More stupid synth tricks...

[Christopher List]

Howdy DIYers,

Well, I've just <finally> finished the "Program Select" input on my sequencer. 
This is a CV input which goes through an A-D converter and makes the sequencer 
jump to one of it's 16 steps (based on a 0-5v input). 

The design was a follows....
CV input goes through a FWR. The output of this goes to a comparator network 
made from 4 LM339s. The (-) inputs on the 339's are connected to a resistor 
divider ladder of (16) 2.2K resistors - the bottom of the ladder is connected 
to ground, the top is connected to a 100K trimmer and then to +15v. When 
trimmed, the top "rung" of the ladder is about 4.7v and the bottom is .3v. The 
16 comparator outputs go to an encoder made from two 4532's and a 4071. This 
goes to the jam inputs on my counter chip. The "enable out" of the "lower bits" 
encoder is connected to the "write-inhibit" pin of the counter. As long as the 
CV input is below .3v, all the comparators will be off, the enable out of the 
encoder will be high and the counter (and the sequencer) will function 
normally. Once the CV goes above .3v, the comparators start turning on, the 
encoder creates a 4 bit binary address and the counter is jammed to that 
address.

I was talking with Matt Haines about this yesterday and he gave me the cool 
idea of running it from an audio frequency sawtooth. The beauty of this being 
that you could get oscillations at frequencies ABOVE the frequency of the VCO, 
as opposed to <<clocking>> the counter from a VCO, which will divide the 
frequency. I tried it last night, and it works! I set all the odd knobs in a 
row to low and all the even knobs to high, and sent a sawtooth (level shifted 
to 0-+5v) into the select input. I got out a square wave 3 octaves above the 
original! To stress test it I cranked up the frequency of the sawtooth and at 
about 8 or 10kHz, it starts to get screwy. It starts shrinking the range - so 
that it only steps through the middle steps of the 16. Keep going higher and it 
locks up. I'm not sure where the lock up comes from, but I'm sure the switching 
time of the comparators has something to do with these problems... 

Changing the wave shape to non-square - by turning the pots in the sequencer 
row - creates the same kinds of sounds as simply clocking the sequencer from an 
audio VCO (I call these "Coleco-Vision Waveforms") - the big advantage is that 
you can get higher frequencies.

Another neat trick with this is running the CV input from an ADSR, running the 
output through a slew lim (or not) and controlling stuff like filter frequency 
with it. The sequencer sweeps back and forth based on the rate of attack and 
decay and stops at the step set by the sustain voltage. Fun fun fun....


On another front, Matt and I worked out some enhancements to that AD + AR 
circuit I told you guys about last week. I just built one of these "enhanced" 
versions last night and it works great! I added a switch that lets it work 
either the way it was originally, **or** as a pure linear slew limiter with 
different CV-able rates for rise and fall. This was a little tricky because the 
original schem only responded to a positive input and I wanted the slew lim to 
be able to work over the + and - voltages...

I also got the min. decay time closer to the min. attack time.

I will be updating my schematics and posting them on the Web somewhere...
(calling Ric Miller - you interested in this one?)

(BTW, there were some errors in the first schem I sent to a bunch of people. 
For one, I mixed up the inputs to the flip-flop in the translation from my 
notes to a computer-drawn schem - there's other stuff. Wait for the updates 
before you start building!)

- CList