AMZ SSM2165 Compressor Builds?

[Arn C.]

Has anyone built this Compressor and if so did you make it exactly like the schematic or did you change a few things/ modify it?
I have one and I am going to build another but maybe alter it a bit.

Thanks!
Arn C.

[~arph]

I buillt one yesterday (though with comp ratio at 1:1, I only need the noise gate). I found that the rotation point and noise reduct pots between pins 11 and 14 and 9 and 14 work better when the values are swapped. Eg. 1 Meg between pin 11 and 14 and 50 meg between 9 and 14. Seems a bit odd, because that would mean the datasheet is wrong. I'll study the DS again today.

Here is the schematic. Nearly a 1:1 copy from the DS.

http://www.muzique.com/ssm2166.gif

(thanks jack)

[Mark Hammer]

I've built it and rebuilt it.

Jack's drawing is essentially the app-notes drawing, with maybe a twist or two.  I think there are several things worth noting and thinking about with respect to the circuit.

It works, but the chip and circuit are really optimized for voice mic and insertion into a larger device, rather than for use with instrument-level input, and as a stand-alone "effect".  So, the input impedance is lower than one would like for a guitar effect, the gain throughout the circuit is a bit heavy-handed, and the output is just hanging there.

This means that:
a) there needs to be a high-impedance buffer on the input if you want to hang onto bandwidth,
b) the gain of the envelope-/rectifier drive stage (set by R1 and R2) needs to be turned down a bit,
c) the gain adjust pot and overal resistance from pin 2 needs some tinkering, and
d) there needs to be a terminating resistance on the output to stop-the-pop.

Note that like the Dynacomp, turning the compression up also increases the signal level...enough that you can't achieve near-unity level balance with the suggested gain control when using more than just a touch of compression.  That can be addressed by reducing the gain of the initial stage via R2.  Presently, the stage has a fixed gain of 2x.  That doesn't sound like much, but it is effectively a multiplier of gain in other stages within the chip.  So, dropping it down to something closer to unity gain will have a discernible impact.  Make R2 39k, and you're closer to that goal.  For gain-setting, you probably want to make R9 470R and R10 a 5k pot.  Either that, or consider sticking a volume pot on the output of C10 as a way of both providing a terminating anti-pop resistance AND a means of bringing the signal level back down to earth.

The number of things one is tempted to stick on this circuit can very quickly make it a mess of wiring and controls, so here is what I would suggest for a simple and functional version:

1) A compression pot (natch)
2) An output level pot.
3) A trimpot on the inside to set a fixed gain (balanced by the output level pot).
4) A toggle for 2 or possibly 3 Avg cap values so you can achieve different time constants and "feels" (page 10 of the appnotes shows the impact of different cap values).  This will be of particular importance for the downward expansion response, as well as the compression.
5) A Noise Gate control pot to set the threshold of where the downward expansion kicks in.
6) Some sort of unit-gain JFET input buffer.

The Rotation Point is the sort of thing you can implement with a trimpot on the inside.  If you want to get über-fancy, you can use a toggle for 2 or 3 Rotation points, but use of just a single well-chosen one will not impede this being a good pedal.

There you go, 3 knobs and 2 optional toggles.  It will all fit nicely in a 1590B.

[WhenBoredomPeaks]

I bought this chip once but it was SMD.

[~arph]

The ones on smallbear are DIP.

Thanks for the great info Mark.

Most of what you said can be derived from the datasheets. Thing is I'm not using this as a standalone unit. It is in the wet path of a pt2399 delay I'm building (with four pt2399 in series). So it's stuck between two opamp stages. I've only been able to put it up the breadboard yesterday and first tinkering with it I noticed that it does reduce noise quite substancially, but that my Noise gate and rotation point values seem off. I understand that all values here influence each other such as the gain resistor and R1 R2. I had the gain resistor at 1k2 and the rest as per the Q&D schem. In this configuration I needed the Rotation point resistance at least at 330k while the noise gate resistor has little influence.

Tonight I'll have som hours to tinker again. I'll post the results if anything interesting comes up.

[Mark Hammer]

Using it to manage the wet path strikes me as eminently sensible.  Pricier than a 570, I suppose, but still sensible when the total tab is compared to the cost of a commercial delay pedal that behaves itself.

[~arph]

Indeed.. and I only need it to do downward expansion.  I tried it with a compander too.. compress the input, expand the output.. still noisy. I got some LM1894's too from ebay to try as well.

With the size of this project I don't care for a $8 chip more or less. (it will even contain an arduino pro mini..)

here's a (bad) image of the current breadboard.

[armstrom]

Mark,

I have used some of your suggestions from this thread and others spread out over a few years (and other forums!) to put together a schematic that I think captures most of your suggested modifications. I've also done a quick PCB layout (as yet, untested!).  Would you mind taking a look at the schematic and let me know if I missed any modifications you would make to the circuit? Sorry that the component values aren't on the schematic drawing (just in a list) I made the layout a bit too small and rather than rearrange everything I just hid the component values so it wouldn't look so cluttered

http://www.mr2-power.com/compressor.pdf

Thanks in advance for any feedback! I'll report back when I have etched the board and tested it out.

