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midiphy Eurorack Expander Modules

latigid on

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What rail is shorted?

I always measure +12V-0V and -12V-0V or any rail to 0V before powering up.

Photos of the board would be useful. Do all op amps have the correct orientation? What about the electrolytic caps?

Check the pin headers J1 and J2 for shorts. 0V pins are in the middle, rails on the outside.

The schematic just has all of the 1206 caps lined up, so it would be easiest to tell you where to look. It is a pretty simple circuit: the +12V is on op amp pins 4 and the adjacent caps (C6/8/10/12), -12V on op amp pins 11 and adjacent caps (C7/9/11/13). 

Check all around the power section C2-C5 and the beads L1/2

R1 is the voltage reference, so check there too. 


Good luck!

Edited by latigid on
minus 12V
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I really don't know how this happened, I managed to miss-orientate all of the opamps! :disgusted: At least a learned now how to desolder smd parts with hot air....

After resoldering them in the correct way, the module shows the two top led in red, the 3rd in green the 4th dim red and 5-8 green again. In the CV configuration page 1st led reacts to cv1, 2nd to cv2, and 4th led to cv3. all the other leds show no sign.

Is it possible that i burnt the opamps? I used the hot air (300 C) for about 1-2 seconds on each opamp.


Edited by workspace
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All good, happens to us all!

Without any input (i.e. DAC is not sending any voltage) all octal LEDs should be red, because 0/2048*gain - 5 = -5V. This also assumes all of the LEDs have the correct orientation. (As a sanity check, you can test the octal board with a binaire board to verify that all LEDs can be controlled.)

It's possible that the op amps didn't appreciate the reverse power or the heat or both. So I would be tempted to replace all TL074s. 

Measure on both sides of R1, do you get a proper +5V reference? If not, consider replacing the LM4040.

Viel Erfolg!


Edited by latigid on
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10 minutes ago, workspace said:

The two sides of R1 have +4.98V and +12.00V so I guess the LM4040 is bad as well.

Thanks for you support!

Sounds about right to me (could be a voltage meter/probe thing)! But if you've already ordered the replacement it wouldn't hurt to swap it.

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  • 1 month later...

Hi everybody,

I’ve finished my aout and everything seems to work as expected until I do calibration procedure:


i begin with the 5v step at 0v using the trimpot .

after that I do 1v, 2v, 3v, 4v steps and when I go back to the 5v step ( with zero calibration correction) the voltage isn’t a 0v anymore.... no matter how many I try to reproduce thoses step. I cannot have a précise calibration and my vcos aren’t tracking properly. ( I use bipolar calibration)

any help will be appreciated :)

Edited by Dimduj
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Not sure I understand fully. You adjust the trim pot to get the scaling as linear as possible, i.e. the trimmer should be set so each octave step is as close as possible to the target voltage. Then you use the SEQ encoders to finely adjust for each volt setting. The DAC can't drive negative bits, so there is a limit to the lowest setting. But the remaining octaves should be very linear.

Did you already see @Hawkeye's video on the calibration procedure?

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Yep, thx for the explanation I thought the trimpot was a kind of offset not a coefficient.

my mistake was trying to set up the trimpot while I’ve values already defined for step 1v, 2v ,... once I setup all the value to zero again calibration was easy and now I got acceptable value for every channel.

is got a precision with 15-20 cent variation between octaves of a channel. Do you achieve more precise values on your side ?

thx for the support 

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@Dimduj can't exactly tell the absolute values, but we performed an "ear-based" tracking test together with TK. - while we've not got the "youngest ears :)", still to us there was no audible pitch difference between a MIDI controlled digital oscillator to a eurorack CV controlled analog oscillator over many octaves tested. So, i think 15-20 cent note variation sounds high and should be audible when listened to closely. We've not yet performed exact measurements (other than those with a DMM) and just can tell what we heard :).

There might be two pitfalls that cause imprecision:

a) Calibration with a DMM, that was not very precise.
b) The analog VCO used as a eurorack module is itself not tracking perfectly, even if supplied with the correct control voltage.

If you calibrated with a good digital multimeter, after calibration, the SEQ should reliably output the "CV" of every octave with a precision of about two millivolts, that's at least what i was able to reproduce when measuring with the DMM. In my super rough :) calculations, if an octave spans over 1000mV, assuming linearity between notes (which is not the case), each semitone of an octave would have about 83mV of "range". Now, if the DAC achieves an output voltage result that is within the range of say +/- 2 mV for every note, the "error" would be within the range of 2/83 or about 2 cent instead of 15-20. That is assuming the conditions a) (good DMM) and b) (good VCO) are met.

More questions regarding your calibration process:

a) did you have to enter high values to trim the actual DAC output voltages of every octave to get close to every octave's target voltage?
I remember not having used correction offsets of values > 20 per octave during the calibration process. If you needed high values there, this would indicate the DAC would loose a bit of its 12bit output precision, as the SEQ would have to do some more internal interpolation to reach the voltage targets.

b) are all of your CV channels behaving similarly? The DAC channels might have slight performance differences.

c) Can you configure a SEQ track to use a CV out channel, then play all of the C notes over many octaves (in SEQ live forwarding mode) and measure again the outputs with your DMM? Are these DMM readings close to the target -4.000V, -3.000V, ... +5.000V volts? This would be a double check outside the calibration menu. 

