Quadrature VCO Module finished

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Fri Sep 8 18:40:00 CEST 1995


For all those who are interested, I got my Quadrature VCO
Module finished last night.
It contains a sin / cos VCO, that is built around 2 LM13600's and
some TL07x opamps. The core is a schmitt-trigger / integrator
triangle generator. A second comparator / ota-integrator pair
generates a second triangle wave with 90 degrees phase shift
(as described in Tietze/Schenk, Halbleiterschaltungstechnik).
The second dual ota forms sine / cosine waves out of the triangles
by slightly overdriving their inputs (like in the Oberheim SEM's LFO).
These otas are used for level control as well. Together with two
opamps they form VCAs. The special thing is an output limiter
in the VCAs, made by 5.1V zener diodes. So, with increasing
Level CV, the waveforms can be overdriven from sine to rectangle,
which acts like a primitive timbre modulator. The level CV is composed
from an Initial Level pot and an AM input with AM Depth pot.
The frequency control section has a 3-way swith with the ranges LFO,
Wide and audio. There is a manual Frequency knob and an FM input
with FM amount pot (+/- 1V/octave Range). I hope that the VCO will
decently track in the audio range; I have a tempco resistor and a
25 turn trimmer for adjustment, but I have not scaled it yet.
There are 4 outputs, 2 for sine/cosine each, to avoid the need of
multiples. There are LEDs for sine and cosine as well. Like most other
LEDs in my Modular system, they glow proportional to the current
output voltage (green when positive, dark when zero, red when negative).
To achieve a propartional brightness and to compensate for the different
brightnes of the red and green system in the dual LED, a simple diode/
resistor network is used together with an opamp.
As this VCO only fills half of a module space, I added some other little
devices. There are +5V and -5V fixed voltage outputs, a 0-15V manual
CV with poti, a voltage inverter with two outputs and two input / thru
jacks (to avoid the need of multiples ...). Last, not least, there is a 
with two inputs, threshold poti, led and output.
First impressions:
Connecting the Qudrature VCO to a Voltage Controlled Amplifier / Panner
(sin to AM, cos to PanMod) gives nice circulating effects (You might know
this from Serge Modules ...). Now doing this in the audio range! Ringmod
and "Ring-Pan" effects. Now use an ADSR to control the frequency and
the amplitude of the Q-VCO, and feed a Pad sound from the OB-8 thru
the VCA/Panner. Sounds like the good old CS-80. Overdrive the VCA
section in the Q-VCO during the peaks of the ADSR curve, and the effect
gets more dramatic due to added ringmod spectral products with a
modulator blending from sine to square and back to sine.
Great fun! I hope the keyboard tracking will properly work when I'll try and
adjust the scale tonight ...


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