[sdiy] Moog VCF: Taurus vs. "Regular"

Tom Wiltshire tom at electricdruid.net
Thu Mar 24 21:03:06 CET 2016


Hi Steve,

The Taurus VCF sounds great. No sonic limitations at all.

As you've noticed, they were able to use half the OTA as the differential amp, and that meant they didn't have to bother with the multiple-op-amps differential amp in the typical design.

The other important thing to note about the Taurus is that a part of the sound comes from the filter driving the VCA (the other half of the LM13700) into its non-linear region. I hesitate to call it "clipping" - it's too soft for that - but it's definitely driven hard.

I did some PCB work for Rick Holt of Frequency Central on a Taurus clone design that he developed for Eurorack:

http://www.frequencycentral.co.uk/?page_id=1361
http://www.frequencycentral.co.uk/wp-content/uploads/2014/04/Raging-Bull-Schematic.pdf

There are sound samples here:

http://matttechmodulardemos.bandcamp.com/album/frequency-central-raging-bull-moog-taurus-vca-vcf-clone

HTH,
Tom

On 24 Mar 2016, at 18:52, sleepy_dog at gmx.de wrote:

> Hey there,
> 
> I recently stumbled upon some EFM circuits, such as their version of the Moog Taurus VCF.
> It is nicely low-parts count compared to the "regular" (MiniMoog, Source, ...) VCF.
> The main difference I spot is that in the "regular" one, the two sides of the transistor ladder are bufferd by each an own opamp, of which the outputs each go into one of the inputs of a third opamp, used as difference amplifier.
> In the Taurus, there's just 1/2 of a LM13700, with its voltage inputs connected directly to the left / right parts of the ladder, and the current input fed by a fixed resistor going to one supply rail.
> 
> I guess the Taurus needed low parts count for it's a pedal.
> And also, it's a bass pedal, which makes me wonder...
> 
> Does the Taurus VCF setup suffer from any sonic / performance disadvantages over the regular Moog VCF? Or would it do just fine in a synth not strictly for bass sounds?
> 
> 
> Regards,
> Steve
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