Noise on microphone cables

We ran 5 microphone cables about 175 feet through PVC pipe underground between two buildings. Shortly after the job was completed we tested the lines. They weren't as quiet as I had hoped they would be but they were usable. Over the past two years they have gotten more noisey. We set up a test today. All cables were open ended, that is unterminated on the other side. We set each input individually so that the VU meter read 0VU noise level. Then doing one input at a time we plugged a mic in. The noise level dropped to -6 on all inputs when checked individually. It was an improvement but certainly still not too good. Next we started out with open ended cables again, established our 0VU noise reference and then shorted all three terminals on the XLR connector together with a piece of foil. I repeated this with all the remaining cables and the results were the same. The noise completely disappeared and the line was clean. I next disconnected the line at both ends and checked each conductor to ground for high resistance. I read infinity from each conductor to the sheild as well as to building ground. Next we took one mic and boosted it to line level at the far end. We ran this line level signal through one of the underground cables to a line level input on the mixer. The noise level is now way below the signal level and the signal was squeaky clean. So now we know that we can definitely retrofit the job and get it to work this way, but this whole thing really puzzles me. Does anyone have any ideas as to why we are getting all this low level noise on these cables? They seem to acting like antennas. But when grounded they are clean. Also since addional equipment is now going to be an added expense to my customer, and one which they will not apreciate, any ideas as to what I might try to remedy the situation other than pre amplifying my mics before they go through the underground cables would be nost sincerely appreciated. Lenny Stein, Barlen Electronics.

Reply to
captainvideo462002
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Probably poor balance at either or both ends.

I'd have started out with star quad cable.

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*On the other hand, you have different fingers*

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

Of what use is an unterminated cable? You say you're getting noise, but that it goes away when you terminate the cable. If I understand you correctly, that's not unusual. A cable without something connected *on both ends* is kind of useless and will act like an antenna...unless of course, it's intended to be an antenna.

Other issues with underground conduit may be at play here as well. Is the conduit environmentally sealed at both ends? Mice just love to play in those things. Water gets in....

jak

Reply to
jakdedert

Balanced and low impedance ? Individual screened cables ?

What equipment (make and model) are the mics plugged in to ?

Graham

Reply to
Eeyore

When tested in exactly what way ?

What's the point of that ? You should fit a 200 ohm resistor between the 2 signal conductors to simulate a microphone's source impedance.

You cranked up the mic gain with open circuit inputs ? What were you trying to achieve ?

Use a 200 ohm resistor !

I'm sorry but your test seems utterly bizarre. As an audio pro for 35 years I can't believe the stuff you're saying. You don't come over as remotely competent never mind professional at all. Your tests are absurd in the extreme. Whatever are you thinking of ?

Graham

Reply to
Eeyore

" snipped-for-privacy@yahoo.com" hath wroth:

Yep. If you look at the "noise" on a scope, it probably will look like an AM broadcast transmitter modulation envelope. Also, the unterminated end point will pickup some 60Hz hum. If the "noise" is on/off keyed, you're near a Navy submarine communications transmitter.

The increase in "noise" level might be due to water incursion, corosion, or dissimilar metals electrolysis.

Running a microphone into an unterminated load is silly. They weren't made to operate like that, they turn the cable into an antenna, and you'll never see it in real life. Install a termination and re-measure.

It's also rather odd measuring the "noise" level with the microphone active. I suspect you're measuring either the background audio noise pickup of the microphone (i.e. road noise), or the added noise of the mic pre-amplifier.

What you've done is described the symptoms, without also supplying much information as to what you have to work with (type of cable, connectors, end points, proximity to sources of "noise", depth of burial, type of microphone, etc. It's rather difficult to guess(tm) the cause without such details.

Also, the term "noise" is subject to much creative interpretation. It could easily be mic audio background pickup, ingres, transformer action, rectification, or RF. If you have a suitable audio spectrum analyzer *AND* an oscilloscope, you may want to take a photo of both and post it somewhere. The frequency and waveforms of this "noise" will offer considerable information as to determining its cause and source. That will supply much more info than a Vu meter.

I'll bet on road noise from the live microphone.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Reply to
Jeff Liebermann

Not on a balanced circuit.

Graham

Reply to
Eeyore

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175 ft cable runs , underground routes , grounding each end , microphone level signals , no line termination , no signal conditioning . noise level measurement's on open lines ..

