MFOS Ring modulator

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dummyplugconspiracy
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MFOS Ring modulator

Post by dummyplugconspiracy »

Weird module. It's not quite what I expected though the MFOS site is pretty candid about its quirks. Here's what seems odd about mine ....

The sine wave is totally inactive until the coarse knob gets to about 1:00. But tops out at about 250hz by the time I get the knob to 3:00. That's an incredibly small range of performance for a seemingly functional potentiometer. I had changed the R46 from 120k to 47k as recommended for 15v builds. I swapped it back to 120k to see if that leveled it out, but no change. Any idea why it's like that? Is it normal? Seems like a limited effect sonically.

I also noticed that when I input a 500hz sine wave for testing I get out a ring tone that is pitched MUCH higher. So much higher that its practically unusable. I can't imagine wanting that effect at that pitch - it's giving me a headache. And turning down the coarse knob has barely any effect since it has such limited sweep. Am I missing something here?

Lastly, I'm not following how input 1 and 2 interact. Input 1 level control only seems to have an effect when input 2 level control is raised too. It appears input one doesn't do anything unless the switch is in combined mode. Is it possible the switch is wired wrong? In the MFOS diagram it shows the resistor tied to middle lug and input tied to bottom lug. I have three lugs since it's a spdt switch so I just ignored the topmost to replicate the diagram. Is that correct?

Seems like all the features are working in at least some configuration but the results just don't seem right. If anyone has some experience with this I'd love your input.

Thanks!
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Re: MFOS Ring modulator

Post by dummyplugconspiracy »

I haven't had a chance to test it yet, but I now see how using the second input as my ring modulation source might have totally different results from using that fixed hi freq Sine oscillator onboard. The switches might make more sense in that dual input configuration too. I will be back at my synth tomorrow to try it. But input is still appreciated!

THANKS
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Paul Cooper
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Re: MFOS Ring modulator

Post by Paul Cooper »

Are you familiar with ring modulators? If not, have a read up on them first as this may help in checking operation. Not suggesting that you have not got a build issue, just that you may be able to make a more meaningful assessment of the functionality.

The sine wave oscillator does not appear to be working correctly but for now, ignore that and use external signal sources. Start by applying a single sine wave into one of the inputs, select combined mode and check for frequency doubling at the output.

Remember that the ring modulator outputs a waveform that consists of the sum and the difference of the two input signals. Thus, in combined mode it simply doubles the frequency as there is no difference.

This module is of proven design, here is the output of a MFOS ring modulator with 100Hz and 400Hz sine waves applied. Note the output contains two peaks of 500Hz and 300Hz, i.e. the sum and the difference of the two inputs. The other peaks have amplitudes more than 50dB below the main peaks as it is a log scale so hardly audible.
MFOS 100+400Hz.jpg
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Re: MFOS Ring modulator

Post by dummyplugconspiracy »

Thanks for the guidance! I'm familiar with ring modulators and I spent a few minutes listening to the audio samples on the MFOS site. I tested with two external oscillators and that configuration seems to be working great. It's definitely being a ring modulator. So I guess my troubles are limited to that internal sine oscillator being way too high and barely controllable. Any ideas?
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Re: MFOS Ring modulator

Post by Paul Cooper »

No ideas except for the usual check your build as fault finding the sine wave oscillator circuit is not so easy. You have not given much for me to go on in way of description, i.e when it starts to oscillate what is the frequency and is the output a good quality sine wave over its operating range?
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Re: MFOS Ring modulator

Post by dummyplugconspiracy »

I will dig into it some more when I can get it back on the bench. The sine wave quality looked perfect on the scope across the whole range. Just very high frequency and super narrow adjustment on the pots. I'll get back to you with specifics.

THANKS
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Re: MFOS Ring modulator

Post by Bodo1967 »

dummyplugconspiracy wrote: Wed Nov 02, 2022 3:57 pmThe sine wave quality looked perfect on the scope across the whole range. Just very high frequency and super narrow adjustment on the pots.
Checking the CVs on the frequency pot wipers and at the CV summing point (that's pin 6 of U1-B) would be a good starting point imho. No need to patch any external CV but just check what the frequency pots do.

The sine looking good indicates that the oscillator as such is working fine and just gets (too) high CV all the time.

HTH
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Re: MFOS Ring modulator

Post by Paul Cooper »

Bodo1967 wrote: Wed Nov 02, 2022 4:13 pm (that's pin 6 of U1-B)
Not there, the inverting input is a virtual ground.
dummyplugconspiracy wrote: Wed Nov 02, 2022 3:57 pm The sine wave quality looked perfect on the scope across the whole range
That's encouraging hence Bodo1967's suggestion of checking the CV circuit.

