Metropolix pitch output increases in voltage when second pitch output is plugged in

If I connect a single Metropolix pitch output to a VCO and tune it, and then plug the other pitch output into another VCO, the original pitch output’s voltage increases by a noticeable amount and I have to return my oscillators.

As a specific example, I’m outputting the lowest C pitch from Metropolix and send pitch output 1 to a Dixie II+. I use a Mordax Data to tune it to C3 at ~130.8 Hz. Now I connect Metropolix’s pitch output 2 to a VCO 1U, and the Dixie’s pitch goes up to about 132.4 Hz. Looking at Data’s voltage monitor mode, the pitch CV initially reads as 0.01V on the Data, and when I plug in the second pitch output it goes up to 0.03V. Note I can’t just plug a “dummy cable” into the Metropolix pitch output. It has to be plugged into another (non-passive) module to cause the voltage shift.

This behavior is fairly consistent regardless of what pitch I am outputting and what I am plugging into, although the exact amount the pitch shift depends on what I am plugging into. For example, running the same experiment as above, if I plug the second pitch output into an Intellijel buffered mult instead of the VCO 1U, the Dixie’s pitch goes up to 132.2 Hz instead of 132.4 Hz. It’s a pretty minor difference but this makes me suspect it has something to do with impedance between the modules. It also doesn’t matter if I patch pitch output 2 first (the behavior is ~symmetrical although the exact voltage numbers probably vary slightly).

I am not too bothered by this behavior because I use multiple sequencers and quantizers and I’ve learned that the only sane way to tune things is to decide your pitch CV → sound source signal paths first, patch them up, and then tune (after the analog VCOs warm up of course). Every quantizer is slightly different, and I have encountered other modules that cause slight voltage shifts depending on how things are patched. However, it is an ongoing frustration with Eurorack that if I decide to make changes to my pitch CV signal path mid-patch, I am going to have to pause and retune oscillators.

I don’t think anything is wrong per se, and I suspect my Metropolix has always behaved like this to a degree, but it wasn’t until I got a Data and started measuring things more carefully that I realized how consistent this behavior is. My Metropolix has changed racks and power supplies along the way, and I’m wondering if my current power situation might be exacerbating the situation. It’s possible it was completely negligible in my old case, but I wasn’t paying enough attention back then to know.

So I’m curious if anyone else experiences this or has any insights into it? Is it normal? Is there something weird with my system? Is there anything I can do to compensate?

Hey! So, I was also using a Metropolix and a Dixie II+, and I didn’t notice what you’re describing (it having anything to do with when things are patched - that definitely wasn’t the case in the experience I had), but I did think the Metropolix’s pitch voltage swung about until I realized that nothing else was doing that, and it was actually my Dixie. So I know you’re saying it happens in reverse, also - meaning whether track 1 or 2 is patched first - but are you also experiencing it on the VCO 1U? I dealt with it for awhile by sync’ing the Dixie to the VCO 1U (whose pitch stability was solid), and eventually unracked my Dixie and remain unsure what to do with it because I’m not sure if there’s actually anything odd about it. Of course this might not be relevant to your case, but I thought I should mention it, as it sounds at least superficially similar.

Sorry, I meant to respond to this but I wanted to investigate more first and haven’t spent a lot of time with my modular the last couple weeks. Thanks for sharing your experience. Anyway, yeah, it happens to me with any VCO. So it seems like it’s Metropolix’s “fault” in my rack. It definitely does not seem like a Dixie-specific problem on my end.

I guess my main question is whether this only became an issue when I changed cases a few months ago. If I get really motivated, I can try putting Metropolix and a couple VCOs in a 4ms Pod case and test in an isolated environment. I’ll report back if I get around to doing that.

I hear you - I was reluctant to even relay my experience as I don’t want to contribute to disinformation out there. I’ve just been too lazy to re-rack my Dixie and figure out what was going on, as that it is an evening of questionable rewards…

Had a similar problem with a used metropolis I bought knowing it was “broken” but bought it anyway suspecting a FW update would do it. The problem was the pitch was only outputting 6.58V or so, all the time. The unit arrived after being in the cold mailbox a few hours, and when i brought it in and tested it , it worked!! for 30min. After going back and forth with the amazing techs at Intellijel, they thought it was most likely the main ATMEga IC on the back of the pcb or the 4066D SMD ic. Of course first, I went for replacing the atmega. nice and easy… that didnt do it, and snd so sent it in. Ended up being that 4066D one but also a couple other small things. all sorted. They mentioned it was probably plugged in backwards at some stage in its life for a bit too long and fried that and EVEN another opamp or two. so yea. send it to them, unless u can test boards and desolder SMD yourself.

Thankfully my Metropolix tracks 1V/oct extremely well. The behavior I am describing simply shifts the pitch voltage by a small (but noticeable) offset, but it still tracks 1V/oct great. So I simply need to turn the frequency knob on the connected oscillator up or down a little when I encounter this situation. Once I’ve re-tuned, it’s fine. It’s a relatively minor annoyance and not a serious problem. Not worth paying for shipping and being without it for a few weeks to round trip from US to Canada for repair.

It just seems odd and I’m wondering why it happens, so I wanted to inquire with the community if anyone else saw anything like this. My experience is once you start putting a lot of modules from different manufactures together in the same rack, weird things can happen. I used to get all worried about this sort of thing and have sent modules back to the manufacturer, who on several occasions tested thoroughly and found no problems. We concluded it may be some sort of power issue in my case. But I have multiple cases with different power supplies, and encounter some strange behaviors in all of them. So it’s probably due to the combination of modules connected to the power supply? It’s all very mysterious :person_shrugging:

What is likely happening is you are changing the ground path impedance when you have connected a new cable. Since this then changes the reference level for voltages, you create a small relative offset.

Modules are connected to a power bus via a ribbon cable (which has impedance) and then into a power bus that has some impedance too. If these were minimised and your module was not drawing much current, then it would be less of an issue.

It gets even more complicated once you start cross patching modules.

Eurorack would also be a lot better if it had dedicated power rails and ground for just the digital modules.

tldr; your module is ok and you are doing the right thing by just adjusting once you make the new connection.

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Thanks for the explanation!