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[Mike Karseboom]

I am trying to diagnose a low level 60Hz ( I think) hum/buzz in an outdoor 70V system. 


Testing the speakers and amp back at the shop there is no detectable hum problem.  The only thing that would be different there would be the power supply for the amps.  The onsite power supply used when the hum is present is a somewhat sketchy GFCI outlet under a utility cover in the sidewalk.  I have tested this for voltage and it is around 109V.  Could the power source itself cause a hum?


The amps are Bogen Black Max 450 70v amps. 


To simplify by looking at 1 wiring run to installed speaker:
- the wire is new 14/2 outdoor/direct burial wires just installed.  There is no foil or mesh shielding nor any drain wire.
- it goes about 500' feet total from the amp to the speaker withh only 1 break joined with wire nuts in a new junction box.
-  routing is up a building, across the street on a guy wire, down a conduit to ground level, underground for a ways, the back up to rooftop and across the street on another guy wire to the speaker on a pole
- there are other active speaker wires from different amp channels of the same audio system that run in parallel at times and/or in the same conduit or junction boxes.  But there are no AC or other foreign wires in these conduits or junction boxes.
-  There may be AC wires in close proximity at times on the sections across rooftops.  In one case there is cross-street guy wire about 1 foot above the speaker guy wire that has an AC wire attached of unknown voltage.


I can pick any of the 4 total separate circuits/amp channels and turn all the others off and get the same hum on the selected circuit.  That is, every circuit seems to have the hum even when I isolate it from all other circuits.  Two circuits are new with new wires and speakers and two are old wires and old speakers.  The example above is the simplest circuit with just one amp channel and just one speaker on it.


Any ideas on what would cause the 60Hz hum?  It is pretty low level but it is noticeable and does increase as the volume is increased.

[David Pedd]

If you hook a speaker up to the amp direct, do you get hum?

I mean, using a 3 ft. speaker cable, hook it up right at the amp.

If you do get hum that way, then take an AC outlet tester and make sure the outlets are wired correctly.

What is the source input for the amps?  Are they plugged into the same AC source?

If the input is AC powered, try using a battery powered source, such as an iPod.

[Mike Karseboom]

If you hook a speaker up to the amp direct, do you get hum?

I mean, using a 3 ft. speaker cable, hook it up right at the amp.

If you do get hum that way, then take an AC outlet tester and make sure the outlets are wired correctly.

What is the source input for the amps?  Are they plugged into the same AC source?

If the input is AC powered, try using a battery powered source, such as an iPod.

David - Thank you for the reply.


I did do the short (10') direct amp to speaker connect in the shop before the install and all was OK at that time.  I tried all the amp channels with a couple of the new speakers with no problem.  Now it is too late for a direct amp to speaker test as the speakers are up around 30' in the air and I have no way to access them.


I did test the AC GFCI receptacle wiring with simple non-contact tester, a plug in tester that showed wiring OK, and Kill-a-Watt tester that showed the low voltage of 109V. 


The input was an iPod with tracks that have not had any issues for me in the past and it was plugged into the charger on the same extension cord as both amps.  The headphone jack outlet went to a whirlwind Isopod stereo to mono combiner with XLR output connector.  From there a simple XLR splitter (all paralleled) was used to split to the "A" channel input on 2 separate amps.  The amps were set for the "B" channel to use same input as the "A" channel so all 4 total drive channels should have seen exactly the same signal.   


I should have unplugged the charger and run the iPod from battery as well as just connecting  to one amp channel  to simplify. It might be a week before I can test that on site.  In general  I have not had any kind of hum from the iPod in the past on other systems whether plugged into the charger or not.


I did try different speakers circuits run from different amps/channels and that made no difference.  The hum was still there.

[Mike Caldwell]

Connecting any 70 volt speaker directly to the amp will work for a test. That will eliminate all of the outside speaker cable, there's a chance it could have a bad piece of insulation somewhere and is shorting to conduit, power cable, another speaker cable, ect. Disconnect all of the speaker cables while doing that test, if there is no hum start connecting the speaker cables one at a time till you find the one that causes the hum.
If there is hum on the test speaker with no other speaker cables connected the problem is somewhere right in front you then.

You may just need to lift the pin one audio ground on the connection between the amps.

As for checking the AC power outlet get one of the basic testers that has the three lights in it that test for proper outlet wiring.


I just re read your post and noticed that you said the volume of the hum changes with level of the amps volume control, with the volume control all the way down is the hum gone. If that's the case that just about rules out any issues with the external speaker lines, but do go ahead and do the direct connect speaker test anyway. You'll at least have a handy speaker to listen to while you go through the rest of the trouble shooting.

[Len Zenith Jr]

Did you get any hum without the Ipod hooked up? Just the amps and speakers? The fact that the hum increases with volume level suggests to me that the hum is present at the source. Perhaps the ipod charger/amp combo caused the loop. If that is the case a transformer isolater would be my fix.

[Mike Karseboom]

Did you get any hum without the Ipod hooked up? Just the amps and speakers? The fact that the hum increases with volume level suggests to me that the hum is present at the source. Perhaps the ipod charger/amp combo caused the loop. If that is the case a transformer isolater would be my fix.

Thanks all for the suggestions.  It may be a week before I can make these tests but I will report back...

[David Pedd]

One last thought.  I looked up your amps, and they don't use XLR's.  Make sure you have your inputs wired correctly.  As you are going from an unbalanced source, it could be easy to mis-wire.  I've seen that a few times.  Bogen uses the center connection as ground.

[James Hicks]

I'd definitely look into the iPod without charger, although in my experience problems there usually manifest themselves as weird buzzes etc rather than 60 cycle hum.

[Mike Karseboom]

One last thought.  I looked up your amps, and they don't use XLR's.  Make sure you have your inputs wired correctly.  As you are going from an unbalanced source, it could be easy to mis-wire.  I've seen that a few times.  Bogen uses the center connection as ground.

After hours of screwing around checking the installed wiring, grounds, the iPod, etc., this turned out to be the problem.  The Phoenix to XLR female adapters I bought were not wired correctly for the Bogen amps.  Seems there are more variables in the way fixed install type gear is wired.  Rewiring the Phoenix end was easyand eliminated the hum.


When will I learn to look for the simple solution first?  Thanks all for the help.

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