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Sims Audio VCA / flat Version
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Sims Audio VCA / flat Version
This is the flat Version of my VCA Sims. The original Elektor Formant VCA used two 3080 in series. In most cases only one is used and the second one only adds to noise and distortion. This VCA is AC coupled only. The THD in the Elektor Formant VCA is about 1% and the noise to voltage ratio is not that good either. Time to look for a replacement. I wanted a decent DC coupled audio VCA to process audio as well as control voltages. And easy available parts (2014/08). I tried different architectures and then decided to separate audio and DC VCA. With specialized VCA it is easier to reach both goals. I made my own implementation of the Sims-VCA introduced by Mike Sims in the EDN Magazine January 1995. With this architecture it is possible to achieve THD of 0,02%. Unfortunately i can not confirm the statement from Mike Sims that trimming the circuit for minimum THD achieves minimum control voltage feedthrough. Trimming for minimum THD causes an constant DC bias at the output. I have had to add a output capacitor to avoid the bias at the output. And you need test equipment to measure the THD for correct trimming.

The documentation for download can be found in my website.

excellent work, as always.

the sims OTA does not have minimum bleedthrough at the minimum distortion point. you can use a second trimmer to trim the two independently, but you do get a large offset at the output (as youve noticed). a third trimmer can be used to get rid of this offset. there is an example of this in figure 29 here:

there is still a bit of distortion in the sims OTA due to the finite Vin required on the OTA to create an output current, so the input does not act as true "virtual ground" as it would with an opamp summing junction. therefore the input current is not a perfect replica of the input voltage for all amplitudes. this can be fixed with an opamp buffer before this summing junction:

i include this image here, rather than the one in the above pdf, as it has a better compensation network that eliminates a bunch of problems i was seeing with the other.
Hello Guest,

thank you. I know your paper, have read it again and appreciate it a lot. Great work. I have build the VCA with the three trimmers as well but I can't get rid of the offset. I think I must have made a mistake in the implementation. Because I decided to split audio and DC VCA this was not not a problem. Using capacitors fixed it for me for the audio VCA. But i need to have a second look at my other implementation.

I used your VCA schematic with the buffer for my limiter/compressor here:

When I build the standalone VCA (fig. 30) I ran into trouble. This one seems to be a bit on the sensitive site. It reacts sensitive to the routing and I don:t find the time to sort it out. But I will try your version given here. Thank you.
yes, the one shown above is much better, but still has some sensitivities as you described. for my latest project i had a spare OTA, and rather than make a dual OTA VCA, i just used the first OTA as a current source for the linearizing diodes. it was lower noise (~6dB) and more stable than the traditional dual OTA schemes, but had higher distortion (~6dB).
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