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Hunting down studio monitor hum

gustav_errata

gustav_errata

pfm Member
I know, I know, another hum thread. This time it's about two active studio monitors. (Edit to note: sorry for the long post but I wanted to provide all possibly relevant details).

In my office/home studio, I have a pair of Focal Shape 50 studio monitors. These have two inputs, each: one balanced XLR and one unbalanced RCA. According to the user manual, "You can connect up to two sources to each loudspeaker using the two inputs available. Both inputs (XLR and RCA) are active simultaneously. In order to draw the greatest benefit from this feature, we recommend switching on just one of the two sources to avoid undesired signal noise." The RCA inputs are not connected to any source. The monitors have fixed gain (which differs by input), so the only onboard controls are EQ related. The monitors are on Isoacoustics stands, so can reasonably be considered to be isolated from external vibrations.

The monitors are connected via the XLR inputs to an Audient Nero monitor controller, which, for all intents and purposes here functions as a preamp. It has two sources connected: my PC's USB audio interface via balanced connection and a Primare Prisma NP5 via coax spdif to the Nero's built-in DAC. The NP5 is always powered on, the PC is only powered on when in use (but its PSU remains switched on).

All devices are connected using the manufacturer-provided mains cables to the same bog standard Belkin surge-protected-but-not-condituoned mains block.

Both monitors hum at the same frequency. I can hear a low hum from the cones but not from the passive radiators on the side. If I put my ear to the side wall though I can hear the same hum with higher harmonics. These are very quiet, mind. The internal hum can only be heard with my ear to the wall. The cone hum can be heard clearly from a few centimeters but I can still sense it from my listening position 80cm away. When I turn them off my whole body relaxes from the absence of the hum. Another note: when the monitors' automatic standby function kicks in, the hum stops.

Here is what I've tried, all without success:
  • Different room: these used to be in a different room, albeit probably on the same circuit/radial/spur/whatever the correct term is
  • Unplug all devices from the mains except one monitor
  • Disconnect the XLR source from the monitor
  • iFi DC Blocker, for removing DC offset
  • iFi Ground Defender, for removing ground-related hum
  • Both iFi gadgets at the same time
The only other idea I had was to try some shorting plugs on the RCA inputs. When I mentioned that in another thread here, a concern was raised that because the inputs are simultaneously active, a shorting plug on the RCA input might short the preamp (monitor controller here).

Can anyone else weigh in with insights on other things to try that I might have overlooked or on the shorting plug question?
 
Apologies for such a tardy response to your long post...

Have you tried, no XLR connected, nothing else connected and shorting plugs in the RCA sockets on the speakers?

Also, when did the speakers start to hum, what changed in the system? I'm assuming that they used to be silent.

And how old are they?
 
chiily said:
Apologies for such a tardy response to your long post...

Have you tried, no XLR connected, nothing else connected and shorting plugs in the RCA sockets on the speakers?

Also, when did the speakers start to hum, what changed in the system? I'm assuming that they used to be silent.

And how old are they?
Click to expand...

Apologies for the lengthy post. I went for as much detail as possible to help anyone who might be more clued in than me.

I don't have shorting plugs yet. My original post was a WTD ad in the classifieds looking to borrow some. I should receive them this week. I'll try what you suggest first as it seems the safest approach.

The monitors have always hummed. They're 3 years old. I just finally have started to look into resolving the hum as it's really getting on my nerves lately. I had more or less accepted the fact of the hum for a while but upon looking into it more I have learned that while a bit of hiss is nothing, a hum generally shouldn't happen.

Oh one other salient note: pro audio studio monitors are sold individually. So the fact that I get the same hum from both suggests to me that it's not a fault but either an intrinsic property of the make/model (but note that I have not read any other complaints about these particular monitors) or something in my setup/environment.
 
Using a phone app, I just measured the hum from the cone at ~151Hz ~1.5dB over ambient noise @ 1cm (as I said, it's quiet...but it's there and it bugs me when the music stops.

Edit: from the sidewall I measure ~200Hz and ~750Hz at maybe 1dB with the mic pressed to the wall.
 
By all means try the shorting plug to see if it kills the noise. Just don't connect anything else to it while the shorting plugs are fitted.
Also, look around the back. A photo from the website shows that to the left of the mains iec input the are 4 decent sized screw heads in a square. These may well be where the transformer is mounted. Is the buzz/hum louder here than the cabinet walls?
Edit: 151Hz is 3rd harmonic so seems more like rectified mains noise that 50Hz hum.
Higher frequencies from cabinet suggest it could be induced noise into the amp or the wall naturally singing ie resonance.
 
I had something similar with a pair of JBL active monitors connected to a Focusrite interface. Tried all the usual suggestions about making sure they're on the same power strip etc. In the end I ended up using an isolation transformer which did the trick.

I wonder if something like an Art DTI would do the trick? Obviously would be ideal if you could try it before buying...

https://www.amazon.co.uk/dp/B0009GUOQA/?tag=pinkfishmedia-21
 
This site contains affiliate links for which pink fish media may be compensated.
a.palfreyman said:
By all means try the shorting plug to see if it kills the noise. Just don't connect anything else to it while the shorting plugs are fitted.
Also, look around the back. A photo from the website shows that to the left of the mains iec input the are 4 decent sized screw heads in a square. These may well be where the transformer is mounted. Is the buzz/hum louder here than the cabinet walls?
Edit: 151Hz is 3rd harmonic so seems more like rectified mains noise that 50Hz hum.
Higher frequencies from cone suggest it could be induced noise into the amp or the wall naturally singing ie resonance.
Click to expand...
Hmm, yep it could be mechanical hum.

