I just built the VOS, and do not understand a few things. According the the schematic diagram V5, the 555 timer is the LFO, and this drives the LED, yet the marking on the case shows the LED as “oscillator 2”.

VCO 1 pitch pot changes the duty cycle of the oscillator so that at the top of the range of the frequency pot the output dies all together. Same with the LFO depth and type. The circuit probably needs some DC decoupling caps in it to stop it from doing this.

The switch marked S3 has no front panel marking at all.

With the exception of the volume pot all of the pots seem to have all of their range compressed at one end. Why are all the other pots logarithmic and not linear?

With all of the range of the pots squashed into such a tiny range the circuit is not much more than a click and squeak generator, tomorrow I am going to replace the pots and do some decoupling to stop DC from killing the output at the extreme range of the pots.

Sorry for being such a dumb-ass, I wasn’t watching what I was doing when I put the panel together, for some reason I had it in my head to swap the LED and switch, so have a laugh at my previous post then ignore it :-)

Looking at some application notes on the 555 to try and see if I can figure out how to deal with some of the DC offset and duty cycle issues, I know that the VOS is what it is, but I can’t help wanting to change a few things to get it a bit mere repeatable and linear, even if it takes away some of the wildness of the circuit. Can’t really de-couple the LFO output as there needs to be some DC passing at low frequencies, and no portamento from caps discharging, but there must be some way to get some of the DC offset out of there somehow.

I’m not really sure what to suggest about the DC offset… are you talking about the DC offset in the final signal? The cap right before the output should deal with that to some extent.

If you want to keep things from dying at the very top of the range, you might try adding another resistor in there so the max knob setting isn’t 0 ohms, but rather 5k or so. Originally the thought on the analog pots is that we hear pitch and volume according to a log, so it most of the time analog pots make more sense for music apps. However, the VOS is crazy. The effect of each knob changes depending on the other knobs… sometimes the effective range is only 10 degrees, sometimes it’s 3/4 of a turn. I’ve been playing around with some 250k linear pots instead of the 500k’s, and some 50’s instead of 100ks. Another part of the picture is that we can’t buy just any value pot, although by putting them in parallel with fixed resistors I guess we could create ideal values. Lots of options.

I’m not sure if you were talking about this or not, but you can still hear the LFO even when it’s off. Putting a 100uF cap across power and ground anywhere will do a lot to fix this (this “defect” happens to be my favorite feature, though). A .1uF ceramic right across the 555’s power ground will really fix the problem. Furthermore, if you power it with a cheap supply instead of a battery, you have to use a cap across power and ground to get it to work at all. This depends on the output resistance of the supply…

We’ve actually been hoping people would start tweaking things and customizing it. That’s the point of open source, anyway :)

Am I talking about the same stuff you are? Eager to see what you come up with on the pots and decoupling.

Yes, tweaking is good, putting some series resistors next to of couple of pots sounds like a good first step, from a quick look last night with the ‘scope it looks like the reason the sounds cut off is because of the duty cycle becoming too short, so maybe a diode across the DIS and THR pins to change the mark-space ratio would be a good experiment, (anode to the discharge pin). I think some DC offsets at extreme settings might be pulling the pitch out of hearing range too, but that is part of the fun I guess.

I haven’t really had time to take good look at everything with the CRO yet and take notes, but it seems like the there is a lot of hot rodding potential here, which would be fun.

Not to sat that the VOS isn’t great just as it is, it is a cool kit and a lot of fun, I just can’t help myself wanting to mess around in there and seeing what other simple mods are possible with this circuit.

Hey SGJ,

We’re totally more than interested in mods and things to the circuit. When I designed the VoS, I was basically approaching things half knowing what was going on with the circuit, and half circuit-bending. I’ve learned a lot since my first few prototypes, and this was the most stable, and interesting configuration of the one’s I’ve built. I’m really interested in any improvements and other mods that can be done to the circuit. I think the VoS has quite a unique sound, due to the nature of the circuitry (no buffering, etc.). Let us know if you find anything cool!

-recompas