I have a customer with a paging system in their building. Over the past few days it has been noticed that there is a Dull Hum coming over the speakers. Some louder than others.
I am not necessarily a "sound guy" but, it is electronics low voltage. I don't have the mfg. model at the moment but it could be a Bogen.
Maybe a 250 Watt at 70 volt with (1) output. > Speakers have various outputs off a transformer at each speaker. > Tapped at maybe 1/4 to 3/4 watts. > All are in hallways in a 4 story building for general announcements off a phone system paging output. > Has 3 used inputs > Est. 20 years old
So far I have tried a little troubleshooting as follows.
Power (twice) cycled for about 10 seconds. No change > Removed each input one at a time. No change > Removed all (3) inputs at same time. No change
I am guessing it is the amp going south.
But was wondering if there are other things that could cause this Dull Hum on the speakers?? Needless to say if I had a second Amp I could just swap it out and see if the problem persist. However, that is not the case.
Main causes are: loss of screen on an audio input deteriorating cap/s in amp deteriorating cap/s in something that feeds the amp removed earth or chassis connection somewhere.
You can distinguish 2 from 1,3,4 by unplugging all inputs
The noise is probably 50/60Hz mains frequency leaking into an audio input.
It could also be 100Hz power supply ripple.
NT has listed two ways in which mains hum can get into an audio system - lo ss of screen (or screen connection to ground) on an audio input cable, or a lost earth/ground or chassis connection somewhere.
Grounding different bits of an audio system to ground at different places c an introduce hum - it's not unusual for "ground" at one point to have a vol t of so mains frequency potential when compared with another ground connect ion. It shouldn't happen, which isn't to say it doesn't.
Deteriorating reservoir power capacitors in a mains to DC power supply some where can increase ripple levels - usually 100/120Hz but it can also be 50/
60Hz.
Looking at the audio output with an oscilloscope can help - you can work ou t the frequency of the hum, which often helps.
If he has a scope, checking for ripple on the power supply rails is also worth a look.
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The hum being different in different speakers could be down to their different bass responses. It probably is power line 60Hz/120Hz where PA speakers have very poor sensitivity.
Your tests have established that it isn't coming through the inputs. That leaves the power and output wiring. Your guess that the amplifier is failing is probably correct, drying out power supply reservoir capacitors being one of the more likely.
You didn't mention checking the output wiring, it is imaginable that a poor connection lets in RFI but rather unlikely to cause a steady hum. There'd be no harm in checking the output connections even though it is only a slim chance.
Also check that the amplifier is actually grounded to a good ground.
My guess is the hum really is power line from an ageing power supply. If the power supply has some short hold up time (i.e. it will continue to produce an output for a fraction of a second after power off) then it may be possible to tell if the hum goes instantly power is cut or continues that very short hold-up time. That would help nail down the source to the amplifier's power source vs coming in somewhere else.
Finally if the amplifier is inspectable you may get lucky and actually see a swollen or leaked power supply capacitor.
Twenty years old , it is almost a sure thing that the power supply caps ar e bad. You could clip in some caps in parallel with the original caps to verify that the original caps are shot. Or power the amp on for say
12 hours and see if the caps reform and the hum abates.
It sounds like a ground loop. Bill Whitlock's a guru when it comes ground loop troubleshooting. Here's a good introductory Whitlock paper:
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There's more papers available with a simple search.
My hunch, based upon your description, is that people may have only recently noticed a long-standing problem. The problem may be caused by a phone system / speaker system mismatch. Allow me to walk through such a scenario. Valcom one-way amplified ceiling speakers, for example, bundle an LM386 circuit with each speaker. Each speaker module uses two twisted pairs, one pair for -24VDC and the other pair for audio input. You need a control unit, or at least a balun, to connect a phone system's audio line out to the twisted pair that feeds the LM386s in each module. The phone system's audio line out may have been directly connected to the twisted pair that feeds the modules by mistake. You might want to look into that.
As an aside, here's my favorite Whitlock story:
A few years ago, Whitlock was asked to help find a long-standing buzz problem in an auditorium sound system at Seahawk Stadium in Seattle. In the auditorium were two suspended 12 kW powered speaker clusters. The power was 208-volt 3-phase.
Power wiring connecting the two clusters consisted of five wires: three phase wires, a neutral wire, and a (safety) ground wire. These individual wires were laid in an overhead channel. Current in each of the phase wires was measured at about 30 A rms with the amplifiers idling. Customary checks confirmed that there were no inadvertent connections between safety ground and building steel at the clusters. Audio was routed to each cluster via a single cable and XLR connector from the performer?s mixing console. It was reported that a buzz contaminated the same channel regardless of what make and model mixer was used. A ground voltage difference, as measured between shield pins of the cables from the two clusters, was measured at about 650 mV rms.
On the hunch that this voltage difference was due to magnetic induction in the power interconnect wiring, I asked the staff to twist, as tightly as possible, all the current-carrying wires (3 phases and neutral) and lay the safety ground wire next to the bundle a few inches away. After this was done, the voltage between shield pins of the two cluster inputs had dropped to less than 3 mV rms (near residual for the meter used at the time). Connecting a test signal source confirmed that the buzz was gone!
Thank you, 73,
--
Don Kuenz KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.
Ok, so based on the collective wisdom of those responders the source of the Dull Hum is either the Caps in the Amp itself or some how the unit lost its ground connection.
To provide some more data, I did check the out going connections and they are/were tight.
With the previous troubleshooting and the new information above, what would ALL say the odds are it is that the Amp needs replaced??
50/50 60/40 70/30 80/20
Sadly, I may not have a big choice to start. Replacing the Amp will prove or disprove it was the right choice or not.
