[sdiy] Unusual BP filter - Voltage Control

[harrybissell]

Sometimes you can run a small load on the heater supply... BUT
beware if some part of that supply needs to be ground referenced.
You could make a lot of hum that way.

With "vactrols" you are theoretically isolated... but probably you'd want the
CV input to be ground referenced, and that could cause you trouble.

I did this in a guitar amp with LEDS to drive channel switching. I ran the
signal to an isolated switch. I did have some problems when I ran the LED
on only one polarity...

H^) harry

david wright wrote:

> thanks to everyone for helping improve this
> great filter - i wish you all had one.
>
> i'm going to try 8 matched vactrols first - four for each pot
> and see how it sounds. this seems to be the most
> in tune with 1950s thinking. i like the idea of EL panel
> alot - but i would have to make an additional 'break out'
> box to house power supply, tubes and controls to power the ac input.
> so i think i'll pass on that for now. this sucker is allready
> a 'space heater' and i live in the desert.
>
> if the 8 vactrols does not sound so hot - i will try the idea
> of pww each of the vactrols' lamps via a PIC. i wonder if
> i can run a pic off the heater voltage? ;)
>
> thanks again
>
> dave wright
>
> ps - neon light bulb multivibrator w/ controls!
> www.carrionsound.com/neonvibrator.mp3
>
> ----- Original Message -----
> From: "harrybissell" <harrybissell at prodigy.net>
> To: "david wright" <bastardita at hotmail.com>
> Cc: <jhaible at debitel.net>; <synth-diy at dropmix.xs4all.nl>; "KA4HJH"
> <ka4hjh at gte.net>
> Sent: Tuesday, September 14, 2004 10:20 PM
> Subject: Re: [sdiy] Unusual BP filter - Voltage Control
>
> > It depends on how fast you like to modulate the center frequency
> >
> > Vactols are good for slower motion, matching is tough but once you do that
> > you could pass anything through it.
> >
> > EL panels could have higher speed, otoh harder to drive.  I'd think whis
> > would
> >
> > be second choice.
> >
> > PWM of analog switches would (imho) change the character of the filter a
> > lot.
> > For a LPF maybe, for a BPF I'd avoid it.
> >
> > Now an interesting idea would be to use the PWM to drive the LEDs in the
> > Vactrols... have 8 separate PWM outputs and a linearization table for each
> > Vactrol. Make the matching REALLY good.   The Vactrol slowness would
> > probably filter the PWM WAY WAY better than the analog switch method
> >
> > H^) harry
> >
> >