Update.
By swapping transistors around I was able to get better and more similar
response from my two Synthacon filters. I removed all 8 transistors and
measured the gain of each, along with some extras that I had in my parts
box. Then I repopulated the boards with matched parts.
What a difference! Before, the FREQ pot of one VCF would not sweep all
the way to the highest frequency. Now both sweep the total frequency
range, and with the same sensitivity. (I am using a 430K resistor where
it shows 220K on the schematic to the FREQ pot, as suggested by John
Loffink.) It is really nice to have the initial frequencies of the two
VCFs be close when the two FREQ pots are in the same position. The RES
pot responses are now closer, as well.
I found out how to measure the gain (HFE) from a reader idea in
Electronotes #39, page 6, from Terry Mikulic. It's simple. For NPN
(2N2222), connect emitter to ground, a 1.5 Meg resistor from base to
+15V, and put a milliammeter between the collecter and +15V. You'll
typically measure between 1 and 3 ma, which corresponds to 100 and 300
HFE, etc. For PNP (2N2907), use -15V instead. I found a pretty wide
range of HFE, low of 67 to high of 246, for the 2N2222s. My batch of
2N2907 ranged from 140 up to 320. I did not have 4 NPNs that matched
closely. However I matched the pairs in each of the VC circuits. One
pair was 158,161, the other pair 240, 246. Then I matched the output
amplifier gains of the two VCFs. I used an HFE of about 110 for the
2N2222s on the output, and chose an HFE of 320 for the 2N2907s. There
had been quite a difference in the HFE of the output stages of the two
VCFs, before I did this. You could also compensate by using the 'level'
pads on the board to trim the output levels with resistors or trimpots,
but it's nice to have the gain matched by the transistors.
By the way, the BC549 series measures much higher HFE, 500 to 600, which
is why they are not suited to the Synthacon.
I wish I had thought to do this during initial assembly. I highly
recommend it. If you are skillful in soldering, it is not too hard to
change the transistors. I used Chemwick to remove the solder
carefully. I even changed a few transistors twice.
-Richard Brewster
Richard Brewster wrote:
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>One thing you'll need to consider is that the Synthacon VCF response is
>highly dependent on the transistors and diodes. I built two of these in
>a single panel and discovered that with identical parts the response and
>sound is still quite different. I did not match the diodes or
>transistors (a few notes on how to do this would be helpful - hint to
>the list). I treat these filters as different critters. When I do a
>stereo patch, the results are not matched. I don't really mind this too
>much. But if you are expecting parallel, matched response with a single
>set of controls, it is not likely to work well without a lot of
>tweaking. This VCF is not a precision instrument. And it's not
>expected to be.
>
>-Richard Brewtser
>
>ps_minor wrote:
>
>
>
>>hi all, i'm new to this list. i just dicovered ken's excellent diy site today after searching
>>around for kits/pcbs for building a simple stereo filter.
>>
>>i'm thinking of putting 2 synthacon pcb's in a box and using just one set of pots/cv jacks
>>for controls. my objective is to process a stereo signal with one set of controls, cv and
>>everything. is there anything i should consider before proceeding with this? can i just
>>hook-up one set of pots/jacks to both boards, easy as that?
>>
>>thanks, -psm
>>
>>
>>