Is there a test tool or procedure with a Volt/Ohm Meter that I could perform on the out going speaker lead to give some indication that the speaker and or wiring is or is not the problem.
Well Dan, I have been running my Security Alarm Dealer Low Voltage company for 29 going on 30 years. I would say my troubleshooting and repair abilities are very good. However, as I stated in my original post that "I am not necessarily a "sound guy" but, it is electronics low voltage". The key point is I rarely get involved with Sound Amps & Speakers. One challenge I have is that I am partially deaf in both ears and it makes it difficult to "hear" sounds effectively enough to follow a problem.
I am sure I could de-solder and replace the Caps. My challenge is which Caps?? How long can I shut down the paging system in the building just to tear the amp down not to mention the time to acquire replacement amps and then install them??
My hope here was to get a hint of what I was guessing was the issue. I have that hint and at this point I need to investigate further as to the Model and Specs so I can search out a replacement to do the job. Then the downtime will be minimal and can be done on a evening when paging is less critical for this 24/7 establishment.
Should it not be the Amp then I will have a bigger task trying to find the issue elsewhere.
And while typing the above it just hit me that I could pull out some old ceiling 70 volt speakers from my storage locker and do a quick test by clipping on the amp output. I will need another body with good ears to make the test work correctly.
Oh, and to answer your other question I am in South Central Pennsylvania, USA.
Replacing big fat capacitors (BFC) on a single/double-sided audio board has got to be a piece of cake compared to recapping a multi-layer motherboard. (Hint: many times recapping only the two/three largest cap families on a motherboard is enough to resurrect it.)
Les, you might pull one speaker out of the ceiling and connect only that one speaker to the Amp's audio outlet to confirm that the noise originates in the Amp itself.
Groundloop floobydust:
You typically use a clamp ammeter, such as the Extech 380947, to measure ground loop current.
More Whitlock wit:
What Does "Ground" Mean * A FANTASY invented by engineers to simplify their work * The "uni-potential" fantasy assumes all ground symbols in schematics are at exactly the same voltage * Truth: Real-world conductors have resistance, causing small voltage drops * Truth: Ground circuits most often serve, either intentionally or accidentally, more than one purpose
Misguided Strategies * Reduce unwanted ground voltage differences by "shorting them out" with massive wires or bus-bars * Reduce noise experimentally by finding a "better" or "quieter" ground * Skillfully route noise to an earth ground, where it disappears forever! * Is an earth ground for electronic systems really necessary? Think about aircraft electronics ...
Common Myths * Earth ground is the absolute zero-volt reference * Fact: Many unintentional currents flow in soil and create voltage drops just as in any other resistance - soil is a relatively poor electrical conductor * Ground wires have zero impedance * Fact: Wires have impedance and cannot make multiple points in a system have an identical "zero-volt reference" * Noise (voltage) exists on a single wire or at a single point * Fact: Voltages can exist only between two points * All voltages are relative or differential * Voltmeters have two probes ... * Always ask "Voltage with respect to what?"
Troubleshooting Ask lots of questions. Troubleshooting guru Bob Pease suggests these basic questions: 1. Did it ever work right? 2. What are the symptoms that tell you it?s not working right? 3. When did it start working badly or stop working? 4. What other symptoms showed up just before, just after, or at the same time as the failure?
Sketch a block diagram of the system. 1. Show all interconnecting cables, indicating approximate length. Note any balanced inputs or outputs. Generally, stereo pairs can be indicated with a single line. 2. Note any equipment which is grounded via its 3-prong power plug. 3. Note any other ground connections such as cable TV or DSS dishes.
Use the equipment's own controls, with some logic, to provide additional clues: 1. If the noise is unaffected by the setting of a volume control or selector, it must be entering the signal path after that control. 2. If the noise can be eliminated by turning the volume down or selecting another input, it must be entering the signal path before that control.
Testing to Find Problem Interfaces Some tests which involve disconnecting cables can now further pinpoint the problem:
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Thank you, 73,
--
Don Kuenz KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.
** Afraid that does not count - you are not experienced in audio system trouble shooting, particularly line voltage systems which are a special case.
** Irrelevant, nearly all modern electronics is low voltage.
** You MUST get someone to help you, with good hearing and line voltage system experience. STOP kidding yourself !!!
** Yeah - right.......
YOU are NOT being realistic !! Like most folk who know only a little about audio, you imagine it is all really quite simple. Nothing could be further from the truth.
** Such guessing is the mark of a novice.
Guessing gets in the way of proper trouble shooting procedures - which you do not know and cannot carry out anyhow with your hearing disability.
Symple. Replace the power supply capacitor; might be 10uF at 450V to drive the plates. This is a typical symptom and obvious problem with obvious repair.
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The BIG ones, usually 2 of them. And no need to desolder them, just hook new caps onto them to see if that fixes anything.
Or get one of those component testers, see if the caps have good capacity & ESR.
It only takes 5 mins to power down & solder caps on. If the factory really can't be without it's PA at all it can be done live by unplugging the iron while soldering, but obviously there are hazards in doing that which you should not attempt.
it would be quicker to just hook some new caps on, not to mention cheaper.
should be pretty easy
or leave the new caps on until next day, see what people say
Yes, where he can expect you to abuse him further there.
Alison is an "electrophobic" - don't listen to him as you can easily do this yourself.
The caps in question are hard to miss. They'll be the large ones in close proximity to the mains transformer about where the mains lead comes in. Just turn the juice off and unplug; couldn't be any simpler.
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Except for all those times when the really big bulk reservoir caps are just fine and it is the smaller ones in supply decoupling the preamp that have failed :)
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