Motorized POT

[Tony Allgood]

>Are probably wrong, but I wouldn't have thought a motorised pot would
be fast enough for a mixer. How does yamaha do it with their "flying
faders" I would have thought everything was done digitally with the
faders providing a visual indication of loudness?

Well, Soundcraft and various flying fader stuff use real faders with
motors. No VCAs or DCAs here, they shag the sound too much. Some use
linear motors, others use a dc motor, and a linear track on the fader as
a position sensor. A servo loop of some sort controls the motor. I used
a class AB amp (ie. a simple audio amp) to drive the motors in the
Soundcraft stuff. You will need a reversed biased diode across each
power tranny. This was quiet and could control the motor from a quick
jolt to a slow crawl. The latter will cause the most problem, smooth
with no judders is tricky to obtain. H bridge can be better, ie. two
complementary class B outputs, the motor wired across the two outputs.
That way you can drive the motor both ways without using a split rail,
but losses and device count is higher. I played with PWM. Motors seemed
to get hotter, and the RFI to the rest of the audio chain was worse. The
problem with all of these is stability of the loop. For the fastest
speed of servo, you will need to tailor the loop gain carefully. And
many cheaper motorised faders will have varying current/speed
relationships depending on age and even fader position. The rotary ones
are better. If speed is not a problem this will be easier. One way is to
have a piece of code to alter the loop gain automatically, and it could
update itself to cope with age etc.

If I were to do an automated desk. I would use a That's VCA with a 20
led ladder to show the position of a virtual slider, next to the real
one.

Regards,

Tony Allgood, Cumbria, UK

Rack mounted moog filter and the TB3030 SuperBassline projects:

http://aupe.phys.andrews.edu/diy_archive/schematics/oakley/