[sdiy] Expo-Con

[Tony Kalomiris]

Ian,

Can you please enlighten me on what you mean by '0' and delta Vbe - do you mean the initial frequency (with no CV input), and the difference between the exponential pairs  Base-Emitter voltages?
As a technician I would agree that HF frequency is more important than tracking (much tougher). I personally like the beating on the low end.
Incidentally I would be curious to know how others are 'tuning' synthesizers these days (my favourite is reference beating and 'listening' (have strobe tuner and counter...)

Appreciate the non-conformist contribution :)

Tony K


On 2015-02-03, at 10:46 AM, Ian Fritz wrote:

> At 07:07 AM 2/3/2015, Rutger Vlek wrote:
>> The last difficulty I had to overcome was how to choose an appropriate reference current. From René's site I got all the equations, and Roman Sowa helped me understand the importance of balancing delta-Vbe (the expo's voltage input basically...) around 0, such that the transistors are always working in a range they're most stable in. However, I manually derived the equation that gave me the actual reference current for my VCF, based on the idea that I wanted optimal stability half-way the exponential control scale.
> 
> I take a slightly different view of this question.  I say accuracy is more important at high frequencies  than at low frequencies.  This is because, relatively speaking, inaccuracies at high frequencies produce faster beat rates relative to a perfect oscillator.  IOW, the frequency in Hz matters, not just the octave-to-octave tracking.  For this reason I set the zero of delta Vbe to correspond to something like 2 kHz.  Higher than this is not often used for fundamental frequency, and at low frequencies beats become too slow to be objectionable. I know this isn't the herd consensus, but it has worked well for me.
> 
> For VCAs stability is not a big concern, since we are not very sensitive to small amplitude changes.  Here S/N is a more important consideration, so I say hit your converter with as much maximum current as you feel comfortable with.  So for and OTA (CA3080, LM13700) maybe something like 1 mA max current, for either log or linear response.  For a discrete design maybe more depending on the transistor.
> 
> Ian 
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