[sdiy] S-trig to V-trig converter questions

[Andre Majorel]

Two essentially identical circuits (the second has a diode in
series with the output) :

  http://machines.hyperreal.org/categories/do-it-yourself/schematics/s-trig
  http://www.blacet.com/S-trig%20to%20gate.gif

I've implemented the second. Powered by +15 V. 2N2222 replaced by
a BC550C.

                                ^
                                |
                                <
                   ^         R3 > 10k
                   |            <
                   <            |
                R1 > 100k       +----->|-----o  V-TRIG OUT
                   <            |   1N4148
                   |   R2    | /
   S-TRIG IN  o----+--^v^v---|<  BC550
                      100k   | \
                                v
                                |
                                |
                               ---

Had to increase R1 to 100 k. The original value of 10 k was too
stiff for the TRIG OUT of my MS-20 ; with it, closing the S-trig
only made the voltage at the base go down a few mV from 667 mV
and the output just went from 60 mV to 80 mV. With R1 = 100 k, the
voltage at the base goes down to 350 mV or so and the output goes
to 14.6 V.

As shown, the circuit works well. If you convert the output V-trig
back to S-trig, you can even do things that wouldn't work otherwise,
like triggering the MS-20 with its pink noise generator or the
output of EG1. However,

- the output is a bit high at 14.6 V. I'd rather have, say, 11 V
  to stay well below a +12 V rail.

- when shorted, the output only sources 0.6 mA, as if its output
  impedance was around 20 k. I'm worried this could limit the
  fan-out.

Why is the output impedance 20 k and not R3 = 10 k ? Is it because
the transistor is not completely closed ?

Would decreasing R3 to 1 k improve that ? But then wouldn't the
circuit sink 15 mA when the S-trig is off ? That seems a little
wasteful for a 1-transistor circuit.

-- 
André Majorel <URL:http://www.teaser.fr/~amajorel/>
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