Q & D compressor SSM2166 chip

[markm]

Those 2 cents are quite interesting actually.
I find problems with that as well.
In looking over Jack's schem yesterday and reading the data sheet on the chip, it seems that it would be quite easy to change resistor values to tame this beast a bit, where Gain is concerned and perhaps the Downward expander as well. It looks as though many of the values used in Jack's schematic were the "suggested values" from AD for use with this device.
Bear in mind though, I don't think they had really intended for this chip to be used exactly in the manner in which we are using it.
I think upping the value of the gain resistor from 1K to....10K?   47K I'm not sure.....will greatly calm it down and make it a more manageable stomper. Of course, this purely my "theory" at the moment but, after reading the datasheet, it makes sense and like I said, most of the values in the schem are "suggested values" and not neccessarily meant for an instrument signal.

[Mark Hammer]

1) As for the "amount" of compression provided by this chip, be sure to distinguish between the "knee" and how far that knee is bent.  The rotation control in the circuit sets the knee or point where limiting is introduced.  It is theoretically possible for very hard limiting (i.e., ratios bigger than 10:1) imposed above a fairly high threshold or "rotation" point to sound like less limiting than a much gentler limiting ratio (e.g., 2:1) with a much lower rotation point.  Keep in mind that the audibility of the limiting is a function of the extent to which the circuit/device has the opportunity to apply it, and that in turn is a function not only of the threshold but of the picking style and output levels it is fed.

2) You might notice that the built-in op-amp stage (see 2166 datasheet) and Jack's circuit show a 10k feedback resistor and 10k resistor to ground.  This yields a gain of x2.  I haven't actually tried it out, but it stands to reason that configuring the op-amp stage as a unity-gain stage might well accomplish what some here seem to want.  Clearly, reducing a gain of x2 down to a gain of x1 is not a night and day sort of transformation, but it may get you in the ball park.

3) The noise reduction (downward expansion) works quite well, and often VERY transparently.  But, like any sort of gating arrangement it needs some practice to set right.  I wouldn't describe it as "twitchy"; merely trying hard to cover such a wide range of settings that dialing in a "perfect" threshold can be tricky in some instances.  This strikes me as being the sort of thing that might require a trimpot and pot instead of the 1M pot and 1k fixed resistor.  Different users are bound to have different contexts/needs which will make this segment or that of the full range of gating/expansion thresholds most useful to them.  I'm thinking that an optimal solution might well be to use a 250k pot and 1M trimpot in series instead of the 1M+1k arrangement shown.  The user selects the general range of possible thresholds by setting the 250k panel-mount pot to max resistance and trimming back on the 1M trimpot until a suitable range is identified.  The pot then dials in a subset of the full range of adjustments determined by the trimpot "offset".

[majormono]

There must be a way to keep the volume down to a level where the limiter threshold doesn't have to clamp the volume (to get 1:1 output) and thus render the compressor useless... by the way, in my case this also makes the "gain" control only audible in the release phase (good ole noise woooosh) so it's only the makeup-gain for the compressor part (which can only kick in after the limiter is below its threshold) and doesn't control the overall volume.

Btw. wouldn't upping the gain resistor would lead to even more gain (maximum gain is with pin open, 1k is 1:1)?!

While it is quite logical this chip has so much volume available due to it's original application interfacing microphones for distribution in speech applications it is strange I don't find an easy way to keep gain down since the application note refers to such a typical application in it's setup example.
Since I've already configured the opamp stage as a buffer and shunting the output volume doesn't work (and would also lead to a noise issue) I had already given up on this one but you had to bring it up again... so I just feel completely incapable ;-).

Hm, iv'e checked my schem as well as the board for errors but the expander doesn't work at all... strange, at one point (with 6dB on the opamp IIRC) I had some gate-like effect but it kicked in much too early to be useful. This usually means I've overseen a most simple error q.e.d..

[markm]

Mark,
Just wondering the course of action you would take to calm down the gain on this unit a bit to make it more useful as a guitar effect?
Can you point us to some exact resistors to replace or modify?

