In praise of the SSM2166

[Mark Hammer]

Put together a full-featured (5-knob) dynamics pedal over the weekend unit, using Matt Armstrong's layout for the 2166 (thanks, Matt!).  Jeez what a terrific limiter/compressor/noise-eliminator! Every control is useful. Clean, transparent, quiet.  Not a cheap chip, but so little else is required to make a fantastic unit.  A damn shame they don't make a thru-hole version any more.

[tubegeek]

And where might a curious pedal builder find more info on this project, Mark?

[Ben N]

Search is your friend: https://www.diystompboxes.com/smfforum/index.php?topic=87300.0. But the link for the schem/layout is dead, so our other friend, Google, took care of that: https://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_id=7372.
There ya go.

My question is whether, where and how to cobble in a sidechain input, since I would prefer to put a comp like this later in the chain, but control it with the pristine signal from the guitar. I guess a shorting jack between C4 and pin 3? Anything else necessary?

I would also note that as a "full-featured (5-knob) dynamics pedal", which happens to run on 5v, this would be an ideal candidate for a MIDI interface by way of digipots--but for that 1M noise gate pot, a value that may well be nonexistent in the digital realm. Any suggestions for how that might be addressed?

[rankot]

I've build two of those (one TTH and one SMD) and never been satisfied with them due to noise.

[DIY Bass]

I think I have one of these still out in the shed.  I built a compressor from a kit.  It had 1/4" in and out jacks so I assumed it was for guitar (this was a long time ago when I knew a lot less and assumed a lot more).  It wasn't.  It was aimed at microphone levels with enough gain to give line level output.  I plugged an active bass in the front end.  Much smoke was let out of my amp.  Never used it since. The chip was socketed so it should be possible to pull it out and put it somewhere else

[ElectricDruid]

I would also note that as a "full-featured (5-knob) dynamics pedal", which happens to run on 5v, this would be an ideal candidate for a MIDI interface by way of digipots--but for that 1M noise gate pot, a value that may well be nonexistent in the digital realm. Any suggestions for how that might be addressed?

The SSM2166 datasheet has a graph (Figure 5) on page 6 which shows the response of the noise gate pin. Values above 200K give a gating action only below 1mV signal level, which sounds pretty much like "basically no gating" to me, at least for guitar signal levels (this thing is set up for mic inputs, so maybe it's expecting much lower levels). If you could accept gating only below 2.5mV RMS as the "least gated" setting, you could make do with a 100K digipot.

[Mark Hammer]

Maybe it's the guitar/pickups I'm using (nothing exotic, really), but I find the downward expansion quite usable.  It virtually eliminates all the "breathing" sounds endemic to compressors that lead to many threads/posts here about a given compressor being "noisy".  Compressors are made to boost noise when you stop playing.  The downward expansion capability makes this one dead quiet.  That said, note that use of a large-value averaging cap will result in a long decay time, such that the downward expansion will take a while to kick in, and THAT will result in audible hiss.  Note as well that the op-amp gain/input stage can be easily tweaked for higher gain by making R3 a little smaller/lower, and less hiss by sticking a small-value cap in parallel with R4.

[Ben N]

The SSM2166 datasheet has a graph (Figure 5) on page 6 which shows the response of the noise gate pin. Values above 200K give a gating action only below 1mV signal level, which sounds pretty much like "basically no gating" to me, at least for guitar signal levels (this thing is set up for mic inputs, so maybe it's expecting much lower levels). If you could accept gating only below 2.5mV RMS as the "least gated" setting, you could make do with a 100K digipot.

That is helpful. But, come to think of it, I am guessing that R_Gate is something that could be set-and-forget, not necessarily changing with every change in compression settings. Or am I missing something?

[Mark Hammer]

If the context the circuit is used in is fixed, sure, why not.  Or simply use an internal trimmer.  If where you insert the unit and what you feed it, is subject to variation, then a panel-mount control may be advisable.  What the value of the pot needs to be is up to you and your circumstances.

[Rob Strand]

THAT and Analog devices have some of the best multiplier type devices that have ever been made.   They get down to the theoretical performance limits and the practical limits of being able to match the internal transistors.    Everything else is a compromise!

[Ben N]

If the context the circuit is used in is fixed, sure, why not.  Or simply use an internal trimmer.  If where you insert the unit and what you feed it, is subject to variation, then a panel-mount control may be advisable.  What the value of the pot needs to be is up to you and your circumstances.

Yeah, I meant panel mount or trimmer, but no need for MIDI changing this setting when it changes attack, threshold, etc.

[MaxPower]

How about a circuit based on the sm2167? Under $3 at Mouser. A simpler but still useful compressor/noise gate.

[PRR]

> I am guessing that RGate is something that could be set-and-forget, not necessarily changing with every change in compression settings. Or am I missing something?

Conceptually: you set up your rig, in a nasty dive, it buzzes/hisses. You trim Gate so the crap just drops-out. In a different dive or different rig the setting is different.

So no, not really about the other knobs, but not something you set once and seal-up.

[pruttelherrie]

Not a cheap chip

{ you didn't get it from me } Analog Devices still gives out samples. You'll need a business though. { / you didn't get it from me }

[PRR]

> Analog Devices still gives out samples. You'll need a business though.

Doesn't SAY you need a business.
https://www.analog.com/en/support/customer-service-resources/sales/sample-products.html
At 2/product, they don't have to count your cows or hold your children in hock. Give them a start-up company-name; MarxATronix or Mark's Bait & MRI.

> Not a cheap chip

Seems like $7 for one. You can't beat that by rubbing oxide on/off sand at home. Yes, if you sell $29 pedals you gonna have to get a big bulk discount.

[tubegeek]

Can confirm - no business needed. I said I was a hobbyist and used tubegeek as my "organization," which is pretty funny if you know how disorganized I am. Just ordered 2 2166's, 2 2167's and 2 AD600ARZ's (Dual, Low Noise, Wideband Variable Gain Amplifier, 0 dB To +40 dB Gain) which looked interesting too. I guess I'll have to etch (or buy) some SOIC footprint adapters though, not ready to etch a whole project yet, still mucking around.

[rankot]

My country is not eligible for free samples  

[PRR]

> My country is not eligible for free samples

Ask them "Why?"

The list includes countries all over the political spectrum. It includes Niue, a teeny island, and Mainland China. Kosovo, Kyrgyzstan, Romania, Slovakia, Slovenia, Tajikistan, Turkmenistan, Uzbekistan.... the frikkin Faroe Islands (do they even have electricity?) It can't be about banking rules (like selling into India is tough) since samples are free.

[rankot]

I don't know, even neighboring countries (Croatia, Slovenia etc.) are non-eligible.

[pruttelherrie]

But the link for the schem/layout is dead, so our other friend, Google, took care of that: https://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_id=7372.
There ya go.

By the way, years ago I designed a single sided PCB for the SMD version based on the above schematic.

Eagle files and images are here:  https://possob.xs4all.nl/heskamp/ssm2166_compressor_eagle.zip , feel free to use.

(Related question: is it possible to attache zip archives or eagle files to posts?)