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Hum again...

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Hum again...

Postby Arpangel » Sun Jun 02, 2019 8:16 am

Yes, that old problem.
In the process of my studio clear out I've had to rewire things, plus I'm using a different monitoring system.
I'm using my old KEF 1970's monitors, which I love, driven by a Quad 405/33 amp combination. Old I know, but nice. I want to integrate the 33 preamp into my monitoring chain because I"d like to plug my CD player and cassette deck straight into it rather than go through mixer channels and use the 405 power amp only, the 33 preamp is just a nice sound to have when not recording and using the mixer.
But I'm getting major hum, I've connected the balanced mixer CR output to an ART Clean Box, to go from balanced to unbalanced, then into the 33 preamp. If I unplug the mixer from the mains and just use the 405/33 on its own the hum dissapears.
I've tried disconnecting leads on the mixer one by one but nothing seems to cure it, I get the feeling that if I use the power amp on its own that may cure the problem, as I did this before some time ago and it worked, but I'd really like to use the preamp, as it has such a nice sound with my CD player. But maybe it's not a good idea to try and integrate the studio and hi-fI together?
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Re: Hum again...

Postby Logarhythm » Sun Jun 02, 2019 9:27 am

There is no reason in principle that this is unworkable - I used a Rotel hifi preamp / power amp setup as my combined "studio" (optimistic term in my case ;) ) / domestic listening setup for many years.
Which Art box are you using? To my not-especially-knowledgeable eyes, it looks like the Pro deals with balanced/unbalanced conversion but doesn't provide any transformer-based isolation*, so a ground loop caused by the mixer seeing a different earth potential to the preamp could theoretically be possible.
How is it all plugged in to the mains - is it all on the same socket?

*There are many people here with more knowledge of such things than I have - hopefully one or more of them will correct this if it's an errant assumption :thumbup:
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Re: Hum again...

Postby Arpangel » Sun Jun 02, 2019 10:12 am

Logarhythm wrote:There is no reason in principle that this is unworkable - I used a Rotel hifi preamp / power amp setup as my combined "studio" (optimistic term in my case ;) ) / domestic listening setup for many years.
Which Art box are you using? To my not-especially-knowledgeable eyes, it looks like the Pro deals with balanced/unbalanced conversion but doesn't provide any transformer-based isolation*, so a ground loop caused by the mixer seeing a different earth potential to the preamp could theoretically be possible.
How is it all plugged in to the mains - is it all on the same socket?

*There are many people here with more knowledge of such things than I have - hopefully one or more of them will correct this if it's an errant assumption :thumbup:

Thanks, I think you may be right about the ART, not having a transformer.
Everything is plugged into the same socket, on two extension boards daisy chained.
Just wondering based on what you said, a better box than the ART may be a good idea.
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Re: Hum again...

Postby Arpangel » Sun Jun 02, 2019 10:58 am

Any reccomendations for boxes? I guess it's good to preserve quallity in these situations.
But it would be a bit of a let down, if I bought a Radial and it didn't work!

:?
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Re: Hum again...

Postby Logarhythm » Sun Jun 02, 2019 10:59 am

Not sure it's a case of "better", but perhaps "different" - adding a transformer isolation stage could help.
It looks like the Cleanbox II does the transformer isolation bit. Personally I'd contact Orchid Electronics - he can almost certainly do something that would do everything you need in one neat little unit at an astonishingly reasonable price.
But this genuinely isn't an area in which I have any great expertise, so I'd wait to see what the actual experts think before reaching for the credit card ;)
These could also be worth a read, if you haven't done so yet:
https://www.soundonsound.com/sound-advice/q-what-ground-earth-loop
https://www.soundonsound.com/sound-advice/q-do-balanced-connections-prevent-ground-loops

There will also be many posts in the forums, but perhaps to the extent that there is almost too much information to know where to start...
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Re: Hum again...

Postby Arpangel » Sun Jun 02, 2019 11:03 am

Logarhythm wrote:Not sure it's a case of "better", but perhaps "different" - adding a transformer isolation stage could help.
It looks like the Cleanbox II does the transformer isolation bit. Personally I'd contact Orchid Electronics - he can almost certainly do something that would do everything you need in one neat little unit at an astonishingly reasonable price.
But this genuinely isn't an area in which I have any great expertise, so I'd wait to see what the actual experts think before reaching for the credit card ;)
These could also be worth a read, if you haven't done so yet:
https://www.soundonsound.com/sound-advice/q-what-ground-earth-loop
https://www.soundonsound.com/sound-advice/q-do-balanced-connections-prevent-ground-loops

There will also be many posts in the forums, but perhaps to the extent that there is almost too much information to know where to start...


