Cat's Purr / SDS

[Mark Hammer]

I've been meaning to make this for a while.  It's the "soft distortion" subcircuit from the Roland AG-5 Funny Cat.  Many thanks to our collective friend RG Keen for drawing this out.



I say "subcircuit" because it is the front end that provides the extra harmonic content to be filtered by the Funny Cat, but is bypassable with a footswitch.  Having built a few clones, using RG's (and Charlie Barth's) tracing of the circuit, I was quite taken with the circuit, that Roland labels as "SDS", for Soft Distortion.

I suppose opinions will vary, but I find it really interesting.  In a sense it is basically a single-op-amp compressor that exploits a weakness.  Q1 is an FET that serves as a variable resistance to ground, and its drain-source resistance is modulated by a poorly-rectified feedback of the output from the op-amp.  As the D-S resistance varies, the gain of the op-amp does as well.  But here's the thing, the modulation of the FET is not at all smooth, and the component values were selected to achieve that.  It modulates gain at a medium-high frequency rate that sounds like distortion.

Forty years back, there was a noise gate project circuit, from Jon Gaines, in Modern Recording magazine.  I built one, and it worked fine.  It used a FET for attenuating/gating the signal.  In a subsequent issue, someone wrote in complaining about what they described as "distortion", and Gaines replied, recommending that the person up the value of the smoothing capacitor in the envelope rectifier portion.  The lights went on for me.  When a rectified signal has insufficient smoothing, it sounds like distortion to us.  And that is precisely what the SDS circuit in the Funny Cat exploits, and didn't leave to chance.

I won't tout it as the answer to anyone's wildest fantasies.  It is NOT a high gain device, and won't allow you to sound like your guitar hero.  It's not overdriving FETs, or using diodes in any way.  It just sounds...different.  Not weird, just different.  And if you were trying to find a lighter distortion sound that was different, this may be your ticket.   And, because of the design, it also provides a sort of compression function.

Just for poops and giggles, I replaced the 470k feedback resistor with 330K in series with a 250K pot, to be able to adjust the gain for different guitars/pickups.  I find that provides a reasonable range of adjustment.  It cleans up when the guitar volume is reduced, largely because a lower input signal leaves very little rectified signal to actuate the FET gate.  It is not intended to provide a "high gain" kind of sound, so I doubt making the gain adjustment pot 500k will provide anything useful.  For whatever reasons, turning the 250k pot down, so that the total feedback resistance is only 330k yields a brighter sound, making a tone control unnecessary.  But it really does "purr".

Took me a while to build because I was perfing it to fit inside a 1590A.  Tight squeeze.  I hate using those things.  I'll try and clean it up to be presentable tomorrow and post a demo, because I don't really see much about the SDS circuit online.  In the meantime, I recommend it as an off-the-beaten-path distortion, and a perfect example of "it's not a bug; it's a feature".

[davent]

In 1991 i went and visited Jon at his shop in Rochester NY and purchase a couple of his patch bays. I'd built that noisegate circuit and failed, in my early circuit building adventures, to get it working. He told me to send it to him and he'd fix it but i never took him up on the offer.

Good visit with a really good guy!

[PRR]

We have clean batteries and we have clean wall-warts; but we also have dirty (100/120Hz buzz) warts. Or we got to the gig without the good supply and have to go rooting behind the bandstand for lost and discarded warts. Collective friend RG teaches like this:

[Mark Hammer]

As promised.

[marcelomd]

As promised.

Yo, Mark, tell me about that box with lots of knobs on the left. What does it do? And what kind of enclosure is that?
Thanks!

[Mark Hammer]

It's a plastic Pactec enclosure.  They are a LOT sturdier than one would think.  The pedal itself is this one.  I managed to fit 6 different 2-knob drives into it, on perfboard, with a rotary switch to select which one is in use.  From left to right, a Shin-Ei FY-2 fuzz, Jordan Bosstone, first-issue Boss OD-1, Silicon Tonebender Mk.1, modified Green Ringer, and my Crank booster.  I tried to pick things that only required two controls, and used a different "engine" from the other circuits.

I haven't opened it up in a while, but what I have done with many pedals I made using these plastic enclosures is to cut out a piece of copper shim material that fits against the inside of the "top" of the enclosure.  I use a hole punch to put holes into the copper shim, for the pots and switches to poke through, and they hold the copper sheet in place.  It provides a nice ground plane and takes solder well...once I buff it with steel wool. 

[marcelomd]

Awesome stuff.
Sorry to hijack the topic. The distortion is... not for me... but that box gave me some ideas for the next version of my preamp (yes, I'll eventually stop talking about it =) )
Thanks!

[moosapotamus]

Nice one, Mark. That sure brings back some wild memories. Thx!

[Mark Hammer]

The distortion is... not for me.

To be expected.  Not because it's you, but because it wasn't really designed to be a distortion pedal, as much as something that made the autowah part "sound better".

Personally, I have SO many drives/distortions/fuzzes that they all start to sound the same after a while.  So something that doesn't try to cover the same territory is welcome for me.  I can't imagine anyone would seek out a Funny Cat, though, and pay vintage prices just to use only the SDS part.  The nice thing about being DIY-ers, though, is that we can make the parts we like, without having to commit to the rest of it, and do so inexpensively.  DIY isn't always cheaper, but when it is, count me in!