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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
4,788 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


almarg's avatar
almarg
9,670 posts
 
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
4,788 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 

almarg's avatar
almarg
9,670 posts
 
Thanks for providing the documents, Joe.  After looking at the schematic I withdraw my suggestion of RCA shorting plugs.  Consistent with your earlier comment it shows the center pin of the RCA connector being wired directly to pin 2 of the XLR connector.  Some unusual things that are done with various resistors account for the unusual differences in input impedance and gain between balanced and unbalanced input modes.

I'm assuming, btw, that the schematic you provided, which is for a GFA-7800, represents the circuitry for each of the five channels of your GFA-7805.

I don't know if the fact that the cables you are using connect pin 1 to the shells of the XLR connectors might factor into the problem.  But in any event I would suggest ordering the Mogami cables I referenced earlier, as I suspect they will sound at least a little bit better than what you are using even if changing to them doesn't help the problem. 
With the XLR cables disconnected from pre-amp. (NO NOISE)
This is different than before.  Perhaps when you tried this previously, with noise resulting, most of the length of the cable was closer to the amp than in the present arrangement that is shown in the photos, resulting in the unterminated cable picking up EMI from the amp itself (as I speculated earlier).

Not sure what else to suggest at this point.

Regards,
-- Al
 

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