Cgs synth

Glad it was useful. Just remember to buy a quantity of 2N2222 and
2N2907 transistors, so you can match them. I suggest 20 or 30 of each.
You can use 2N2222A, as well.

-Richard

ps_minor wrote:

>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
>>>
>>>
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