[sdiy] Moog Taurus Filter envelope / contour generator

Tom Wiltshire tom at electricdruid.net
Sat Dec 10 12:59:18 CET 2022

Thanks for the confirmation, Michael.

This is the behaviour I've seen in the simulation, but since it contradicts the service manual, I wasn't sure.

> On 10 Dec 2022, at 03:02, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
> I own a set of Taurus pedals and this is indeed how the envelope generator behaves.  Dave Luce designed the Taurus EG to emulate a string bass, that's the reason for its peculiar action.
> The filter EG is an AD EG with zero sustain, while the amplitude EG is an AR EG with full sustain.  Yes there is a sustain control but it does not control the sustain stage of the EG, it is merely a volume level to balance the user setting VARIABLE with the presets.
> Bob Moog told me that Dave Luce lifted the filter right out of the modular.  The solid fundamental of the Taurus is unique among the other Moog synths I own.
> When the reissue Taurus III was designed in 2009, Cyril Lance was maniacal about making a faithful reproduction even down to the quirky EG behavior.  The EG behavior of the Minitaur is more standard and still packs the gut punch of a Taurus.
> MC
> On Fri, Dec 9, 2022 at 5:56 AM Tom Wiltshire <tom at electricdruid.net <mailto:tom at electricdruid.net>> wrote:
> Hi All,
> I've been having a read-through of the Moog Taurus schematics. It's an interesting beastie! For one thing, it uses linear V/Hz oscillators.
> However, my query is about the filter contour generator. This is a very simple design, or it would be without the presets. The schematic is attached. A 15V gate pulse comes in from the top-left. This is differentiated by the HP network C401/100n, R401/1M which gives a time constant of 100msecs. Assuming we're not using one of the presets, we look at the top path to the 4016, path A, and we can ignore everything below that, paths B, C, and D. R403/1K sets the filter amount by setting the maximum voltage the filter contour cap can charge to. R402/10K sets the Attack time, but the longest attack is a 56msec time constant, so roughly 1/4sec. R410 deals with the Decay. R416 prevents the peak current drawn by C402 from frying the 4016, and C402 is the filter contour cap. The envelope output is buffered by the Q402 darlington and goes off to the filter CV mixing.
> The service manual claims that the 100msec time constant is "sufficiently long to allow the controur capacitor C402 to reach nearly full level even when the attack time constant is set to its maximum value of 56msecs". However, it seems that this is not true. I've done a sim of the relevant parts, just the top path, and the results are shown below - Trigger pulse at P5-2 is in red, envelope outputs for attack and decay at 0%, 20%, 40%, 60%, 80%, and 100% shown in yellow. As you can see, increasing the attack time lowers the output level significantly, to about half what it would have been. This remains true (although not as bad) if we increase C401 to 220n to bump up the time constant a bit.
> Can anyone confirm that this is true? Does the Taurus really behave this way, and why would the service manual claim otherwise?
> Thanks,
> Tom
> <FilterContourOriginal.png><FilterContourOutput.png>
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