[sdiy] op amp integrators

[Magnus Danielson]

From: "Thom Ash" <thom.ash at ntlworld.com>
Subject: [sdiy] op amp integrators
Date: Wed, 13 Oct 2004 15:52:00 +0100
Message-ID: <000c01c4b134$35174630$6401a8c0 at HAILESELASSIE>

> hi everyone.

Hi Thom,

> you might remember a while ago i posted about a module i made that
> generates a Lorenz attractor [www.aovt59.dsl.pipex.com/chaos.html for
> more info/mp3s]. anyway i've been thinking it would be cool to add a
> speed control to it and... well, i need some help :/ 
> 
> the speed of the circuit is determined by the size of the three caps;
> each of these works with an op-amp to form a integrator like this
> ['scuse the ascii]...
> 
> 
>       +----||---+
>       |         |
>       |   |\    |
> dx    | G-|+\___|__ x
> dt ---+---|-/
>           |/
> 
> 
> i've been told that for a tuned circuit, the time constant is determined
> by a capacitor and a resistor... where would i put a pot to alter the
> speed?

Inbetween the dx/dt voltage input and the minus input of the op-amp. The output
will balance the minus-input to be at the same voltage as the positive input,
that is at 0 V. The resistor from the voltage input will then have the current
through it proportional to the voltage deviation from 0 V. This current then
charges the capacitor whose charge Q is the integral of current. Please notice
that this type of integrator has a sign-reversal built into it.

A while back I think Jürgen perfected a non-signreversal integrator, but I
don't recall the details. I recall being a part of the discussion but I don't
remember if I where to blame for anything or not.

> also, the fact that there are three seperate integrations that would
> ideally be controlled by one pot is confusing [to me anyway...] would
> this basically require it to be voltage-controlled and have the pot just
> generate a cv? if so, could someone point me in the right direction?

The ASM-1 VCF should give you some good inspiration:

http://rubidium.dyndns.org/~magnus/synths/friends/stopp/

Schematic:
http://rubidium.dyndns.org/~magnus/synths/friends/stopp/asm1vcf.new.pdf

Here you see that the OTAs (CA3080) is used as variable resistors into the
two integrators.

Cheers,
Magnus