[sdiy] Fwd: [funwithtubes] Vacuum Tube Multitasking - The Erla Superflex

[Gordonjcp]

On Sat, May 22, 2021 at 06:18:34PM +0200, cheater cheater via Synth-diy wrote:
> Guys just to clarify I'm asking about a semiconductor filter. Which
> filter uses bias to set frequency as well as the audio itself on the
> same transistors?
> 

Strictly speaking anything single-ended using transistors as "variable resistors".  The original Korg MS20 filter had interesting behaviour where if you drove it hard the cutoff would vary with the incoming signal - so a low-frequency really loud sawtooth would make a distinct pewpewpewpew sweep.

This is because the cutoff is controlled by the current through the trannies, and that's a function of the base-emitter voltage, so if you vary the base voltage you vary the cutoff.  But if you alter the emitter voltage, say by playing some audio into it, you vary the base-emitter voltage by moving the emitter around!

-- 
Gordonjcp