[sdiy] Expo-Con

[Rutger Vlek]

Hi Busby (and René),

I have been working through the same information lately, trying to understand how to set up a new expo convertor for a discrete OTA VCA and VCF. This made me wonder for what kind of application you need the expo? Perhaps that can help me or others make more specific suggestions?

Also, I found that experimenting with different layouts in Spice (circuit simulation program) can help build up an intuition for how such circuits work (but it takes a while to become accustomed to Spice).

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.

With a risk of hijacking this thread, I'd like to ask: how to choose a suitable reference current for a VCA that can switch between linear and exponential response? Half-way the exponential scale is actually very very very low down the linear scale. Is this really the range around which we like the VCA to be most stable? I've had a look at the VCA shootout on Jurgen Bergfors' website, and as far as I can tell non of them has expo's balanced around this half-way-expo point.

Best,

Rutger



On 3 feb 2015, at 11:12, René Schmitz wrote:

> Hi Busby,
> 
> this is only a principal schematic, in practice you would have to have extra circuitry to make it work.
> 
> I0 would be your reference current, which is assumed here to be constant. I is your current output, and Vb is the voltage input to the exponentiator.
> 
> In order to really keep I0 constant, one often uses an opamp. Keeping the collector potential constant and closing the loop on the emitters.
> 
> Maybe the NPN/PNP arp style exponentiator is more what you want.
> Have a look at the vco4069 at my pages for an example.
> 
> Cheers,
> René
> 
> Am 03.02.2015 um 06:11 schrieb Busby Bergson:
>> Hi There -
>> I'm a new user to the group and a new(ish) builder of synthesizers.
>> I've been trying to understand exponential conversion.
>> 
>> I looked at Rene Schmitz's explanation at:
>> http://schmitzbits.de/expo_tutorial/index.html
>> 
>>> From that, I know that I'm not necessarily interested in heat
>> compensation or accurate tracking. So the double-transistor on Ian
>> Fritz's site makes the most sense to use:
>> http://home.comcast.net/~ijfritz/trans1.gif
>> 
>> But I'm not sure how to use it. There are three unspecified
>> connections here. I'm a little confused about what goes where.
>> 
>> Can anyone help?
>> 
>> Thanks,
>> BB
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> 
> 
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