Moog ladder filter

[Rick Jansen]

On Analog Heaven there have been several discussions
on the Moog transistor ladder filter. Wether the transistors
should be matched, what value capacitors to use etc etc.
As this seems to be THE classic synthesizer filter I felt it
necessary to have one :)

My circuit is based on the MiniMoog and Prodigy, which have 
about the same filter. The exp converter uses the cheap and 
widely available transistor array LM3046, and is substrate
temperature controlled (luxury!). The 3046 contains five
transistors, two of which are used as a differential pair,
one transistor is used as a heater, another as the temperature
sensor, and the last as a clamp to protect the heating 
transistor. An LM329 voltage reference is used to set the 
temperature to an absolute value. The LM329 itself is very
well temperature stable. Optionally the temperature control 
can be omitted. (Temperature control of a filter isn't as
necessary as in a VCO.)

>From discussions on Analog Heaven and the Prodigy schematic
it was clear the top and bottom transistor pairs in the ladder 
need to be matched within ca. 2mV, which is ensured by using a 
second 3046 array.

I have designed a pcb for the filter too. The first version
is up and running well. I have made some minor changes still,
and am awaiting delivery of that revised board. When that's ok 
I could order more in case anyone is interested. The board is 
a eurocard (16x10cm), which is an industry standard at least 
here in Europe. I have used a DIN41617 31-pin connector, as 
that's the one used in the Formant too, and I wanted this filter 
to be a replacement if necessary. I don't know about the 
availability of eurocards and DIN41617 connectors in the US though.

Power supply required is +15V and -15V.

Gifs of both schematic and board can be found on my WWW pages,

    http://www.sara.nl/Rick.Jansen -> electronic music projects 

In my setup the filter seems to work nicely. It oscillates well
with Q turned up to max, and happily stays in tune with the 
Formant VCO's over at least 5 octaves. This makes me think the
exp generator based on a LM3046 isn't THAT bad, even though that
array isn't particularly suited for this purpose if you look at 
the specs. That'll be a later dicussion though.

Rick Jansen
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rick at sara.nl 
_____________________________________________
S&H's a module and s&h's looking good