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

[Paul Burns (Fitvideo)]

Hi there , I'd see TubeOhm's thread on Mutable Instruments.

Andre built a Moog Ladder Filter from scratch not long ago.

http://www.mutable-instruments.net/forum/discussion/2716/ladder-filter-for-s
hruthi/p1

cheers Paul


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Jean-Pierre
Desrochers
Sent: 16 February 2014 16:12
To: 'synthdiy diy'
Subject: [sdiy] Question about Arp 4012 vs 4035 VCF sub-modules..

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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