[sdiy] Envelope following -> grafting to knobs

[Paul Higgins]

Believe it or not, Mesa/Boogie used pairs of LDRs driven with opposing 
CVs to substitute for pots in their TriAxis MIDI-controlled tube 
preamp.  Mesa _loves_ using LDRs; they're all over the place in their 
designs, usually as "soft" audio switches.  IIRC, my Quad Preamp has 32 
of them.  However, the higher impedances of tube circuits are much more 
forgiving of LDR deficiencies (such as "on" resistance that is nowhere 
near zero).

Check out Craig Anderton's phase shifter and envelope follower design 
in "Electronic Projects for Musicians" for more LDR madness.  Also the 
phaser in Barry Klein's "Electronic Music Circuits" book (I think 
that's the title).

-PRH

On Sunday, January 25, 2004, at 04:55 AM, nN AAt e e wrote:

>> So replacing them with voltage controlled devices can be hard...
>> usual suspects are
>>
>> 1) several photocells
>> 2) several fets
>> 3) one or more VCAs
>> 4) motor operated pots
>>
>> The biggest issue is whether the pot is used as a three
>> terminal device (pot), or as a two terminal (variable resistor)
>>
>> You will really have to look at each individual circuit to see what
>> are the most important attributes of the design to find a suitable
>> replacement.  Send a link to a particular schematic and we'll take a 
>> look.
>
> most of the designs are three terminal, im probably going to be kind of
> hacking some designs that are already available on most of the pedal 
> sites.
> i look at things from a much more 'circuit bending' kind of view, with
> pretty limited actual schematics understanding etc... i can understand 
> flow,
> components, and usually what is supposed to do what... but get into 
> the math
> and thats almost all over my head... [seventeen year old algebra 2 
> student].
>
> im just more curious about replacing pots with vc right now. i hadnt 
> even
> though of using photo resistors in pedals [which ive done a few times
> bending]. a zvex sort of pedal with photo resistors would be cool 
> [including
> one per control on the effect... some pretty weird alesis AIRfx kind of
> stuff going on...?] i will probably get more into actually building 
> stuff
> later when i get some free time to practice... and when my friends 
> realize
> that buying semi-custom effects at cost is a rediculously good deal.
>
> right now theres a 50% chance ill pull a moog and just do electronics 
> for my
> next year and a half of highschool...
>
> - nate - www.robot-dreams.com
>
> [i have a note book full of drawings of gear that would be awesome if 
> people
> would make]


Paul R. Higgins
email: higg0008 at tc.umn.edu