Adventures with the Tilman modulator

[Tony Allgood]

Hi all,

I have just spent a fruitful weekend trying to get this variable duty
cycled sawtooth circuit to work. I has abandoned my original current
steering method, which was alright, but the frequency shift could not be
easily compensated for. The key word is easily, it could be done, I'm
sure, but with too much electronics for it to be practical for a VCLFO.
So I went for Don Tilman's twin OTA circuit that takes the arithmetic
minimum of two varying sawtooths. This indeed seems to show great
promise.

I used his circuit design but made a few changes to to give a better
result. Firstly, I used a single 13700 without linearisation diodes as
my OTA. These are easier to get than the 3280s. I converted the OTA's
output to voltages with a transimpedance amp instead of resistors. I
tried Don's method of finding the arithmetic minimum, but this presented
the OTA's output with a changing load, so inserting the op-amps solved
that one. But the offset introduced by this method was too much for me,
so I decided to use a comparator and an analogue switch to connect the
lowest output to the final opamp buffer. This worked very well, and the
comparator's output can be used as the pulse output, which follows the
changes in the saw's duty cycle perfectly.

Well, the final results are quite spectacular. The duty cycle can be
changed from about 10% to 90%. I'm going to be using it in a VCLFO, but
it sounds quite good. When you modulate with another LFO, the sound is
similar to PWM but not quite. There is also a pronounced change in
pitch, more of a phase change I think.

I have still some more work to do on it, the VCLFO core is not quite
right, but I will rack it up and put it in my growing modular to give it
a real test. However, I don't think the amount of circuitry justifies
the end result for a LFO. Perhaps I should have made a 'Tilman
Modulator' module instead, that way I could have used it for any saw
input. Anyway, a worthy experiment, thanks Don and all the others that
helped me with this one. I was hoping to make a PCB for this one, I
certainly wouldn't like to build another one on the tiny piece of Vero I
used for this one. But not yet, I have other things to do at the moment.

BTW: I have managed to a build a rather nice waveshaper last week...
inspired by the Serge Wavemultiplier. All voltage controllable.
Schematic coming soon, and I will have a PCB for that one, because it
sounds great... really great.

Regards,

Tony Allgood  Penrith, Cumbria, UK

Modular synth designs, midi-CV convertor, TB303 clone and Filter Rack

http://www.techrepairs.freeserve.co.uk/projects.htm