[sdiy] TB-303 clone

[Tony Allgood]

>The SC1583 and SC2291 are getting pretty scarce these days and I want
to replace these with CA3046 arrays.

The 1583 is still being made by Toshiba and is a worthy component to
use. The 2291 is more difficult, but you only need it in the filter,
where you can use a 3046 for the top and bottom of the ladder with good
results.

>I was planning on using one array for the VCO and using a
transistorised heater scheme to keep the temperature in the VCO at a
constant elevated temperature.

The Prodigy uses this principle, but the current draw is quite heavy and
was not considered viable for a battery powered unit. Besides a temp co
resistor is a very good choice due to space and cost.

>And I want to use one array for the VCF if possible. I want to do this
by replacing the output diff. amp (around Q21) with an opamp.

If you do this, you will alter the sound of that filter. Use the 1583 or
any other matched pair. The diff amp has a limited frequency response
due to the caps and various impedances in the amp. You can replicate it
with an op-amp circuit but its easier to stick with the TB303's original
design.

>Is this possible and if yes why didn't Roland do this then in the first
place?

Opamps were more expensive than 1583s back then, and they take up more
room. However, Roland did use opamps in the later SH-2000 filter which
was a forerunner to the TB303's diode filter. I think space was the
deciding factor with the TB303.

Regards,

Tony Allgood  Penrith, Cumbria, England

Oakley Modular Synth and TB3030:
www.techrepairs.freeserve.co.uk/projects.htm
My music: www.mp3.com/taklamakan