Fast analog sw. as VC resistor

Christopher_List at Sonymusic.Com Christopher_List at Sonymusic.Com
Tue Jan 7 16:36:53 CET 1997





DIY  -

First off thanks to Mark for reviving this idea with such enthusiasm. It
got me all psyched to do some experimenting...

I went home last night and checked out my ASM-1 VCO. It maxes out at about
120kHz with <OK> looking waveforms. When they are square they're alright,
as you shrink the pulse width they get rather pointy, and as you get the PW
really low they freak out and oscillate wildly, then they finally lock onto
+/- V. I connected a wire from the output of the PWM comparator (+/-13.5
output level) and used that (with a diode) to experiment with a 4053.  Note
that my VCO's use AD712 opamps that have a 17v/us slew rate and this is
what is both buffering the sawtooth output and actiong as a comparator.

I wired up the 4053 as an attenuator with 100K resistors. It seems that the
4053 can't be set up with Vd = 15v, Vs = 0v, and Ve = -15v...  which is
what I would have liked - is this correct? My data sheet kind of hints at
it, but doesn't really say. Anyway, after frying a 4053, I set Ve = 0v and
it worked - this meant that my input signal had to be positive. On the
output I put an RC filter - 1.8K and .01uF. I ran an LFO (level shifted
into the positive range) into it.

The output looked great and the attenuation worked very nicely as I turned
the PW knob on the ASM1. I was impressed. Even with the crappy PWM action
at this frequency. At lower pulse frequencies it worked even better - too
low and (obviously) the PWV wave showed on the output. I tried it with
pretty high frequency inputs and got very results (sorry this is so
subjective - I didn't do any linearity tests).

The RC network was tuned too low for higher audio frequencies, so, just for
laughs, I whipped up a 2pole butterworth filter, with a 20kHz cut off for
the output. This worked well, but it didn't take too much magnification of
the waveform to see the ripple caused by the pulses on the switch. I think
you really need either a pulse frequency higher than 100kHz, or a higher
order filter on the output if you want to use this for audio stuff (like
the Q pot in a filter).

I wanted to play around some more and see if I could get a better, faster
oscillator, so I made a simple triangle osc. around one of those AD712. I
set it up for 150kHz - unfortunately this is a little too fast, the
waveform was pretty curvy at the top and bottom. I think a simple, quality
setup would be using the sawtooth generator from the ASM-1 for your
reference - as it's very fast.

It also occurred to me that you could get a log pot reponse (good for the
ADSR) without too much trouble. A log pot is, afterall just two linear
sections. So, you attenuate your sawtooth, then run it through an amplifier
with some zener diodes that will amplify it by 1 when it's below the
threshold, and by 10 when it's above. Set the threshold right in the middle
of the wave and presto - you've got a log pot reference!

I will be experimenting with a faster VCO and a LM311 comparator tonight
(unless Mark beats me to it!) - the LM311 is about 6 times faster than the
339.

Keep up the good work guys!

- CList


ps.
Someone mentioned a replacement FET for the ASM-1 with a different pin-out
could you repost it? - Those NTE466's are $2.50 a piece!

Did anyone get my post about the quantizer? It seems to have been lost on
the way to the list....





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