[sdiy] MAX275 Filter Chip - Any Good

[Aaron Lanterman]

On Tue, 8 Mar 2005, Bill Berzinskas wrote:

> Just came across a few Max275 Filters..  I was wondering if these had
> any musical use?  maybe a vcf with selectable poles and routing?  just a
> thought...  anyone know about this?  if i can't use it for VCF, i assume
> i could at least use it for Audio ADC/DAC Filtering?

I was thinking about that too. Take a look at the diagram on p. 11; it
looks like a state variable sort of filter. R3 connects from an output of
an op amp set up as an inverting voltage follower, so you can think of BPO
as a "voltage" output. The input goes through R1 to virtual ground, so R1
essentially turns voltag eto current, so you could think of IN as being a
"current" input. Hence I'd imagine you could replace R3 with an OTA and
get voltage control (actually, current control) that way.

You can apply similar reasoning to R2; hook the input of the OTA to R2,
and the output to BPI, and get VC that way.

Hmmm, you could do the same to R4; hook the output of BPO to the input of
the OTA, and hook the output of the OTA to LIP, and that would give you
control over that.

There would be no advantage to replacing R1; it just turns the input
voltage into a current.

I'm a DSP theoretician who just started re-learning theory a couple months
ago after not looking at it for twelve years, so take anything I say with
a grain of salt.

- Aaron

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