How the heck does the MS-50 filter WORK?

<<marjan>> urekar.m at EUnet.yu
Sun Jul 19 11:51:59 CEST 1998

Sean Costello wrote:

> How the heck does the MS-50 filter work?  I'm looking at the schematic
> (which you can find at my site - and

There was nice explanation of this by Juergen Haible and Martin Czech
last year,so here's paste of their mails (if only newer archives exist) 
to save time :

start quote

by JH
You might look at it from another point of view. Don't start with
a cascade at all, but with a simple one-opamp / positive feedback
LPF. The one with a non-inverting amplifier, two resistors in series
from the filter input to the opamp +input, a cap from the opamp
+input to gnd, and a second cap from the resistor connection point
to the output of the opamp. You surely know what I mean. Now replace 
the two (floating) resistors with diodes for voltage control. Use a
symmetrical configuration at the diodes to avoid offset voltages.
Keep the levels down (both from the input and from the feedback 
path), to keep distortion low. Voila !

by MC

The filter control voltage is differential amplified into a positive
and negative voltage (IC 7 a and b, there's a mistake : two times a
?).  These are fed into two diodes of the CA3019 diode array in order
to convert the voltage into an exponential current (pins 4 and 1). This
is the bias current for the diode ring. Note that the control current
is differential in contrast to the normal ladder aproach.  The signal
comes from amp ic6b via coupling capacitor into pin 6. The diodes
between pin 6 and 5 or 6 and 8 serve as current controlled resistors
(see my last mail "diode ladders"). But unlike a normal cascade, the
first stage capacitor is not differential mounted, there are two
capacitors c11 and c12 to ground (almost, there's a 100 Ohm to
ground).  This is necessary because the audio signal is not
differential, again in contrast to the normal ladder. So the signal at
pin 5 and 8 is lowpass filtered in the first stage.  The second stage
is comprised by diodes pin 5 to 2 and pin 8 to 8 and cap c13. The
signal is amplified by 221.8/1.8 (amp IC4f), the amplitude is limited
to four times 0.6V by the diode feedback network D5-D12.  So far this
is a two stage filter, 12dB/oct. The feedback or resonance is
controlled via pot vr10 (bypassed by R50 to get some comfortable
nonlinear characteristic). The feedback is fed via R48 and C15 into the
first stage. This is why C11 and C12 don't go to ground but to R46 with
100Ohm.  C15 seems to provide more feedback for higher frequencys,
maybe to overcome some bandwith limitations of IC4f?

I've done some spice simulation which shows that all this explanation
makes sense.  But : I don't understand the substraction circuit arround
IC6b. It seems always to be the same signal as "Peak", but 6dB larger.
Is this substraction to limit the amplitude for high resonance in order
to get a more "even" signal under all resonance settings? Depending on
the setting of VR10 the circuit sometimes gives a phase reversed
Peak-Signal, sometimes not.

However, a very different circuit. I wonder if it also sounds so
different.  CA3019 is no longer, the other Harris diode arrays seem to
be not suitable, but it should be possible to select 6 matching diodes
out of my box of hundreds of 1N4148.

end quote


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