Using SSM2120 for a Boscorelli project

[1978]

I'd like to try my hand at the "Sustain-O-Matic II" in Boscorelli's book.  But...it's built around the SSM2120, which is unavailable.  Does anybody have any suggestions for sourcing "obsolete" parts like these? 

Or, since the project only uses half the chip, any suggestions for a workable replacement?   Googling and searching this forum led me to THAT2252 and SSM2166, but I'm not sure I understand what other changes to the schematic would be necessary. 

best,
Kevin

[Mark Hammer]

Even when available, they were not cheap.  PAiA uses them for a couple of projects (compressors, downward expanders) and you can apparently order some from there ( http://paia.com/parts.htm ), but they aren't cheap.  I managed to pick up a couple a half dozen years ago.

Perhaps a better choice is to get a THAT2180 from Small Bear and attempt one of the variety of simple-to-full-featured circuits that THAT company has on their website: http://www.thatcorp.com/appnotes.html

[1978]

$22.50 -- ouch!  (Although I long stopped pretending that I was building stompboxes in order to save money!)

I had been looking at the app notes on THAT's website--a few of the projects look interesting.  I'm looking for a versatile dynamic effect--upward and downward compression, something unusual and with a little more control than my dynacomp clone...

I also wanted my next project to be just out of my current range of knowledge, so here's a followup question to see if I'm ready for it:  if I do make the sustain-o-matic, I'll need to somehow "trim the DC offset" without using a scope (which I don't have).  Should I rule the project out on this basis alone, or is this something I could tune by ear/or with DMM?  (I know it's a kind of a stupid question since it's a fairly particular application).

[Mark Hammer]

The SSM2166 is a very nifty chip that does a lot more than compression.  In some respects it forms the heart of the "dream compressor".

How?

Compressors normally work by aiming for a very high "default gain", reducing it as necessary when the input level goes up, and slowly increasing it back to the default level as the input signal declines/decays.  Unfortunately, stompbox compressors are not smart enough to tell the difference between background noise/hiss and low input signal levels, so they crank the gain to max when you stop playing.  If you have a pristine input signal that is blissfully hiss-free, you'll have no worries, but if you have a teeny bit of hiss, the compressor will boost the daylights out of it, leading many compressor users to complain bitterly about the noisiness of their compressors.  It's not the compressor itself, but the compressor simply "following orders" and exaggerating an already sub-standard input source.

The 2166 has a built in downward expander that can act like a smart noise gate, turning down the gain - wayyyy down - if the input signal falls below a certain preset level.  The datasheets explain more abut this function.  But the nice thing is that you get the noise benefits of a terrific gate, without interfering with the pleasing squish of the compressor part.  The chip itself may be a little pricey, but when you add up all the other things you'd need to do to achieve that quality of inter-note silence, it more than pays for itself several times over.  As well, when you consider how few extra components are needed to make a viable compressor/gate, compared to what it costs to score the parts for, say, a Dynacomp/Ross, it's actually a lot cheaper than you'd think, based on the price of the chip alone.