Whole thing reads like a TV repair man , mending a radar set....

G ..

Reply to
Graham

There are certainly some issues that need addressing wrt 'technique' there !

Graham

Reply to
Eeyore

BINGO!

Reply to
Meat Plow

I thought that I would provide just a little more information for those of you who seem to think that I'm totally incompetent. The mixer is a Shure M67. The cable is two conductor AWG. 18 with a tight braided tinned copper sheild. Its Mil spec instrument cable which I've used on microphone jobs before. In fact we've done longer runs without incident. There are 5 XLR female jacks mounted on an aluminum plate. The 5 sheilds are connected to their respective jacks but are not tied together, nor are they earth grounded at this end. The open cable test was just to compare all the lines to each other and thats all. The microphone plugged into one line with all the others turned down should have balanced that line and ideally been noise free. I think that I know the difference between ambient room noise and buzz and hum. This isn't my first barbecue. All I was asking was for opinions as to why a balanced line terminated with a low Z microphone might be noisey. If you feel that you must insult me further to speculate as to the cause of this please don't bother to respond. Thanks, Lenny Stein, Barlen Electronics.

Reply to
captainvideo462002

What you should do, is first make sure that the wire is a high quality shielded type. What can happen is that if the pipes are of PVC and not of a metal with magnetic shielding properties, it will not offer external shielding against electromagnetic fields from electrical distribution wiring. The longer your cable run, the more it is susceptible it is to pick up other radiated signals, or noise.

What I do with long microphone runs, is put pre-amps in line at the sending end. I then send the audio at line level down the cables rather than at mic level. Line level is normaly +4 dB with professional equipment. The mic level is normaly at -50 to -60 dB.

At the receiving end, which I assume in your case is an audio board, you can then have the inputs set to line level. With this method, if there is any noise pickup, then it is extremely severe.

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JANA
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Reply to
JANA

I can't understand why you're using that. I've not used ordinary pair balanced mic cable for over 30 years. Star quad is so much better. It can change an unusable mic circuit into a satisfactory one - the difference is that great.

--
*Do infants enjoy infancy as much as adults enjoy adultery? *

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

I really can't understand why you guys haven't discovered star quad cable. With a deal of experience of TV work it's unusual to need to raise the mic level when using star quad - even with cable runs of a mile or so. And a mic to line amp capable of handling anything any mic can throw at it without being able to adjust the preset gain is a rare and expensive beast.

--
*Reality?  Is that where the pizza delivery guy comes from?

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

The screens on such cables are made of copper. Their action has NOTHING to do with magnetic shielding.

Graham

Reply to
Eeyore

What the guy has done is entirely normalpractice in profesional audio. That's NOT a problem.

Heck, I've actually run UNBALANCED mic signals 60 metres without trouble. Obviously that needed some care with the grounding arrangements.

Graham

Reply to
Eeyore

Star Quad should only really be needed in areas of high interfering field strength, where the extra rejection it offers comes in handy.

Graham

Reply to
Eeyore

" snipped-for-privacy@yahoo.com" hath wroth:

Ok, I'll bite. You're measuring "noise" with a Vu meter. Got an audio spectrum analyzer in your war chest? If so, use it and tell us what you see. Is it hum, buzz, crackle, AM radio station, submarine communications, power supply switching noise, or alien signals from the overhead flying saucer?

True. However, the way I recognize experience is by the numbers. You didn't supply any. How many dB was the "noise" increase? What frequencies was the "noise"? What manner of equipment was used?

There's some question as to whether terminated at one end with a microphone constitutes proper termination. In my never humble opinion, both ends need to be terminated to prevent the coax from turning into an antenna, transformer, or linear capacitor. What manner of "noise" you're seeing will also help isolate the cause and mechanism.

Sometimes, insults and haranguing are the only effective way to get people to disclose sufficient information to supply an intelligent answer. My asking for additional information does not constitute an insult, although I can see how it might be construed as such.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Reply to
Jeff Liebermann

The correct method is to 'terminate' the line with 200 ohms at the microphone end (don't use an actual microphone) and the 'termination' at the other end is provided by the input impedance of the microphone amplifier. Since it's not operating as a transmission line, the characteristic impedance of the cable is not relevant (it would be about 100 ohms FWIW).

Graham

Reply to
Eeyore

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braided tinned copper sheild. Its Mil spec instrument cable which I've used on microphone jobs before.

Reply to
Graham

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