[sdiy] HP from LP ? -> Moog 904B

[Andrew Simper]

Non-linearities in analog gear when altering the current to charge a
capacitor (or voltage for inductor) are equivalent to modulating the cutoff
frequency of the filter.

>From the point of view of the equations consider a basic OTA type low pass
stage, which looks something like:

vcap += cutoff * tanh (input - vcap)

if you look at that you can see that the cutoff current is being scaled by
the tanh function downwards, so this is frequency modulation towards DC. In
particular it is the derivative of the tanh function with respect to the
input voltage that is the modulation shape, which is sech^2, and it is
kinda like a gaussian distribution with a lump in the middle and it tails
away with large negative or positive input:

https://www.desmos.com/calculator/y59uk9mrxe

In the Moog 904B the transistors form a non-linearity which causes the
cutoff to increase. This also happens in an MS20 mk1 high pass filter if
you construct in "properly" by swapping the positions of the transistors
and capacitors.

Cheers,

Andy


On 5 August 2017 at 19:18, Michael Zacherl <sdiy-mz01 at blauwurf.info> wrote:

>
> On 5.Aug 2017, at 7:47 , Andrew Simper <andy at cytomic.com> wrote:
>
> > I've never got my hands on an actual 904B, nor built/breadboarded one
> myself, but I have done a dsp model of one and it does sound very cool.
>
> > The cutoff is modulated upwards when driven,
>
> what do you mean by that?
>
>
> > which to me sound really good for a high pass filter.
>
> m.
>
>
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