From bruce@ashbysolutions.com Sun Mar 30 04:55:06 2003
Subject:Re: An Improbable Question But Hey, I Gotta Ask

Hi Ben,

>This question is aimed at the more technically savvy members of the group.
>Going back to our discussions comparing clones to the real thing and how the
>lower frequencies on the Hammonds do not rise or fall as much in volume as
>the upper ones when you use the expression pedal, the question is this:
>
>Would there be certain types of pots that you could put into a volume pedal
>to emulate this characteristic? I imagine that engineers would actually aim
>for the opposite effect when designing the pots, but if there would be
>specialty designs out there then that could alleviate the problem of the bass
>rising & falling in direct proportion to the upper frequencies. Or for
>another approach, would there be an electronic component, e.g. resistor,
>capacitor, etc., that you could attach to the pedal circuit that would give
>it the effect of attenuating the lower frequencies less, and could that be
>done without seriously compromising the overall level of the signal coming
>out of the pedal?

I've never heard of a pot that is frequency dependent. First off, I don't know how it would be done, and secondly, it's counter to the design goal of a pot; i.e., a purely resistive device.

There are several ways to do this:

- 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.

- Many organ manufacturers use photocells, because they have a built-in lag; i.e., they are better able to pass slow signals than fast ones. I think the V3 used this method, and I know that most '60s combo organs like Farfisa did.

- Using active (op-amp) circuitry, it's possible to model all kinds of passive devices, including one whose bass response doesn't vary as much as its treble. This opens up a whole new world -- and can of worms -- because it requires a circuit board in the pedal, plus battery power.

- Just as a capacitor soldered across the terminals of a pot will allow some highs to leak through (a trick used on guitars), an inductor across a pot will allow some lows to leak through. Selecting the right inductor can be tricky, though, because the parts tend to be large, and are subject to the effect of saturation.

Regards,

-BW
--
Bruce Wahler
Ashby Solutions™ http://music.ashbysolutions.com
978.386.7389 voice/fax
bruce@ashbysolutions.com