555 vs. 4046 based VCOs

[Dr. K]

Some time ago I built a 40106 based NAND drone synth. 3 oscillators drive 3 NAND gates. Ie, Osc. 1 drives NAND 1 and 3. Osc. 2 drives 1 and 3, and Osc 3 drives 2 and 3.

With careful tuning, it makes some interesting wavering kinds of sound.

The plan for a next step was to rebuild the device, but using VCOs in place of the 40106 inputs, so that I could drive it with a sequencer.

I was going to make a bank of 3 4046 VCOs, and 3 555 VCOs, and have them all switchabe. IE, you could select a 555 or a 4046 VCO for Osc. 1, 2, or 3.

The idea was that with a 555, you could set a base frequency, and the CV would have the pitch go up from there. With the 4046, configured as in the image, the base frequency would be zero. But after reading more carefully, it seems that you can modify the base frequency by placing a resistor on R2. If R2 is greater than R1, the base frequency is greater than zero. (here, maybe a 1Mohm pot)


Is there a reason you would pick a 555 or 4046 VCO over the other? (I'm not, at the moment, concerned with tracking, V/oct tuning... Simply trying to make the pitch go up with a control voltage, and down without one).

The original idea--having 3 of each--would be straight forward in design, but still, that's 3 extra switches, 3 more pots, more stuff to cram into an enclosure. If the 4046 and 555 oscillators did essentially the exact same thing, the whole thing could be greatly simplified.

I like the idea of being able to set the 4046 oscillator's frequency to 0.

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[PHOBoS]

unlike the 555, whos CV input is directly connected to the oscillator circuit, the 4046 has a high impedance CV input.
I believe the 4046 has a much wider frequency range but if you want to control it with a sequencer that might not necessarily be practical.
Of course you can configure it to have the range you want.
Also the 4046 will give you a fixed duty cycle (50%). With a 555 the duty cycle changes when you adjust the frequency,
at least when you use the standard astable configuration. Not sure if that would matter much for what you want to use it for.

The phase locked loop function of the 4046 can also do some fun things but you might want to keep it simple for now.

[Cynosure]

You could wire up a pot as a voltage divider and mix that with the CV from the sequencer. That way you can set the base frequency, and the sequencer's CV will be added to that.