[Mark Hammer]

If that on-board op-amp is a *real* op-amp, then replace the 10k resistor (R2) and 1uf cap to ground (C2) with....nothing.  You could replace the 10k feedback resistor (R1) with a wire link but the key thing is to make the resistance to ground as large as possible so as to turn it into a unity-gain stage.

Although the gain control CAN be used for level setting, what seems to have escaped most of us is the option to preset the gain via trimpot and tack a 100k log output level pot after C10.  Setting the gain level will be a pain in the arse, but do-able.  The trouble is that, unlike a number of more familiar compressor designs, on this one output level goes up as compression amount is increased.  So, you'd need to set the compression to max, the added output pot to max, and the gain level to what you think you can tolerate.  That will be the max output level attainable.  A/B it with bypass and adjust to taste.  The added output level pot can now attenuate below that max level.

[majormono]

Some more cents (I'll stop when it reached an Euro, I promise).

... the problem is that even with the opamp set as buffer (R1 bridged) it is still much too much and as I tried to put a pot after C10 it just worked as an on/off switch as stated before and it would also up the noise (well, relatively sice you get the noise from the preceding, now "wasted" amplification). I ended up with a fixed 2k2 but it had no effect at all. Been there, done that, failed miserably...

R2 and C2 are used as a 16Hz HP filter, no? Since they did not lower the volume I left them in. This is also explained in page 11 of the application note, also note "step 1" on page 12.

To give you an impression on the difference (and to make sure I don't talk nonsense) I recorded a litte demo; clean; 2166 with Gain, Expander and Ratio 0, Limit 10; 3080 for comparison. Crude (flying wires et al) but you can hear or even easier see (in an audio editor of your choice) the difference.
What really teases me is that it sounds damn good anyways, just waaaay too loud.

www.grol.de/DIY/compdemo.mp3

[markm]

First of all, A real BIG THANK YOU to Mark Hammer.
My Gain dilemma with the Q&D comp has been forever solved.
I made the changes the Mark suggested in his above post, and that I read in the data sheet as well, (Build first ask questions later ) and this thing became a complete and totally usable, not to mention GREAT Sounding guitar compressor.
It's not over-the-top with gain any longer, it still has some gain in reserve just like a Ross/Dyna would but, it's not like it was before where I had to have the gain pot completely CCW for unity! 
You have made my day once again Mr. Hammer and I just would like to acknowledge you on this.
KUDOS to you my friend and thanks for bailing me out on this one.....I feel like I owe you bigtime!!
See this------->  <------------ I am happy my friend.
Thanks,
   MarkM

[Mark Hammer]

De nada, my friend.  No payment required other than good news.

[markm]

 
Thanks there bud.

[theman]

Hi Markm,

Can you tell us which mods or suggestions you implemented? I saw in the datasheet on page 11:

"To configure the SSM2166's input buffer for gain, provisions
for R1, R2, and C2 have been included. To configure the input
buffer for unity-gain operation, R1 and R2 are removed, and a
direct connection is made between the –IN pin (Pin 6) and the
BUFOUT pin (Pin 5) of the SSM2166."

This makes it unity gain as opposed to a stage that has 6 dB of gain with the configuration when R1=R2=10K.

Regarding the noise gate mods ... I like the idea Mark had regarding a pot and a trimmer. The noise gate threshold is not linear at all. For 10 mV threshold, R_gate = 20K, 4mV --> 50K. For 1 mV, R_gate = 250K. For 0.5 mV, it's 500K. It starts to look a little linear at thresholds below 1 mV.  So I guess there are two separate ranges. Thresholds above 1mV, and those below.  So you can use a 500K or 1M trim to dial in the range, and use the 250K to cover the range subset.

[markm]

Here's what I've done so far;

replace the 10k resistor (R2) and 1uf cap to ground (C2) with....nothing.  You could replace the 10k feedback resistor (R1) with a wire link but the key thing is to make the resistance to ground as large as possible so as to turn it into a unity-gain stage.

