connected to the ground) therefore the CMRR is less significant here is assume

 

[abbey road d enfer]

What i mean is that my gain stages are not outputting differential signals, so that 1646 is getting a monolithic input (the inverted input is connected to the ground) therefore the CMRR is less significant here is assume

Why do you do that? Seems silly to me to use a good balanced driver and use it unbalanced, when a simple voltage follower could have done it better.

 

[aviel]

I use it balanced. It is outputting a balanced signal.

 

[abbey road d enfer]

I use it balanced. It is outputting a balanced signal.

OK, now I understand what you mean. The 1646 receives an unbalanced signal so , yes, is second input is grounded; that's the normal operation in most cases.
But I was talking about output CMRR. Having a non-zero inpedance driving the non-inverting input of the 1646 is a cause for impairing its output balance, which in turn results in poor CMRR.

 

[aviel]

OK, now I understand what you mean. The 1646 receives an unbalanced signal so , yes, is second input is grounded; that's the normal operation in most cases.
But I was talking about output CMRR. Having a non-zero inpedance driving the non-inverting input of the 1646 is a cause for impairing its output balance, which in turn results in poor CMRR.

Quick update:
I added a TL071 buffer to drive the line driver and indeed the squeelibg dissapeared! Might be related to other stuff, since i moved some parts on the layout to make place for the buffer..