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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
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almarg
9,670 posts
 
Please explain your reasoning for when connected to a preamp that is powered off being the same as the cable not connected to anything, open ended.
Hi Jim,

While the output impedance of an unpowered solid state component is probably unpredictable in most cases, especially without detailed knowledge of its design, I would expect that in general it would be very high. Very high output impedance = essentially no termination at that end of the cable.
I have had instances when a power amp was left powered up and the preamp was turned off. There was not any hum/buzz coming from the speakers. Of course I have never owned a 300 watt per channel amp either.
Not relevant; different components.  And chances are those preamps had 3-wire power cords, as opposed to the OP's AVR (see the comments in my first post of today about "floating").
I still can’t wrap my head around that the Adcom amp is quiet when nothing is connected to any of it’s inputs, but when an XLR balanced cable is connected to the amp it hums/buzzes. I have been assuming the hum/buzz is only from the speaker connected to that channel of the amp.
If the unterminated cable is not picking up radiation from some external source of EMI, perhaps it is picking up such radiation from some part of the amp itself. And if so, that pickup would probably not be an issue when the input end of the cable is terminated by the very low output impedance of the powered up AVR. I would expect that the very low output impedance of the powered up AVR would load down and "kill" any such pickup.

Best regards,
-- Al

jea48
4,395 posts
 

jea48 said:
I have had instances when a power amp was left powered up and the preamp was turned off. There was not any hum/buzz coming from the speakers. Of course I have never owned a 300 watt per channel amp either.


Not relevant; different components. And chances are those preamps had 3-wire power cords, as opposed to the OP's AVR (see the comments in my first post of today about "floating").

Hi Al,

The safety equipment ground would not have any relevance as its' purpose is to provide a low resistive path for ground fault current to return to the source.

Two things happened immediately when the Marantz was connected to the Adcom by a wire IC.

 The Signal ground of the Adcom was extended through the IC to the chassis/signal ground of the Marantz.

Second, assuming the signal ground of the Adcom is connected to the chassis of the Adcom then the safety equipment ground is connected to the Marantz chassis as well.

Jim


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almarg
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Hi Jim,

Please consider the paragraph in my first post of today in which I used the word "floating."  And in doing so, re your comment that:
... the Signal ground of the Adcom was extended through the IC to the chassis/signal ground of the Marantz.
consider that the Adcom amp may connect pin 1 of its XLR connectors to chassis, rather than to its signal ground (the former, in fact, generally being best practice).  And its signal ground and chassis may be connected together through a significant impedance (that also generally being best practice).

My point being that the amp may not "see" the balanced pair of signals it is receiving as being tightly referenced to its own signal ground.  Resulting in the possibility that the amp might see those signals as having some degree of 60 Hz common mode noise riding on them, that may in turn somehow couple into its signal path to a degree that results in hum.

Best regards,
-- Al
 
jea48
4,395 posts
 

Thanks Al for the response. Hopefully jo3533 will pick up some RCA shorting plugs.


jo3533,

Does the hum sound like one of these?

60 Hz.
https://www.youtube.com/watch?v=E_Sf7rSOU78


120Hz.
https://www.youtube.com/watch?v=pC4Uzt0qm2E




jo3533 OP
18 posts
 
Here is a link to some photos of the following testing, and the schematics of the ADCOM 7805. 

https://www.dropbox.com/sh/1rdxqg82pw3ecq9/AAATCANt3v5Vc3kMZ0taSa1na?dl=0

As you can see in the photos I pulled the units out of the rack and set them up as far apart as I could get them. Plugged them into separate outlets, and used XLR ICs.

With everything plugged in and the amp on, pre-amp off. (NOISE)
With Amp on and pre amp on. (NOISE)

With the XLR cables disconnected from pre-amp. (NO NOISE)

With the pre-amp not plugged into the wall at all, and with the XLR ICs connected. (NOISE)

I took the XLR cable apart on both ends and both end have pin 1 connected to the shield ground of the plug itself. You can see it in the photos.

Ok, thats all I can do for tonight. I worked 13.5 hours today and its time to just sit and eat.

Thank you all for so much help and input........soon I hope to get little feedback on my thought of cheap cables causing the problem?

I need to check chassis current and a grounding wire between the two (although I have done the grounding wire before) but I just can't bring myself to complete it tonight.

Thanks Again 

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