Triangle thru zero VCO
A thru zero oscillator is one that can go
forward and backward. This allows for deeper frequency modulation. Sonically there are
interesting a little yamaha-ish sounds possible. This oscillator sounds somewhat dirty
when the modulation index is time variable. But clean non-time-variant timbres are
possible. I've put up several samples here, to give you an impression
of the sounds I obtained.
When building it make sure you use plenty of decoupling caps. Take care about the supply voltages,
several of the ICs are run of +/-5V ! I think it is a good idea to use the on-card stabilization
even when you have a dual 5V supply available in your system. I used a BA6110 OTA as integrator.
This was simply because it is a single inline device. If you have trouble getting these a LM13600 or CA3080
will probably work fine too. The adjustment is very tricky. Before you begin you should measure the supply voltages.
Set the voltage trimpot for the +5V supply so that it is equal in magnitude to the negative 5V supply.
Then set the linear tune pot (the one connected to the 2,2Meg resistor) fully clockwise. You should see a triangle at the output.
If not then try turning the expo tune pot until you have one.
Now the symmetry can be adjusted. Let the unit have some time to warm up! You can now set the V/oct trimpot using the
known procedure. Now set the linear tune pot to the center position
(or somewhere near that, where the oscillator stops). Then apply an input signal to the linear FM input.
Trigger your scope on the input signal, if it is difficult to get a stable image. There is probably a square
wave superimposed on the triangle signal. The DC levels are different in forward and backward mode.
This can be trimmed away with the trimpot labelled output compensation.
You will perhaps have to readjust the panel linear frequency pot, and the symmetry pot,
because they interact with each other. However there will be a " sweetspot " near the center of the frequency pot.
When you have found it, readjust the symmetry. But using a modulation signal this time.
The examples were done using a sawtooth input from one of my VCO4s, I patched an ADSR into the expo
input of this circuit, and used the same pitch control voltage for both VCOs. The output of the circuit went
into my minimoog filter. This was the basic patch I used for all of the files. I've used the same ADSR for the
index, filter and VCA envelope. Seperate envelopes will be more flexible. Cross or selfmodulation patches are also possible.