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
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

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

I would try the cable fix . They ran the power and cable through the same hole in my house . I had a 20k new system and there was a slight hum . I took some speaker wire , strip the ends and twisted it around the cable splitter and connected the other end to a water pipe with vise grips . Problem solved . 
GOOD EVENING,

I have been busy all day, but I stayed up late last night putting everything together. Here is what I found works....not 100% sure I understand why yet, but I think I understand what has happened.

jea48, The link you provided to www.rane.com coupled with discussions about grounding and ground loops really helped solve this. Also, some dumb luck played a part.

After spending an hour reading through www.rane.com/note110.html and subsequent pages I began to understand the difficulties manufactures face with using balanced and unbalanced connections on the same unit.

I ended up modifying all of my XLR interconnects to match "cable 1" at www.rane.com/note110.html. I did this to one cable first, and after doing so the noise that was being produced changed greatly. Not is was much louder. It also sounded more like a 120hz hmm instead of the 60 cycle buzz.

It was good to have something different and right away I thought to now chases ground the two units together. Since the amp and pre-amp were so far apart, and I was in a hurry to test my theory. I just plugged an RCA cable into the unbalanced connection point above the XLR IC on the amp and then touched the outer ring against the chases of the Marantz. The following silence was only broken from my cheering. No more buzz or hum. Just a very soft hiss that I find to be common in systems.

Right after this I posted the success and then went to work reinstalling my system.

After installing all my components and plugging the amp and pre-amp into the power source, I checked to verify the noise was still gone. I made a custom lead to go from the frame of my amp to the frame of the pre-amp. Hooked up just one speaker fired it up and.....BZZZZZZ. "What the heck?" I think to myself.

Taking a step back and reviewing the changes the main thing that has changed, other than the proximity of the components, is where I am grounding the two components together.

Grabbing an old RCA IC I cut it apart and plug into the amp unbalanced input, and connect the shield wire to the frame of the Marantz. For some reason there is still a 60 cycle buzz.

"Wait", I think to myself. I have plugged into a different channel than what channel is hooked up to the speaker. So I switch the RCA plug to the same channel that the speaker is hooked up to and voila. The noise is completely gone again.

What I ended up doing is jumping all of the RCA outer rings together and then linking that to chassis of the Marantz.

Keep in mind I am using the XLR ICs for my sound path. The RCA outer ring is just for grounding to the Marantz.

I think by doing this I have bonded all of the signal and chassis grounds in the amp and joined the Marantz eliminating any kind of ground loop effect.

The one thing that is still present is a pop or clicking sound when I turn on the system or mute the pre-amp. Don’t know why.

My system has been running all day, and it has never sounded better.

Thank you all again for your help! Especially jea48 and almarg! Your awesome!

P.S. My back is just fine.

Jo3533,

Great news! I am glad to hear the dreaded hum/buzz is finally gone.

With that said,...... Curiosity killed the cat. Would you try one more test for me?

Would you disconnect the wire that you connected to the Marantz chassis and instead connect it to the metal chassis of the Adcom amp.

Post back the results.

Jim