arp avatar and 4075 filter probem

[Joachim Verghese]

Hi there,

> As part of my plans to mod the avatar (and make it patchable) , I have
> build and installed the truly excellent 40235 filter...

That's good news, thanks for the vote of confidence.

> But I also like the sound of the 4075 so I have kept that on the
> filter board and installed a three way slide switch on the front panel
> to switch between the 4075/4023/4035 filters.

Good decision. However, there's a potential problem when using the
40235 and not removing the original filter. Namely, the audio and
CV inputs of all these filters are _low-impedance_ summing nodes and
cannot therefore be directly connected together without altering
the gain structure of the final circuit.

If you want the option of using the original filter, you have to
switch the audio and CV inputs as well, so that only one filter
(4075 or 40235) is connected at a time. Chances are of course
that this is what you have done.

Internal to the 40235, I've worked around the problem by buffering
the audio and CV inputs with op-amps, so that these signal do not
need to be switched between the 4023/4035 circuits. Incidentally,
the buffer outputs could be used to drive the 4075 too, in which
case connections could be made as follows:

40235 "SIN" signal via 56K resistor to pin 1 of 4075.
40235 "FBK" signal via 56K resistor to pin 2 of 4075.
40235 "FCV" signal via 100K resistor to pin 4 of 4075.

Needless to say, the above 4075 signal connections would have to be
removed from the main board.

> I guess I could play around with adding additional resistors to the
> freq slider when it feeds the 4075..but I am wondering if its 
> as simple as that ..or is it a design problem with the 4075 itself?

Hmm, the only related problem I know of is the somewhat limited cut-off
range (12 kHz, approx.). This can be fixed by changing four of the
4K7 resistors on the 4075 board to 2K2.

It would be interesting to see how you have connected the 40235 and
4075 to the main board in more detail.

Best,

-joachim