Monday, January 21, 2013

Neat Synth Tricks using a 555

I should probably start by saying that this isn't so much about audio purposes, but how a 555 can be used for all sorts of neat stuff for controlling analogue synths and their sound.

First, an oscillator:

How it works is pretty simple: the cap C1 (can be any value, as can all the components shown) gets charged and discharged in a motion similar to that of a saw wave. Physics lesson over.
Not that that was much of a physics lesson. ANYWAY, so what can this be used for? The saw is the same frequency as the square wave, so it can't be used for fat analogue bass sounds, or -- or, can it?
The neat thing about a 555 is that it can be used as a frequency divider. Pull up the datasheet for a 555 and you'll see. If you input this oscillator's square wave into a monostable 555 setup (which outputs pulses on trigger) whose timing cycle (frequency) is set just right, you can make a divide by two, three or more circuit. Actual values would vary depending on your oscillators. Output the monostable's saw wave (same deal as above) and you have a saw below the first oscillator.
More typically, you'd output the above oscillator's saw wave, then use the lower square wave to fatten up the sound. Think Moog bass, but about $999 cheaper. More or less. This is musical, too: say you are playing an A at 440 Hz, the other oscillator will be at 220Hz with a divide by two circuit. That is an exact A, one octave lower. You can use this for sub-oscillators, if that's your thing.
Speaking of sub-oscillators, if you make one I recommend a switching system. You can feed your signal to the timing resistor into a multi-way switch. Each switch point has a certain size resistor on it. Switching between these resistors (after you calculate their values) will allow you to create a switchable-octave sub-oscillator. Not many instruments have this, so it would be an interesting feature.
Neatly enough, this same monostable circuit can be used as your trigger signal from your analogue keyboard. First, set up a comparator circuit to compare the keyboard signal to ground. When you play a note, the voltage will go high, thus creating a gate signal. If your gate/trigger needs are met with this, great! If not...
You can then input this gate signal into a monostable 555, and get a trigger output! Now you have analogue CV (keyboard voltage via the standard resistor chain), gate (comparator), and trigger signals. And, given that we know that those are the three typical signals in an analogue synth, we now have full control.
If you've been reading this blog for long, you also know that I use 555's as normal audio oscillators. This is great if you need saw, triangle, and square/pulse waves, but what if you need more interesting waves?
Yep, a 555 can help you there, too. Used in typical oscillator fashion (as above), it can be used as a timer (which is actually the intended use of a 555). Here is where we delve into lifehacking and such fun.
Grab two 555 oscillators and an N-channel JFET (quite common). Run the first oscillator into the Gate input of the FET. Then, wire the Drain output of the FET after the timing resistor of the second oscillator. Wire Source to ground. Voila, you suddenly have a (rather shoddy) FM oscillator! The first is modulating the second. Be sure to monitor the FET. They like to get hot when you externally switch them a lot.
Or, secondarily, you can take the same two oscillators and wire the outputs in series along a wire, to the output. I guarantee you will get some neat sounds. This is the same premise as my 'HardMix' module.
Heck, combine these two to get just totally freaky. And, well, noisy. Oh, and of course, feel free to use the saw outputs in these situations as well. Very interesting stuff.
I know these are somewhat basic 555 functions, but I've been playing around with them lately. THEY'RE FUN, OKAY??

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