[sdiy] Expo-Con

[Rutger Vlek]

Hi Ian,

Thanks for sharing your opinion. I agree with your view on stability in the higher range and I'm pretty sure Jurgen Haible would have too, given his work on linear 'constant beat rate' detuning. I'll give it a try and see how it works out in a real world prototype.

Best,

Rutger



On 3 feb 2015, at 16:46, 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