New AMZ SSM2166 Compressor Question

[kbibs]

Hi all,
I just recently finished building the SSM2166 Compressor from Jack's site using his new board and design. It's a very nice compressor and very capable from very subtle to full out limiting and no loss of tone. The controls took a little while to figure out and I guess that's the problem with too many knobs to turn! It does have a couple of quirks though. I've  noticed some hiss or noise at the initial attack of a note which goes away very quckly. It doesn't make sense to me but I can get it to go away if I turn up the noise gate almost all the way up, the note still comes through but the his on the attack is greatly reduced. But I can't run it this way because the notes die out prematurely. Does anyone else hear this with their SSM2166? ... Does anyone have any ideas on how to eliminate this or what could be wrong if anything? Is it just me? ... it seems to work as advertised other than this. Another little quirk I hear is when the noise gate starts to kick in there's a little oscillation almost like it goes above and below the threshold of the gate as the input decays but I can understand and live with this.

I just recently lowered the gain on the input gain stage by increasing R6 to 47k from 10k, so the gain is now 1 + 10/47 ~= 1.2 down from 2.0. That didn't seem to change this hiss on the attack much if at all. I reduced the gain because I was hearing some limiting on the input even with a ratio of 1:1.

Thanks,
Ken

[Govmnt_Lacky]

Link to project??

[chptunes]

Link to project??

http://www.muzique.com/tech/2166.htm

I haven't built mine yet.. ears peeled.

[Mark Hammer]

All compressors will begin to hiss when you stop playing.  The reason is that they cannot tell the difference between background noise, and a soft signal, and ratchet up the gain to "help them out", whether you want it or not.

What makes the SSM2166 such a great chip is that it includes downward expansion on board, and permits you to more readily differentiate between background hiss and soft signals. 

Part of what makes the SSM2166 a slightly perplexing chip for some is that it was intended for use in mic control strips, so it anticipates a) a fairly low signal level and b) a low-impedance signal source.  Happily, Jack revised his original design (which was essentially derived from the appnotes) and made it better suited to guitar, by including an input buffer stage to keep more top end, and not using the internal op-amp stage for gain.

What also makes it perplexing in its operation is the interaction of the controls.  I recommend reading the appnotes for a better idea of how they can interact.  For example, if the Rotation point is set low, and the Compression cranked, then you end up with a sort of all-or-none output, where either the signal is downward expanded, or it is at close to max output all the time.

Ideally for most unobtrusive effect, one would set the noise gate (downward expansion) as low as is feasible, set the Rotation point as high as is feasible or desirable, and set the compression amount as modeztly as possible.  BY setting the gating threshold and the rotation point as far apart as possible, that will make whatever compression you do apply as predictable as it can be.

Another thing to consider is the use of a toggle to select between different averaging-cap values.  Some may be better suited to certain contexts than others.

[chptunes]

Another thing to consider is the use of a toggle to select between different averaging-cap values.  Some may be better suited to certain contexts than others.

Which Caps are the 'averaging-caps'?

-Corey

[Mark Hammer]

There is only one averaging cap, and it is the electrolytic cap between ground and  pin 8 of the chip.  Its role is identical to that of the various decay/smoothing caps to ground you see in just about any auto-wah.  Smaller value = faster response but greater risk of ripple.  Larger values = less ripple but slower response/recovery.

[chptunes]

Ah, cool.. now I see that Mr. Orman even put the phrase "AVG Cap" on the schematic. 

[kbibs]

Thanks Mark! ... yeah that's one of the nice things about this compressor is the downward expansion/noise gate, it removes all the hiss when you stop playing. It took a little while in playing with it and reading the datasheet to figure out the controls. I usually end up with the controls where you mentioned, i.e. rotation point high, noise gate low and ratio low (2:1 max). The hiss I'm referring occurs at the initial picking of a note and decays very quickly and could very well be the attack time of the compressor. It's silent up to the point where the signal is larger than the downward expansion point and after that the compressor gain is high based on the ratio I guess and then gain drops?

It's more perplexing than annoying and that's why I was wondering if others hear something similar or is it just my build.

- Ken

[Mark Hammer]

So-called "breathing" - the gradual onset of noise when one stops playing - is a common phenomenon in compressors.  Indeed, it is, and has been, a regular source of people posting here over the years about this or that compressor being lousy or noisy, or worrying that maybe there was something wrong with their compressor.  And, as such, it has presented one of the best illustrations of how useful it can be to understand the internal workings of one's pedals, in order to get the best performance out of them.  I certainly don't say that as any criticism of anyone, since some kinds of effects are more obvious in the functioning than compressors, and don't require as much background knowledge to make them yield to your will.

One of the best things you can do to optimize noise performane in a compressor, whether this or any other, is to feed it as hot a signal as is feasible to do without pushing the rectifier portion too hard.  This is because the breathing occurs as the unit ratchets up the gain, following a peak that has dramatically reduced the gain.  If the signal-to-noise ratio at the point where the envelope is detected is poor, then gain will be applied to the noise accompanying the audio signal.  Ideally, you want the noise to be such a modest component of what you feed the compressor that negligible gain is applied to it as the gain recovers from peaks.

In the case of the SSM2166, you see the op-amp stage included with the 10k feedback resistor and 10k ground-leg resistor.  That provides a gain of 2x.  Making that stage unity gain (no ground leg resistor and straight wire feedback loop), and adding some variable gain to the input stage outside the chip can serve to nail the ideal S/N ratio and input signal level, so that breathing is kept to a minimum.  And, as I think I implied earlier, if the gain recovery is made faster, via the AVG CAP, one doesn't hear any residual noise gradually increasing, since gain is reduced in response to peaks, and then comes back near immediately.  That may or may not yield suitable outcomes, but presents an alternative.