My new VCO

[Ingo Debus]

Hi Juergen,

this reminds me of my very first "serious" DIY music project, the
Tuenker "Sound Organ". Oscillator was with SCR too, no VCO though. The
four sawtooth footages were generated the same way with resistor
networks. No ICs, two frequency dividers on one PCB.

Anyway, I have some questions (sorry if some of these have been covered
before):

What is the usable frequency range of the VCO itself (without frequency
divider), using the V/Hz input? Are the frequency dividers just to
select the footage or do their outputs have to be switched by the
keyboard, when a larger (88 keys) keyboard is used?


I think it's very useful when the footage outputs could be mixed too.
Especially if there were tri/sine converters for each footage, a organ
drawbar thing could be done.

However, if only one footage needs to be available at a time there's no
need for a complete resistor network for each footage. To get the next
lower footage just add the next lower square wave with double the
amplitude of the previous one. The Reset input of the frequency divider
IC could be used to disable/enable the squares. With a GAL as frequency
divider, the footage could be selected with static digital signals;
useful if several VCOs are ganged for a polyphonic system.

Of course the amplitude of the saw is twice as high then (could be
compensated with another switch scaling down the output level).
Instead of the resistor network, a multiplying DAC IC could be used. Not
necessarily cheaper, but maybe space saving. The top saw had still to be
added with a discrete resistor.

Even if all footages are available the same time, with an output buffer
for each footage, only two voltages need to be added for each footage:
the saw of the next higher footage and the square.


Another question: Is the + input of the TL071 really connected to two
caps only?

Ingo