FMing of VCO

Jeff Foster jfoster at
Fri Jul 10 02:23:51 CEST 1998

Eric Barbour wrote:

> >I wonder if there is a way to do FM on the tube synth vco (by Eric)
> >posted here a bit ago?
> Yes. Just:
> --build two VCOs (their outputs are weird-looking sawtooth waveforms)
> --connect input of each to an op-amp driver so you can mix 2 or more CVs together.
> --Then take one VCO's output, run thru a 10-1 voltage divider (to protect the
>       op-amp from damage by the high voltages the VCO thyratrons make)
> --and run it into the input op-amp of the other VCO.
> --Do vice versa for cross-coupling.

ok, thanks for the suggestion, I'll take this into account while I design the front
of my tube based unit (1 vco, 1 bpf, 1 vca) now I have to make room for a second vco..

> >Tubes are non-linear to some degree
> That, sir, is a FALLACY. You have been reading the incoherent and
> ignorant statements made by half-baked designers who don't know
> any better.

nope, I have never read any tube statements except the ones posted here.I have only
looked and to some degree understood the schema present here as well,
regarding tube circuts.

I am under the impression that the "tube sound" is in part caused by there non
I guess I am wrong or confused or both :-)

hope the book idea regarding tubes ( as noted on the Engineering musician site)
I could use it.

> Many of them have never actually tested a variety of
> tubes for distortion. They make this kind of statement based on the
> cheap, low-quality 12AX7s (which is the only tube type they are familiar with)
> which they tend to use in their cheap, sleazy audio processors.

I'll take your word for it.

> In fact, some small-signal triodes are MORE LINEAR THAN ANY
>         I CAN PROVE IT!!! Any takers?

> The tubes used in these VCOs are nonlinear, which makes them
> useful for unique FM behavior. A low-distortion tube VCO IS possible.......

so the nonlinear nature is in the type of tube, but no tubes in general. That is,
onecan design for degrees of non linearity?

feel free to educate me further, but be kind ;-)

> >combining this with crossed-patched FM might lead to a
> >good sounding chaotic sound system generator.
> In fact, I've done this with my experimental modular system. It works
> amazingly well, and VERY strange things happen as you change the
> tunings of the two VCOs.
> >Can the existing sync in be used some how?
> Yes, but you would have better results by mixing CVs with
> an input op-amp, then going to the grids of the thyratron.

thank for the ideas and clarifications Eric,


Jeff Foster - 415-662-1254
System/Network Engineer, Skywalkersound, Lucasdigital Ltd.

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