Filter expo converter


am looking to use the LP2 filter design (the MS 20) as part of another project that would include LFOs (two) and a random stepped generator (0 - 5V). I’m confused by the issue of not inputting a negative voltage to the expo when I want it to accept +/- voltages from the LFO which could be made to give -2/+2V.

Can anyone shed any light on this or is the 0-5V only thing from the digital board?

What is “the issue of not inputting a negative voltage”?

The project as it stands runs off +/-5V, has two simple VCOs (LM13700 single amp design - outputs just a triangle wave), two LFOs based on the Roland System 100 LFO and a random stepped generator based on 40106 logic gates (effectively a Psycho LFO as in CGS). What I want to add is expo control for the VCOs, something to extract gates and triggers from the random generator, AR envelope generators, two VCAs and two VCFs. This is all analogue. I’m not too worried about 1V/octave currently (that’s for the next project!) but want to use expo converters for the VCO and VCF to make it more “musical”. It’s basically a music box that “plays” itself from combinations of the LFOs and random generator.

I’m attracted to the LP2 filter as it fits my power supply and has an expo converter, hopefully one I can hack for the VCOs, but I’m confused by the idea that it will only accept positive voltages so any further info on this would be great!

Hi Pichenettes,
everything I’ve read about this filter says all CVs should be in the range of 0-5V. Other stuff I’ve possibly mis-read seems to suggest that negative voltages would cause damage.

The circuit is calibrated for producing a cutoff of ~10Hz at 0V and 15kHz at 5V. That’s because the filter is controlled by a digital circuit whose output range is 0…5V.

It won’t blow up if you provide less or more than that, but do you really need a filter with a cutoff that low or that high? That’s why I mentioned that the CV should be in the +0 to +5V range.

Anyway, it’s not a big deal to change a few resistors values to add an offset or increase/decrease the range.

Er… of course I need that range, doesn’t everyone all the time? (joke) But yes, that’s why I’m looking at this design. As you say I’ll have to play with it and make it work for me.