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9700 MIDI2CV8, TRS connector, ring circuit (transistor o.c.)
https://www.paia.com/talk/viewtopic.php?f=4&t=17
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Author:  PAiA-Scott [ Fri Jan 30, 2009 2:10 pm ]
Post subject:  9700 MIDI2CV8, TRS connector, ring circuit (transistor o.c.)

giulio wrote:
> Hello Scott!
> The trouble has been shot!
> The issue was Q4: I removed Q8 (which I think is useless if I don't use a trs connector) and on my multimeter its hfe was about 120 . I then removed Q4, and its hfe was about 50. I put the transistor that used to be in Q8 in Q4 , and now velocity and gate do not affect pitch anymore!
> I've already managed to tune the midi2v8 and the VCO, so I'll be testing the synth in the next days, and maybe I'll be back with some more bugs...
> I still can't understand what the "rings" of the connectors are intended for...
> Thanks
> Giulio

Hello Giulio.

Good news! But these transistors should not affect the voltage output. They do share a connection with the output voltage through a base resistor, so if the transistor has trouble or a short-circuit, this might be a reason for the voltage output not being normal.
These transistors are Open Collector circuits. A voltage load connected to them is switched to ground or zero-volts when the voltage corresponding to a gate or pulse trigger is active, or, they conduct an amount of current to ground from a voltage source for a varying amount of voltage at the output. When interfacing with a Moog, for instance, the S-trigger connector can be wired to the Ring (transistor o.c.) and the Sleeve (ground) of the TRS (tip ring sleeve) connector on MIDI2CV8 so a gate trigger or pulse trigger ' operates this Switch-trigger input (pulls the voltage present on the input to zero). Or, as shown in the example in the MIDI2CV8 manual page 19, an LED connected through a resistor to a voltage source can have the current through it controlled by the o.c. transistor.

When only using the voltage outputs, you should only wire to the Tip and Sleeve circuits of the MIDI2CV8 trs connectors. It is OK to use a mono TS tip sleeve plug in the MIDI2CV8 trs sockets. This simply connects the ring with the sleeve and the o.c. transistor is joined with ground--it does nothing.

Thank you.

Sincerely, Scott Lee

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