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

[Dave Manley]

Andre Majorel wrote:
> 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.
> 

The MS-20 trigger output uses a voltage divider on the output of an 
opamp (configured as a comparator) to drop the trigger output voltage 
from 15V to around 5.5V.  The divider is made with 4k3 and 2k7 resistors
so the original 10K pullup is too strong, and is pulling up the strig 
output so the swing instead of 5.5V -> 0V, is around 7V -> 3V.  The 
MS-20 circuit really isn't an strig output like the typical circuit in a 
Moog, it is really just an inverted voltage output trigger.

With your circuit (100K pullup) the output shorted and strig low, the 
output current should be around 1.4mA and the collector current should 
be 0.  Try measuring the collector or emitter current with the output 
shorted and see what your circuit is doing.  If you see any current, 
perhaps the transistor is bad, or the resistor values are incorrect.

The circuit works as expected in spice using a 3904.

To drop your output voltage you could add a voltage divider like the 
MS-20 assuming the following trigger input has a high impedance.


-Dave