Recording problems with eurorack?

Anything modular synth related that is not format specific.

Moderators: Kent, luketeaford, lisa, Joe.

Post Reply
User avatar
nickajeglin
Learning to Wiggle
Posts: 20
Joined: Thu Mar 07, 2019 7:41 pm
Location: Omaha

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.

User avatar
Pelsea
Ultra Wiggler
Posts: 925
Joined: Thu Apr 19, 2018 11:46 am
Location: Santa Cruz CA
Contact:

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 on modular synthesis at http://peterelsea.com
Patch responsibly.
pqe

User avatar
nickajeglin
Learning to Wiggle
Posts: 20
Joined: Thu Mar 07, 2019 7:41 pm
Location: Omaha

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:

Arneb
Veteran Wiggler
Posts: 692
Joined: Thu Jul 04, 2019 4:50 pm

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?

Gonzbull
Learning to Wiggle
Posts: 13
Joined: Sun Nov 23, 2014 6:00 pm
Location: New Zealand

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.

User avatar
thetwlo
Super Deluxe Wiggler
Posts: 4973
Joined: Wed Apr 16, 2008 4:05 pm
Location: East Bay, CA US

Re: Recording problems with eurorack?

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

get a real audio interface?

User avatar
smithjohn
Common Wiggler
Posts: 206
Joined: Thu Jan 31, 2019 4:01 am
Location: Bat country

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.

User avatar
nickajeglin
Learning to Wiggle
Posts: 20
Joined: Thu Mar 07, 2019 7:41 pm
Location: Omaha

Re: Recording problems with eurorack?

Post by nickajeglin » Wed Mar 11, 2020 8:24 pm

Hi again,

I've done some testing, and excluded a couple of things.

1) UMC is ok with non-eurorack sources, as are the monitors, so it's not either of them.
2) Eurorack causes problems with both of the above, so it's probably it.
3) Ripple coming out of the power supply is heterodyning between 50 and 110mV Pk-Pk. Frequency is around 50Mhz. This is consistent even if I disconnect the eurorack from the power supply.

I have not yet tested the audio signal chain with various modules patched in and out.

Power supply ripple is obviously not great, but could potentially be filtered out by inductors on individual modules. Of course most of my modules just have bypass caps, which I dont' think will filter 50Mhz noise. Also I don't think it would cause the audio problems that I have. I'd be happy to be corrected.

Questions:
Could the power supply ripple cause this kind of problem?
Could this be an impedance thing? My diy modules have buffered outputs, but some of them don't have resistors after that. I'm imagining a scenario where reflections from impedance mismatch could do bad things to the ouput opamps.

Next steps: patch random stuff together while listening and monitoring the audio signal on the scope...

Edit: Just to get ahead of some people, the power supply ripple is clearly a problem. What I'd like to know is if it's germane to the audio issues I'm encountering. Don't tell me "your power supply sucks, and you do too" unless you can also give some constructive input.

User avatar
cornutt
Fig juggler
Posts: 1389
Joined: Sun Jan 25, 2009 10:06 pm
Location: Rocket City USA

Re: Recording problems with eurorack?

Post by cornutt » Mon Mar 16, 2020 2:48 pm

To answer your question, I think so. Is your supply linear or switching? If it's linear, it sounds like it's having some kind of regulation problem. Take a look at how the supply's outputs are connected... I know that on my Condor open-frame supplies, there's a "sense" pin that needs to be connected to the output pin or the supply won't regulate.

Switching supplies are known for being noisier than linears. However, I've never heard of one oscillating at 50 MHz. Someone smarter than I am about high frequency will have to advise you on that.
Sequence 15 -- sequence15.blogspot.com

User avatar
nickajeglin
Learning to Wiggle
Posts: 20
Joined: Thu Mar 07, 2019 7:41 pm
Location: Omaha

Re: Recording problems with eurorack?

Post by nickajeglin » Mon Mar 16, 2020 6:03 pm

Thanks for the information. My power supply is very similar to those condor open frames. There are sense pins, but they are connected directly to the + and - 12V outputs. The supply is very old as well, so it may have some parts that aren't operating correctly.

50MHz also seems very high to me. I'm wondering if it's an artifact of bumping against the limitations of my oscilloscope. I need to pull out the manual and make sure that the measurement is accurate.

Someone above mentioned attenuating the input to the UMC. It just occured to me that if I'm overloading the input, then no amount of attenuation at the output is going to help. This might be as simple as adding an attenuator between the rack and the interface...

User avatar
starthief
Super Deluxe Wiggler
Posts: 4605
Joined: Tue Oct 04, 2016 10:39 am
Location: St Louis, MO
Contact:

Re: Recording problems with eurorack?

Post by starthief » Mon Mar 16, 2020 6:46 pm

I use a UMC1820 and it has no noise problems at all, can easily handle Euro levels, has rock solid drivers etc. The only downside of it is a tendency to run hotter than other interfaces I've used -- maybe from packing 8 preamps in a small space?

Not sure if all their UMC line is equally good, but I wouldn't dismiss it just because of the Behringer name.

User avatar
c0rpse
Wiggling with Experience
Posts: 299
Joined: Mon Jun 02, 2014 12:54 pm
Location: Denver, Colorado

Re: Recording problems with eurorack?

Post by c0rpse » Tue Mar 17, 2020 6:32 pm

Maybe your output needs to be buffered better? Maybe the impedance difference with the headphones does some filtering?

It seems a bit unfair to call this thread "recording problems with Eurorack" and not "recording problems with DIY." You might have more luck in DIY forum.
VINO MALO | Designer and Software Developer for WMD

Post Reply

Return to “Modular Synth General Discussion”