[sdiy] Adapting exponential convertor for single supply

[paula at synth.net]

Tom,

  if you don't need an exact log response, I'd go with a square, i.e. 
input*input, it's not that far off a log curve and you can do it with a 
few transistors - 
https://commons.wikimedia.org/wiki/File:Transistor_Multiplier.svg

Paula

On 2017-07-27 10:27, Tom Wiltshire wrote:
> The rest of the circuit is a 12dB/oct VCF (and as Paul P said, there
> are thousands of LM13700 designs that run on a single supply, so
> there’s no problem there) modulated by a PIC-based LFO - a sort of
> auto-wah pedal thing. There’s a simple “Cutoff Freq” control which
> sets the basic position from which the modulation is applied.
> 
> The problem I’m having is that the frequency control and the
> modulation response are linear, so it’s rather touchy at the lower
> end, and then not very responsive at the higher end. So I thought “Ah!
> No problem! I’ll just stick some simple exponential convertor design
> in there” and started hunting for something that might be suitable, or
> which could be bashed into shape.
> 
> As such, it doesn’t have to have brilliant temperature stability or
> accurate tracking. It just needs to turn a linear response into a
> roughly log/expo one to improve the pot feel and the modulation
> response. That diff pair circuit looked like it might be pretty much
> what I was after, although it’s all wired for dual supplies. But
> there’s no *requirement* to use a dual supply for a diff pair, so that
> should be easy to fix.
> 
> Perhaps I should re-phrase my problem: What’s the simplest exponential
> convertor that can accept a 0-5V input, runs from a 0-9V single
> supply, and produces a current suitable for a LM13700?
> 
> Thanks,
> Tom
> 
> PS, Paula: Yeah, I did wonder about sticking an extra look-up table in
> the code to do a expo conversion before I output the LFO, but I don’t
> have any pins left to add the Cutoff control on the digital side, and
> I wouldn’t be able to maintain the full mod depth across the range.
> 
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
> 
>> On 27 Jul 2017, at 06:38, Paul Perry <pfperry at melbpc.org.au> wrote:
>> 
>> There are thousands of single supply guitar effect pedals using 
>> LM13700 &
>> LM13600 op amps, on 9V DC supplies, so it's certainly possible to use 
>> them.
>> 
>> The problem is connecting up with the rest of the circuitry - in audio 
>> gear the
>> coupling caps solve most problems. But with synth stuff it's usually
>> direct coupled & there will be tears. TI actually make a chip 
>> described as a "rail splitter"
>> for this application - not sure if it really is extra useful or just a 
>> way to get a few extra $$.
>> http://www.ti.com/product/TLE2426
>> 
>> paul perry Melbourne Australia
>> 
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