onsdag 26. desember 2018

Minimoog filter research

I'm trying to figure out a bit more about the minimoog filter, to be able to use it in my XM8. I've started by simulating a circuit. I first drew up the Yusynth version, but thought the results were a bit odd so I switched to the Schmitzbits one. I later realised that I probably should have used a 0-10V CV with the Yusynth one to cover the full frequency range, this worked well with the Schmitzbits one.

Anyway, first off I will have a look at the current-to-frequency ratio. Both filters are trimmable so that the initial freqyency offset may be set. To chose a sane offset I will figure out how the core responds to currents. This is the simulated circuit:



I got the following measurements. The -3dB point has been read manually by setting the CV and
running ac analysis with frequencies from 20 to 20kHz in LTSpice. I tried reading the current simultaneously but it seems to vary a bit with frequency. For consistency I did a new run with a fixed frequency of 1kHz and read the current from there.


Plotting the Hz/uA vs frequency gave this:

What we see is that the Hz/uA is a bit higher at the start and then stabilises at around 36Hz/uA (or 0.027uA per Hz).

I tried validating the results by connecting a current source to the common emitters:


Current is what I set the current source to. Expected freq is what I would get if the control current was linear and 27nA/Hz. Measured is what I got. It matches the previous measurements closely.

There is however an important weakness to my measurements. At low frequencies, there is actually a dampening for all frequencies. At 0.84uA the top of the response curve is at -10dB. Similarly, at high currents (500uA) the top is at 1.39dB. This has not been accounted for in the table above. The -3dB falloff is from the TOP of the curve for that current.

Response at 840nA

Response at 500uA



I still think we get usable results, something we can use to set a nice offset current with U3.

Variations

I was curious what would happen if I changed the cap values from Schmitz to Yusynth, i.e. 22nF to 47nF. Turns out that by doubling the capacitance, you also double the amount of current needed per Hz. Not really that surprising when you think of it at it takes twice the current to charge the cap with the same speed.

In practice, this means that by doubling the capacitance, but leaving the rest of the circuit - including the control voltage - the same, the cutoff frequency will drop by one octave - see the last column of these measured values:


I then changed the Yusynth variation to use the input/output stages from the Schmitz version to see how all the small changes affected the circuit. Here is a screenshot of the results:


and the circuits for reference:


Quite interestingly, the Yusynth version has a 5kHz cutoff at 10V CV with similar settings as the Schmitz. Tweaking U3 will change this however, for example, a wiper setting of 0.85 instead of 0.92 will result in a cutoff at 14.6kHz instead.

I noticed that I had unintentionally kept the 56k resistor from the Yusynth circuit in the output part of these. Removing it has no effect on the output.

Replacing the 27k on the input with the 10k from the yusynth on the other hand, increases the output from 1.5dB to 9.9dB. It does NOT change the frequency response however, so attenuation on the input and gain on the output will not change the measurements in this post.

Caps and resistors

In general, increasing or decreasing a capacitor value will change the absolute cutoff value a certain control current will result in. But it probably still holds true that doubling the current will double the frequency and thus give a cutoff one octave up. I assume the same is true for changing the resistors between the steps in the ladder.

The Yusynth calibration scheme tells us to make sure the base of Q16 is at 18.2mV when the CV is at 0V, and that it increases by 18.2mV for every additional volt. This is presumably similar to the 17.5mV I have written about in my doc on the exponential converter, but with a slightly different temperature (without temperature compensation, the expo converter requires 18.12mV per octave at 30 degrees celcius).

When one has this dialed in, we know that one volt increase means one octave up. We then need to set the correct starting point by adding a current, which is done using the U3 potentiometer. In other words, the 1V/oct scaling is independent of the caps and resistors used.

Tuning range

By setting the tuning pot to its extremes, we get:

Yusynth with Schmits i/o (47nF):
wiper 0, CV 10: LP: 31.2kHz (HP: 23Hz)
wiper 0, CV 0: LP: 94Hz
wiper 0.96, CV 10: LP: 94Hz
wiper 0.90, CV 0: LP: 31Hz (but with an initial 9dB attenuation)

Schmitz (22nF):
wiper 0, CV 10: LP: > 100kH (HP: 21Hz)
wiper 0, CV 0: LP: 1.1kHz
wiper 0.96, CV 10: LP: 937Hz
wiper 0.94, CV 0: LP: 37Hz (but with an initial 10dB attenuation)


CV control of the emphasis (resonance)

I originally thought about controlling the resonance using an OTA-based VCA. While googling this, I came across a circuit that appears to be from the ASM-1. Initially, I could not get it working with an LM13700 in place of a CA3080, so I started thinking about other synths with moog filter and patch storage. The Memorymoog came to mind after I failed to find the schematics for the little phatty. I found the sheet, and guess what - it is almost the exact same circuit as the ASM-1. I simulated it and it looks good. The resonance is a bit low but I need to try it in real life to see if it is sufficient.



The filter core itself has a nA per Hz of about 33nA/Hz, or 30Hz/uA. The tracking seems quite linear all the way up to around 5kHz.


There is a difference between the response curves of the minimoog and the memorymoog filters. At high frequencies, the minimoog has a HPF part as well, as high as 100Hz for the most extreme settings. This is not there on the memorymoog.

The memorymoog (right) has no visible HPF part above 20Hz

Filter for the XM8

I've tweaked the ASM-1 filter a bit, to get unity gain, 0.5V/oct response from 0 to 5V (to get 10 octaves from a 5V DAC) and a fairly high resonance. I have not yet breadboarded and tested this but it looks promising.

Resonance
Suggested circuit

I also tried adding an OTA in place of the output opamp, as per the memorymoog circuit. While it works, I get massive distortion at the output, even with very low control currents. I will have to simulate this to see if it is real. The non OTA output may have to be tested to find a suitable gain too, but the values above look promising.

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