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Author Topic: Microwave stops hum in stereo  (Read 595 times)

Brian Larson

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Microwave stops hum in stereo
« on: January 17, 2019, 12:56:27 am »

My stereo system in my apartment has had  a noticeable hum for as long as Iíve had it. Recently I moved the microwave to the same outlet as my stereo and while microwaving something while the stereo was on the hum completely stopped. It only stopped as I could hear the magnetron in the microwave kick on which was a little after clicking start.

Why would this happen and whatís the non-microwave equivalent to this that I can leave on all the time to kill this hum?


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Art Welter

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Re: Microwave stops hum in stereo
« Reply #1 on: January 17, 2019, 11:40:45 am »

Why would this happen and whatís the non-microwave equivalent to this that I can leave on all the time to kill this hum?
It could be a poor neutral connection and an imbalanced load across the two hot legs in the apartment, the power drawn by the microwave "balancing" the load across the two hot legs.
If that is the case, any load of around the same wattage would balance the circuit, and add around 1000 watts of heat to your apartment and a dime or so per hour to your electrical bill.

Microwave ovens usually are pretty noisy, the hum must be really bad to hear it over that racket. You should check out what's wrong with the AC or the stereo that is causing the hum- usually a ground loop is responsible.

Good luck!
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John Roberts {JR}

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Re: Microwave stops hum in stereo
« Reply #2 on: January 17, 2019, 01:48:09 pm »

+1  could be weak floating neutral ground bond at the panel... can you go to your fuse box and meter the two legs wrt ground/neutral? If you are not comfortable doing this, dont.

My microwave blows a fuse if I run it at the same time as my electric kettle, so yes they draw several amps.

JR
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Brian Larson

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Re: Microwave stops hum in stereo
« Reply #3 on: January 17, 2019, 02:21:15 pm »

Electric was just redone about a year ago. Everything from the main building disconnect to our apartment sub panel down to the branch circuits was replaced in a gut renovation. Iíll stick a meter on it tonight but my feeling is that itís something with the stereo, not the house wiring.

The setup is pretty non-traditional, a mix of antique and DJ rack gear fed by an AppleTV. Hereís the signal flow:

ATV HDMI out to and HDMI audio stripper
RCA from audio stripper to IR volume control
RCA from IRVC to Rane MP2S mixer
1/4Ē from MP2s to Ashly SC66 PEQ
1/4Ē from peq to Furman TX2 crossover
Hi out from crossover to Harman receiver
Lo out to crown D75A (sub)

I think the hum started when I added the Ashly PEQ which has some other issues. It probably needs to get looked over by someone who knows about these older units (any suggestions in the NYC area?) but it just seemed interesting that the microwave of all things stopped the hum. I never thought ADDING RF would LOWER the noise floor.


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Ike Zimbel

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Re: Microwave stops hum in stereo
« Reply #4 on: January 17, 2019, 02:27:29 pm »

Electric was just redone about a year ago. Everything from the main building disconnect to our apartment sub panel down to the branch circuits was replaced in a gut renovation. Iíll stick a meter on it tonight but my feeling is that itís something with the stereo, not the house wiring.

The setup is pretty non-traditional, a mix of antique and DJ rack gear fed by an AppleTV. Hereís the signal flow:

ATV HDMI out to and HDMI audio stripper
RCA from audio stripper to IR volume control
RCA from IRVC to Rane MP2S mixer
1/4Ē from MP2s to Ashly SC66 PEQ
1/4Ē from peq to Furman TX2 crossover
Hi out from crossover to Harman receiver
Lo out to crown D75A (sub)

I think the hum started when I added the Ashly PEQ which has some other issues. It probably needs to get looked over by someone who knows about these older units (any suggestions in the NYC area?) but it just seemed interesting that the microwave of all things stopped the hum. I never thought ADDING RF would LOWER the noise floor.


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You're not adding RF, you're more likely lowering the voltage at that receptacle. If the Ashly has a power supply that is struggling to regulate the voltage that's coming in, it could be causing 120Hz hum. When the voltage drops because of the current draw from the microwave, the power supply gets a little break.
Some older Furman gear did not have regulated power supplies, so the crossover might need a look too. I will PM a tech I know in NYC.
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Brian Jojade

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Re: Microwave stops hum in stereo
« Reply #5 on: January 21, 2019, 12:59:25 pm »

You're not adding RF, you're more likely lowering the voltage at that receptacle. If the Ashly has a power supply that is struggling to regulate the voltage that's coming in, it could be causing 120Hz hum. When the voltage drops because of the current draw from the microwave, the power supply gets a little break.
Some older Furman gear did not have regulated power supplies, so the crossover might need a look too. I will PM a tech I know in NYC.

+1 on the voltage drop likely fixing the issue.  See if you can get something else that you can plug in to the same outlet that pulls a decent amount of current, such as a space heater to see if the problem gets fixed that way as well.
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Chris Hindle

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Re: Microwave stops hum in stereo
« Reply #6 on: January 22, 2019, 08:29:20 am »

+1 on the voltage drop likely fixing the issue.  See if you can get something else that you can plug in to the same outlet that pulls a decent amount of current, such as a space heater to see if the problem gets fixed that way as well.
I've always found a 1K Par does the trick when a load is needed.
Makes a nice heater too.
-20C up here this morning, -31C with the wind.
That's -24 or something for the non-metric of the world...
Where's a bar-o-six when you want some heat ?
Chris.
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John Roberts {JR}

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Re: Microwave stops hum in stereo
« Reply #7 on: January 22, 2019, 08:44:58 am »

Might as well meter the outlet voltage with VOM too... While rare it is possible for too high  mains voltage to saturate a marginal transformer.

JR
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Tim McCulloch

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Re: Microwave stops hum in stereo
« Reply #8 on: January 23, 2019, 05:57:46 pm »

I've always found a 1K Par does the trick when a load is needed.
Makes a nice heater too.
-20C up here this morning, -31C with the wind.
That's -24 or something for the non-metric of the world...
Where's a bar-o-six when you want some heat ?
Chris.

Or back when big frame analog consoles were king, and you could warm the FOH tent with the desk and its power supplies.
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Chris Hindle

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Re: Microwave stops hum in stereo
« Reply #9 on: January 23, 2019, 08:07:22 pm »

Or back when big frame analog consoles were king, and you could warm the FOH tent with the desk and its power supplies.
PM-3K tourpack and a couple of racks of goodies kept me warm on many a show....
Chris.
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