Cgs synth

me too, i am most grateful for this piece of info. thank you richard!

i've been deliberating for weeks on which way to proceed, ultra-compact stereo sythicon
w one set of controls or start a new frac-rack. i've decided on the frac-rack so i'll be
building 2 complete modules. looks like i'll be ordering my pcbs and bits next week.
can't wait to get started. cheers, -psm

--- In cgs_synth@yahoogroups.com, Larry Hendry <hendrysr@y...> wrote:
> 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@n...> 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