Posted by & filed under Filters, Synth DIY, VCAs + Amplifiers.

I typically use the 12-bit MCP4822 DAC to produce a CV for the SSM2164 (or CoolAudio V2164, it’s the same thing) . This is a good, cheap combination. However, the 4822 produces an output of 0-4.095V, whereas the 2164 needs 0-3.3V.

It is possible to use a simple voltage divider to reduce the voltage to the correct level, and it is also possible to add a capacitor to give an RC 6dB/Oct rolloff whilst we’re doing it. However, it’s not entirely straightforward, so this document is to remind me of the process.

The situation is this:

FilterDivider

We have an input from the DAC, a voltage divider, a capacitor across the lower leg of the divider, and the impedance of the 2164’s CV input.

Step One

RbAndImp

First we pick a (speculative) value for Rb, and calculate the actual value in parallel with the impedance. We’ll call the actual value Rc.

The SSM2164‘s CV input has an impedance of 5K. Assuming 100K for Rb gives us:

Rc = (100 x 5) / (100+5) = 500 / 105 = 4.762.

So Rc is basically equivalent to a 4K7. This is the bottom part of our voltage divider.

 

 

Step Two

DividerCalcThe next step is to calculate the top part of the divider, Ra.

We want to divide 4.095V to get 3.3V, so we need a ratio of 3.3 / 4.095 = 0.806.

Our Rc value of 4.762 gives us:

Ra = (4.762 / 0.806) – 4.762 = 5.908 – 4.762 = 1.146

The closest practical value is 1K2, so our divider is 1K2 and 100K.

 

 

Step Three

RfilterThe final part is work out the impedance that the capacitor sees. This is the parallel combination of the top and bottom of the divider, Ra and Rc. We can then use our usual RC filter calc to determine a good capacitor value.

Rf = (1.2 x 100) / (1.2 + 100) = 120 / 101.2 = 1.186

This is very close to our 1K2 value.

Say we’d like a cutoff of 1000Hz:

C = 1 / (2 x Pi x 1186 x 1000) = 134nF

So the nearest values might be 100nF (1341Hz), 150nF (895Hz), or 220nF (610Hz).

Possible filters for the SSM2164

The end result is simple enough. We take 0-4.095V input, we provide a 0-3.3V output, and we have a 6dB rolloff. Any of these filters can go between a MCP4821 or MCP4822 DAC and a SSM2164, or you can design something similar for your own situation using the method above.

FinalFilters

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