How to measure If there I’d DC on the mains

[DaveR]

Is the noise coming out of the speaker or the amp?

[Pelliott321]

first thing we did
we have gone through all possible check points for ground loops.
disconnected everything, leaving only power amp and speakers.
no hum in main system but hum in subs
cheated mains on sub amps, no help.
cheated mains on power amp no change.
added each component back in one at a time
no change with either cheating mains or not
we feel there is a problem in sub amp, but want to check out possible DC on mains first

[DaveR]

So with the subs plugged into the mains and no other connections, you hear a noise. Is the noise coming out of the speaker or out of the amp on the back side?

[Pelliott321]

yes

[DaveR]

Yes, noise is coming out of both the speaker and the amp? If the amp has a toroidal transformer, it can be buzzing but the speaker will be silent. This is the problem I had. Does the volume control on the sub have any effect on the noise? This question assumes the noise is coming out of the speaker.

How old are the subs?

[SoundMods]

first thing we did
we have gone through all possible check points for ground loops.
disconnected everything, leaving only power amp and speakers.
no hum in main system but hum in subs
cheated mains on sub amps, no help.
cheated mains on power amp no change.
added each component back in one at a time
no change with either cheating mains or not
we feel there is a problem in sub amp, but want to check out possible DC on mains first

You are not going to have D.C. on the mains. From the power company steam-turbine-driven generators to your receptacles -- there is not going to be a D.C. component in evidence ever!

But noise? You bet. Florescent lighting ballasts, LED power supplies, refrigerator motors, A/C motors, cell phones, Wi-Fi networks, TV power supplies, neighbor's power tools, automobile ignition noise, etc.

Plus to add insult to injury -- you live in a neighborhood where you get your power from pole-mounted 13-kV to 120/240-volt 45-kVA step-down transformers.

Those old neighborhoods typically had eight to ten houses on the same transformer -- all connected to the secondary in parallel.

HUM? Welcome to the wonderful world of chasing ground loops! I finally won that war, but not after many frustrating battles.

[Pelliott321]

the guy I am trying to help lives in the country with frequent brown and black outs

[Pelliott321]

the subs are servo subs and servo amps, part of a large speaker system.
I am thinking to figure out how to hook up a known good amp and e-xover to test.
he is a bit leary about this. I want to prove that the plate amps are the problem so he can have them repaired by the manufacturer

[DaveR]

If the plate amps are not connected to anything except AC for power and the speakers and they hum they have a problem. I assume that a plate amp is mounted in the speaker cabinet so all the wiring between the amp and speaker is contained within the cabinet. If it's not, it is possible that the wire from the amp to the speaker is routed near AC wiring and is picking up the 60Hz telling the amp the speaker needs correcting so it tries to counter the 60Hz it sees as feedback.

[SoundMods]

the guy I am trying to help lives in the country with frequent brown and black outs

When I got my sub-woofer that had a built-in plate amp. I bought an 8-meter interconnect designed for the purpose.
The interconnect came fitted with a floating shield over the co-ax shield with separate ground connections on each end.
So -- one manufacturer (Audioquest) recognized that there could be a problem like that and designed an interconnect to deal with it.
It works as advertised.

Attachments

Audioquest Sub-Woofer Interconnect.pdf

(306.73 KiB) Downloaded 352 times