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Mains Hum from Neumann KH 310A

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Mains Hum from Neumann KH 310A

Postby jodaki » Fri Sep 25, 2020 6:47 am

Hi,
Ive been noticing earth-type noise in 2 of my valve mics, also in an ISA 430 and channel 1 of a ISA 428 (both only at +40db gain). I have methodically turned off all other equipment in the studio and have tracked it down to my monitors - a pair of neumann KH 310A,

Ive toggled the ground switch on the back of the monitors - no difference. Ive unplugged their signal cables - no difference. The only thing that seems to quieten my studio down again is to turn them off. Has anyone got any ideas about what I can do to fix this?

Other things Ive tried: moving the mics away - no difference. Putting equipment on a separate mains ring to the monitors - also no difference.
thanks, John
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Re: Mains Hum from Neumann KH 310A

Postby ef37a » Fri Sep 25, 2020 7:19 am

I have determined that the power supply for the KH310A is a universal 100V to 250V SMPSU so you cannot be getting mechanical buzz from a mains transformer.

Do both monitors hum and at the same level? Does the sensitivity control affect the hum?

Can you record the hum? Any decent LDC should be up to the task.

Dave.
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Re: Mains Hum from Neumann KH 310A

Postby jodaki » Fri Sep 25, 2020 8:35 am

ef37a wrote:I have determined that the power supply for the KH310A is a universal 100V to 250V SMPSU so you cannot be getting mechanical buzz from a mains transformer.

Do both monitors hum and at the same level? Does the sensitivity control affect the hum?

Can you record the hum? Any decent LDC should be up to the task.

Dave.
Thanks Dave,

I have a feeling the right monitor is slightly louder than the left. I will fiddle with the sensitivity control and record some for you.

thanks
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Re: Mains Hum from Neumann KH 310A

Postby jodaki » Fri Sep 25, 2020 1:03 pm

jodaki wrote:
ef37a wrote:I have determined that the power supply for the KH310A is a universal 100V to 250V SMPSU so you cannot be getting mechanical buzz from a mains transformer.

Do both monitors hum and at the same level? Does the sensitivity control affect the hum?

Can you record the hum? Any decent LDC should be up to the task.

Dave.
Thanks Dave,

I have a feeling the right monitor is slightly louder than the left. I will fiddle with the sensitivity control and record some for you.

thanks

Here's a recording of the system with the speakers turned off and then turned on. Its not silent at the start but you can definitely hear when I turn them on.

https://www.kilroyjames.co.uk/ftp/mains_hum.aif
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Re: Mains Hum from Neumann KH 310A

Postby ef37a » Sat Sep 26, 2020 11:54 am

I had hoped someone with much better ears than I would have had a harken by now?

No matter, I honestly don't hear or see "hum" in that signal. Noise yes but no specific frequency. I am wondering if you have a resonance in the room that resolves the LF noise as a hum? The 'Sea shell effect'.

Of course, at the listening position you should hear nothing at all from active monitors.

Dave.
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Re: Mains Hum from Neumann KH 310A

Postby Mike Stranks » Sat Sep 26, 2020 12:17 pm

I've had a listen and heard nothing that makes me think something's awry.

Trying to record an apparent issue like this is a pretty fruitless task as it's taking the noise out of context and we also have no reference 'room sound and level'.

And Dave: although it shouldn't be the case, in a quiet room you can hear a faint hum/hiss from some speaker sets - fairly respectable ones too!

I don't know the characteristics of those Neumann speakers so can't comment on what they ought to sound like when quiescent.
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Re: Mains Hum from Neumann KH 310A

Postby ef37a » Sat Sep 26, 2020 12:42 pm

Mike Stranks wrote:I've had a listen and heard nothing that makes me think something's awry.

Trying to record an apparent issue like this is a pretty fruitless task as it's taking the noise out of context and we also have no reference 'room sound and level'.

And Dave: although it shouldn't be the case, in a quiet room you can hear a faint hum/hiss from some speaker sets - fairly respectable ones too!

I don't know the characteristics of those Neumann speakers so can't comment on what they ought to sound like when quiescent.

Mike, YOU might hear some self noise in a pro studio but I would never expect to in my house*! My Tannoy 5As are dead silent from where I sit although a few years ago, one did develop a buzzy transformer but that seems to have stopped. (I have pretty normal hearing below 400Hz)
I note you say "...should not be the case" Bloody right! 100W power amps with a -100dB S/N have been with us for decades. Maybe a slight hiss is permitted for less then top end monitors but nothing should hum!

*My el cheapo SPL C meter gives me a noise floor of ~28dB but I suspect that is as low as the meter will read.

Forgot! I asked for a recording assuming the 'hum' was gross enough to be noticeable. I always expect folks to give us a reference noise? Never do, silly me !

