[sdiy] CV inputs with bipolar "amount" knobs

[Harry Bissell]

The pitch bend on the Prophet V used diodes for a deadband
circuit. It works well but you'd need to make a CV and then
use a pair of VCAs or a four-quadrant multiplier to get the
inversion and level correct. 

VCAs or multiplier will have feedthrough and offset troubles,
maybe not worth it...

H^) harry

----- Original Message -----
From: Matthias Herrmann-DEU <Matthias.Herrmann at jeppesen.com>
To: Synth-diy at dropmix.xs4all.nl
Sent: Tue, 27 Jul 2010 02:34:08 -0400 (EDT)
Subject: Re: [sdiy] CV inputs with bipolar "amount" knobs

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of David G. Dixon
> Sent: Tuesday, July 27, 2010 8:04 AM
> Subject: Re: [sdiy] CV inputs with bipolar "amount" knobs
>

<snip>

> and it's sweet!  No trimmer, no detent; the response is linear and the
> centre is calibrated by lining up the pot knob with a line on the panel
> (real high tech!).  However, it adds loads of fun to a filter having
> this on
> there (if I'd only known sooner...).  My next one will probably have a
> detent and a trimmer.

in my experience a potentiometer with center detent could cause another problem: you might not always have the same resistance value at the detented position. it may be different each time you set it back to zero. however, for a VCFs control voltage input it might not be crucial, and it may depend on the quality of the potentiometer - I tried alpha and alps with no luck.

I think this is why harry suggested a center tapped potentiometer. I never had one, though.

I believe you could alternatively use diodes at the potentiometers wiper to create a kind of 'window', but than you would lose a lot of control range where you need it most probably.

cheers,
matthias
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-- 
Harry Bissell & Nora Abdullah 4eva