At 8:50 PM -0700 12/16/01, Scott Juskiw wrote:
>
>Has anybody bothered to tweak their 410 so that the filter
>frequencies are all the same when the FREQ pots are all set to the
>same value?
>
>I've noticed that if I pump a 1000 Hz sine wave into my 410s and then
>adjust the FREQ knobs so that each filter output is at its loudest,
>the FREQ knob settings vary between 3.0 and 5.0. That's quite a range
>and makes it rather difficult to correlate a FREQ knob setting to a
>Hz value when trying to dial up formants for speech synthesis.
>
>Since the filter resistors are inside the Vactrol they can't be
>changed. Changing C6-C8 is unlikely a solution since caps are not
>available in a wide variety of values with 1% tolerances. It looks
>like the best solution is to tweak the Vactrol LED current through
>R54, R57, R60.
>
>I was wondering if anybody has already done this on their 410, or has
>another solution. Or am I just being a nutcase?
Possibly, yes, to all three questions :)
I could see why they wouldn't be the same. The Spectrol 149 pots have a
10% tolerance. You also have a number of resistors, a cap, two
transistors, and a vactrol, for each filter. None of which are matched nor
precision parts. I'm no EE, but I've also read that vactrols aren't
perfectly linear. So even if you tweaked the resistor values to get the
knob settings to be the same for one set of settings, it might not be
temperature stable or matched up for some other setting.
Rather than monkeying around with a mess of 1% resistors and possibly not
getting anywhere, maybe you could simply adjust the position of the knob in
relation to the pot's shaft. That would be a much less invasive procedure.