OTA distortion?

[vdm]

Hi,

My msg just got deleted by silly IE, so this is short and sweet as i'm off to band practice 3mins ago 

simply I want to overdrive an OTA (3080) or 2 into distortion. This is to investigate the effect of turning up the gain control on a moogerfooger pedal.
I'd love any kind of info you could give me, or simple OTA gain stages that can be OD'd.

Thanks,
Trent

P.S. if anyone has a Korg MS20 schematic or board pics of any moogerfooger pedals i'd be very appreciative!!!! i have a few rare schems to trade with if you'd like some encouragement - PM me

[Paul Perry (Frostwave)]

MS20 schem? Here ya go! 
http://www.korganalogue.net/korgms/service/ms20/m20s.html

and more links here: http://www.sonicstate.com/synth/korgms20.cfm

[Mark Hammer]

Take a look at issue 4 of DEVICE (I think) at http://hammer.ampage.org (around page 9 or 10).  It has a 3080-based voltage-controlled distortion project.  The CA3080 has a fairly low distortion threshold, or rather poor signal-handling capability, that is exploited in this project.  There might be some useful ideas there.

[Paul Perry (Frostwave)]

Not to forget Moog patent 4,180,707 towards the bottom of this page (thanks Don Till!)
http://www.till.com/articles/moog/patents.html

[vdm]

Thanks very much,
I had seen those before, but its almost comforting to think that two people I hold in such high regard in the FX world keep coming back to this, that there *must* be something in what they post.
Cheers,
Trent

P.S. Still looking for innards shots of a moogerfooger

[PharaohAmps]

Take a look at issue 4 of DEVICE (I think) at http://hammer.ampage.org (around page 9 or 10).  It has a 3080-based voltage-controlled distortion project.  The CA3080 has a fairly low distortion threshold, or rather poor signal-handling capability, that is exploited in this project.  There might be some useful ideas there.

The 3080 (and to a lesser extent the LM13700 OTA's) sound great in distortion.

http://www.pharaohamps.com/Pharaohamps/Sounds/cheetah_1.mp3
http://www.pharaohamps.com/Pharaohamps/Sounds/cheetah_2.mp3

I've been meaning to talk to Francisco P. about some projects / layouts for Tonepad.  If anyone wants to build  a pedal that sounds like this, let me know.



Matt Farrow

[jrc4558]

I would really like to build something like the pdal on the Cheetah soundsample!
I also like the paint on your pedals! What brand is it, if you dont mind me asking? I was trying to get that sort of finish many times, but it always looks just like shiny nail polish, whereas your have this good texture to them...

[vdm]

Matt,
The cheetah samples are nice. Personally I'm more for a more subtle overdrive sound, but I'd be very interested to know how you setup the 3080 in a clipping stage.
I was thinking I might drive it with a mosfet boost of some kind because I like the sound and attach some kind of filter.
So I'm after quite a different beast, but I don't understand how the OTA works properly yet.
Any tips would be great and I'll go and experiment.
Trent

[PharaohAmps]

I would really like to build something like the pdal on the Cheetah soundsample!
I also like the paint on your pedals! What brand is it, if you dont mind me asking? I was trying to get that sort of finish many times, but it always looks just like shiny nail polish, whereas your have this good texture to them...

Thanks for the complement.  The pedal is a pretty simple design, just a 4558 and a 3080.  With the demise of the 3080, I've redesigned the circuit using a single 13700, but it has a different kind of sound.  It's similar, but I have to mess with the bias current on the input diodes to match the same distortion that I got from the 3080.

On an interesting note, I've been messing with JC Maillet's "Super Full-Wave Rectifier" to use to drive one section of the 13700.  It makes a great envelope detector for a compressor, or if you invert the response it can cause an OTA to act like an expander.  The truly slick thing to do (Mark H., you'll appreciate this) is to mix inverted and non-inverted envelopes with a single pot.  In the center of rotation, the two signals would cancel.  At one end, you'd have a compressed signal, and at the other you'd have an expanded signal.  The compression would let you get more "saturation," and the expansion would restore some of your dynamics.  If you want to go ape over the controls, you could even select pre- or post-distortion sources for the envelope signal.

All of this is gonna take more than my customary three knobs, Wild Mouse box standard design.  We'll see where it ends up.

The paint is Rust-Oleum Hammered finish, which I buy by the quart and spray using a Sears spray gun.  They sell the stuff in spray bombs, too, but it's harder to apply that way. 

Matt Farrow

[Vaum]

I'm pretty much a noob when it comes to this sort of thing so I'm probably embarrassing myself, but hey, I tried...

Will this work? If not, can someone help me to fix it.



Where can/should I add a tone control?

Basically I'm trying to simplify this design...



Thanks.

[gez]

Vaum, don't understand why that 10M is so large in value.  Mirror the other input and try a 100k?  It seems a little convoluted.  Can't remember the way RG does trims for bias, but a short click to GEOFEX will tell you.

Also, you need a resistor from the output of the OTA to ground to develop a voltage at the output - OTAs output current.  For the same reason (high output impedance) you need a buffer.  If you're using a 9V supply, that amp used for Vref could be wired as a follower and its + input connected to the OTA's output (no components needed, just a wire from out to - input of the amp).  Both 47K resistors would be referenced to the centre of a divider (decoupled by a cap), as would the resistor from the output (to develop a voltage).  You could just ditch the 10M.  If you're not modulating, then the trimpot is probably a waste of time.  Just a simple resistor (try 20K?) from output to Vref is usually suficient for decent bias (keep the divider resistors low-value).

It would be an idea to use a stop resistor in series with a pot to the Iabc pin (control gain this way).

The ratio of the 100k to the 47k is a little low (doesn't divide down by that much), but if you're trying to distort the amp that's one way to go.  Again, if you're trying to overdrive the input of the OTA add some gain to the input stage.

[Paul Perry (Frostwave)]

The 10M looks low to me as well.
Interestingly, in the original circuit I think the trimpot is to make the behaviour of the VCA as symmetric as possible - but, since we're making a distortion device here, you want to be able to get a fair way over to one side. So I'd definitely go lower. And call it a 'tilt' knob or something.