Yes, and not only so much info, but I guess everyone's situation is slightly, or, massively! different.
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Re: Hum again...

Postby Hugh Robjohns » Sun Jun 02, 2019 11:10 am

The principle of isolating grounds is the right one, and a decent transformer isolation box should cure the obvious ground loop problems you're experiencing, so two possibilities immediately spring to mind:

1. You're not actually using a transformer isolation box.

For example, the ART Cleanbox Pro is a +4/-10 level converter with active electronics and no isolating transformers -- so no ground isolation.

You need to use either the ART Cleanbox II or the ART DTI units -- or one of the transformer isolator boxes from another manufacturer (like Orchid). Personally, I really like the DTI for its good quality/price ratio, and its amazing versatility in connectivity -- I have 5 of them!

2. You are using a transformer isolator box, but it's been placed too close to a mains transformer and is consequently picking up a radiated magnetic field resulting in hum being injected into the audio.

So if you have put the transformer box on top of the Quads, for example, try moving it further away!

H
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Re: Hum again...

Postby Arpangel » Sun Jun 02, 2019 2:36 pm

Hugh Robjohns wrote:The principle of isolating grounds is the right one, and a decent transformer isolation box should cure the obvious ground loop problems you're experiencing, so two possibilities immediately spring to mind:

1. You're not actually using a transformer isolation box.

For example, the ART Cleanbox Pro is a +4/-10 level converter with active electronics and no isolating transformers -- so no ground isolation.

You need to use either the ART Cleanbox II or the ART DTI units -- or one of the transformer isolator boxes from another manufacturer (like Orchid). Personally, I really like the DTI for its good quality/price ratio, and its amazing versatility in connectivity -- I have 5 of them!

2. You are using a transformer isolator box, but it's been placed too close to a mains transformer and is consequently picking up a radiated magnetic field resulting in hum being injected into the audio.

So if you have put the transformer box on top of the Quads, for example, try moving it further away!

H

Thanks Hugh, the box is about 2ft away on top of a cassette deck, as you say, it's not transformer isolated so...that may be a problem. I'll check out some others.
I've unplugged the 33 preamp, and I'm using the 405 purely as a stand alone power amp for my monitors, it's as quiet as anything, no hum, and I'm still using the Clean Box as well, but as soon as I reconnect the 33 preamp to the mixer it's back again, giant hum, it's not the 33, it works fine stand alone with the 405.
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Re: Hum again...

Postby James Perrett » Sun Jun 02, 2019 2:46 pm

Have you tried disconnecting the screens at one end in the cables running between the mixer and the Cleanbox? Also check that there is no hum when the inputs to the Cleanbox are completely disconnected.
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Re: Hum again...

Postby Hugh Robjohns » Sun Jun 02, 2019 3:03 pm

Arpangel wrote:I've unplugged the 33 preamp, and I'm using the 405 purely as a stand alone power amp for my monitors, it's as quiet as anything, no hum, and I'm still using the Clean Box as well, but as soon as I reconnect the 33 preamp to the mixer it's back again, giant hum, it's not the 33, it works fine stand alone with the 405.

These things are always hard to diagnose remotely... but it sounds like the 33 is grounded via the mains safety earth, but the 405 isn't... which is somewhat disturbing!

Do you have access to a multimeter? If so, it would be worth using the continuity mode to find out which individual elements of your signal chain have a direct connection between the safety earth pin on the mains plug and the audio connector shields. Once you know that, you can devise an appropriate way of connecting everything together without creating ground loops.

An alternative to the transformer isolation box is, as James says, to make up a bodge cable from the mixer in which only the hot signal wire for each channel is connected to the appropriate Quad inputs, with the cold and shield/screen wires isolated (at the Quad end, ideally). It's not the most elegant or technically ideal solution, but it may well cure your hum problems.

The other potential issue to bear in mind is that the original Quad 33 had very sensitive inputs -- the nominal level for the standard radio input is around -20dBu (100mV) rather than the +4dBu your mixer is probably throwing out, while the standard tape input card can be configured for +2dBu (1V), -6dBu (400mV) or -20dBu (100mV).

H
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Re: Hum again...

