Linear and Exponential control for a 2162 VCA

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MrKathode
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Linear and Exponential control for a 2162 VCA

After a lot of time, I've decided to finish my first VCA module. I'm using the THAT2162 for the advantage of having two control pins (EC + and EC-), with the idea of using the second pin in the future by a panoramic control for a dual VCA.

The circuit I present here, It's based from various topics that I've seen here about VCAs based on the SSM2164 IC and Irwin linearization. Despite being a bit confusing, seems to work relatively well in simulation. I have broken my head a little looking for a way to get a control between a linear and exponential response that works at least on the simulator. More than math, I use some logic and a lot of bruteforcing values to get results.

One of the problems I see (I am not sure it is really a problem) is that with 100% exponential control and maximum control voltages I get 11.2V pp at the output.

I would love to receive your suggestions, improvements or corrections before testing the circuit in real life on a PCB.
Muff_VCA_sch1.gif
I found the datasheet and the spice models on That co site:

http://www.thatcorp.com/datashts/THAT_2 ... asheet.pdf
http://www.thatcorp.com/Device_Models.shtml
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MrKathode
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Re: Linear and Exponential control for a 2162 VCA

I post the graphics of the response. The CV input is a triangular wave from 0 to 10V (cv imput pot at 71%).
Muff_VCA_lin100.gif
Muff_VCA_explin50.gif
Muff_VCA_exp0.gif
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CLee
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Re: Linear and Exponential control for a 2162 VCA

I'm working on the same thing and just sent out some boards for folks to test. I'll post a schematic when I get a chance

CLee
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Re: Linear and Exponential control for a 2162 VCA

One thing I noticed is that if you use the Coolaudio chip instead of the THAT one you'll need a 51R resistor from SYM to EC+, otherwise your negative going wave will be distorted

MrKathode
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Re: Linear and Exponential control for a 2162 VCA

CLee
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Re: Linear and Exponential control for a 2162 VCA

Here's the circuit I'm working on. The outputs drive the EC+ and EC- inputs of the VCAs in my panner circuit. The dual pot blends from linear to exponential. The diode and transistor keep the voltages between +/- 600mv. The trimmer adjusts the gain of the expo circuit at unity gain. Some component values still need to be firmed up, particularly the 1meg resistor.

There's no reason that my approach is any better than yours, just throwing it out for you to check out.
VC Mixer Channel_PG2 copy.png
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MrKathode
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Re: Linear and Exponential control for a 2162 VCA

It is very nice to see different ways to achieve the same thing I've been working for. I haven't experienced your circuit in simulation yet, but as the cool audio datasheet says, to combine both controls with differential drive is advantageous in order to get better attenuation.
At first I tried to limit the control voltage in EC + to protect the IC using diodes in parallel, but I realized that this affects the attenuation in 0 external voltage. Now, in the worst case, I get -550 mV in EC +. Do you think it is risky?

CLee
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Re: Linear and Exponential control for a 2162 VCA

-550mv should give you around 80db of attenuation, you'd have to see what your real world results are. Using both control ports would be better, like you say, but then you can't use the opposite port for panning. I'm using the transistor like a diode because it's lower leakage. Using a diode there was keeping the voltage from going low enough to get good attenuation but still keep the chip from being damaged.

CLee
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Re: Linear and Exponential control for a 2162 VCA

Wow, MW is acting strange right now...

You can also get more attenuation by having gain through the 2162 and having the opposite attenuation at the current/voltage op amp

MrKathode
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Re: Linear and Exponential control for a 2162 VCA

You are right, it is better to test in real life and then decide if I need to protect at the cost of less attenuation...
Wow, MW is acting strange right now...

You can also get more attenuation by having gain through the 2162 and having the opposite attenuation at the current/voltage op amp
I've already tried that but it didn't work very well, I started to see distortion in the response. I don't know why, but I haven't thought much about that.