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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
almarg's avatar
almarg
8,673 posts
 
Hi Jim,

Although we can’t be completely certain, I’m thinking that R49 and R47, which are explicitly noted in the parts list in the service manual as being "jumpers," are most likely just short lengths of wire. And I’m envisioning that during the design process the 1/4 watt 0R00 designations were just used by the electrical designers to specify to the mechanical designers that the corresponding holes and pads in the printed circuit board should be spaced and sized such that they would be suitable for insertion of 1/4 watt resistors. The intent being to provide the flexibility of being able to install 1/4 watt low value resistors later in the development process, if it proved to be necessary.

Best regards,
-- Al

jea48
2,944 posts
 

Al,

Yes, I found it in the parts list of the service manual.


"R20 ,R21 ,R47 ,R49 ,R51 JUMPER"

Thanks.


While waiting for Joe to post how he fixed the hum problem I have been looking at the other PDFs he posted last night.

I also found a picture, (I believe), of one of the five channels used in the amp.
http://www.adcomparts.com/m/pic/36007805B%20003.jpg

The clip seen on the top left side of the board, do think that is used to bond, connect, the signal ground to the chassis? Or do you think a bonding jumper wire was used?

Note the jumper wire in the R49 spot? Note the little guy in the R47 spot?

(Look at the service manual PDF of the channel amp's parts/components layout.)


Jim

almarg's avatar
almarg
8,673 posts
 
Good find, Jim. Consistent with your comment, very close to that clip are a couple of locations that appear likely to be designed and designated for resistors, but appear to have jumpers installed. (I expanded the photo to 200% to be able to see that better). My guess (which appears to be confirmed by the diagram you referred to in the service manual) is that one of them is R49, and that there is a connection within the board from one of the R49 pads to the pad the clip is attached to. With the other end of the clip being physically connected to chassis.

Although as you indicated it appears nearby that there is an actual resistor installed at what the service manual diagram depicts as R47 (the pin 1 to analog ground connection). So the parts list and the photo are inconsistent.

Best regards,
-- Al

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