MFOS Quad Timbre Bank, Eurorack

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Midiot
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Re: MFOS Quad Timbre Bank, Eurorack

Post by Midiot » Fri Oct 30, 2020 8:53 pm

Now that we're beyond that.....

Can anyone explain the need for the 1uf NP (non-polar) "electrolytics" ..??
If these are to simply remove the DC between stages.... what other capacitor types will work ? The smallest BP electrolytics I found were rather "tall", like 11 millimeters tall.
??
Over at MFOS, the page describes...
"Capacitor C8 (1uF aluminum bipolar) capacitively couples the VCA's output to the inverting output mixer (U12-A) via R30 which provides a gain of two due to R126 200K used as U12-A's feedback resistor. Output mixer U12-A's output is capacitively coupled to the output jacks (J9 and J10) wired in parallel.............."
https://passive-components.eu/capacitor ... lications/

It seems electrolytics are not the only choice here.
"We'll be living in all the oceans now."
(type of music I make.... drone atmospheres, deep late-night beats.)

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KSS
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Re: MFOS Quad Timbre Bank, Eurorack

Post by KSS » Fri Oct 30, 2020 9:34 pm

You can make serviceable NP elcaps by putting two -of double the desired capacitance- back to back. Seen often in audio gear.

Or you can use film types. But often these will be as large or larger than the elcaps, and almost always will be more expensive than mfd NPs or the paired 'DIY' version.

Edit: NPs are tall -or long in the case of axial types- because they're simply two caps in the same case. Wired back to back, just like* the DIY version.

*Some *are* slightly different than this simplistic description, but in this use there's not much functional difference. /edit

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Midiot
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Re: MFOS Quad Timbre Bank, Eurorack

Post by Midiot » Sat Oct 31, 2020 1:32 am

KSS wrote:
Fri Oct 30, 2020 9:34 pm
You can make serviceable NP elcaps by putting two -of double the desired capacitance- back to back. Seen often in audio gear.
This is very true.
But is this method useful in this situation ? , because I am trying to use the best caps for the duty, and in the smallest size. This feels like a film type, not electrolytic. I think the design was made when Film caps were not so plentiful ?? maybe ??
"We'll be living in all the oceans now."
(type of music I make.... drone atmospheres, deep late-night beats.)

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KSS
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Re: MFOS Quad Timbre Bank, Eurorack

Post by KSS » Sat Oct 31, 2020 7:23 am

Film caps were more expensive and were larger-sized. Now everything's smaller. But good film caps will still be large in that value. And probably cost more than dual elcaps.

"Best" is a funny word. Especially when it relates to audio coupling on a bank of filters. Fully intended to color the results.

You say puh-tay-toe, I say puh-tah-toe.

Results will be different.<--Though maybe not as much as you expect. "Better" is in the ears of the beholder.

If you can find decent film caps in a size that works, at a price that makes sense, then go with them. But if cost and size matters, dual elcaps might be what you're looking for. Back to that potatos thing. Or is it potatoes? ;)

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Midiot
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Re: MFOS Quad Timbre Bank, Eurorack

Post by Midiot » Tue Nov 03, 2020 9:38 am

""But good film caps will still be large in that value.""

1uf ?? .... not that large. This board asks for 2.5mm leads.
Let's see.....
https://www.mouser.com/ProductDetail/WI ... 6KHQ%3D%3D

I am going to try it (1uf film caps) and we'll see, because I have an identical board with 1uf BP 'lytics already installed.
I appreciate your help and interest !! 8-)
"We'll be living in all the oceans now."
(type of music I make.... drone atmospheres, deep late-night beats.)

jonnyjupiter
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Re: MFOS Quad Timbre Bank, Eurorack

Post by jonnyjupiter » Tue Nov 03, 2020 5:02 pm

Midiot wrote:
Thu Oct 29, 2020 8:47 pm
"The module draws 30mA from both the +12V and the -12V supply."

...seen on this page....
http://musicfromouterspace.com/analogsy ... REBANK.php
Thanks! Couldn't see it for trying. Still can't find the draw for the ASDR EG or VCA though.
Circuit board populated. All the parts for MFOS stuff are stocked by Soundtronics in the UK. I imagine Synthcube will do the same in the US.
5U modular gradually taking shape...

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Midiot
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Re: MFOS Quad Timbre Bank, Eurorack

Post by Midiot » Tue Nov 03, 2020 7:07 pm

I will test the actual current draw, and post the results here.
+30mA seems rather small or weak for eight active chips, hmmmmm. Maybe that's at idle ??
We could look at the opamp spec sheets, and do some maths..... ??
but our best guesses would be surpassed by measuring the actual working chips/ unit.
"We'll be living in all the oceans now."
(type of music I make.... drone atmospheres, deep late-night beats.)

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Midiot
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Re: MFOS Quad Timbre Bank, Eurorack

Post by Midiot » Sun Nov 08, 2020 8:42 am

Circuit board populated. All the parts for MFOS stuff are stocked by Soundtronics in the UK. I imagine Synthcube will do the same in the US.
5U modular gradually taking shape..
Following your build... I hope it works out OK
.
"We'll be living in all the oceans now."
(type of music I make.... drone atmospheres, deep late-night beats.)

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Re: MFOS Quad Timbre Bank, Eurorack

Post by johnstilton » Mon Nov 09, 2020 10:45 pm

I am also following. I built one and am going to start the troubleshooting this weekend because it’s self oscillating uncontrollably

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Re: MFOS Quad Timbre Bank, Eurorack

Post by jonnyjupiter » Wed Nov 11, 2020 2:01 pm

I always procrastinate with the wiring to the front panel.
Busy this weekend, but will get to it shortly.
Will let you know if I have any self-oscillation issues. Any particular filter or the whole device? At all resonance levels? BP and/or LP?

