[sdiy] Question about Arp 4012 vs 4035 VCF sub-modules..

Jean-Pierre Desrochers jpdesroc at oricom.ca
Sun Feb 16 17:12:03 CET 2014


I'm about to use Arp's VCF sub-modules design
for my next filter DOTCOM modules.
In the past Arp copied the famous Moog transistors ladder filter
design and put it in some of its first 2600 synths.
I read a lot about the very good sounding of this design (Moog's)
so I looked around for some schematics and examples..
I found Moog filter's chematics all around but no PCB's layout
and the hard to find parts values, specific supply voltages drove me to
Arp's sub-modules.
I found out this web link:
http://www.till.com/arptech/modmain.htm

I found 2 possible choice of designs:
The 4012 and later 4035 VCF's.

Here is 4012 VCF description:
http://www.till.com/arptech/pdf/4012.pdf
.4-pole low-pass transistor ladder 
.Temperature compensated exponential control 
.Cut-off frequency range approx. 0 - 35 kHz 
.DC-coupled signal path 
.Replaced by 4072 module

Here is 4035 VCF description:
http://www.till.com/arptech/pdf/4035.pdf
.4-pole low-pass transistor ladder 
.Temperature compensated exponential control

It looks like the only differences between both designs
is the use of 4 x 3086 transistors arrays vs lots of hand matched TZ-81
transistors for the 4012..
Which one should I use ?
The hand matching time I would spend with the 4012 design
is not a problem to me.
I'd have a taste for the 4012 but..

And the last question is:

4012's output buffers AD3958 matched FETs
and the 4035's  output buffer 2n5459 FETs pair...
Could I replace both types with an opamp with FET inputs
like the TL071 or one with more hi impedance front ?
The FET design was created in a time HI impedance FETs input opamps
were hard to find..

Thanks for your help.




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