Postby James Perrett » Sun Jun 02, 2019 4:11 pm

Hugh Robjohns wrote:An alternative to the transformer isolation box is, as James says, to make up a bodge cable from the mixer in which only the hot signal wire for each channel is connected to the appropriate Quad inputs, with the cold and shield/screen wires isolated (at the Quad end, ideally). It's not the most elegant or technically ideal solution, but it may well cure your hum problems.

I was thinking of doing it at the input to the Cleanbox - keeping the grounds connected on the unbalanced side.

I've just found some pictures from when I modified my 405 and, as it comes from the factory, the mains ground is definitely connected to 0V in the amplifier.
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Re: Hum again...

Postby Hugh Robjohns » Sun Jun 02, 2019 4:24 pm

James Perrett wrote:I was thinking of doing it at the input to the Cleanbox - keeping the grounds connected on the unbalanced side.

Yes, good thinking, James. :) If it's necessary to use the Cleanbox to reduce the console's output level to feed the Quad (from +4dBu to -10dBV/-8dBu), then running the balanced desk output to the Cleanbox, but isolating the cable screen at the Cleanbox end should work to break the ground-loop...

I've just found some pictures from when I modified my 405 and, as it comes from the factory, the mains ground is definitely connected to 0V in the amplifier.

Yes, it is... I have a 405 here myself and the power amp reference ground is most definitely tied to the mains safety earth.... which is why I was slightly perturbed by Arpangel's descriptions of the symptoms earlier.
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Re: Hum again...

Postby Arpangel » Mon Jun 03, 2019 11:14 am

Hi all, interesting, I'm using the clean box to reduce the level into the 405, which is absolutely necessary. All works fine, completely quiet, no hum.
The strange bit is that when I then bring in the 33 the signal from the mixer is if anything, too low through the clean box, I'm plugging into the tape input on the 33, no difference in level if I go into the radio input I may ad. I don't actually need the clean box with the 33.
Hum is alaways there though.
I'm going to disconnect screens today as suggested, I'll report back later.
My mixer just for the record is a Behringer QX2442,
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Re: Hum again...

Postby Arpangel » Tue Jun 04, 2019 7:54 am

Hugh Robjohns wrote:An alternative to the transformer isolation box is, as James says, to make up a bodge cable from the mixer in which only the hot signal wire for each channel is connected to the appropriate Quad inputs, with the cold and shield/screen wires isolated (at the Quad end, ideally). It's not the most elegant or technically ideal solution, but it may well cure your hum problems.

I'm doing that today, but, I'm not connecting the mixer directly to the Quad 33, I take it I have to disconnect the screen on the inputs to the ART box, I'm using balanced quarter inch jack (mixer end) to XLR's (ART) for that, so I just pull off the screen at the XLR input end?
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Re: Hum again...

Postby James Perrett » Tue Jun 04, 2019 12:07 pm

Arpangel wrote:I'm using balanced quarter inch jack (mixer end) to XLR's (ART) for that, so I just pull off the screen at the XLR input end?

Yes, that should work.
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Re: Hum again...

Postby Arpangel » Thu Jun 06, 2019 12:35 pm

Hi, OK, screen off, hum has reduced a bit, and changed character, but it's still there, not as annoying though.
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Re: Hum again...

Postby Arpangel » Mon Jun 10, 2019 9:26 pm

Amazing! The hum has completely gone, and I'm now able to use the 33 as part of my system. I de-soldered the screens, but one was still touching the pin! I put some heat shrink around both and clamped them out of the way, just in case I heed to connect them again in the future. But honestly, now no um at all, and it seems a lot quieter.

:thumbup:
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Re: Hum again...

Postby James Perrett » Mon Jun 10, 2019 9:29 pm

Great news :thumbup:
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Re: Hum again...

Postby Hugh Robjohns » Mon Jun 10, 2019 9:32 pm

I love it when the engineering bit of audio engineering proves its worth! :thumbup:
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Re: Hum again...

Postby Martin Walker » Tue Jun 11, 2019 11:28 am

Hooray! :clap:

There's nothing more satisfying than finally resolving a background noise is there?

I recently solved the very occasional buzz on certain loud bass notes in one of my monitor speakers - I unbolted the woofer, lifted it out, couldn't see anything lose, then bolted it back in, and the noise hasn't happened since.

I suspect it was the dressing of the loudspeaker wires, and simply moving them solved the buzz. Yay!


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