Cgs synth

Richard, This is a GREAT piece of information. Thanks for posting
it. I have not built my Synthacon yet. But, I will be sure and
follow your advice, adlong with the stuff sent my John. Looks as if
I will be the benefactor of your R&D. :-)
Best,
Larry


--- Richard Brewster <pugix@...> wrote:

>
> 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:
>
> >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
> >>
> >>
> >>
>
>
>
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