Moog drive circuit?

[mlabbee]

I was playing around with a Mooger Fooger Murf the other day and found that just cranking the drive produced the most amazing, warm and creamy overdrive (no other effect applied).  Does anyone have any idea what the drive circuits in these things look like? I'd love to build one . . .

[Paul Perry (Frostwave)]

I expect the filters are based around the LM13700 dual OTA, but I have never looked inside. Anyone able to pop the hood?  :wink:

[travissk]

I looked up all the moog schematic information I could, and the only stuff I could find was for synths, neither with a drive circuit.

The manual for that specific moogerfooger is freely available but it doesn't mention the drive.

[gez]

Maybe the filter is still engaged with no effect and is just shifted into a higher range to allow the majority of the signal to pass?  I did something similar with a fuzz-wah not so long ago, it turned a pretty awful fuzz sound (tapped the signal from a half wave rectifier and fed it to the filter) into something quite wonderful.

[travissk]

From my (limited) experience with a hardware Moog filter, it sounds very nice when you sort of "overdrive" it... perhaps the filter section remains on and unshifted, but the transistors present in the filter "ladder section" do something nice when you feed it an amplified signal?

I was playing through a synth, so I can't say if that's the guitar tone you're after. The Moog is also a lowpass, 24dB per octave filter, so  it would cut the harshness out of the signal, resulting in a creamier sound.

This might be something interesting to experiment with; I might give it a shot once I get back to school in September.

[aron]

My micro Moog used to get a great, warm, overdrive.... There's something magical about that filter.

[puretube]

My micro Moog used to get a great, warm, overdrive.... There's something magical about that filter.

more on that in DIY synth sites....

now on with vacation  

[Paul Perry (Frostwave)]

I could be wrong, but I would be very suprised if the MURF actually had the Moog ladder filters in it. For one thing, the ladder is a low pass filter, and the MURF looks AFAIK like a graphic wiht the individual elements automatically controlled.
The MS20 style filter (using LM13700s for example) has an overdrive characteristic many people like. In fact at one time Moog had a patent for a distortion unit, using OTAs in 3 different ways, I think.

[mlabbee]

Intersting - thanks for all the ideas.  I found the patent (I think) you referred to - it's US Patent No. 4,180,707.  You can find it on www.uspto.gov.  He suggests using the RCA CA3080 OTA.  I'd like to take a shot at building the circuit - it looks like mouser has them - #570-CA3080E - I wonder if it's the same thing.  Anyone know these OTAs or a good substitute?

[Paul Perry (Frostwave)]

Yeah, that CA3080E would do fine. Or a LM3080, if you  find one.