Pre-distortion Diodes for Moog Ladder Filter

[BJ]

Magnus Danielson wrote:
> 
> >>>>> "MBI" == Michael B Irwin <mirwin1 at istar.ca> writes:
> 
>  MBI> Hi,
>  MBI> Has anyone thought about or tried adding linearizing diodes to the audio
>  MBI> input of a Moog VCF ? This would be similar to the predistortion circuit
>  MBI> used in the CA3280 and LM13600. Presumably this could be used to change
>  MBI> the filter performance from the traditional warm-sounding gentle
>  MBI> distortion ( with plenty of background hiss ) to a cleaner, higher
>  MBI> signal-noise ratio mode ( more like other VCF's). It would be neat to be
>  MBI> able to switch between these two modes.
> 
> Well, I have considered these diodes, only to figure out that they
> would probably not really help all that much. They would help to
> compensate for the tanh distorsion of the input transistor pair.

Agree, I'v tested in reality and it does not gain much, its just a
hassle
to del with it, try instead to balance the ladder instead to minimise
the noise!
 
> However, this is not the same as compensating the distorsion of the
> transistors higher up in the ladder. In Dr. Moog's filterpatent it is
> clearly stated that the signal level is kept so that the signal
> current is much lower than the modulation current in order to keep
> distorsion down. What happends is that the signal will not only
> experience direct distorsion, but also modulate the poles in the
> filter. This is a non-linear filter responce, but it may be nice to
> the ear.

Well nothing new in that, the Moog filter is highly non linear allready
and distorting the filter has allready been done long time ago
whit the results of a "not so nice to the ear" responce" because of 
all the added noise that does to the cirquit!
What seems to bee an gained parameter in theory is in reality drovned in
noise!!

Thats the problem with the Moog filter, "its noisy" as hell and 
as son as levels are shifted you add noise.

Take the MS20 VCA, if that one is feed with a to strong signal it 
either rectifying the signals on various ways or it makes a tri wave
into a sine wave, for those who need a voltage controlled tri to sine
shaper! 


BJ