VL1 phat philter envelope idea/question...

[Dj Task Manager]

I like your thinking!  I have spread my electric-butter too thin and thus far have not even opened my VL1. 
Keep us posted on your progress, this could be a damn nifty mod!

[Gordonjcp]

You've got loads of space to fit a Gakken SX1 filter in there.

[airburst]

Thanks for the encouragement folks, I'll certainly report back if I get something working.

I hadn't considered the Gakken filter circuit. I need to take a closer look at it.  Most of what I've read about the Gakken has said the filter is pretty awful, but I guess these things need to be taken in context – given that I'm trying to bodge a simple filter into a cheapo synth I'm not expecting Moog-like marvels. At the moment I'm still favouring some variation on Tom Escobedo's design (on the basis that the "PhatPhilter" sounded pretty neat in the SK8).

My main concern right now is whether the unused half of the op-amp IC is suitable for use in the filter circuit. If Tom Escobedo's circuits are designed for a 9v supply, will any of the component values need changing for a situation like the VL where things are running at lower voltages?

The chip in my VL says LA6355 on it but I can't find a datasheet for that. However I do have a datasheet for the LA6358, which is what is marked on the board. I'm guessing the two are pretty similar and Casio probably just substituted the LA6355 late in the day for supply reasons.

If the datasheet is right then it's a chip that's designed to cope with a pretty wide range of supply and input voltages. So not too much problem there as long as I can sort out the offset of the –3v to –1.2v signal (mentioned earlier in this thread)

However the datasheet also says the output voltage range is 0 to Vcc–1.5v

As I understand the VL, it has a 6v supply but is set up internally to have a sort of 'artificial zero' reference voltage at 3v, with rails at +3v and –3v either side of that. So I'm wondering whether that max out of Vcc–1.5v will be a problem.