Dave.
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Re: Mains Hum from Neumann KH 310A

Postby jodaki » Sat Sep 26, 2020 12:56 pm

Thanks for the replies, however I think I may have caused some misunderstanding.

The noise is NOT in the room per se, it is internal to the preamps or mics - I hear it through headphones when recording at high gain.

It is not coming out of the speaker cones - but it is CAUSED by the speakers somehow.

The noise is outputted from equipment such as preamps and valve mics and is noticeable when the preamp gains are high (over 40 db)

If you listen to the clip I posted there is about 10s of high gain mic signal followed by 10s of the same but with the speakers turned on. There is a roughly 10db increase in the background mains-type hum. If it needs to b clearer I can record it again with higher impedance on the preamp to exaggerate th effect.

I’m assuming I need some sort of isolator or ‘conditioner’ (forgive any incorrect terminology - I have no clue what I need) for the speakers but I really dont know anything about that sort of thing so am looking for advice.
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Re: Mains Hum from Neumann KH 310A

Postby James Perrett » Sat Sep 26, 2020 1:59 pm

Yes, there's a buzz on that recording - pulses with probably a 50Hz repetition rate.

Have you checked the ground in the mains cable to the speaker - I'm just wondering whether it could be a mains filter issue. Are you using any filtered mains sockets?

Is it an acoustic noise from the speaker or is it purely electronic? If it is purely electronic can you record it without the ambient background noise?
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Re: Mains Hum from Neumann KH 310A

Postby Tomás Mulcahy » Sat Sep 26, 2020 3:07 pm

Yes it's a buzz, not a hum. Sounds suspiciously like a lighting dimmer that's nearby (radiating into the air rather than on the supply line) or a dodgy phone charger (i.e. a failing SMPSU nearby). But that's just an (educated) guess. You'll have to do a bit more detective work in the room. Or the room next door!

This article covers pretty much all of the possible sources:
https://ethanwiner.com/dimmers.html

It's good advice. I built the 20A mains filter because there's only one ring mains circuit in my new house, and it has the fridge and the water pump on it. You'd get nasty crackling when the motors switched on. The mains filter described eliminated that. Probably because my Quad 405 does not have a mains filter inside.
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Re: Mains Hum from Neumann KH 310A

Postby Arpangel » Sat Sep 26, 2020 3:42 pm

I heard a definite buzz, quite high, and an increase in overall noise when you switched the monitors on, but I’d like to know what volume level you’re listening to all this at?
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Re: Mains Hum from Neumann KH 310A

Postby jodaki » Sat Sep 26, 2020 4:53 pm

James Perrett wrote:Yes, there's a buzz on that recording - pulses with probably a 50Hz repetition rate.

Have you checked the ground in the mains cable to the speaker - I'm just wondering whether it could be a mains filter issue. Are you using any filtered mains sockets?

Is it an acoustic noise from the speaker or is it purely electronic? If it is purely electronic can you record it without the ambient background noise?

Hi James, I now believe I have 2 separate problems so Ill post about the other problem separately. Regarding the mains hum:

1. yes it's electrical ie. I can record it without mics attached.
2. after lots of testing it seems more complicated than I previously thought. Im still trying to understand it properly to reproduce it reliably.
3. It is definitely worse with the Neumann speakers turned on.

It's taking me a while to understand it as I said so I will upload a clip this evening which hopefully will explain it clearly. To summarise though - I think I have a ground problem somewhere but it is made worse when the Neumann's are on.

Thanks again for all the input. John
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Re: Mains Hum from Neumann KH 310A

Postby jodaki » Sat Sep 26, 2020 7:51 pm

Arpangel wrote:I heard a definite buzz, quite high, and an increase in overall noise when you switched the monitors on, but I’d like to know what volume level you’re listening to all this at?

It's an electrical noise not acoustic (It's in the audio signal - so its not that the speakers are buzzing/humming it's that they are playing a buzz/hum present in the signal). I hear it via headphones at very low level but as mentioned, the mic pres are at almost full volume. The noise starts to appear using the focusrite pres at gain levels above 35db (ish).

Im finding the problem more complex than I first thought and will have to carry on looking tomorrow..
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Re: Mains Hum from Neumann KH 310A

Postby Hugh Robjohns » Sat Sep 26, 2020 8:02 pm

There is only one way to solve wring/grounding problems, and that is to start with the simplest possible system all powered from one mains socket with correct grounding.

To ensure correct grounding you need to know which items are Class-1 (grounded) and which are class-2 (double-insulated or ungrounded).

So the simplest system is typically a computer and an interface. If that sounds clean on headphones, then connect one monitor speaker and test again, then the second and test... And then carry on with other equipment.

Any unwanted noises should be dealt with and resolved fully before extending with more gear.

Everything should be powered in a star-arrangement from just one wall socket.

All audio cabling should be balanced, and kept well away from any mains cables and, especially, any line lump PSUs.
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