micromoog s-trig to v-trig

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nateflanigan
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micromoog s-trig to v-trig

Post by nateflanigan » Fri Jun 26, 2020 9:03 pm

Hey, I'm trying to mod the s-trig out to be v-trig on my micromoog.

I started with the set up in this thread but it doesn't work for the mm.
viewtopic.php?t=28086

Thing is, the mm seems to have a kooky implementation of the s-trig circuit. Resting I measure 13vdc on the s-trig out connector, with a key depressed it drops to 0.2 but it does not have continuity with ground. Disconnecting the s-trig wire from that rig I can fake it by clipping the "s-trig point" to ground to close the transistor and get a v-trig from the + supply.

So, is anyone familiar with how the micromoog trig circuitry works? On this very blurry schematic I've attached it looks like it internally converts v to s. Wondering if I can just grab the signal at c701???
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Synthiq
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Re: micromoog s-trig to v-trig

Post by Synthiq » Sat Jun 27, 2020 3:44 pm

First, you can find a better schematic here:
http://dl.lojinx.com/analoghell/MoogMic ... matics.pdf

Technically you have the correct signal at the collector of Q702 as you suggest. My concern is if you connect that node to a jack you will expose some internal nodes to the external world and what could happen if you by mistake connect it to another output. That node also go to the reset inputs of the 556 timer and I'm not sure what would happen if you applied an external -15V signal to the chip when it is supplied from ground and +15V.

To isolate the v-trigger output I would either duplicate the R716/R717/Q702 inverter to drive the jack or use a modification of the last circuit in the post you linked to. Since the s-trigger output is only 13V when high, there is still 2V between the input and the positive supply so the pnp transistor will still turn on so the signal has to be divided down. I would try a 47-68kohm resistor between the input and the base and a 10kohm resistor between the base and positive supply. If powered by +15V, it is unlikely that another output could pull it higher than that so the diode can be removed but I would replace it with a 1kohm resistor to limit the output current when the output is shorted to ground.

nateflanigan
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Re: micromoog s-trig to v-trig

Post by nateflanigan » Sat Jun 27, 2020 4:25 pm

That's excellent information, thank you. I pulled the pcb and fastened a wire to the c701 node. I am indeed getting a +15v signal when I depress a key now. I'll experiment with your modifications to the suggested isolation circuit, but wouldn't a diode in line with the gate signal protect it as well? Also, I'll likely wire up a fixed voltage divider to knock the new v-trig out down to 10v.

Thanks so much for your advice.

Synthiq
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Re: micromoog s-trig to v-trig

Post by Synthiq » Sat Jun 27, 2020 9:32 pm

If you decide to skip the buffer, you can reduce the output voltage to 10V by placing a 20kohm resistor between the collector of Q702 and ground. This will also affect the reset level to the 556 and the inverter stage for the trig inputs, but I believe they should still work.

I think I wasn't completely clear about the buffers and that they are both inverting stages so the input should be taken from the collector of Q801 and not from the collector of Q702.

With a npn buffer, the amplitude can be reduced the same way as above. The output impedance will be 10kohm in parallel with the 20kohm resistor so around 6.7kohm when the output is high and almost zero when low. Both will increase by 1kohm if you place a 1kohm protection resistor between the collector and jack.

To reduce the amplitude of the pnp buffer to 10V, just split the 10kohm collector load into a 3.3kohm and 6.8kohm resistor and take the output between them. This stage will have a 2.2kohm output resistance when the output is high and 6.8kohm when low. Since the loads are normally connected to ground, it is more important to have a low output resistance when generating a high than a low level so the pnp buffer can drive 3 times more load than the npn buffer and doesn't require a separate 1kohm resistor to the jack to limit the output current. The diode only protects the buffer for signals above +15V but not for shorts to ground or negative voltages but it's fine to leave there if it makes you feel better but it will reduce the output voltage somewhat so you may have to reduce the 3.3kohm resistor or increase the 6.8kohm resistor. This stage also has the advantage of only drawing current from the supply when the output is high while the npn buffer draws some current all the time, especially when the output is low.

nateflanigan
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Re: micromoog s-trig to v-trig

Post by nateflanigan » Sun Jun 28, 2020 5:26 am

I think I wasn't completely clear about the buffers and that they are both inverting stages so the input should be taken from the collector of Q801 and not from the collector of Q702.
Got it, that makes more sense to me now. I'm not sure if I have these parts on hand or not, but you've given me plenty of options to play around with, much obliged.

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