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Richard
Reply with quote  #1 
Hi everyone, I'd appreciate some advice with my ground loop issue.

My Son of Beast is silent at idle.  My Masterpiece is silent when connected to a different amp.  However, with the SoB turned on, and with the output to it as the only MP connection, as soon as the power earth connects as I go to plug the MP in, I get audible mains hum through both channels at the speakers.  Last night, I briefly disconnected incoming mains earth from the masterpiece chassis, and the problem was solved.  If I connect mains earth to the interconnect ground, bypassing the Masterpiece, I get no issue.

What is the best, (and safe) way to solve this issue?  I'm not prepared to leave the Masterpiece chassis without a direct earth connection.

I've read about a couple of solutions:

1) 10ohm resistor in the interconnect ground at the SoB end
2) Separate chassis ground from earth ground in one or both pre and power amp.   [earth-f4] 

and I'm also wondering if I could:
3) Fit a 2 way IEC socket to one chassis and use the output to power the other - [c13-c14-iec-connector] 

If I connect the chassis ground point on both together and get no hum, I think that suggests option 3 would work.

Are any/all of these viable solutions?  Is there possibly a problem with my build which has created this issue?  I don't really have physical space to relocate equipment, although I might be able to temporarily.

Thanks





Michael
Reply with quote  #2 
Richard,
I look forward to the response because I have the same problem with my SoB / Masterpiece set up. 
One thing I haven't verified yet is ensuring that RCA cables and power cables are separated from each other outside equipment, because if they are together the 60 Hz hum could be coming from power cable into the RCA cable..

Mike
Kelvin Tyler
Reply with quote  #3 
Richard and Michael.
These ground loop problems can sometimes be tricky to track down. Much depends on the individual situation.
On the Masterpiece the chassis is used as a ground plane.  The chassis, the RCA socket grounds, the signal and PS grounds, and the 'house ground' on the IEC mains socket, are all connected together. On the SOB the chassis is only at connected at the input RCA sockets and to the 'house ground' at the mains inlet.
Under these conditions, the interconnect potentially completes a ground loop. To break this loop try lifting the ground connection near the input sockets on the SOB. This will leave both chassis safely grounded. By no means  a guaranteed solution, but easy to try.
Kelvin
Michael
Reply with quote  #4 
Hi Kelvin,

Thank you for this advice I feel like that could be the issue because I only hear the hum when the SOB itself is active and idle.  

I also did a little digging on the net and found this kit on glassware:

The House-GND kit is a simple circuit that helps reduce hum in audio equipment that is attached to a dirty house-ground connection. The diode bridge only connects the two grounds when the voltage difference between them exceeds about +/-1.4V. The small capacitor allows high-frequency noise to find a path to the house ground. The 10-ohm power resistor makes a DC connection between grounds, while still offering some isolation between grounds. The kit is quite small, the PCB being only 1 by 1.4 inches big. Includes all parts, including two hex standoff assemblies and instruction sheet.

Perhaps if removing the ground from the RCA input, its possible that house-ground isn't very good, and this route could be explored.
Richard
Reply with quote  #5 
Hi guys,

firstly Michael, please don't take this the wrong way, but I'm glad you have this issue too [smile]  I hope we can both find a fix.

Kelvin, thanks for your help.  I've tried a couple of things:

With earth disconnected from the Masterpiece I connected the 2 chassis, and the hum reappeared.

Complete disconnected the Masterpiece and connected the earth of its mains cable to the ground of its output interconnect.  No hum.

Reconnected the Masterpiece mains earth to chassis and as you suggested, cut the ground from the SoB rca input to the chassis.  Sadly, the hum returned.

So I think I need to be looking inside the MP for a solution.  I'll ponder on it some more.

Thanks for your help.
Kelvin Tyler
Reply with quote  #6 
Richard. Sorry to hear that your problems with hum persist.
From what you say, ("Complete disconnected the Masterpiece and connected the earth of its mains cable to the ground of its output interconnect.  No hum."), I rather get the impression that the output RCA grounds may not be properly connected to the chassis. The primary connection to the chassis for these and other key grounds is at tag-strip T13B. This tag has a good number of connections to it. It is vital that all connections are good here and the screw making the chassis connection is really tight. Then make sure that there is good connectivity between the external ground barrels of each RCA connector and the chassis.
These hum-loop issues can really test one's patience!
Good luck,
Kelvin


Michael
Reply with quote  #7 
Hi Kelvin,

the hum is gone from my system after I disconnected the "star ground" to chassis from both RCA outputs and inputs for SOB. The mains earth ground is still connected of course.  
Kelvin Tyler
Reply with quote  #8 
Michael. Good to hear you have cured your hum problem. Not quite sure what you mean by RCA outputs and inputs on the SOB. The RCA connectors on a standard SOB are inputs. Probably just my misunderstanding.
Regards,
Kelvin
Bruce Rozenblit
Reply with quote  #9 
Michael,
What you have done is to ground the chassis through the power cord ground which is then re grounded back to the circuit through the cable shield.  The cable shield is grounded to the chassis of whatever equipment it is connected to, which is grounded to the power line ground thereby completing the circuit.

