[sdiy] CV mixer trouble

Harry Bissell harrybissell at wowway.com
Wed May 16 13:16:12 CEST 2012

I'd say using the power supply rails might be OK if you ~own~ the rails (like you say you do, Tom...)
Then you always know what is connected and how stable and clean they are.

The MOOG ROGUE uses its +/- 12V for VCO references...

Other than that, its a really bad idea. I don't ever use the rails of my own system, even to drive
the pots on the front panel :^)   Precision references are not very expensive these days.

----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: Neil Johnson <neil.johnson97 at ntlworld.com>
Cc: synthdiy diy <synth-diy at dropmix.xs4all.nl>
Sent: Wed, 16 May 2012 06:02:12 -0400 (EDT)
Subject: Re: [sdiy] CV mixer trouble

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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