Recording problems with eurorack?

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nickajeglin
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Recording problems with eurorack?

Post by nickajeglin » Sat Feb 08, 2020 7:34 pm

Hello everyone, I'm having a hard time getting high quality recordings out of my system.

I have a eurorack setup that is entirely DIY modules. It's fairly noisy because I learned a lot about noise prevention as I gained experience, but for some reason, it's much worse when I try to record or output to a pair of monitors. Here's my basic process, and where I have issues.

I normally setup a patch with headphones plugged directly into 4->1 mixer that's in my rack. I use a mono to stereo adaptor made by soldering a mono jack to one end of a cable, and a stereo jack to the other with both channels paralleled onto the mono signal. In this configuration, there is some low amplitude noise, but nothing objectionable. I can twist knobs, patch wherever I want, etc, and the noise stays roughly the same.

Once I am happy with the patch and want to record, I remove the adaptor and run a mono cable from the mixer to a behringer UMC202HD through a 1/8 to 1/4 adaptor. This is plugged into my laptop via USB. In this configuration the noise is much worse, even at very low signal levels. Adjusting the output level on the UMC causes loud bursts of noise, crackles, and pops as I twist the knob. The bursts seem to be consistently dependent on knob position and the direction I turn it. Additionally, I can hear clipping as well as bleed and crosstalk from other modules that wasn't apparent when I was using the headphones. This is possibly a result of poor module design on my part, and the headphones just aren't sensitive enough to hear it.

These issues also happen (to a lesser extent) when I plug the system into a pair of micca bookshelf speakers that I use as monitors. The issues are also the same if I remove the computer from the loop by plugging the UMC into a USB power brick and listen directly from the output with the headphones.

Does anyone have any suggestions? I recently purchased a 1/8 to 1/4 cable in hopes that removing things from the audio path might help, but I haven't had a chance to try yet. I don't want to start another flame war over safe grounding procedure here, but it is a potential issue. Everything in the system is isolated from ground. The computer (or monitors, or USB power to the UMC202) are powered by 2 prong plugs. The synth power supply case is earth grounded, but the "0v" rail going into the system is not connected to this ground.

Hopefully someone can point me in the right direction to start troubleshooting. I don't have a ton of experience with audio recording equipment, so I'm not even really sure where to start looking.

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Pelsea
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Re: Recording problems with eurorack?

Post by Pelsea » Sat Feb 08, 2020 10:40 pm

No need to go into grounding here—your setup is not optimal but that’s not the cause of the problems you hear. Three possible causes come to mind:

There is some DC offset coming out of your DIY mixer. The Behringer should be able to deal with that, but I’ve learned not to overestimate the quality of that brand.
There is high frequency oscillation out of your system. This often the result of omitting feedback capacitors from an opamp circuit. It’s not necessarily the mixer that’s doing it, because that junk has a way of propagating on the power buses. The culprit may not even be in your patch. You’ll need a scope to hunt that down.
The Behringer is not up to the job. You may be hitting it with too much signal (you didn’t mention an attenuator) or it may have flaky pots or a blown capacitor.
Books and tutorials at http://peterelsea.com
Patch responsibly-
pqe

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nickajeglin
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Re: Recording problems with eurorack?

Post by nickajeglin » Sun Feb 09, 2020 12:43 pm

Thanks Pelsea, DC offset did occur to me.

My DIY mixers are layed out so that you can cut some copper jumpers and add caps on the inputs to make them AC coupled. I did that with one of the 2 mixers, then immediately patched up something in stereo, so one of the channels would still have the offset, I'll switch that one over to AC coupling as well to see what happens.

HF oscillation also is a possibility I haven't considered. All of my modules have 10u (or 100u, I don't remember) bypass caps at the power inlets. My rough understanding is that this should filter out high amplitude, relatively low freq oscillation. From studying other people's designs, I see that some of them also include ferrite beads to filter broadband HF noise. I do have an oscilloscope, so I will investigate further here. How much HF oscillation would it take to cause problems? What kind of amplitude on the scope should be cause for concern? Even though my modules are DIY, they're all fairly clean PCB layouts, so I don't really want to perform surgery to add ferrite beads if I don't have to.

Feedback caps: I am missing them. I only recently learned why they can be important. This is an easy fix, as I can just stack SMD caps on top of the feedback resistors where needed.

I am worried that I'm overloading the behringer. The output level of the synth is probably around +/-5V, potentially spiking up to 12V, not including any DC offset. What I'm hearing does sound similar to the clipping I hear if I overload the diy mixers I made. For some reason, I assumed that the input level pots on the behringer would provide attenuation. Is that not correct? There are 2 switches for each input: line/instrument and "pad". My assumption was that those were just swapping higher value resistances into the signal path to attenuate the input, so there's nothing to overload. If they are doing something more complicated with a pre-amplifier or buffered input behind the panel though, I suppose that could be an issue.

I will read the manual for the behringer. I'd like to set up an A/B test with signals of known and acceptable amplitude to make sure that there's not a warranty claim in my near future.

Thanks for the pointers. I'll keep blundering along until I get it right! :cloud:

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Re: Recording problems with eurorack?

Post by Arneb » Sun Feb 09, 2020 1:09 pm

Maybe the UMC interface is just being shit. Does it work fine with non-Eurorack sound sources?

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Re: Recording problems with eurorack?

Post by Gonzbull » Thu Feb 13, 2020 12:58 am

If you're getting bursts and crackles and noise when turning the knob on the UMC then theres the problem. Id maybe look into the SSL2 or Motu M2 as an interface upgrade. Ive got the SSL2 and its a pretty handy device and it sounds great.

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thetwlo
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Re: Recording problems with eurorack?

Post by thetwlo » Thu Feb 13, 2020 1:31 am

get a real audio interface?

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smithjohn
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Re: Recording problems with eurorack?

Post by smithjohn » Sun Feb 16, 2020 2:43 am

The UMC series seems to be pretty well regarded? :despair:
I'm not having problems with my UMC1820 when recording eurorack, maybe OP has a busted interface or some other problem as speculated above. Probably there are better interfaces out there yes, but wouldn't call the UMC series totally shit.

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