-Matt

[snoof]

did you end up building this? how did you like it?

[armstrom]

I actually just got a chance to test it out yesterday. It worked quite well! I made a few small changes to the schematic and layout so I'll need to update the document if anyone else is interested in building one.  I'll try to get a new pdf uploaded today.
-Matt

[armstrom]

Ok, I've updated the pdf in my first post to reflect how my prototype was built.  You may have to manually refresh the page if your cache still has the old pdf stored. To make
sure you're seeing the correct version ensure that you see R9 listed in the BOM (old version only went to R8).

Schematic and PCB transfers:
http://www.mr2-power.com/compressor.pdf

Datasheet:
http://www.analog.com/static/imported-files/data_sheets/SSM2166.pdf

Here are some additional thoughts:
-Play around with the values of C6 and C7. They are the averaging caps for the VCA and RMS level detector. C6 is always in the circuit and SW1 switches C7 in parallel resulting in a slower response from the circuit. If you only want one response "speed" from the circuit simply omit C7 and SW1 and adjust c6 to taste.
-VR3 controls the compression ratio. 50K is the largest reasonable size but you can always go smaller if you don't want as much compression. Using a 25K pot will limit your maximum compression ratio to about ~6:1 vs the ~10:1 maximum provided by the 50K pot
-The gain control is a little touchy on the low end. If you look at the datasheet figure 6 explains why this is. Below about 2K on the gain pot the gain quickly drops from 4dB to zero dB (unity gain). You can add a 2K fixed resistor (off board) to avoid hitting unity gain, but I wanted to keep the full range of the control so I just live with the sudden drop-off.
-The rotation point trimpot is wired kind of "backward". The way to think of it is that turning the pot clockwise will result in MORE limiting (lower limit voltage). I would start with the trimpot just a few degrees off its minimum rotation (CCW) and see how you like it. The limiter, while not terribly useful in this application does affect ALL of the other adjustments so it's better to be set once and left alone.
-You can replace R9 with a jumper if you want more output from the circuit.

I think that's about it! If anyone builds this let me know how it turns out for you I will try to get some sound clips eventually (not that there's much to hear from a compressor sound clip, it's pretty transparent.)

[Edit: added a link to my pdf here just to avoid confusion]

[snoof]

i breadboarded it yesterday, i quite like it!  The noise gate is not the best, but i'm digging the comp.  the rotation and the C6 value are the keys to getting the attack/release/feel right.  Thanks for the schematic!  I'll have a look at your revisions...

[Mark Hammer]

Actually, I find the downward expansion to be one of the most appealing features.  Having a noise-reduction circuit explicitly dedicated to what can be one of the most noise-susceptible parts of one's signal path is a really nice feature, particularly if one has single-coil pickups.

[armstrom]

Yeah, the noise gate is pretty nice. When my friend was testing out the circuit yesterday we did notice one odd behavior though. If you're using a large averaging cap then you get a short period of hiss from the circuit just after you mute the strings. The compressor is trying to ramp up the gain due to the lack of any real signal and the noise gate hasn't yet killed the gain. It's a bit strange but easily "dialed out". I may even change the size of the noise gate pot since there's pretty much no way to use the full range of the pot as most guitars won't put out enough signal to get out of the downward expansion zone when the pot is rotated fully clockwise.. but I guess it's good to have that much range just in case you have some active pickups or some other really hot signal.  

One thing to keep in mind is that this compressor has automatic make-up gain. You can think of it as compressing from the "bottom up" rather than from the top down. The net effect is that your quiet signals will get louder when you turn up the compression rather than your loud signals getting quieter.. if that makes any sense. Mark touched on that a bit in one of his earlier posts in this thread. I think some other (more basic) compressors work the other way and have another gain setting to help bring your overall level back up, however I'm not too sure about that as I've never used any other compressors

[snoof]

that's probably what i was hearing in regards to the noise gate behavior.

[armstrom]

could be. Another thing to keep in mind is that when the compression ratio is up very high and your noise gate is set low the circuit will pick up all of the handling noise of the guitar as well as noisy pickups. The compressor can be really unforgiving and expose every little noise you make on the strings if you're not careful

[armstrom]

I've posted a sound clip of my compressor design in this thread:
http://www.diystompboxes.com/smfforum/index.php?topic=87300.0

[snoof]

Looks good man.  I'm gonna try your PCB out as soon as I can get one together.

[snoof]

used your PCB armstrong, worked great!

[armstrom]

Cool, glad to hear it worked out for you One thing you can try if you want more compression is to swap the 50K pot for a 100K. That will get you up to the maximum compression available from the chip (15:1) vs. ~10:1 you can get from the 50K pot.
-Matt