I did this modification to set the op amp section to unity gain.
This completely took care of the over abundance of Gain/Boost/Output/Volume that it had originally and made the comp very useful on it's own. I do plan on trying some of the other options available that Mark H. mentioned in this thread as well.
You are correct abou the info in the data sheet, there are alot of possible "adjustments" to be made to this circuit but for now, I'm just so tickled with getting rid of the Overkill Volume that this circuit had that I've not gone any farther......I've been just playing my Tele through it most of the night playing some killer Chicken Pickin' country licks and they sound absolutley fabulous through it!

[theman]

Here's what I've done so far;

replace the 10k resistor (R2) and 1uf cap to ground (C2) with....nothing.  You could replace the 10k feedback resistor (R1) with a wire link but the key thing is to make the resistance to ground as large as possible so as to turn it into a unity-gain stage.

I did this modification to set the op amp section to unity gain.
This completely took care of the over abundance of Gain/Boost/Output/Volume that it had originally and made the comp very useful on it's own. I do plan on trying some of the other options available that Mark H. mentioned in this thread as well.
You are correct abou the info in the data sheet, there are alot of possible "adjustments" to be made to this circuit but for now, I'm just so tickled with getting rid of the Overkill Volume that this circuit had that I've not gone any farther......I've been just playing my Tele through it most of the night playing some killer Chicken Pickin' country licks and they sound absolutley fabulous through it!

Thanks for the info and glad to hear that it worked out for you. I plan on building it soon ...

[markm]

Thanks for the info and glad to hear that it worked out for you. I plan on building it soon ...

After performing the "surgery" on my Q&D last night, and playing through it for a couple of hours I must say that it's effortless to get a good tone out of this....infact, you have to work at it to make it sound BAD!
That's kind of a nice change of pace ya know?
Anyway, build it! It's worth it.
I plan on messing around with some of the comp values and rotation a bit but even as it sits right now on the bench, it's fine.
As a matter of fact, any other "mods" I do will probably take place later on as I am quite content with it as is now.

[Arn C.]

Markm,
     I am looking at the schematic right now and I was wondering if you took out r1 also and put a jumper in its place, also, when you removed c2 and r2 did you  still run the remaining circuit to ground?

Thanks!
Arn C.

ps  I am asking because I have had one build in a rackmount for several years now and want to try this out.
     

[markm]

No problem Arn,
I removed R2 and C2 completely, no need for jumpers.
I replaced R1 with a jumper and that's all.
Instantly tamed and usable Gain.

[Arn C.]

Markm,

Like this?




Arn C.

[jimbob]

I do plan on trying some of the other options available that Mark H. mentioned in this thread as well.

I cant wait. Ill give you time to post those than Ill build it. Thanks. I have been looking for a different kind of compressor lately. Sounds like a good one. Markm- How do you compare it to the Dynacomp?

[markm]

Markm,

Like this?




Arn C.

Yes sir!

I do plan on trying some of the other options available that Mark H. mentioned in this thread as well.

I cant wait. Ill give you time to post those than Ill build it. Thanks. I have been looking for a different kind of compressor lately. Sounds like a good one. Markm- How do you compare it to the Dynacomp?

Don't wait for me, I may not do anything else with it right away-- if ever!
I think I may like it more than the Dyna but, I am very traditional and me and the Q&D are still in the honeymoon phase!
The Q&D can get just as squishy as the dyna but it seems to have a much faster recovery time.
It is a unique sound similar yet very different from a dyna......strange but true.
It has the ability to get that real squishy twangin' Tele type sound yet, the true guitar tone is still there + even with high compression settings, it does have the same drop-out on the lower strings that the dyna has.
I like it!

[Stephen]

I am out of place here but I like the gain...Dont need a pedal.... distortion!!

I will do the second chip I have like this though!!

[Arn C.]

Thank you Markm!!!!!!!!

Peace!
Arn C.