Juno/jupiter VCA and resonance CV

I just discovered something surprising.

First of all, my Juno CV runs the output VCA at very low currents, less than 0.2mA, while the LM13700 can be driven up to 2mA.

I have started researching this to see if I can change the CV to get unity gain at 5V on the JP6. For some reason I expected the control circuit to be converting a linear CV to an exponential one, because I didn't think the Juno had it in it to do this elsewhere. But it makes perfect sense I guess, this lets us generate both linear and exponential envelopes in software.

I took a closer look at the juno 106 schematics and discovered that the CV present at R37 in my schema is not 0-5V. It is actually an adjustable voltage, amplified right after the DAC. It is adjustable from min 0.2V/max 10.3V to min 0.32V/max 10.42V.

Now, this means that the max current at the control of the VCA is about 0.3mA, It is not a major change but it is something at least. And without having to worry about changing the response curve of the VCA CV control, I can change the max control current easily.

As for VCA gain adjustment, I can either chose to add a slightly higher than unity gain and adjust this in software, or add a adjustment pot. A CV summing point may be a good idea anyway

PS: Without adjustment, CV below 0.6V will give 0V out due to the diode drop of the transistor.

A closer look at responses for the Jupiter 6 filter

NB: This post was written before i discovered a missing 33k feedback resistor. Results are not correct

To make a good state variable filter with 12/24dB HP, LP and 12dB BP I simulated the JP6 filter and tapped it at various places to look at the responses.

For my simulated circuit, setting cutoff CV to 0.2V (this has nothing to do with 1V/oct CV btw, look at the circuit to see exactly how it is wired) and Resonance CV to 10V (=off, see this post for details), gives the following results:

24dB LP:
- Cutoff: 507Hz
- Resonance (3V CV): 14dB at 953Hz

24dB HP:
- Cutoff 2kHz
- Resonance (3V CV): 13.7dB at 1.05kHz

12dB HP + 12 dB LP
- LP cutoff: 1.77kHz
- HP cutoff: 574Hz
- Center: 1kHz
- 10dB attenuation on top
- Resonance (3V CV): Top has 13dB amplification

12dB LP + 12dB HP
- LP cutoff: 1.77kHz
- HP cutoff: 574Hz
- Center: 1kHz
- 10dB attenuation on top
- Resonance (3V CV): Top has 13.5dB amplification

12dB LP with connection to second stage
- Cutoff: 666Hz
- Base attenuation 1.35dB
- Resonance (3V CV): Top has 6dB amplification over base at 885Hz, base is -1.26dB

12dB LP without connection to second stage
- Cutoff: 666Hz
- Base amplification 1.6dB
- Resonance (3V CV): Top has 7dB amplification over base at 885Hz, base is 1.6dB

12dB LP with connection to second stage
- Cutoff: 1.38kHz
- Base attenuation 1.2dB
- Resonance (3V CV): Top has 6dB amplification over base at 1kHz, base is -1.18dB

12dB HP without connection to second stage
- Cutoff: 1.4kHz
- Base amplification 1.7dB
- Resonance (3V CV): Top has 7dB amplification over base at 1kHz, base is 1.7dB

Moog style ladder filter for the XM8

I have simulated a ladder filter for the XM8. It is based on various designs found around the net - I've studied the original moog circuit, the Schmitzbits one, the Yusynth one, the modified ASM-1 for emphasis voltage control and not least the Memorymoog (which is probably the source for the ASM-1) for the output VCA.

Cutoff CV is 1V/octave, using a 100k resistor to the mixing point at the negative terminal of U10. The point labeled cv in the schematics has a 0.5V/octave response, to allow 10 octaves of control using 5V CV.

U9 tunes tracking, it should be approx 18mV (17.5 - 18.2mV, depending on temperature) for a 1 octave increase.

U4 sets initial frequency (freq when CV is 0). Adjust to whatever you like.

U11 sets output symmetry.

I have suggested two alternatives for R16 and R24 - choose the one with the range you need for the input voltage. the 10Vp.p alternative will probably distort for higher amplitudes while the 20Vp.p is probably more noisy as it attenuates and amplifies more to run the core at the same amplitude.

Simulated circuit

Juno filter for the XM8

UPDATE: This filter circuit has some limitations to its maximum cutoff frequency and high frequency tracking. I will post an updated version that adresses these problems.

I have simulated a Juno filter, based on various designs found on the net. I have made a few deviations from the original Juno filter:

- Frequency tuning happens before the frequency 1V/oct mixing point. The Juno has if after, which means that chaning the tuning also changes the tracking. My change may have messed up other things, no guarantees there. My version makes it easy to set the base frequency over several octaves. NB: The resistor and pot values in my schematics may not be ideal.

- Input audio is attenuated heavily. In my simulations, anything above 1V p.p gets distorted. I have chosen to allow +/- 10V audio. R29/R30 do the attenuation, and changed values for R27 and R26 make up for it in the output stage.

- Resonance control is 0-5V but highest possible resonance equals what we would get from 10V in the juno. This corresponds to a maximum control current of 0.25mA, approximately what I found in the specs in my post about the ba662. I still need to see how this works out i practice. The control curve is different too.

- Output is now in phase with the input

- Filter has unity gain at 5V CV (or slightly above it).

- Freq CV in the schematics is 0.5V/oct to get a 10 octave range from 5V CV, the summing point has 1V/oct when using 100k input resistors.

I have suggested two alternatives for R26, R27 and R38 - choose the one with the range you need for the input voltage. the 10Vp.p alternative will probably distort for higher amplitudes while the 20Vp.p is probably more noisy as it attenuates and amplifies more to run the core at the same amplitude.

R26 may be replaced with a 100k resistor and a 200k potentiometer (for the 20Vp.p version) to be able to tune the output VCA max amplitude.

U15 adjusts tracking, U14 sets initial frequency, U16 adjusts output symmetry.

PS: The filter may be tapped after the second pole (the output of U4) for 12dB instead of 24dB response.

Simulated circuit

0 - 10V resonance (original circuit from Juno 106). 1V/step

0 - 5V resonance, modified circuit. 1V/step

Still to do: check response curve for the VCA CV.

Filter fact

If current per Hz is constant, a doubling of current gives a tone one octave up, because it will double the frequency.

This is independent of what current gives what frequency, i.e. what cap/resistor values are used. Those are only used to set the base frequency/frequency at a given current.

The filters I've researched do not have a quite constant current/Hz ratio. It is however flat enough to use this fact even though the actual cutoff will be slightly off.

(and by frequency I mean the cutoff frequency, where attenuation is 3dB).

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.