Simple ohms law calcs will show that with a +/-15V supply, the wiper of the fine pot should have a voltage change of +/- 2.3V. The coarse pot has a voltage change of +/- 9.75V.

The voltage gain of U1-B for the coarse pot is x0.018 and for the fine pot is x0.0009.

If you measure the voltage at Pin 7 of U1-B you should see a voltage change of approx 350mV for the coarse pot (+/- 175mV). For the fine pot, the change is much smaller at around 5mV also checked at Pin 7 of U1-B.

You can repeat the test at Pin 1 of U1-A and you will see the same voltages but with a polarity inversion. i.e. +/-175mV for the coarse pot becomes -/+175mV at Pin 1.

The voltages are theoretical and ignores the parallel effect of R20 & R26 so don't worry if the voltages differ a bit.

If you don't get these readings then check U1 is in the right way around and the resistor values in and around U1-B and U1-A.
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Re: MFOS Ring modulator

Post by Bodo1967 »

Paul Cooper wrote: Thu Nov 03, 2022 10:05 am
Bodo1967 wrote: Wed Nov 02, 2022 4:13 pm (that's pin 6 of U1-B)
Not there, the inverting input is a virtual ground.
We seem to be looking at different schematics then...

U1-B is definitely the summing stage where all CVs from the coarse and fine pots plus the external CV inputs are joined and inverted. Ray did it this way in many designs including the MFOS State Variable Filter from which the sine oscillator in question is derived.
MFOS Ring Modulator module description wrote:The sine oscillator in the SONIC MULTIPLIER is a two integrator and inverter [...] If you look close you will see that this is the state variable filter made to oscillate permanently
MFOS State Variable Filter module description wrote:The summed control voltages are inverted by U1-A
MFOS Sound Lab Ultimate description wrote:The coarse pot provides between about -10V and +4.5V which is fed via the wiper of R3 to voltage summer (op-amp U1-A) input resistor R12.


State Variable Filter
MFOS SVF CV summing excerpt.jpg
Ring Modulator
MFOS RM CV summing excerpt.jpg
Sound Lab Ultimate VCO1
MFOS SL Ultimate VCO1 CV summing excerpt.jpg
MFOS SL Ultimate VCO1 CV summing excerpt.jpg (50.42 KiB) Viewed 302 times
Sources for quotes and schematic excerpts: musicfromouterspace.com (Ring Modulator and State Variable Filter module descriptions; SoundLab Ultimate VCO1 description)
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Re: MFOS Ring modulator

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Bodo1967 wrote: Thu Nov 03, 2022 11:41 am U1-B is definitely the summing stage where all CVs from the coarse and fine pots plus the external CV inputs are joined and inverted. Ray did it this way in many designs including the MFOS State Variable Filter from which the sine oscillator in question is derived.
I don't disagree, it's just that you cannot physically measure the summed voltage at the inverting input U1-B Pin 6 because it is at a virtual ground potential (probably within a few mV of true ground). This is because the non-inverting input is grounded and the virtual short that exists between inverting and non-inverting inputs when in a closed feedback loop circuit like this one. I'm sure there are some clever people on the forum who can explain it in a much better way than I.

Remove U1 then yes, you can measure there but better still, measure at the output and take into account the gain of U1-B (attenuation in this case) which is what I described.

Does that make sense?
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Re: MFOS Ring modulator

Post by Bodo1967 »

Paul Cooper wrote: Thu Nov 03, 2022 12:14 pmI don't disagree, it's just that you cannot physically measure the summed voltage at the inverting input U1-B Pin 6
I could've sworn I had done just this in the past when adding a pitch wheel to my SoundLab Mini :hmm: .
Paul Cooper wrote: Thu Nov 03, 2022 12:14 pmRemove U1 then yes, you can measure there but better still, measure at the output and take into account the gain of U1-B (attenuation in this case) which is what I described.

Does that make sense?
Measuring the output is in any way good for seeing the actual CV, but for finding a mistake in the pots (solder bridge, whatever) it might still be necessary to check the input side :despair: .
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Re: MFOS Ring modulator

Post by Paul Cooper »

Bodo1967 wrote: Thu Nov 03, 2022 12:34 pm Measuring the output is in any way good for seeing the actual CV, but for finding a mistake in the pots (solder bridge, whatever) it might still be necessary to check the input side .
Indeed but one step at a time. The OP is still finding their way in fault finding, initially I'm just trying to isolate the fault to an area before getting them to dive in deeper.

Regardless, thank you for your welcome input.
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Re: MFOS Ring modulator

Post by dummyplugconspiracy »

SOLVED! Somehow I managed to reverse polarity on the transistor at Q2. A mistake so dumb that I could only outdo it be repeating it on Q3. I must have been half asleep. A messy repair since I didn't have spares. But man .... This thing sounds amazing.

Thanks!
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