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Reply with quote  #31 
Kelvin, Thanks for the suggestions. First off, in all of my tests for all previous posts, and how my power distribution is set up, I have all three of the devices connected to the same power distributor. So I was mindful if this possible aid as well.

I have disconnected the ground lug in the SOB as you suggested, and it did reduce the ground loop humming a little, but not completely. I then proceeded to also disconnect the star ground point in the MP at T13-B, and whilst this did also reduce the humming further, it did not completely go away. So the third step I did was to lift the ground pin in the power cable for the DAC that the humming totally stopped.

However, and this is the spoiler, there was a gradual build up of a static noise that got louder and louder over 10 mins, until it became too loud not to ignore. I'm guessing that this is probably because of the floating grounds in either or both of the SOB or MP, perhaps a static electricity build up of some sort.

Do note that the Live and Neutral pins in the power cable of the MP is still flipped throughout the above tests.

So I am probably going to revert back to reconnecting the ground lug in the SOB and the star ground point T13-B in the MP, and try to figure out if I should just live with a single Earth connection in one of the devices and lift the earth pins in the power cables of the other 2 devices. Not ideal, but what else can I do? Fortunately,  have a circuit breaker for the whole home as an added safety measure.

I do wonder though, how they earth the HiFi systems in Japan where they only have a two pin power system domestically? Seems to me they don't seem too concerned about the safety issue?

Thanks once again for all the help!

Kelvin Tyler
Reply with quote  #32 
Mel. On the MP, T13B is not the only connection of the signal circuitry to the chassis. I would reconnect T13B, leave the SOB input ground lug disconnected, and keep your DAC ground floating. As I am sure you know, it is generally best to only make one change at a time.
Good luck.
Bruce Rozenblit
Reply with quote  #33 
Do not disconnect the internal ground connections inside the Masterpiece.  Only disconnect the power cord ground.  The system is still fully grounded through the RCA cables back to the one piece of equipment. My equipment is safe to float the ground because it is fully transformer isolated.    
Reply with quote  #34 
Thank you all. I have reconnected the MP internal ground connection at T13-B, and also in the SOB at the ground lug (i.e. restored all ground connecting points including for the RCA inputs in the SOB).

While reconnecting the ground lug in the SOB, I also decided to change the four 0.1uf 400V coupling caps which in my kit, were supplied as reddish-brown generic polyester (I think) caps instead of the original polypropylene caps shown in the manual. I don't particularly like polyester caps as coupling caps unless they're well made and have a good reputation, such as MKTs from Philips, Vishay, EROs, etc. I changed them to some 0.1uf Russian K40Y-9 PIOs that I already have on hand.

I was surprised that after changing the above caps, and reconnecting all internal ground connections in the MP and SOB, the ground loop humming has gone down to a much more faint level with both the MP and SOB power cord Earth pins connected to the mains, and only the Earth pin in the DAC lifted. Before, as mentioned previously, I had to disconnect the Earth pins in 2 out of 3 devices in order to eliminate the ground loop humming. Now, with only the DAC with Earth pin lifted, the ground loop humming is faint.

I don't think the brown polyester caps are defective in any way, perhaps the re-soldering may have done something... I really don't know. I had previously checked all solder joints and everything looked good, so I don't know what to make of this. Anybody else has done something like this and had a similar result?

(Side note: the sound of the 0.1uf Russian K40Y-9 PIOs does have an impact on the sound but they have not yet been burned-in. I do notice that the bass is clearly more articulate and is more defined, and goes deeper too without being bloated or boomy. The SOB is truly amazing! Ref: Buster Williams double bass; Hans Zimmer sound track of The Dark Knight Rises, just to name two.)

Reply with quote  #35 
Just to update this post on what I have settled on in resolving the ground loop issue, I have decided to follow Bruce's indication that I only connect one device to Earth safety ground. It is not ideal from the prescribed safety practice (or regulation) perspective, but after reading far and wide, and digging deeper in the forum postings here, I have come to the conclusion that it is an inherent problem facing audio systems that adopt the "un-balanced" interconnecting signal cable approach (i.e. RCA interconnect cables in our case).

It is virtually impossible, as far as I can tell, to fix ground loops in an audio system where all devices are connected to the power mains with 3-pin power cables with an Earth ground pin connected to the mains power supply. There is bound to be an imbalance in the ground voltages between each device, and it is a question of degree whether this imbalance is slight or high. Even slight imbalances can result in relatively loud humming.

It is possible to reduce any ground loop humming by careful design, layout and sequence of the ground points and using star grounding schemes, but from what I can see and from what I have read, Bruce has already thought of everything from a design perspective to reduce this to a minimum in the case of the SOB and MP. It is in the nature of the beast (oops, no pun intended) that un-balanced interconnect systems will have differences in ground levels between two or more devices, and it can't be fully resolved unless you move to a fully balanced interconnecting audio system.

Just slight imperfections in the winding of power transformers can and almost always does, result in the imbalances between two devices. Since there is no such thing as a perfect transformer (or a perfectly wound one), there is little hope to avoid this inherent problem.

It is possible to "fix" the humming by using hum eliminator devices, and there are apparently some pretty effective ones out there, but the problem never really goes away completely, and you will be adding "filters" or "isolators" in the signal pathways.