This eliminates the ground loop between the chassis, power line ground and cable shield.  If you float the power line ground now, the circuit should hum, especially if you touch the chassis, as it is floating and will act as an antennae.     
Richard
Reply with quote  #10 
Well I've been through everything in the Masterpiece and couldn't find anything wrong.  Apart from a whole load of zero resistance from output signal to ground which had me totally confused for a couple of minutes until I realised I'd left the relay plugged in [crazy]

Whatever hum there is is now much quieter, but it does seem to come and go.  I could definitely live with it if it stays like this, but I'm gonna go through the SoB when I next have some time.
Stephane Bureau
Reply with quote  #11 
I have a device called the Hum X by Ebtech (about 60$) and it gets rid of the ground loops I get sometimes when there are 2 or more devices plugged on the same circuit. I like it very much.
Mel
Reply with quote  #12 
I have the MP and SOB combo as well and just completed both builds. I am also having this ground loop hum problem.

Reading that Michael seems to have resolved his ground loop issue by disconnecting the RCA ground wire from the Ground Lug in his SOB, I wanted to ask whether this is perfectly fine and causes no other issues before I do the same.

In troubleshooting this issue, I have found the following:

1.  I am only looking at three devices in my system, the MP, the SOB, and my DAC.

2.  The humming stops if I disconnect the Earth ground pins of both the SOB and the DAC. Either alone does not resolve the problem. Because of this, I am deducing that all three devices are forming this ground loop since all three have Earth ground pins attached to chassis and/or RCA jacks of all three devices, and all go to Earth ground in the power mains.

3.  However, and this is partly why I asked if there are any other issues above, I notice that disconnecting the power cable ground pins of the DAC and SOB results in the sound quality having noticeably less air, "openess" and "emotional feel" to the music. This is subtle but noticeable to me. (I wonder if anyone else has noticed or experienced this before?)


I definitely want to keep the safety Earth pins connected for all three devices and this is only done as part of troubleshooting the ground loop problem only.

Therefore, if I disconnect the RCA ground wire from the Ground Lug in the SOB, will this have the same effect of degrading sound quality? Or create any other issues? I know that it would mean the shields in the RCA interconnect cable won't be grounded to the SOB chassis but it would be connected to ground at the other end (at the MP RCA input jacks). Is this a bad thing? Would it pick up EMI like an antenna?

Wolfgang
Reply with quote  #13 

If everything else is 100% ok with the amps, pre and DAC and there is no problem hiding somewhere I would try to change the power plug connections. Power transformers are not symmetrical and their isolation resistance is never 100%. This means that some tiny little amount  of the mains power can go into the audio circuit. Depending on the position of the power plug this “leaking” can be a little more or less. If it’s low enough or more or less equal  potential in all connected  amps, DACs  it doesn’t matter and there is no (audible) current  through the interconnects (hum) because there is no potential difference.  If it’s significantly more (1V for example) in one gear there will be audible hum.

The easiest way to find out is to change hot/neutral connection of the  power transformer. With 3 prong plugs and keeping the earth ground safety connection this means resoldering .

I must have even a little circuit somewhere which can indicate  this problem with an LED if it exists. If there is some general interest for building it (opamp and some diodes, resistors) I can try to find and post it.  But first I would try all combinations with the power plug.

Arvis
Reply with quote  #14 
Hi Wolfgang,
if it is not of much hassle I would be interested in seeing the small circuit posted.

thanks,
A.
Mel
Reply with quote  #15 
Quote:
Originally Posted by Wolfgang

If everything else is 100% ok with the amps, pre and DAC and there is no problem hiding somewhere I would try to change the power plug connections. Power transformers are not symmetrical and their isolation resistance is never 100%. This means that some tiny little amount  of the mains power can go into the audio circuit. Depending on the position of the power plug this “leaking” can be a little more or less. If it’s low enough or more or less equal  potential in all connected  amps, DACs  it doesn’t matter and there is no (audible) current  through the interconnects (hum) because there is no potential difference.  If it’s significantly more (1V for example) in one gear there will be audible hum.

The easiest way to find out is to change hot/neutral connection of the  power transformer. With 3 prong plugs and keeping the earth ground safety connection this means resoldering .

I must have even a little circuit somewhere which can indicate  this problem with an LED if it exists. If there is some general interest for building it (opamp and some diodes, resistors) I can try to find and post it.  But first I would try all combinations with the power plug.




Thanks for the suggestion. In my case scenario, since lifting the Earth ground pin on 2 out of 3 of the power cables stops the humming, would this not be a clear case of ground loop induced humming? Am not sure if what you described applies in my case since the power cables are the same except with the ground pins disconnected, but it could be that I'm not understanding what you mean, so forgive me if I misunderstood.

It appears that this is a very common problem and I am guessing that almost everyone seems to experience this problem if they adhere strictly to having the safety Earth ground connected to the chassis ground point, on all devices within the system. This would result in multiple routes to ground in the interconnected system, and hence the ground loop issue arises, correct?

I would have thought that by now, there would be a standard approach to include a ground loop blocking circuit as part of the overall design of, e.g. a tube amp or a tube preamp, or a commercial DAC, etc. I found the following one at the Elliot Sound Products (ESP) website:

[earth-f4] 
I wonder if anyone has tried this or any similar ground loop blocking sub-circuit in the SOB or MP?

And, would I be correct that the "Zero Volt Line" above would, in the case of the SOB, refer to terminal T2-D where the black (ground) wire from the 250V transformer is connected?

I might just try this out if someone knowledgeable enough can confirm this please.

Thank you!
  





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