[LAU] setBfree: Gain sweller from lin to log characteristic

Adam Sampson ats at offog.org
Wed Dec 15 01:01:36 CET 2021

Gerhard Zintel <gerhard.zintel at web.de> writes:

> Does anyone of you know if the original swell pedal of the B3 is
> changing the volume in a linear or logarithmic way?

Here's a quick recording from the output of my C3 (same internals as the
B3 in a different case), with the drawbars at 88 8888 888, holding down
Cs across four octaves and moving the swell pedal as smoothly as I can
between the endstops:


This looks more or less linear to me by eye. Note it doesn't go all the
way down to silence.

However, in terms of the preamp circuit it's not quite that simple --
the swell pedal isn't a variable resistor, it's an unusual variable
capacitor with two fixed plates and a moving one, in between two valve
buffer stages. One fixed plate is fed directly from the previous buffer,
and the other is fed through a filter circuit -- so effectively the
pedal is crossfading between the full signal and a filtered and
attenuated version of it (simulating a real organ's swell box with the
shutters closed, I guess), which means that there's some variation in
the frequency response as well.

This also disregards any nonlinearity in what you're playing the organ
into -- if you're doing the traditional overdriven Leslie thing, then
there'll come a point where it gets crunchier rather than louder...


Adam Sampson <ats at offog.org>                         <http://offog.org/>

More information about the Linux-audio-user mailing list