[sdiy] voltage controlled filter???

[harrybissell]

Hiya synthplayer...

synthplayer88 at spymac.com wrote:

> Hi guys,
> I was wondering if anyone can tell me whats the different betwen using
> transconductance amp like the CA3080 and the LM13700 or the use of balanced
> modulators/multipliers to control the cutoff frequency of say a state variable
> filter, instead of dual rev log pot?

why a dual ?    Also, the reverse log pot is usually a feature of the Moog ladder
resonance control... the SVF probably does not need the reverse log because they
work opposite.

Moog filter has maximum resonance when there is 100% feedback   (well high, anyway)
SVF has maximum resonance when there is 0% feedback (well low, anyway)

> Is is because dual rev log pot are hard to find?

yes.

> In order to use transconductance amp or multipliers don't you need to generate a
> rev log voltage in order for it to work like a rev log pot?

The reverse log pot is used so that the resonance does not come on suddenly.  You'd
have to
experiment if you are using a VCA (or OTA) to see if you like how the 'feel' is when
you turn
the pot.   You can generate any curve you want... you could use a linear current
source and then
change the shape of the voltage you apply... or use an exponential current source or
a log current
source etc...   you need to play and see what you like best.

I'd start with a linear current source, and see if you like it. Its probably the
easiest to make.

> I am guess people are using dual gang linear pot wiring it like this
>
> http://home.comcast.net/%7Esbernardi/elec/og2/partsub_pots.html
>
> to get the rev log voltage

yes you can do it that way....

> then control a pair of the transconductance amp or
> multiplier with it?

A pot has to be turned by hand... the OTA could be controlled by any voltage
source... LFO,
Env generator, etc...

OTOH the pot has zero DC  offset voltage, zero distortion, unlimited signal handling
ability (well
almost :^) and it is dead simple...

>
>
> Isn't it much straight forward to use a dual gang rotary switch  and work out the
> resistance for cut off frequency?

cutoff frequency and resonance are not the same. If you mean cutoff freq, the SVF
needs two
resistors to tune so you could use a dual gang rotary switch or pot...

>
>
> I guess my question is, why do people use VCF in synth?

subtractive synthesis. Take a harmonic rich waveform and remove the harmonics you
don't want...

> why voltage controlled?

how many hands do you have ???   how fast can you move them all... maybe in opposite
directions
different amounts at the same time :^P

> For modulation prupose?

> yes.
>
> Howcome I don't see people mention the use of balanced modulators, VCAs and
> multipliers instead of the CA3080 and the LM13700?

when many of these circuits were first built, analog multipliers were VERY expensive
and
suffered from a lot of problems.  The OTA had the advantage that it was quite
inexpensive...
the only thing that really matters (at the chip level) is that all the transistors
are matched, something
that comes almost for free in a monolythic IC.

Some folks DO use Multipliers to control VCFs, Roman Sowa (of our list) for example.

H^) harry