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Re: MFOS Quad Timbre Bank, Eurorack

Post by johnstilton » Wed Nov 11, 2020 6:01 pm

jonnyjupiter wrote:
Wed Nov 11, 2020 2:01 pm
I always procrastinate with the wiring to the front panel.
Busy this weekend, but will get to it shortly.
Will let you know if I have any self-oscillation issues. Any particular filter or the whole device? At all resonance levels? BP and/or LP?
The whole device is. No control of anything just a rail to rail trapezoid full of harmonics. Actually sounds kind of wicked. I havn't even gotten the opportunity to double check the wiring yet. Hopefully this weekend.

There are so few of these out there it is kind of timely that we have incomplete builds at the same time so that we could share readings (if necessary). A couple of wigglers just helped me to troubleshoot a MFOS State Variable Filter so I have a fresh perspective of how Ray's implemented the OTA's in his filter designs .

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Re: MFOS Quad Timbre Bank, Eurorack

Post by johnstilton » Sat Nov 14, 2020 7:17 pm

So it turns out I plotted my pots “backwards” so the outside legs are reversed on all twelve of them. So everything, reequency, resonance, amplitude was full screaming when I thought theY should be off.

One thing I noticed right away was that my signal at BUF is amplified. Did anyone notice that the input buffer has a gain of 10 (?!).

I think that maybe as built that this circuit may not be expecting 5Vp-p signals ? Maybe this as is if for guitar or something. Anyone know?

With my input attenuated and the the controls turned the other way I have been experimenting and it seems like everything is more or less working even though the resonance goes from just there to screaming which also makes me think the initial gain isn’t correct.

I’ve made some cool sounds though: modulating the frequency inputs at audio rates and then sending quick envelopes to the VCAs make for all kinds of complex gongs clangs chimes and pings!

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Re: MFOS Quad Timbre Bank, Eurorack

Post by jonnyjupiter » Sun Nov 15, 2020 7:26 am

No, I didn't realise it had a x10 gain until you said. Went to the MFOS page and it says:
"You can raise the maximum gain of the input buffer by reducing the value of R128 or increasing the value of R129. You can lower the maximum gain of the input buffer by increasing the value of R128 or reducing the value of R129."
Maybe experiment with those 2 resistor values and see what happens? Be sure to let us know.
I'll hopefully get mine wired up next weekend - will pay careful attention to pot pins 1 and 3! Thanks for the warning.

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Re: MFOS Quad Timbre Bank, Eurorack

Post by johnstilton » Mon Nov 16, 2020 3:09 pm

Taking a closer look at the circuit after BUF the signal is attenuated by a voltage divider comprised of R10 680K and R14 1K which is a magnitude of only 0.0014. So if you input a 5Vpp signal the Input buffer tries to amplify it to the rails. Backing off the input level to something closer to the original waveshape (let's say 5Vpp again) will get attenuated to only 4mV at the non-inverting input of the OTA.

This all seems to be as-designed as far as Ray's documentation goes, as he has written that the input to the OTA has to be small to avoid distortion, and with my recent observations with the State Variable Filter: https://muffwiggler.com/forum/viewtopic ... 4#p3386044. My signal on my SVCF at the OTA inputs was only like 1.4mV and that was after altering its own input buffer to be unity gain.

So yeah, I guess the think to do would be to experiment to see what is the best gain factor for the input buffer and then change the value of R10 (and duplicate for the other channels) to find the "best" working level at the input of the OTAs.

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Re: MFOS Quad Timbre Bank, Eurorack

Post by johnstilton » Sat Dec 12, 2020 8:23 pm

jonnyjupiter wrote:
Sun Nov 15, 2020 7:26 am

Maybe experiment with those 2 resistor values and see what happens? Be sure to let us know.
I'll hopefully get mine wired up next weekend - will pay careful attention to pot pins 1 and 3! Thanks for the warning.
JJ did you get your module wired up? If so what did you discover?

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Re: MFOS Quad Timbre Bank, Eurorack

Post by jonnyjupiter » Sun Dec 13, 2020 8:02 am

Not yet - got crazy busy with work. I get off on 18th, so will be back to it that weekend.
I'll post my findings as soon as it's finished.

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Re: MFOS Quad Timbre Bank, Eurorack

Post by johnstilton » Sat Jan 09, 2021 10:23 pm

I left mine alone for a bit to work on other things and recently got back to it.

One of the things that doesn’t seem correct about mine is that the on the CV inputs of the all channels, both the amplitude and cutoff CVs , do not result in a “zeroed” control if the input voltage to that CV if patched to 0V.

For example, a cutoff CV patched to common will wield a 1830HZ sine wave at full resonance (although the HP/BP channel is a lower frequency). An amplitude CV patched to common will yield a bipolar output of 1.7Vrms

The manual controls seem entirely functionally normal though (although as I write this I note that the pots are still wired CCW and I wonder....)

Anyway....maybe something is floating. I’ll try to figure out what this issue is and post if I find anything.

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Re: MFOS Quad Timbre Bank, Eurorack

Post by johnstilton » Sun Jan 10, 2021 12:33 pm

I resolved the potentiometer wiring. The CV issues are all still there. Making some measurements, it doesn’t seem like any amplitude or frequency reaches “zero” unless the input is less than -4VDC.

I’m going to have to get to know these transconductance amplifiers better.

At this point I am also wondering if anyone else has one that experiences the same behavior.

I also wonder if this was how the operation was intended. Maybe it was designed to turn off only near the bottom of a bipolar waveform? LFOs seem to work well in the CV inputs. Also, the AR envelope on my MFOS ultimate is bipolar and rests as a negative voltage, for example. But it’s like of useless for unipolar signals

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