Moog 904A lowpass filter Question

[Dana.Scott]

I really should look at the schematic before I answer the question, But I'll
answer from memory at the risk of sounding stupid.  :-)
One of the pins of the 3086, pin 13 I think, is connected to the substrate
of the IC.
For the transistors in the 3086 to work correctly, none of the other pins of
the same 3086 should ever be more negative than the substrate pin. The extra
transistor should be connected  to pass current to the substrate pin in an
attempt to keep it as negative as possible if there is a circuit fault that
severely unbalances the ladder.

-Dana Scott

> > do you or anyone else know why the ARP 4034  and 4035 modules, which
> > contain the classic Moog filter ladder, have two diode connected
> > transistors connected from one side of the ladder to the other side,
> > across the lowest and third from lowest capacitors?
>
> I think Juergen posed the same question some time back, but we
> couldn't come up with any good explanation for the inclusion of
> those two diodes.
>
> As you say, the signal levels in the ladder are much too low
> to make the diodes conduct, even with a hard-driven signal input
> (as in the Pro Soloist).
>
> Perhaps the board layout is simpler with the extra transistor
> wired like that (3086 pin 13 run via pins 1 and 2) -- can't think
> of any other reason.
>
> -joachim
>