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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
Post removed 
Post removed 
jea48
4,672 posts
 

Al,

R47 looks like a thermistor to me.

Puzzling to me though it is installed in the R47 circuit board location instead of the R49 location. Unless the circuit trace on the underside of the board was changed before the actual circuit board production putting it in series with the chassis bonding, connection, grounding clip.


Picture of amp board
http://www.adcomparts.com/m/pic/36007805B%20003.jpg


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

Jim

almarg's avatar
almarg
9,670 posts
 
Good call! Yes, it appears that it could be a thermistor. Ironically, though, I believe that the symbol for R55 shown nearby on the schematic (connecting digital ground to chassis) is one of the symbols that are used to represent thermistors, yet R55 appears in the photo to be just a jumper. Also, I don’t see any listing at all for R55 in the parts list.

So if the photo is considered to be gospel, as opposed to the schematic and the parts list, it appears that XLR pin 1 is connected directly to chassis (via the R49 jumper); digital ground is also connected to chassis; and analog ground is connected to those points through a thermistor.

Given the flexibility that is provided by the board design for how these circuit points are utilized I doubt that there would have been a reason for them to modify the related circuit traces under or within the board. But given the inconsistencies between the various documents it’s all hard to figure.

Best regards,
-- Al

jea48
4,672 posts
 

Al,


"Ironically, though, I believe that the symbol for R55 shown nearby on the schematic (connecting digital ground to chassis) is one of the symbols that are used to represent thermistors, yet R55 appears in the photo to be just a jumper. Also, I don’t see any listing at all for R55 in the parts list."

(Note: I quit using the Agon insert quote feature. Half the time the thing doesn’t work or doesn’t work right! Maybe it’s my old Windows 7.)


You are correct R55 location on the picture of the amp is a jumper wire.

As for R55 not being on the parts list, I couldn’t find it listed either.

It could be the Adcom 7807 is not built exactly as the 7805. One may have come before the other.

Edit: And we don't know how many times the schematic wiring diagram, jo3533 supplied, may have been revised.


Where do you suppose Joe, (jo3533) is?

I sure hope he didn’t throw his back out, or worse, trying to put the Adcom amp back in the rack.

Jim

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