Minimoog VCF transistor question

[Mick Bailey]

How does the Hfe of the ladder transistors affect the way the filter behaves? I used matched pairs of BC 547B (Hfe worked out at 317 after selecting the transistors from a batch of 30)

I have a batch of 100 2N5088 with a Hfe of 480 which I could use instead.

The reason I ask is that I'm fairly happy with the filter, but the resonance control has a very narrow range of operation even with the trimmer at its minimum setting. It will self-oscillate though.

I wondered if swapping out the transistors would be beneficial to the way the filter performs.

[yusynth]

The Hfe of the ladder transistor has nothing to do with the response of the resonance.
What is important and gives a progressive resonance control is to use a reverse audio (or reverse log) 50K potentiometer, that is the C curve, as stated in the BOM. Using a linear or an audio (log) potentiometer (A and B) will not give the good resonance control and all the range gets concentrated over a very short range.

50KC pots can be purchased at banzaieffects, musikding and smallbear electronics

[Mick Bailey]

Thanks for the advice on the transistors. I'm currently using a 47k log pot, but wired so that maximum resonance is fully anti-clockwise (control operates backwards). This is a temporary arrangement until I build up a list of items to make placing an order with Banzai worthwhile.