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[Stephen Swaffer]

In conjunction with Pete's question, what happens when you short the input?

If you think about it the components don't know the difference between 60 Hz "noise" and 60 Hz "power", so I don't see any way 60 Hz "noise" is getting through a power supply specifically designed to convert (filter) 60 Hz into DC.

What is acting as an antenna to pick up the noise?  The only thing connected is the power cable?

[Henry Cohen]

I've been troubleshooting what I thought was a dirty power problem, as I've tried seemingly everything to get rid of a 60 Hz hum (with harmonics, visually verified with Smaart) in some powered speakers, but nothing - not even running from an online UPS (and with, to be certain, it unplugged from the wall) and with nothing plugged into the speaker - has worked. This leads me to look at the hum being RFI/EMI-induced. Approached from that direction, what options should I consider? The speakers in question are Yorkville C120P, NX550P, and NX200S, and then, for troubleshooting, I tried connecting a pair of 1st gen Mackie HR 824s. It should also be noted that the trouble was only noticed after the power company either installed/replaced/something what must be a good-sized transformer not too far away (I haven't tracked it down for a visual look, but I've heard it HUMMING, though it doesn't do so continuously).

A relatively good test for suspicion of EMI or RFI is to take the DUT, powered on and with gain turned up enough to hear the noise, and move it about in the space, changing planes (horizontal versus vertical, forward vs back, etc.). If you hear the noise change in any way (not of course due to the fact your ear may now be off axis of a speaker DUT). RFI or EMI is likely a culprit.

For RFI, using a spectrum analyzer with good dynamic range, look for strong signals in the usual bands: AM broadcast, FM broadcast, as well as doing a series of scans for high noise floors <1GHz.

If EMI, take 4' circumference open loop of metal wire (don't connect the ends together; a wire hanger or a piece of THHN solid conductor works well) and suspend it in space, not touching anything. Use a good DVM with the test leads twisted together to measure any induced current and voltage in the open loop of wire. Depending on the strength of the EM field, the voltage and current maybe at micro- levels. Again, change the orientation of the loop to see if the vlotage or current changes. As long as a significant difference between the loop measurement and with the DVM probes shunted is seen, EMI is likely.

[Charles Johnson]

So with just a powered speaker, with the volume turned totally off still has noise?

Correct, though, oddly enough, the NX550P has ceased to hum, though it still has some noise, regardless of the volume level. The C120P still hums and has noise.

In conjunction with Pete's question, what happens when you short the input?

To be determined - I need to make up a connector first

What is acting as an antenna to pick up the noise?  The only thing connected is the power cable?

I don't know - that's one of the reasons that this is so puzzling!

A relatively good test for suspicion of EMI or RFI is to take the DUT, powered on and with gain turned up enough to hear the noise, and move it about in the space, changing planes (horizontal versus vertical, forward vs back, etc.). If you hear the noise change in any way (not of course due to the fact your ear may now be off axis of a speaker DUT). RFI or EMI is likely a culprit.

I'll try changing planes - I've tried multiple locations, but always with the speaker in the vertical position. The odd thing about the NX550P that has stopped humming is that I haven't moved it (from where I last tested it). A C120P, set right on top of it (the 550P is on the floor), however, still hums. The noise levels in both are still the same and the input gain setting does not affect them. I'll take down the other C120P from its mount and see if it's the same (that is, sitting atop the 550P).

For RFI, using a spectrum analyzer with good dynamic range, look for strong signals in the usual bands: AM broadcast, FM broadcast, as well as doing a series of scans for high noise floors <1GHz.

I don't have an RF spectrum analyzer, but I can say that the broadcast radio bands around here are quite congested.

If EMI, take 4' circumference open loop of metal wire (don't connect the ends together; a wire hanger or a piece of THHN solid conductor works well) and suspend it in space, not touching anything. Use a good DVM with the test leads twisted together to measure any induced current and voltage in the open loop of wire. Depending on the strength of the EM field, the voltage and current maybe at micro- levels. Again, change the orientation of the loop to see if the vlotage or current changes. As long as a significant difference between the loop measurement and with the DVM probes shunted is seen, EMI is likely.

I'll give that a shot. Different locations and different times of the day, too.


Charles Johnson

[Keith Broughton]

This may have been covered already but what is the voltage at the AC mains supply at the speaker?

[ProSoundWeb Community]