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Help With 60Hz Hum PLEASE

Greetings,

I am having some issues with noise coming through my speakers at low volume. The noise sounds like a 60hz cycle hum, and can be heard clearly when the volume is all the way down. I have tried several things to quiet down this noise, but nothing has worked so far.

Before I give details of action items I have tried, let me inform you of the components I am running.

My system consist of:

·     AV PRE/PRO MARANTZ 8801

·     AMP ADCOM GFA-7805

·     SONY 300 DISC CD CHANGER

·     SPEAKERS B&W CDM9NT/CDM CNT/CDM1NT

·     SUB VELODYNE SPL1000 SERIES II (NOT HOOKED UP AT THIS TIME)

·     USING XLR INTERCONNECTS BETWEEN PRE-AMP AND AMP (MONO PRICE)

·     DEDICATED OUTLET CONNECTED TO 20AMP CIRCUIT BREAKER

·     FURMAN POWER CONDITIONER (EVERYTHING PLUGS INTO THIS)

·     I SEGREGATE OUT THE AC POWER LINES FROM THE SIGNAL AND SPEAKER CABLES


So this is what I have done to check the system so far.

1.   First off I separated all of the components power sources, plugging each component into its own outlet with the amp plugged into the dedicated outlet. (NO CHANGE, NOISE PRESENT)

2.   Unplugged any no essential item leaving just the pre-amp and amp plugged in (NO CHANGE, NOISE PRESENT)

3.   Removed the amp from the system, plugged in single source and speaker set (NOISE IS COMPLETELY GONE!!!)

a.   FOR ABOVE TEST I USED MY IPHONE WITH A SET OF RCA INTERCONNECTS STRAIGHT TO THE AMP.

                                              i.   I really suspected the amp at first because about a year ago I had the unit repaired for a standby condition. The repair center I used also repaired the CENTER CHANNEL BOARD while they had the unit. I noticed that the center channel has almost no noise coming through, but after the previous findings I decided this might be a symptom not the cause of the noise.

4.   Hooked back up system and tested (CRAP, THE NOISE IS BACK)

5.   Unplugged all of the XLR interconnects from pre-amp side (NOISE IS GONE, YEAH!)

6.   Plugged in one interconnect into pre-amp (noise comes through that channel)

7.   Switched between XLR and UNBALANCED (THE UNBALANCED CONNECTION WAS A BIT WORSE)


My conclusion is that something is happening between my pre-amp and amp that is allowing this 60hz signal though. I am wondering if it could be the interconnects themselves because I am not using a high quality interconnect. All I am using is Mono Price XLR cables. Although, why would there be no noise when the cable is plugged into the amp and not the pre-amp? Might the Marantz 8801 be the culprit due to the fact that when I plug in the cable into the pre-amp is when the noise comes through? Or maybe my first instinct was correct in that the amp is the problem?

Any help figuring this out is appreciated.

Thank You Kindly,

Joe

jo3533
jea48
4,449 posts
 

Joe, (jo3533),

Thanks for doing the test for me. Had I known you had to pull out the rack I would not not have asked you to try it. Thanks again.


AL,

Looking at the Adcom Amp schematic wiring diagram again, trying to figure out why when all the outer contacts of the RCA jacks of the 5 amps of the Adcom are connected together and then connected to the Marantz chassis by an extended wire, the hum/buzz is eliminated.

Wiring diagram.
https://www.dropbox.com/sh/p7w1zmad9dwhx3o/AABK0x1GA9mMJZ_-hz-V--a1a/GFA7800AMP_REV082.pdf?dl=0

Without the added wire connected to the Marantz chassis, (connected signal ground), to the signal grounds of the Adcom’s 5 separate amplifiers, the XLR cables/circuitry (either at the Marantz or the Adcom), the signal grounds of the two pieces of equipment are not connected together by the XLR cables alone.

Am I right?

So when the two pieces of equipment’s signal grounds were not bonded, connected, together, (by the added wire connection circuit), why was the sound, hum/buzz 60Hz?

Jim

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almarg
9,670 posts
 
Looking at the Adcom Amp schematic wiring diagram again, trying to figure out why when all the outer contacts of the RCA jacks of the 5 amps of the Adcom are connected together and then connected to the Marantz chassis by an extended wire, the hum/buzz is eliminated.
Your guess is as good as mine, Jim.  Although I'll add that if the modules in the Adcom conform to the photo you supplied (as opposed to the schematic), as we said earlier it would appear that the Adcom's analog ground (which is common with the ground sleeves of the RCA connectors) is connected to XLR pin 1 through a thermistor.  And if that is the case then the solution Joe arrived at resulted in a reduction of the impedance between the Adcom's analog ground and the chassis of the Marantz Pre/Pro (which you had indicated is common with the signal ground of the Marantz), from whatever impedance the thermistor has during normal operation to essentially zero.  The introduction of that more direct connection between the signal grounds of the two units would seem consistent with a reduction or elimination of hum, that might have been caused by small amounts of AC leakage to chassis in either component.

In any event, considered purely from an intuitive standpoint it seems to me to be very plausible that what Joe did would result in a solution.

Best regards,
-- Al
 
jo3533 OP
18 posts
 
My thought comes down to two words. Faraday Cage.

I believe what I created was a EMF cage around both the Adcom and Marants. That is why the noise comes back when I only connect to the chassis of the Adcom.

I could be completely wrong, but its what I think is going on. Again I am a mechanical engineer not a electrical engineer. I leave that stuff to the smart people. 
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almarg's avatar
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almarg
9,670 posts
 
I suppose a Faraday Cage effect is a possible explanation, but a couple of factors seem to suggest otherwise.  First, the problem didn't seem to be sensitive to the positions of the components, which lessens the likelihood that EMI pickup (that would be prevented or minimized by a Faraday Cage) was responsible.  Second, a similar shielding effect was apparently present all along, since the two chassis had always been interconnected via the shield of the XLR cables.  The shields of the cables having apparently been connected to the chassis of each component both via pin 1 and via the jumpers in the cable which connected pin 1 to the shells of the connectors.

Putting my previous speculation in more general terms, I suspect basically that the grounding approaches in the two components, possibly in combination with some condition-related issue (such as a leaky line filter capacitor), somehow caused an interaction between the two components resulting in small (and perhaps expectable) amounts of AC leakage to chassis in one of the components (perhaps occurring via stray capacitances in a power transformer or via a leaky line filter capacitor) to be seen by the amp as signal.

Best regards,
-- Al
   
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