stand alone build questions

stand alone build questions

Postby Altitude » Wed Dec 11, 2013 6:14 am

so i scoured the manual and this looks like a patchers paradise. My idea is to simply add a control surface board ala MI anushri with regulated power, pots, and jacks. The immediate thing that jumped out as a problem was the waveform select, could that be rigged with a pot to mix the two waves?
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Re: stand alone build questions

Postby guest » Wed Dec 11, 2013 9:01 am

thats an interesting question. im assuming you want to do it all in analog, and not mix via a digital signal. so, the 2 waveforms come out on j6, and you could put a 100kB mix pot on there, and take a 220k resistor from the wiper and send it through a 1uF cap to the VCF input on j8. there are 2 downsides to this approach: 1. the volume wont be entirely consistent for the full rotation of the mix pot. you will get 220k + pot resistance, and the pot resistance changes as you turn it. this might not be a big issue, and you could vary the pot and 220k value to get better results. a smaller pot value helps here. but, 2. there will always be a bit of bleed with this method. the output resistance of the waveform generators is not 0ohms. i dont know exactly what it is, but its less than 10k. so worst case senario, when the pot has one waveform full off, it is still letting a bit through at the ratio (output resistance / pot value). so the worst case would be 10k/100k or 1/10th its original value. i think the output resistance of the saw is much lower, probably closer to 1k, but the square wave is closer to 10k.

the other option is a reverse wound dual pot. so as you turn one down, the other gets turned up. this would eliminate the bleed through problem. you take a 220k from each wiper as before. the pot could go a bit lower, 50k, but to go much lower might affect how the square wave sounds. in this case the, the other end of the pot should go to 5.333V.
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Re: stand alone build questions

Postby guest » Wed Dec 11, 2013 9:09 am

but, if you did want to do it in digital, you could get some other cool effects. setting up a potentiometer controlled ramp wave oscillator (opamp integrator with schmitt trigger, 555, etc) would allow you to vary the frequency, and then run that to a comparator with another pot controlling the comparison voltage, which now controlls the duty cycle. basically a simple pwm oscillator circuit. you send one signal to one waveform select, and then invert it and send it to the other waveform select. you can have a full on or full off by putting a resistor divider from the oscillator into the comparator, so the comparison voltage can go above and below the oscillator voltage. not only do you get lower bleedthrough on the signals, but now you can turn the switch down to audio frequencies and get some crazy audio switching sounds. if you could tune that switching frequency to be a controlled by a sub-oscillator, it might be absolutely ridiculous.

this is the main reason i havent done a case or anything yet. there are just way too many options, and i would be tempted to do them all, and you would need a wheel barrow to haul the synth around.
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