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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
jo3533 OP
18 posts
 
jea48,

I bought the ADCOM used. One channel was not working, and it would not turn on at all when first purchased. The tech was not very forthcoming on what exactly he replace. He did mention the bridge rectifier however he never confirmed if he replaced it. I have tried to contact him several times and have not received a response......Dunno whats up with that. There are no traditional fuses on this unit. It has had some mod completed by ADCOM that uses some other sort of safety.

YES, I listened to the two links and it sounds just like the 60hz buzz! I mean its dead on with the recording.

I do have a diagram that shows the DC power supply and I will post it in the shared dropbox folder.

I want to point out that the transformer itself produces an audible 60 hz buzz that can be herd while standing next to the unit. I have read this is normal.

Using my multi meter I checked for continuity between the chases and the #1 pin on the XLR jack and yes, there is continuity. There is also continuity between the chases and the outer shell of the RCA jack.

I went doen to Stereotypes in Daytona Beach, FL and they were nice enough to allow me to borrow a set of TARA LABS XLR IC for a small deposit.

I plugged in the Tara Labs cables and the noise was still there! Rats....it was worth a try.

almarg, the 7800 series uses the same amp boards for the GFA-7805/7807. You will see the same markings for the power supply schematics. Link to service manual below.

https://www.dropbox.com/sh/p7w1zmad9dwhx3o/AAC1ZY5kCxocLwKCn_nfTgBNa?dl=0

Post removed 
jo3533 OP
18 posts
 
NEWS. Dead silence....AT LAST!

I will post what I did to fix the problem later. Right now I am trying to put my system back together and confirm it still works after its all back in the rack.

THANKS SO MUCH

Especially to jea48 and almarg. You pointed me in the right direction. Thanks!
almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
Wow! Congratulations! Can’t wait to hear what the solution was.

Jim, sorry for not responding to your post of yesterday morning sooner. It’s a busy weekend here. Your points were well taken, IMO, and as you’ve seen Joe subsequently confirmed that continuity existed per your suggestion.

I would highlight, though, that R49 and R47 are jumpers, not resistors, as indicated by the zero ohm value noted on the schematic ("0R00"), and as stated explicitly in the parts list in the service manual. So analog signal ground and chassis/safety ground are connected directly together in this design, rather than through a low value resistor (which would lessen the chances of ground loop issues, as Atmasphere has stated in a number of past threads here).

Also, as indicated in the Rane paper you linked to earlier, ideally XLR pin 1 should be connected to chassis ground and NOT to signal ground, although in this case that is probably a moot point since both grounds are jumpered together.

So although the design is not ideal in a couple of respects, in ways that could conceivably contribute to hum problems depending in part on how grounds are handled in the component the amp is connected to, and perhaps also depending on whether XLR or RCA interconnections are used, my understanding (consistent with what is said in the Rane paper) is that lots of other designs are similarly flawed.

But we’ll soon see what the upshot is in this case.

Best regards,
-- Al

jea48
4,424 posts
 

Hi Al,

Interesting comments regarding the R49. At the time of my post Joe had not posted the service manual PDF.

I wonder what component R49 is actually made of? It’s not a fuse and technically it is not a jumper. "R" usually designates a resistor. But as you pointed out it does not have a resistance value. What ever it is made of it is rated for 1/4 watt. I assume that if a DC B+ voltage fault to chassis event happened the 1/4 watt device would blow open. (Note R47 connection is upstream of R49. R49 is the gate keeper to the chassis connection.)

ARC uses an actual 10 ohm 1/4 watt resistor to connect the power supply neg DC rail on one channel to the metal chassis. Another 10 ohm 1/4 watt resistor is used to connect pin 1 to the chassis. (ARC VT50)
http://www.audioresearch.com/ContentsFiles/VT50_SchemPL.pdf


Even though when Joe checked for continuity from pin 1 to chassis and he posted he measured there is, I was still going to suggest he still run a jumper from pin 1 of one of the amp’s XLR female connectors to the chassis. I first wanted to look at the other PDFs he supplied last night.

Jim

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