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In Reply to: RE: Dedicated lines = new hum posted by jea48 on June 21, 2007 at 12:57:31
the three lines are all terminated on the same panel, on the same side of the panel. when i unplugged all other equipment except for the amps, i still get the hum. when i unplug the interconnects from the amp though, the hum goes away. do i need to ground my preamp, or is something wrong w/my interconnects?
i'm scared to stick a volt meter into an outlet... is that dangerous?
Follow Ups:
> > i'm scared to stick a volt meter into an outlet... is that dangerous?
> > > > > > > > > > > > > > > > > >As long as you keep you hands away from the metal probes you will not get bit.....
If you feel safer shut off the 3 breakers. then insert the 2 test probes in to two different circuit recepts. turn back on the breakers and check for a reading on the meter....
Most electrical panels, not all.......
(1)L1 .......(2) L1
(3)L2 .......(4) L2
(5)L1 .......(6) L1
(7)L2 .......(8) L2
(9)L1 .......(10)L1
(11)L2 ......(12)L2
ect ......... ect
"All on the same side" does not necessarily mean on the same 120V leg. In an electrical panel, as you go down one side, the breakers are on alternate legs.
there are two lines on L1 and one line on L2.
again tho, when i disconnect the interconnects, but leave the amps on and connected to the speakers, the hum disappears...
Ground loop
can't friggin believe this. spent a boatload of money on dedicated lines and i get this?
i guess i could just not use that other dedicated outlet, right? plug everything into the two outlets on L1...
Call the electrician back to install all the lines to the same leg of the AC.
However, you should not have gotten hum even if one of the circuits was intalled to the other leg. Check all the outlets for correct wiring: hot (smaller slot) to ground should be 120 volts, neutral (larger slot) to ground should be zero volts.
Connecting your audio system to outlets wired to both AC legs increases its exposure to RF noise and degrades the purity of the sound, but, if all the neutral wires are installed correctly, you should not experience any audible hum. Something else appears to be wrong.
Al a reversed polarity at one of the recepts sure could cause his problem. Hard to believe the electrician would not have checked for proper polarity before he left the job though.
At any rate easy to check as you stated in your post. Steve is a little hesitant to use a meter on 120V. He could go to Home Depot and buy a cheap plug in polarity and equipment ground checker. That will tell him if the polarity is reversed on any recept.
away for the wkend, will have to troubleshoot next week. i'm ok using the volt meter on the recepticle, i'll just turn off those circuits before i do so, then turn them back on... basically i'm looking for a 0, right?
It sounds like the surge unit (cheap) is leaking on both the neutral and earth. If that it the case check for excessive leakage on the earth (the number one culprit for hum). I'm sure the polarity is fine. Unless the guy is a moran, it's pretty hard to screw up the polarity...especially for a pro who has done this more than once.
If it is the surge unit, there are three ways to fix it. One, rip out the surge unit and replace it with a quality magnetic breaker. Two, lift the ground at the outlet (not advised...but it is what the surge circuit believes you are going to do). Three, lift the ground by using either isolated or split phase (balanced) transformer on all your a/v circuits.
To clarify about lifting the ground at the outlet...if you have a copper cold water pipe near the a/v duplex, you could run a 12ga insulated ground to the pipe and have a good grounding set up. Take all of you dedicated lines and star ground them at the pipe clamp. Just make sure you scrub the pipe surface clean with Caig Progold 100% solution before installinth the ground. Also make sure the clamp is solid copper and not something cheap like zinc if you want a solid ground.
Alan
The notion of a separate ground for the audio circuits has been discussed before, but is worth repeating in this context.
The AC "ground" in USA-type wiring is a safety system. It is a separate conductor in each circuit that is tied to the neutral at the entrance panel. Its purpose is to carry fault current arising from a short from hot to exposed metal on the attached equipment until the circuit breaker opens, without allowing a lethal voltage to appear on the exposed metal. This means the resistance of the safety wiring has to be low.
By connecting the audio outlet ground terminal to a water pipe or other independent earth electrode, the safety circuit has the resistance of the earth between the independent electrode and the ground electrode to which the neutral is connected inserted into it. Fault current passing through this extra resistance could cause a lethal voltage to develop on the exposed metal of the faulty equipment and the circuit breaker might not even open. This could cause a fire.
The neutral and "ground" wires are all connected to earth at the service entrance to help protect against lightning-induced common-mode surges. If there should be a nearby lighting strike with a separate audio earth connection, the lateral currents that flow through the earth between the two electrodes can cause a very high voltage to develop between the neutral (tied to earth at the service entrance) and "ground" (tied to earth by the independent electrode) at the audio outlet. The outlet will flash over at about 6000 volts, but the equipment may be destroyed and set on fire in the process.
Agreed Al...which is why I told him not to pursue this option...instead I told him to lift the neutral using a isolation transformer.
Alan
One more clarification....the best option for you if you decide to keep the surge is to isolate your components with a transformer...either unbalanced or balanced is fine. This is the safest way to lift the neutral in order to stop the hum and remain grounded to the house.
Alan
It is not dangerous if you are careful, but I've always been wary.
When testing things like this, I get an extension cord or two, plug the voltmeter into the ends of the extension cord, then plug the extension cords into the wall.
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