From james_eaton@btopenworld.com Sun Mar 30 17:05:05 2003
Subject:Re: An Improbable Question But Hey, I Gotta Ask

--- In CloneWheel@yahoogroups.com, Bruce Wahler wrote:
> - As mentioned previously, Hammond used a variable capacitor,
which coupled with the impedances in the organ created the right
response. This is a little more difficult than it sounds -- at
least, as some sort of D.I.Y. kit. Variable capacitors of the size
required for audio frequencies are pretty large.

The B-3 solution uses a capacitor (actually three halves of a
capacitor where the third half moves between the other two) that
fades between two circuits:
- one that sends the signal directly to the output
- one that filters out the middle using a low and a high pass
filter to achieve loudness compensation at low volumes roughly
corresponding to the Fletcher-Munson loudness curves.

As the pedal is pushed down, the output fades from the filtered
signal to the direct signal, and gets louder in the process.

Later L-100s used light dependent resistors, and this circuit can be
applied to clones without too much difficulty as it is passive.

However, it is not necessary to go to even these lengths as it is
possible with a few passive components to approximate the loudness
compensation on a B-3 pedal using something like a boss FV-200.

I did this and published the circuit on this list a long time ago,
but I don't know what the subject line was.

Needs a low pass filter in parallel with the pot, using two
resistors and a capaictor, and a high pass filter using a single
capacitor and a variable resistor.

Cheers
James