Using the SSM2166 chip

[carlmart]

I am willing to build a balanced microphone preamp with a limiter, and as the SSM2166 seems to do both, I wonder if anyone has tried it.

My guess is it might be possible, setting the compressor for a fixed position and disabling the noise gate. But I'm not sure if it's so and how to do both things.

[Mark Hammer]

Oddly enough, a guitar compressor is NOT what it was designed for.  It was intended for a mic strip, so you are clearly on the right track.

First off, the noise-management part is not a gate, but rather downward expansion (i.e., the proportional attenuation of low-level signals).  You can set the threshold for expansion pretty low; low enough that you won't really hear its action unless you have the amplifier volume cranked.

The chip has essentially two "knees".  One is the boundary where downward expansion is applied, and the other is the boundary where peak limiting is applied.  Both the downward expansion and peak limiting have fixed ratios.  In the simplest possible configuration, expansion would be at the floor, compression at 1:1 (i.e., none) and the "rotation point" where limiting begins moved around to attain different dynamic ranges, to be adjusted via the gain control so as to maintain similar output amplitude (minus the peak excursions).

The more complex configuration would vary the rotation point, the compression ratio below it, and the gain.  And the most complex version would allow for downward expansion to be introduced at different thresholds, same with peak limiting, compression between those points be varied, and gain to be adjusted to offset any loss of output amplitude.

The question is whether the internal op-amp in the chip is suitable for turning into a balanced in mic preamp.  I guess that depends on how hot your mic signal is to begin with.  I would imagine that in some instances it is better to use an external circuit to provide initial pre-amping of the mic, and then use the internal op-amp for a little more gain.  But that's just a hunch, not anything borne out by empirical evidence.

[carlmart]

Thanks, Mark.

I am looking for chip that could be used as a limiter, and by chance I found this one.

You're right on two things: noise is not too low for low level mic signal; and second it's not really balanced, as I thought it was.

It might be interesting trying it just as limiter, but it's not that cheap just for that.

If you know of any other limiter chips, please do tell me so. There's one by NJM, but the application notes are so poor that I wonder if it's worth trying.

[~arph]

You can use the ne570/571 as a limiter I have seen a schematic for that somewhee

[armdnrdy]

You can use the ne570/571 as a limiter I have seen a schematic for that somewhee

Many ICs manufactured by the THAT corporation are suited for this application as well.

[carlmart]

Yes, we designed a limiter using THAT's parts, but I was looking for something slightly cheaper.

[Mark Hammer]

It's expensive for a chip, but ridiculously cheap for a high-quality flexible limiter.  You'll likely spend more on the box or pots/knobs than you will for the chip itself.