[sdiy] Yves Usson's Minimoog Filter

Gordonjcp gordonjcp at gjcp.net
Sat Jun 10 11:59:02 CEST 2023

On Sat, Jun 10, 2023 at 03:37:49AM -0400, Ben Bradley via Synth-diy wrote:
> I have two thoughts. First is the schematic says (at each transistor)
> BC547C, with the C suffix indicating higher HFE. The Onsemi/Fairchild
> datasheet shows the C grade to have Hfe of 420 to 800. From my study
> of the Moog filter, the original "special" TIS97 transistors had a HFE
> of around 800. This is higher than the average 2n3904, but not WAY
> higher, so I don't know if it would make a significant difference.
> I've wanted to make this kind of filter with transistor sockets so I
> could easily find out, especially if there's really any magic related
> to the "special" transistors as some have claimed.

Unlikely, to be honest. You'd maybe get a bit of a difference but nothing really drastic.

> The other thought is the input signal level. I recall part of the
> filter's sound has to do with the input being slightly or moderately
> overdriven (with the Minimoog mixer level at full volume), so try
> increasing the input level until the tone changes, and see if that's
> more to your liking.

This does make a big difference, because a long-tailed pair has a lovely soft saturation curve. In the Minimoog in particular it's driven absolutely balls out, getting right up into that curve.


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