arp avatar and 4075 filter probem

[steven thomas]

Hi Joachim..thanks for your valuable information.

>===== Original Message From Joachim Verghese <jocke at netcontrol.fi> =====
>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.

well I only did this last night ..and to be honest in my haste to test it I 
didnt worry about buffering the inputs to the filters..but clearly thats
the way to go. I noticed that the combined filters were not as responsive to 
cv
as when i hooked them up separately...  Also the three way switch I have has 
quite a few pins, but I am not sure it can cope with switching  3 signals on 
three filters..
so buffering the inputs as you suggest looks like the best option.

>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.

great ..I'll try this.

>> 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.
I did this mod already (from the info on your website)
I didnt hear a dramtic change..but it did sound better.
>It would be interesting to see how you have connected the 40235 and
>4075 to the main board in more detail.

well I will add the buffers and see if i can improve things.
I'll let you know how I get on.


thanks again..steve
>Best,
>
>-joachim

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