vco8 (Axxe VCO)

[Joachim Verghese]

On Sun, 30 Jan 2000, tomg wrote:

> I may have stumbled over something you want.....It's the vco from
> the Arp Axxe.

> http://www.xavax.com/efm/synth/cbook/pics/vco8a.gif

Hi,

This is the VCO design that ARP used for practically all their
synths from 1973 onwards, or so. It also appeared as the 4027-1
submodule in the model 2600; schematic at:

http://www.netcontrol.fi/~jocke/arptech/pdf/4027.pdf

A few questions about tomg's version:

1. Are you sure you got the schematic right in regard to the
retrace FET's gate circuit?

2. ARP used a 3086 array for the comparator NPN pair, which
helps keep the trip point stable. You're design apparently
uses two discrete devices. This might theoretically introduce
stability problems, especially if the two devices aren't
thermally coupled. Have you compared the performance of your
circuit to one that uses an array?

Best,

-joachim