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

[Michael E Caloroso]

The AC coupling between the VCF and VCA includes a 20z corner boost.
Combined with driving the OTAs into non-linearity, these features are
responsible for the solid beef of the Taurus architecture.  Bob also
told me that the AC coupling between the stages was also optimized low
frequency response.  You can't replicate this with an EQ.  These
features are unique to the Taurus and is not in any other Moog
product.

Some years before the Taurus 3 was released, I was having lunch with
Mike Adams who envisioned using the Voyager circuitry in a future
Taurus reissue.  I explained to him the differences in the circuits
that led to the unique Taurus sound and that the Voyager would not be
a faithful reproduction of the Taurus.  Bob was still around and
agreed with this assessment.  Thankfully they heeded my advice when
they reissued the T3.  It certainly sounds like the Minitaur retains
these features, haven't yet played a SubPhatty yet.

MC

On 3/24/16, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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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