Have a nice weekend!
Best regards,


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On 03/10/2019 at 2:54 PM, TK. said:

A downstripped MBHP_CORE_STM32F4 module in Eurorack format would be nice-to-have as an alternative solution to the Euroreceiver - maybe with a SCS (6 buttons, encoder, display) - this would allow to run MBCV which gives us a lot of LFOs, Envelopes, sequencers, modulation matrices, etc.

Best Regards, Thorsten. 

On 03/10/2019 at 11:09 PM, TK. said:

Must-have requirements:

  • STM32F407VG (1MB flash)
  • SCS with 6 buttons, 1 encoder and 1 GLCD (so that we can also use the display as a scope)
  • USB Device
  • SD Card
  • SRIO (J8/9)
  • J19 for AOUT
  • 50mm depth
  • HP doesn't matter as long as it still fits into the Pod40X case together with the Expander modules


  • up to 3 additional MIDI IN/OUT (could also be provided as an optional module)
  • USB Host
  • 4 on-board LEDs
  • Audio-DAC

Potential extension modules:

  • SRIO based encoder/ledring modules
  • AINSER64 module (with at least 8, but maybe 3x8 INs?) - inputs should be buffered, protected and maybe also amplified and level-shifted for +/- 5V

Such a module could also cover MBNG for script based processing.

Best Regards, Thorsten.


On 03/10/2019 at 4:04 PM, Antichambre said:

I'm working on something like this, I try to do not exceed 50mm depth. I need it for the HAARP,

And adapt the SCS to the euro format is also a good idea, @latigid on ?

I plan to create another CS based on the work I made for @Phatline with a pot for each parameter and TFT screen, like I did for the HAARP.

Best regards,


I did it... And it works.

  • STM32F405RG (1MB flash), dipCoreF4 based.
  • USB Device/Host(OTG FS)
  • USB Host(OTG HS in FS mode)
  • SD Card(Micro)
  • SRIO (J8/9)
  • J19 for AOUT
  • J15A for serial LCD, 2 CS lines.
  • J18 CAN
  • J4 I2Cx2
  • J5 ADCx2
  • 2 Leds
  • User/Reset Button.
  • USB/EURO Power, auto switching with Euro Prior.
  • Power switch acts on both power source.
  • 44mm depth
  • 12HP



Best regards

Edited by Antichambre
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Just now, latigid on said:

@Antichambre maybe you should start your own thread as to not mix up things, thanks.

It was to complete the discussion about an Euro Core module, discussion which was started here. There's no mix, just a parenthesis. This is fully compatible with your Euro Modules and even made for.
Have a good Sunday my friend.

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  • 7 months later...


I'm about to finish my second A1 CV expander module. So I'll have 8 bipolar (-5 to 5V) outputs, and 8 unipolar outputs (0-10V)

I have 5 digital multimeters laying around, 4 won't give 2 decimals at 49.9K ohm, and the fifth gives a bad output at measuring resistance. So I cannot mach the resistors as described. But for the unipolar outputs I need to mach only 2 resistors instead of 3, and all the resistors are 49.9K. So I think it's good enough, what do you think?!

And do I need to do something special for getting output 9 to 16 working, like chaining from CV1 19a? or jumper to rc2 on the DAC board?

Thanx and a good 2021 to everyone!

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What meters do you have? Some have a "high-res" function that is activated by holding the backlight button (Fluke meters). Another option is to buy resistors with better tolerance (e.g. 0.1%) but they will be more expensive. In the end, the CV is calibrated on the SEQ and has a variable gain with the first op amp stage. So you should be fine.

For more than eight outputs, simply chain the J19 data from the superDAC board. RC1 should still be set for all DACs. 

Best of luck and happy new year!


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Hi, thanks for the answer. I have a Fluke meter, but it's an older one and it doesn't seem to have this high-resolution function. My latest 'china' one did have the function, but it fails on resistance lately. Buying 0.1% was an option if I knew this before but I wanted to finish the modules today. I just soldered the 49.9k in and I think its good :)

I just checked the new CV and gate modules by swapping them with the 'older' ones and they seem to do it on all outputs, so that's great. But if I put everything together I can't get CV2 working. I changed the HW4 file, so the clocks and trigger modules are corresponding with the right shift registers but I don't see an option for activating CV2, I only see this: CV_GATE_SR1   10  The second 'CV2' module seems to spit out a copy of CV1, and the second gate modules seems to do nothing. Am I missing something?

edit: stupid of me, but I chained CV2 from 19 of CV1, not 19a. Now I did chain it from 19a, and CV2 is working ! I activated the corresponding gates by adding;

CV_GATE_SR2   11   in the HW4 file

Edited by Elektruck
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  • 3 months later...

Hi all,

I've been having trouble with my eurorack modules build (couple of mails to Andy and Peter, thanks for the help).

The remaining problem was an incorrect VREF1 on the transmute8 pcb. Now I've botched the desoldering and lifted the trace.

Can someone tell me where I sould connect the VREF1 to? 


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Me again, I've managed to solder the VREF1 on transmute 8. The CV module is working well for a couple of minutes and suddenly begins to jitter. When it does, all the eurorack modules become unstable and send random (I guess) very fast signals. Any help appreciated.

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It is important with older euroceiver boards that the J0 jumper is installed. This grounds the two systems. Is yours grounded? 

To rule out the line drivers, you can bypass them but you'd have to open the SEQ case of course...

Something else to try would be a different power supply e.g. the USB+5V instead of the regulator etc.


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