[sdiy] Moog Taurus Filter envelope / contour generator

[Tom Wiltshire]

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

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