Pre-distortion Diodes for Moog Ladder Filter

[Magnus Danielson]

>>>>> "B" == BJ  <zzynt at swipnet.se> writes:

 >> 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.

 B> Agree, I'v tested in reality and it does not gain much, its just a
 B> hassle
 B> to del with it, try instead to balance the ladder instead to minimise
 B> the noise!

Makes sence.
 
 >> 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.

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

Certainly, there are some "nice" properties in the Moog ladder
especially with late 60thies standards, but several waves of competent
OTAs, VCAs etc. has flooded the market since.

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

Rigth. Say that you dimension the signal to peak at 1 % of the
modulation current, then your signal is -40 dB of the modulation
current. Also, think of the varying noise in the curcuit as we
modulate things. What a mess basically.

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

Oups. But then, I like my MS-20 as it is ;)

Cheers,
Magnus