[sdiy] EMS VCS3 VCO

[Magnus Danielson]

From: Yves Usson <yves.usson at imag.fr>
Subject: [sdiy] EMS VCS3 VCO
Date: Sun,  2 Jan 2005 20:59:04 +0100
Message-ID: <1104695944.41d8528834d32 at webmail.imag.fr>

> Hi and happy new year !

Hi Yves and a happy new year to you too!

> I intend to build myself a clone of the EMS-VCS3 and before going into
> the hardware, I tried to simulate the behaviour of the VCO1
> with Spice3. The simulation is based on the schematics available at
> Graham Hinton's site. Unfortunately, the bloody thing doesn't oscillate 
> at all ! 
> 
> Has somebody in the list ran such Spice simulation for the same circuit
> and did it work ? Has somebody built the oscillator from this schematic
> and what are the pitfalls?
> 
> Thanks for you advices.

OK, I haven't simulated it in Spice, but I would break it up into pieces and
simulate the circuit in separate parts.

My understanding of the curcuit in short:

Q59 & Q60 - CV input op-amp with Q61 & Q63 for expo-responce.

The current charges the capacitor and Q64 & Q65 buffers and amplifies the
signal. Q67 & Q68 forms the schmittrigger amplifier and Q62 is the feedback
transistor.

Q66 is the saw output amp. Q69, Q70, Q71 and Q72 forms the sine-shaper.

Anyway, I would toss all the output amp and shaper stuff for the moment.
Then I would simulate that the schmitt-trigger works as intended (feed with a
triangle wave). I would also ensure that the CV-input mixer has the propper
output current to charging.

I have been comparing the schematics online with what I have, and difference is
mainly readability.

While being fairly obvious, I hope this little comments never the less are
usefull in having you looking through the simulation model. I could check
against the real thing if you really need to. Toss me the simulation model if
you don't figure it out.

Cheers,
Magnus