[sdiy] 3046 vco help?

[Harry Bissell]

this looks like the bastard love-child of the ARP 4027 VCO and a Moog Rogue (iirc).
ARP used a FET reset, Moog used the BJT (as is used here). The expo converter looks
like the Rogue (iirc).

Beware, this VCO is (or should be) designed to be connected to sources that do not
get disconnected (as you would in a modular). Floating one or more of the (alleged)
'summing' resistors changes the whole tuning, as there is not a true summing junction at all.
Moog used a 100ohm on the transistor base (iirc) this one will be worse with a 1K.

You'd need to either drive this with an inverting summing amp followed by an inverting opamp,
or buffer each CV in with a voltage follower. I chose the second path when confronted with this trouble,
as it fit my already completed circuit better

H^) harry


----- Original Message -----
From: Magnus Danielson <magnus at rubidium.dyndns.org>
To: synth-diy at dropmix.xs4all.nl
Sent: Thu, 18 Aug 2011 15:08:35 -0400 (EDT)
Subject: Re: [sdiy] 3046 vco help?

Hi Dan,

On 18/08/11 18:34, Dan Snazelle wrote:
> I have been messing around with the EFM 3046 vco
>
> http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/3046_VCO.pdf
>
> i would love to understand how the core circuit works.
>
> (minus the square shaper)
>
> I see 2 differential amps, a fet, a cap and a pnp....how does it work though?
>
> I figure if i could get my head around a simple core like this transistor core, it might make understanding other discrete vcos easier to grasp.
>
>
> Thanks for any help!

The lower left transistor pair is the expo-pair. The collector on pin 1 
pulls current through from the 10 nF capacitor which is then being 
buffered by the J112 FET which essentially acts like a emitter follower.
The output of that is being tossed into the upper pair which acts like a 
comparator which with the 2N3906 acts like a reset of the sawtooth.
Finally, the 2N3904 is the output comparator for the square signal.

There it is. Not too odd setup. The "benefit" of this is the lack of 
high gain in the feedback path, which can make it warmer due to the 
inherent trigger jitter, but explaining that can be a bit tricky, but I 
could go into that if you need me to.

Fun thing. Does it sound good?

Cheers,
Magnus
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-- 
Harry Bissell & Nora Abdullah 4eva