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Common diagram of ARP 4034/4035 VCF wrong
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Common diagram of ARP 4034/4035 VCF wrong
ricko
For anyone interested. The most common schematric floating about the internet for the ARP 4034/4035 VCFs (used in the ProSoloist. Explorer 1 and early Odysseys) shows that the modules has 4 CA3086 transistor arrays.

What is unusual is that for three of the 8086s, pins 1,2,3 (one of the tied transistors) are shown as formed into a diode and going over to the other side of the ladder. This was pretty puzzling to me: what is the purpose? Limit self-oscillation? So I looked at the traces on an actual 4034 module, and that schematic is not what my board has: all that happens is that pins 1,2,3 are shorted together and not used.

So the circuit simplifies to (in the ladder) the same configuration as used in the 4012, which is the same as the ARP 2500's 1006, which is a vanilla Moog ladder.

(I guess the variation between them is in what transistors they use, the signal levels, the impedance the ladder drives. And the capacitor values: if the Moog ladder has decreasing resonance as the cutoff frequency lowers, then a filter using bigger capacitors will presumably pull the resonance response curve more towards lower frequencies, which certainly would account for an audible difference in tone between different vendor's filters: is that what the difference between the Minimoog and Taurus filters is? Using .1uF rather than .68uF caps would all other things being equal move the claimed resonance-fall-off down by half an octave, I think.)

[[Update: I see this schematic problem has been covered in http://www.electro-music.com/forum/post-212483.html ]]
ricko
Actually, after writing that above, I notice there is a potential thing that may make a difference.

The CA3086 datasheet says

" The collector of each transistor in the CA3086 is isolated from the substrate by an integral diode. The substrate (Terminal 13) must be connected to the most negative point in the external circuit to maintain isolation between transistors and to provide for normal transistor action. To avoid undesirable coupling between transistors, the substrate (Terminal 13) should be maintained at either DC or signal (AC) ground. A suitable bypass capacitor can be used to establish a signal ground"

I don't see that ARP has done this. Maybe it is implied, info from someone with a better grip would be nice.

To me, it seems that these "integral diodes" give the circuit in the attached diagram, which may or may not be relevant at the signal strengths listed. Am I going off the rails here?
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