[sdiy] CV mixer trouble

Tom Wiltshire tom at electricdruid.net
Wed May 16 12:02:12 CEST 2012

On 16 May 2012, at 08:24, Neil Johnson wrote:

> Hi,
>> So, now, the question is, how do you get a 5V reference from the -15V rail?
> Well, if you're going to use the -15V rail as a reference (dumb idea
> but lets follow it) then you just need a single resistor.

You're suggesting using a input resistor that is three times as big to mix -15V into the mixer, thereby giving -5V at the mixer, and +5V at the output?

I take your point about using the rails for reference voltages. There are several reasons why it isn't so stupid as it might seem in this particular situation:

1) It's a PIC circuit and the analog power rails are actually the 'clean' power, whereas using the digital 'dirty' +5V rail would be a very different business.
2) The control voltage that this CV mixer produces is quantized to MIDI note values 0-60. So I have a good degree of noise immunity. If I'm in the middle of the 83mV per semitone of a typical 1V/Oct CV, I could probably go 30 or 35mV up or down before I get dangerously close to a semitone step.
3) It's a prototype, so it's a proof of concept at this stage, not a carefully-perfected final design. I can easily put a proper reference on the final one. I probably would have done already, but I don't have one in my parts bin.


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