Output phase, SVF Cell 1 and 2

Cell 1

LP, BP and HP are in phase with the filter input (which is NOT the cell input, but IS the summing point for feedback).

Notch, however, is reverse before and after the notch point. This means it will not work with feedback. I address this further down in this post.

Allpass is in phase before and after the cutoff frequency, and 180 degrees off at the cutoff frequency. I am not sure if this is wrong and has to be corrected, I need to check the expected behaviour, though I believe that adding AP and dry should give a notch, which is what would happen here as the 180 degrees off at cutoff would cancel out the dry signal.

Cell 2

Cell 2, all outputs are in phase with input

The outputs of Cell 2 (as seen by the mux) is in phase with its input (Holds true for LP, BP and HP, and of course for the Cell 1 direct as this is the same as the input to Cell 2).

The op amp after the mux inverts the signal so the output phase of the filter is inverted. 

This affects both the output itself and the filter feedback. The output phase is selectable so that's taken care of there. The feedback on the other hand, is inverted back in the feedback VCA (positive input + inverting op amp).

Feedback VCA inverts signal

Alternative notch

Combining dry (inverted) and BP (unity gain) can give me notch without having to add another op amp (as described here: http://atosynth.blogspot.com/2019/01/jupiter-6-all-pass-filter.html)

Simulating this shows a big difference in the offness of the notch - green is LP + HP, red is dry + BP.

From the simulation, LP + HP results in a -1.2dB down to -36dB notch. Dry + BP results in -19.5dB.

Measured in my circuit I see approx +/-150mV for LP+HP, though it looks a bit noisy. Dry + BP is +/-375mV.

Also, gain when no notch is a little low:

The output is +/-5V. To boost it to +/-5.5V which is the range of the other waveforms, we can use 56k + 3.9k in op amp feedback, making the gain 1.07

Here is the alternative notch circuit:

Output levels, SVF Cell 1 & 2

Cell 1

I've measured the output levels of three different circuits for cell 1 of the SVF.

The results are fairly stable, within approx 0.05-0.1V between cells.

Output of LP, BP, HP and Notch are all around +/-5.5V when input is +/-5.0V. 

As I mix BP with input, and the output should be 2 x BP + Input, we have to reduce the input resistor for input to boost it from 5 to 5.5V approx. 

BP output is between +/-5.3 and +/-5.4 for all cards, so gain needs to be between 5.3/5 = 1.06 and 5.4/5 = 1.08 Experimentiation showed that using a 51k and 1.8k resistor in series, combined with the 56k feedback resistor, gave a good result. The calculated gain for this is 1.06.

Cell 2

I did the same for Cell 2, testing device 2, 3 and 4 against each other, and again they match fairly well.

Raw output is around +/-3.4V with a little offset. BP, however, was too low even after the 100k/67k boost and came out around +/-3V. After some experimenting I chose to replace the 67k resistor with 4.7k + 56k, which brought BP up to the same as the others.

The output after final amplification is +/-5.4 to 5.5V which is approx the same as from cell 1. Not sure if I should adjust this back to +/-5V, as the input is 5V, I guess unity gain is a nice feature so perhaps worth the extra work.

CMOS switches and current switching

After my previous post where I discovered that Douglas Self has some examples where he's switching mixer inputs at the op amp virtual ground, and after talking to the guy on Reddit that made me discover the CH446Q (ARabidSquid, Architeuthis Flux on Tindie), I decided to test it in practice.

I hooked up a DG418 on a breadboard, and used an 78L05 and 79L05 to get a +/-5V supply for the DG418.

I then used a TL072 op amp with +/-12V supplies, with one op amp to buffer the input from the function generator and the second to "sum" (just invert really) the output from the switch, with a 47k negative feedback resistor. 

Version of circuit where input resistor is before the switch

I could now experiment with having the op amp input resistor BEFORE the switch (placing the switch at the virtual ground) or AFTER the switch

An initial test with a 10Vpp (+/-5V) signal showed that it worked in both configurations.

Resistor after the switch, 10Vpp

Resistor before the switch, 10Vpp

I then placed a 20Vpp signal on the version with the resistor after the switch. As expected, the switch clips the signal since it sees the full 20Vpp, but exactly how it clips is interesting. Measured at the input pin (i.e. the output of the buffer), the INPUT is clipped - the top is at 9.16V and the bottom at -5.85. I didn't expect it to be asymmetrical.

Resistor after the switch, 20Vpp. Output (and input) clips

Next up, I moved the resistor to before the input. The switch should now see no voltage swing (though a current passes through, +/-0.2mV at its peaks). This also worked as expected, the output is also 20Vpp (though inverted because of the op amp configuration).

Resistor before switch, 20Vpp passes nicely

This looked very promising. Next up was turning the switch OFF. 

Again, for the 10Vpp signal, it worked as it should for both variations.

Switch off, 10Vpp

I then tried cranking up the signal for the variation with the resistor before the switch, but then "disaster" struck: Once the input signal got higher than the DG418's supplies, it started leaking! With a 13Vpp input, it blocked everything up to 10Vpp. The remaining, however, was passed through, giving the output small "bumps"! 

Resistor before switch, switch off, 13Vpp. Anything above approx 5.1V is passed, giving funny little bumps on the output.

When showing the result to ARabidSquid, he confirmed that this was how CMOS operate, it cannot block anything higher than it's supply voltages.

After seeing this result, I remembered that I read something similar yesterday - someone posted about a similar issue, where a current input made the switch leak when it was turned on. Someone suggested using an SPDT (or two switches in a crosspoint switch) to sink the current. This could be an option, to use a separate bus to sink all input signals that are not in use. However, I think I'd rather stay within the operating limits and keep my signals at 16Vpp max.

Mix bus and channel muting

I'm designing a variation of my pre filter mixer using the CH446Q 8x16 crosspoint switch, and while doing so I started wondering if having the switch after the mix input resistor was a good thing or not. 

Douglas Self to the rescue, seems that switching directly to the virtual ground of the mixer summer is the way to go.

From "Self on Audio" by Douglas Self - Figure A seems to be the way to do things which is what I've done in my new design.

My mixer