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

Ben Bradley ben.pi.bradley at gmail.com
Mon May 24 22:35:03 CEST 2021


No, actually the operations of transistors (specifically BJTs) are
quite different from tubes. Here's a generic description of that
"superflex" tube circuit - the main or only difference between the
audio and the RF going through the tube is the frequency range:
https://en.wikipedia.org/wiki/Reflex_receiver
The tube grid input is very high impedance (almost) regardless of the
signal level. Unlike a control voltage or control current, neither one
"controls" the other, they're both just different signals going
through the same tube, used as a (mostly) linear amplifier. You could
do the same thing with a transistor but it would be a different
circuit due to bias considerations.

A transistor's input (base-emitter) impedance is inversely
proportional to the DC current into the base. This impedance varies a
lot with voltage, so the signal level into it has to be very low to
keep distortion low. That's the too-short version of how the original
Moog ladder filter works.

At this point it's not really a control voltage, but rather a control
current. The control current is actually injected into the emitter
(and the base current is proportional).
You can find a lot by googling:
ladder filter patent
The first link of course the actual patent, and other links describe
its operation in various amounts of detail.

On Mon, May 24, 2021 at 3:47 PM cheater cheater via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> ok, so essentially all modern filters derive from the superflex circuit?
>
> On Sun, May 23, 2021 at 9:15 PM Gordonjcp <gordonjcp at gjcp.net> wrote:
> >
> > 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
> >
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