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Moog One synth (and FX) ground loop noise - help!

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Moog One synth (and FX) ground loop noise - help!

Postby bigster » Mon Aug 26, 2019 12:29 pm

Hi all. I'm desperately trying to understand a problem I'm having with a new Moog One synth, which seems power-related, maybe a ground loop issue. Moog support and my retailer (PMT) are being helpful, and really I'm just trying to understand the situation better before going back to them. I'd be really grateful for any thoughts and advice.

To cut a reasonably long story short, the Moog has prominent mains hum/whine-like spikes in its noise floor. They are there in main, sub and insert outputs, and are more pronounced when using an unbalanced lead - though are still there with a balanced connection. They fluctuate when adjusting hardware-related aspects of the synth, like LED or screen brightness, or changing cooling fan mode. They're audible if you crank monitoring levels, though at normal levels are sufficiently low in the noise floor that it's not a deal-breaker (even if it is a big irksome in principle - not a single other synth I own, analogue or digital, has anything like this).

Here's the real problem though: when I run the Moog through any external effects pedals (so inevitably using an unbalanced connection) the spikes get much more pronounced. For example, with a Strymon BigSky reverb, even though that is dead quiet by itself or when used with any other synths. Here's what I see on my MOTU 1248's analyser - absolute loudest signal peaks from the Moog in this setup go to about -36dB, average patches perhaps to around -50dB, so the S/R ratio is not good. Somehow those noise spikes are very insistent too:

Image

Even weirder, some days ago I accidentally jiggled the synth's mains inlet plug (it's a lockable 4 pin XLR-like plug) while repatching some cables, and the spikes totally disappeared. I also discovered that they changed somewhat when simply touching the locking release button. Both of which suggest to me a ground loop issue of some kind. Via Source Distribution PMT sent me a replacement power brick, as the soldering in the original was pretty rough... But the new one does not improve the situation. With the same micro-jiggling it can still produce a spike/whine-free audio output (though never in the plug's fully inserted and locked position, so it's not practicable). This is the 'clean' analyser grab, that gives me a good 70-80dB S/N ratio even for quiet patches, and what I suspect the synth should be producing all the time:

Image

In troubleshooting this I've tried multiple cables, unplugging all other mains in the studio, plugging the One, the effects, the 1248 and my mixer into different mains sockets, trying different IEC cables, comparing it with known good synths (Moog SUB37, Grandmother, VC340, Nord A1 - all quiet as mice), etc etc. Hours of headscratching. Only the One is misbehaving, so I can only really conclude it has a fundamental grounding issue - and then the question is whether it's a faulty unit or inherent in the design (which I could not live with, for my requirements).

Perhaps the only way to sort this out is to compare my synth with another One, and see if they behave differently.
bigster
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Re: Moog One synth (and FX) ground loop noise - help!

Postby Hugh Robjohns » Mon Aug 26, 2019 1:07 pm

If you suspect a ground loop -- and that is quite likely from your description -- you'll need to break the ground path connected via the output output cable screens.

I'd suggest experimenting first with a single output connection, just to test the concept.

If you're using its balanced output, you can isolate the audio ground simply by opening one of the connectors -- ideally the one at the destination end (ie, mixer or interface end) -- and snipping the cable's screen connection which goes to pin 1 (if XLR) or the sleeve (if TRS). Make sure you isolate the screen wire to ensure it can't short against the plug body or the other audio connections.

Alternatively, if you don't want to butcher your cables, invest in a transformer isolation box like the ART DTI or similar.

With the audio cable screen disconnected, the only grounding path can be via the mains power supply's safety earth, and so there can be no loop... So if it is a ground loop issue via the Moog this test should cure it. If so, proceed to modify the other balanced output cable or plug into the other channel of the DTI box.

The same techniques -- cutting cable screens or using an isolation transformer -- should work with the unbalanced outputs.

I'm slightly perturbed that the problem goes away if you wiggle the DC power connector on the synth. Presumably the reason is that the ground connection in the power supply plug is temporarily lost... but it doesn't say much for the integrity of that connector!

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Re: Moog One synth (and FX) ground loop noise - help!

Postby bigster » Mon Aug 26, 2019 1:21 pm

Really appreciate that Hugh. I'm borrowing an isolation box from a colleague tomorrow so that should yield up some more info.

I mustn't give the wrong impression about the power connector. The 'jiggling' aspect came from good old-fashioned user error, whereby I'd pushed in the plug but it hadn't fully locked. When it is locked, no amount of jiggling will disturb it.

Looking at the situation more broadly, and assuming that lifting an audio ground does solve the problem, would you say it's weird that just this one synth is involved in a ground loop. All others here produce no such problem. Should I have to be messing around with lifted earths on a £7000+ synth? Or to put it another way, is this reasonable behaviour, or does it point to an underlying problem with the individual unit?
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Re: Moog One synth (and FX) ground loop noise - help!

Postby Hugh Robjohns » Mon Aug 26, 2019 3:04 pm

Glad to hear about the power connector.

Ground loops are almost guaranteed when using unbalanced connections between Class-1 (mains-earthed) devices within complex studio setups with lots of signal paths. The only thing a synth designer can do about that is make the device class-2 (double insulated) and ground free -- and many do exactly that.

Balanced connections shouldn't suffer ground loops problems though -- that's one of the benefits of using them -- so it's odd that you're having issues when connecting the balanced outputs (I assume you didn't have any other connections to the synth at the same time?).

However, if it really turns out to be a ground loop problem it's more likely that the heart of the problem lies in the rest of your system rather than the synth itself -- quite possibly a mixer or other interface which has what's known as a 'pin 1 problem' whereby the internal earthing arrangements aren't as they really should be.

This used to be a very common issue -- input cable screens were routinely passed onto the input preamp circuit board, rather than being taken direct to the chassis, which allowed any circulating ground currents to enter the preamp's ground reference point directly, resulting in unwanted noises of the type you're complaining about. Most modern equipment has better internal earthing arrangements these days and is far less prone to the problem, but it can still occur.

Let's see what happens with the ground isolation first, but once we know whether you really are dealing with a ground loop issue we can investigate the rest of your system's wiring to come up with a practical fix.

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