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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
4,423 posts
 


This is driving me crazy. Next I am buying longer interconnects and moving the amp farther from the pre-amp. Ill let everyone know the results.

You never said how long the XLR cables are. I assume they are at least 1 meter.


That should be plenty long for a temporary equipment set up test. It may require moving the heavy Adcom amp though. (You will probably need put the Adcom on the floor. If the floor covering is carpet you may need to either sit the amp on a hard surface or slightly elevate the amp above the carpet if the amp has air vents on the underside of the amp.) (You will need to reconnect, solder, the shield back on the XLR connector)

.


Set up:

Rearrange the Adcom so you can place the Marantz 8801 on the floor directly behind the amp. The rear panels of the two will be facing one another. Space the two as far as possible from one another limited only be the XLR cables.

For this test plug the Adcom and Marantz directly into the wall duplex receptacle 20 amp dedicated circuit. Unplug the Furman power condition from the AC power. You do not want anything AC powered near the Adcom or Marantz equipment.

Do not connect anything to the inputs of the Marantz 8801.

Isolate the Adcom speaker wires as they leave the amp as far as possible from the Marantz 8801 as well. Basically you want the Adcom and Marantz 8801 setting all alone by themselves on the floor.

Make sure the AC power cords of the two units are kept away from the XLR cables.

Turn on/power up the Marantz 8801. Mute the 8801.

Next power up the Adcom amp.

Check for hum.



Adcom GFA 7805 specs:
Continuous power: 300Wx5 into 8 ohms (24.8dBW), 450Wx5 into 4 ohms (23.5dBW); both at <1% THD, 20Hz-20kHz. Input sensitivity for full output: 1.61V RMS. Voltage gain: 29.67dB. Input impedance: 500k ohms (RCA), 10k ohms (XLR). THD+noise (typical): <0.135% (8 ohms), <0.3% (4 ohms). Frequency response: 10Hz-20kHz, +0/-0.25dB at 1W, 8 ohms. Power bandwidth: 3Hz-130kHz, -3dB.Signal/noise: >112dB, A-weighted. Damping factor: >500.

Read more at http://www.stereophile.com/content/adcom-gfa-7805-five-channel-power-amplifier-specifications#jVM8Mi...

jo3533 OP
18 posts
 
Jea48,


My XLR cables are very short. 1 foot only. I ordered some loser cables and when they come in I will be doing this test and let you know the results.

Thanks!
jo3533 OP
18 posts
 
OK.....so I never made it all of the way to the testing sequence as stated above. I started to set up for the test, but when I get the 100lb. amp out of the equipment rack and set it on the floor. I decided to try a couple of things before I removed the rest of my equipment.

In order to shorten this a bit I will cut to what I did the gave me a reason to pause and reflect.

For this the amp is plugged into a dedicated outlet with NOTHING else attached.

I ended up plugging in my iPhone directly into the amp using 1/8" to RCA adapter. Voila, no noise at ALL!. 

So with the iPhone plugged in with RCA connectors there is no 60hz hmm. So I decided to go ahead and plug in the xlr cable into the same channel that the RCA cable is plugged into. NOTE: the XLR cables were not connected to anything for this test, they were plugged into the amp only. The noise was back, although much softer. So then I flipped the "Balanced/Unbalanced" switch to "Balanced" and the noise was back. Unplugged the xor cable and the noise is gone. 

Bottom line. Every time I plug in the xlr cables the noise comes back. Even if they are not connected to anything on the other end.

I am now, more than ever, thinking the problem is within the amp.

Any thoughts? Any suggestions? 
 
jea48
4,423 posts
 

jo3533,

You cannot connect the XRL cables to the amp without the other ends of the cables connected to a load/source.

If they are not connected to a load, like a preamp, and are just laying there open ended you basically have 5 antennas picking up RFI and sending the noise to the inputs of 5 power amps.

If you have the amp on the floor that’s is definitely the hard part of the job.

The Marantz 8801 is considerably lighter.


But, before you pull the Marantz out of the rack do you by chance have an old DVD player? Or you could use your Sony multidisc player.

For a test connect a pair of RCA interconnects from the audio out of the player to the front right and left channels RCA jacks on the Adcom amp.

With the Adcom amp turned OFF,

Plug in the power cord of the player and turn it on.

You will not be playing any music through the player. DO NOT press start to play.(If you are using an old DVD player don’t put a disc in it.) By chance if a disc is in the player and you were to press play the music would be loud but not ear deafening. I still would not try it!

Next flip the toggle switches on the back of the Adcom for the front right and left channel amps to single ended. Turn on the amp.

Just a guess you will not hear the dreaded hum.

Unlike when you were using the iPhone for a test which is battery powered, the player is 120V AC powered. The player is the termination for the single ended RCA interconnect cables. Both pieces of equipment are powered from your house’s 120V mains power.


Post back your findings.

jo3533 OP
18 posts
 
Ok, so I hooked up an old Koss CD player to my amp. fired up the CD player and then the amp. One note about this unit is that it only has a two prong plug much like my Marantz.

The noise was GREATLY reduced. I could still hear it, but is was very faint.

Just out of curiosity. Would better xlr cables help clean up some of this noise? I am using an inexpensive cable set currently.

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