[sdiy] MOTM300 is an interesting design

[Rob]

Hi,
This sounds very similar to the Rossum 1972 sawtooth core VCO. The Emu
version uses a dual FET 2n3958 buffer into a LM311 comparator, with a 2n3644
BJT around the capacitor. The LM311 can be gated off and on, to stop the
whole VCO...a cool feature! Theres a fast LM318 as an output buffer which I
replace with an SMD AD825. I'm still working my way through an Emu
waveconverter that translates saw into tri, sine and pulse. Its needing
quite a bit of rework, and the sine wave (from a CA3080) looks more like
ocean waves right now...

Rossum used a lot of dual FET buffers before JFET Op Amps came out, as did
ARP. They can cause problems, so check the soldering and remove any flux
from the PCB. I have been through quite a few 2n3958's whilst prototyping,
so I now put them in 6pin DIL sockets. 

Rossum went onto to redesign his VCO for the Audity in 1980 and replaced the
dual FET with a JFET Op Amp, plus some other minor changes such as trimmer
based HFT and microprocessor based autotune.

Regards - Rob
www.emulatorarchive.com


As a side note - Paul's VCO is a sawtooth core, but the integrator isn't 
quite like anything I've seen. There is no integrating op amp. The cap 
just sort of hangs out, and is buffered by this dual-FET uberbuffer 
between it and the relatively low-impedance input LM319 (it reminds me of 
some early Buchla stuff where he puts FETs before BJT op amps).

And the intergrating cap it discharged with... a 2N4402 BJT???? Not a FET 
or MOSFET? Strange and interesting...

The summer stage looks top notch... all OP275s and such...

- Aaron