abbey road d enfer

Re: "squeeling" noise in mic preamp
« Reply #20 on: January 01, 2019, 06:50:58 AM »


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.

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Regarding the That 1646, i didn't actually know that.
It's in the datasheet...

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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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


aviel

Re: "squeeling" noise in mic preamp
« Reply #21 on: January 01, 2019, 10:53:08 AM »
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

Re: "squeeling" noise in mic preamp
« Reply #22 on: January 01, 2019, 11:55:44 AM »
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)?

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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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

aviel

Re: "squeeling" noise in mic preamp
« Reply #23 on: January 01, 2019, 12:15:36 PM »
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

Re: "squeeling" noise in mic preamp
« Reply #24 on: January 01, 2019, 03:32:11 PM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

aviel

Re: "squeeling" noise in mic preamp
« Reply #25 on: January 01, 2019, 05:00:28 PM »
I use it balanced. It is outputting a balanced signal.

abbey road d enfer

Re: "squeeling" noise in mic preamp
« Reply #26 on: January 01, 2019, 10:45:42 PM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

aviel

Re: "squeeling" noise in mic preamp
« Reply #27 on: January 07, 2019, 01:57:28 PM »
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.. :)


 

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