Converting V-Trig (gate) to S-Trigger

[reve]

Hey gang. So I'm in a bit of a quandary and I'm hoping to draw off your collective expertise.

I built a PAIA Fatman that spits out linear pitch CV and a positive gate. 0v off, 5v On.

I have a Yamaha CS-15D that expects linear pitch CV and negative (moog style, s-trigger) gate. 5v Off, 0v On.

If I hook them up and control the Yamaha through the Fatman's MIDI interface, all is well except that the gates are backwards. Hold a key down, the VCA turns off. Let go of the key, the EG triggers and the note sounds.

So I need to convert positive gate to negative gate. Seems simple, right? My favorite search engine said I should make this transistor-resistor concoction:

http://www.doepfer.de/faq/gen_faq.htm#S-Trig

... but it didn't *quite* work (see below). In desperation, I added a switching diode that I don't think really does anything, as illustrated in this nearly identical schematic:

http://www.synthesizers.com/moog_trigger-convert.jpg

When I say it doesn't quite work, I mean that when a key is depressed the voltage drops from 0.5V to 0V. So in a sense, it's doing what it should. But the 0.5V idle voltage isn't enough -- I reckon it's gotta be closer to 5v for the Yamaha to fire. Furthermore, I don't understand where the idle 5v that the Yamaha wants to see would come from without an external power source.

So I'm trying to figure out whether:
a) this is expected behavior,
b) these transistors are bad, or
c) something else funky's going on.

I would also like to what the heck I can do to make something that works.

Thanks all!

- RM

[blue hell]

reve

the diode is just there to protect the transistor I guess against reversed polarity, doesn't make too much sense to me, the transistor should survive anyway with 100 k.

Anyway, when you just connect a plug to the trigger input (and nothing connected to the plug) what voltage do you then read over the plug pins?

There should be something like 5V I'd think, if not there doesn't seem to be an internal pull up in the yamaha ... which seems to imply it wants an external 5V source ... or it's defect maybe ...

[ian-s]

I have used the moog circuit (without diode) to connect an MC202 to a Minimoog.

The S-Trigger is not so much 5V->0V, more like 'floating'->connected to Gnd, for moog and korg s-triggers at least.

cheers

[reve]

Hi Blue Hell! Thanks for the response. (Long time listener, first time caller here...)

I sorta figured it was a polarity protection diode but like I said, I was deperate to make things happen.

The S-Trig INPUT of the Yamaha floats at -0.78V with nothing connected. I don't think it's a defect; when you plug something into the input the gate produced by the internal keyboard is disconnected. If I press keys nothing happens because it's looking for an external siginal. However, if I manually route the trigger out to the trigger in, it works again. Which to me implies that the Yamaha is performing as expected -- though that may not involve playing nice with other manufacturers' equipment. If other manufacturers float at 5v, that sure serves to answer my "where's the 5 volts coming from?"

g2ian:

But wouldn't it have to drop a certain amount? Wouldn't I need if not 5, 2 volts or something?

[blue hell]

It's good to read when people say they lurked for a long time

But ... hey ... the thing can self test! and it works as well, great!

So when you connect the trig in to the trig out what voltages do you measure for trig-on and trig-off conditions?

There really has to flow some current somewhere in order to be able to detect a trig from a non trig situation, so there has to be a voltage changing as well and a trig input needs some robustness so there should be something very visible under the right conditions.

[reve]

[blue hell]

It looks like you need an external 5V indeed then ... too bad ...

edit: and the pull up resistor on the collector as well of course.

[ian-s]

The S in S-Trig stands for 'switch'. It is not simply an inverted V-Trig.

If you have a normally open foot switch, plugging it into the S-Trig input will trigger the envelope when pressed. No voltage required, just close the contact. The transistor in the moog circuit is acting as a voltage controlled switch. You could use a relay to do the same thing but it would cost more and make little click noises

[blue hell]

[reve]

[blue hell]

Good work

There is no need I think to get it below 0 volt, and it would require a +/gnd/- dual power supply which is even less attractive ... 0.03 V means the transistor gets saturated heavily, I would have expected something like 0.3V, anyway, you'll not get it lower. 1k not working is because not enough current flows into the transistor's base (due to the 100k base resistor), but there is nothing wrong with 4k7.

Maybe you can modify the Yamaha and bring the 4k7 resistor into it connected to it's power supply, it probably has something around 5V somewhere. I don't think it would interfere with the original working.

[reve]