[sdiy] Expo converters, lin log etc.

[Magnus Danielson]

From: Karl Ekdahl <elektrodwarf at yahoo.se>
Subject: Re: [sdiy] Expo converters, lin log etc.
Date: Sun, 4 Feb 2007 21:40:27 +0100 (CET)
Message-ID: <909024.53349.qm at web26212.mail.ukl.yahoo.com>

> Hi list

Karl,

> Basically what i was doing was toying with a
> multivibrator, it seems however that it's perhaps not
> suitable for a VCO..

You can do it, but you need to do it properly. You need to ensure both sides
is controlled from the expo, and with no (or small) DC shift. If you use the
CCO core out of a 4046 as inspiration you are closing in.

The multivibrator isn't that suitable normally for CV/CC control. What makes
the 4046 approach interesting is that it mirrors in the current and the state
controls which side of the same capacitor it is being pulled through and with
the help of protection diodes it works. BJ showed that this was possible.
All you really had to do was to sense the capacitor diffrential and the then
you got the triangle and the squarewave comes out the normal VCO output.
This triangel core can then be used in the normal sense.

So, it is possible to go that route, but unless you go down a route similar to
BJs route, it is not very interesting.

You can also shape this core up into a sawtooth core by using the very neat
trick of the Pearl Syncussion SY-1. By feeding the squarewave to the expo-
input the high-period of the squarewave will fast-forward the phase of the
triangle-wave and thus act as the reset mechanism. Naturally would a de Franco
compensation be required, but what else to expect? ;O)

If you don't want to play around with a 4046 for this, it becomes a bit messy.
The "normal" sawtooth method (see for instance my ASM-1 schematics as well as
say MiniMoog schematics) is simpler. You ought to learn yourself that one.
The "normal" triangle variant (see my Buchla schemas as well as the Bergfotron)
is a variant of the sawtooth with a slightly more complex twist, but not an
essentially different solution. The 4046 solutions is an implementation variant
of these.

Cheers,
Magnus