You could try the old trick of, with a largeish screwdriver, placing the point against the back of the cabinet, around where we think the transformer is mounted and the handle of the screwdriver against your ear...a mechanic's stethoscope :)

That might help localise the hum down to the transformer rather than the speaker.
 
@gustav_errata can you confirm that you get the same noise from just one speaker with nothing connected?
I hope the shorting plug fixes it TBH. If so it could simply need the internal wires dressing.
 
I used a contact microphone and an frequency analyser to get some better readings than my phone dB meter app.

Front panel, bottom left side:


Front panel, bottom right side:


Left side-wall, bottom-front corner


Left side-wall, bottom-rear corner


Right side-wall, bottom-front corner


Right side-wall, bottom-rear corner


Rear panel in the middle of the 4 screws mentioned by @a.palfreyman:
 
paulfromcamden said:
I had something similar with a pair of JBL active monitors connected to a Focusrite interface. Tried all the usual suggestions about making sure they're on the same power strip etc. In the end I ended up using an isolation transformer which did the trick.

I wonder if something like an Art DTI would do the trick? Obviously would be ideal if you could try it before buying...

https://www.amazon.co.uk/dp/B0009GUOQA/?tag=pinkfishmedia-21
Click to expand...

Thanks, I had come across Art's various devices in my initial research but I forgot to follow up with it. I did try a DC blocker, but that didn't do anything (on the big assumption that the device was fit for its advertised purpose, of course).

I'll see if the shorting plugs do anything and then I'll try one of these Art DTI's. I need to return the iFi devices first (which, btw, each inline iFi device costs £80-90, so this Art device is more cost-effective anyway).
 
This site contains affiliate links for which pink fish media may be compensated.
Sorry, I had to set up a mic boom and omnidirectional mic to measure the cone whilst also working from home.

Here's a frequency analysis of the omni mic pointed directly at the center of the cone, about 1cm away.


You can see peaks at approximately 50Hz, 120Hz, and the most prominent one at 150Hz. There are also minor peaks around 250Hz and 350Hz.
 
So...if we assume that the transformer is indeed mounted on the back panel (which I think is probably correct as I swear I've seen a picture of the back panel removed at some point), then given that the contact mic measurements back there were quietest it seems to me that I'm not dealing with transformer hum. The contact mic measurements were loudest from the front panel, just below where the cone is mounted. Both the cone and the front panel frequencies are all multiples of 50, with the cone at 1x, 3x, 5x, etc. and the panel at 2x, 3x, 4x, 5x, and especially 6x.

So, is the most reasonable explanation that the cone is humming (for some electrical reason) and the "case hum" that I'm hearing are the walls and/or the cone housing (whatever that's called) singing along?
 
gustav_errata said:
Here is what I've tried, all without success:
  • Different room: these used to be in a different room, albeit probably on the same circuit/radial/spur/whatever the correct term is
  • Unplug all devices from the mains except one monitor
  • Disconnect the XLR source from the monitor
  • iFi DC Blocker, for removing DC offset
  • iFi Ground Defender, for removing ground-related hum
  • Both iFi gadgets at the same time
Click to expand...

Is the hum there with absolutely nothing bar the mains connectors connected to the monitors? Is there any scenario aside from off in standby mode where you do not get the hum?

gustav_errata said:
Using a phone app, I just measured the hum from the cone at ~151Hz ~1.5dB over ambient noise @ 1cm (as I said, it's quiet...but it's there and it bugs me when the music stops.

Edit: from the sidewall I measure ~200Hz and ~750Hz at maybe 1dB with the mic pressed to the wall.
Click to expand...

These are not frequencies I would expect to be caused by a conventional installation issue, e.g. ground loops (50 Hz in the UK). I’d not rule out a fault with the speakers themselves. Maybe even a feature; some active speakers are not as quiet as one would hope!
 
Any piece of conducting metal. Screwed up cooking foil, a small piece of wire, paper clip, even your fingers might do it. You just need to short the centre pin to the sleeve.
 
Tony L said:
Is the hum there with absolutely nothing bar the mains connectors connected to the monitors? Is there any scenario aside from off in standby mode where you do not get the hum?
Click to expand...

Yes, the hum is always there, no matter what the connection situation is. I've only heard the absence of the hum with the speakers powered off or in standby.

I hope it's not a fault. I'll have to get in touch with the vendor eventually. I just want to exhaust all at-home possibilities first.
 
BTW, I remember someone else here on PFM has some Focal Shape monitors in their office setup. I wish I could remember who it was so I could ask if theirs have a hum!
 
gustav_errata said:
Yes, the hum is always there, no matter what the connection situation is. I've only heard the absence of the hum with the speakers powered off or in standby.
Click to expand...

In that case as far as I’m concerned you have ruled out anything that is in your domain, the noise exists within the speakers. Basically if the noise remains after you have disconnected all your stuff it can’t possibly be your stuff! Time to contact Focal/the dealer.
 
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