variable Low-Cut, High-Pass & High-Cuts

[TobWen]

Hi there,

I am looking for a variable Low-Cut/High-Pass for a mic-preamp project.

Since I don't have much time for the project at the moment, I have asked a friend to send me an idea.
He has sent me this one (it looks like something from RANE).
This is is about 12db (I think).

My questions are now: Where do I connect this one and is it correct?
IMHO, I directly it after the amp-IC (INA217 for example).
Or can I hook up it on the main-output, which is line level?

Is this a good solution at all, or do you have some better ones?

Best regards,
Tobias

[sek]

Hi Tobias,

He has sent me this one (it looks like something from RANE).

It definitely looks like coming from Rane, yes

IMHO, I directly it after the amp-IC (INA217 for example).
Or can I hook up it on the main-output, which is line level?

Not shure if I understand you correctly.
Just after the INA217 (for example) you already have got line level
Basically, you want such a circuit past any stages with gain, because otherwise the noise of the opamps in your lo-/hi-cut circuit would be amplified, too.

My educated guess is: you can safely put it between the instrumentation amp and the output driver, but in case you have some EQ or filter stages in between, you should decide depending on noise gain (or attenuation, as in the hi-cut).

is it correct?

Well, the circuit in itself doesn't look wrong. But one could do it differently, e.g. with less electrolytics in the signal path (or none at all). I'd guess if you design such a circuit into a mic preamp using a Burr-Brown instrumentation amp and a differential output driver using opamps, you might end up with a whole copy of a Rane circuit

In that case, I would stick to the Rane circuits at all.
Nothing an 'audiophile' would prefer, but then, perfectly working, fool proof industry designs (inexpensive, reliable, good quality).

Hope this helps,
Sebastian.

... who is currently designing a measurement mic preamp mainly based on Rane schematics...

[TobWen]

the actual MS1b is based on a INA163 ... I don't know why.
I don't wanna use it ... I have heard too much bad things about it.

btw: let's write an e-mail to me (tobwen@gmx.de) then we can talk in German!

[cunningham]

This looks to me like any basic active 40dB/dec band-pass filter. The only difference is that it has two ganged pots to adjust the high cuttoff, Fc-h(second filter) and the low cuttoff, Fc-l(first filter). You can change the RC component values & the pots to adjust the bandwidth. The only thing to keep in mind when calculating your R & C values is that you don't want the high freq. cuttoff to overlap the low freq. cuttoff. I suppose the bandwidth is whatever you make it. You might also want to use an OP-AMP that has a good slew rate to help with harmonic distortion.