So, in the end, I have settled (at least for now), on having the safety Earth pin connected in the power cable for the MP, and lifted in the two power cables for the SOB and my DAC. The result is that I no longer have any "humming" and the system is very, very quiet when at idle with the volume turned up to the max in the MP. There is only a very slight "hiss" from the tweeters of my speakers, but I know from experimenting with different components (mainly caps and perhaps resistors in the signal path) that this is usually due to component choice (e.g. types of capacitors) rather than the design. It is amazing and a real tribute to Bruce that there is no humming at all in a tube system (without ground loops). And to think that the output tubes are in direct contact with the speakers, no transformers and no capacitors, not many would have believed this is possible but for the fact that these amps and the MP preamp exist.

I will perhaps post in a fresh post my observations on my experiments with different coupling caps as this has had a *very* significant impact on the sound quality of the SOB and MP, as I realise that not many folks believe that the choice of e.g. capacitors used have a big impact on the sound quality of amps and preamps. Capacitor choice alone has made a significant difference to how quiet the SOB and MP are in my case at idle.

Thank you to everyone who were very patient and helpful in assisting me to try to resolve the ground loop issue, especially Wolfgang and Kelvin.


David Loader
Reply with quote  #36 
There is one guaranteed way to break up ground loops - 1:1 line transformers at the power amp inputs.  I know it sounds a bit perverse when the whole point of OTL's is to get rid of transformers in the audio path, but the distortion from a line input transformer is nothing like as bad as that from a typical power output transformer (unless very expensive).  Think of the great classical recordings from the 1960's - it's probably safe to say there were several line transformers in the signal path.

This is what I decided to do with my new Beast builds as I've had trouble with ground loops in the past.  Yes it's true that the transformer will degrade the sound slightly (I'm using Jensens to minimise this), but I would argue that ground loop noise is also a degrading factor.
Reply with quote  #37 

XLR or balanced connections can have ground loops as well under certain conditions. They are not used by design for eliminating ground loops but for eliminating noise that is picked up by long audio interconnects or noise that feeds into the audio signal from outer sources through the mains. Mostly studio setups and recording live events suffer from these kinds of things.

Good design & filtering can minimize ground loops caused by ripple current within the psu/amp and they will not/hardly be audible in the audio signal (but are still active in the background). In addition every gear has some different potential to some degree  depending on the mains polarity of the transformer and it can be precisely measured (10mV to 1,2V in my audio system before optimizing ). Even if it’s too weak to be heard as hum (normally ground loops below 1V ) it affects the clarity and openness of the sound which can be clearly noticed when it has been removed. Both sources of hum should be eliminated as best as possible for best SQ.

PP amps cancel out audible psu hum/ripple current  by design. It’s basically the same working principle as that of  XLR connections. Big advantage  to SE amps.

David’s solution of complete galvanic insulation is the best if there is a problem that cannot be cured by any other means. Audio transformers are  normally used for “impedance matching” in the recording process or in a really bad case for breaking the ground loop (1:1windings). A second best and cheaper solution is using opto couplers. Both version will change the sound a little (sometimes even to the better in case of audio transformers) but everything is better than a ground loop audible or not.

Reply with quote  #38 
Thanks David and Wolfgang,

Yes, I did come across the idea to use isolation transformers and I believe companies like Ebtech use a combination of isolation transformers and filters to tackle ground loop humming problems. There is a hit to the sound quality and the more transparent the audio system the more clearly this is perceived.

Wolfgang, with your experience and knowledge, how do I troubleshoot and eliminate such ground loop issues without compromising SQ? in particular with reference to both the SOB and MP? There are two power transformers in my DAC, a 250VA toroidal and 60VA toroidal. It has both balanced (XLR) and unbalanced (RCA) in/outputs.

Since any two out of three devices in my case result in ground loop humming when they are hooked up in any combination, how do I resolve the differences in ground voltages or other causes of this imbalance?

Btw, could you explain further what you mean when you said "Good design & filtering can minimize ground loops caused by ripple current with the psu/amp and they will not/hardly be audible in the audio signal (but are still active in the background)."

Are you saying that this is essentially the source of most ground loops?


Reply with quote  #39 


you would need to measure each transformer separately for lowest leak voltage  in case of more than one transformer in one amp/DAC. I would simply start measuring each gear and adjust the power cable for lowest voltage readings. I also would always use the original  RCA connection from DAC to MP never the XLR with pin 2 and pin1 shorted at the RCA end that connects to the MP. That has no benefit but means only extra trouble.

Sometimes other ways than star grounding the psu lead to better  suppression of ripple current. How good this works depends on the psu design and the circuit itself. There are also (uncontrolled) circulating currents (loops) through the psu that can create hum and noise and reduce SQ which should  be kept at a minimum by good circuit design. Unless one uses DC from batteries there will be always some amount of ripple current whether it's  audible or not and it will affect SQ (not everybody will agree with this statement, though). I only mentioned this point in order to acknowledge the fact that there are more sources for hum/noise problems than typical ground loops or simply  insufficient filtering in the psu. Sometimes it’s not easy to figure out what it is and we only think it’s this or that. It's almost impossible to say anything meaningful around this topic on a general basis as possible problems and improvements change from circuit to circuit.

But I can give a simple example that points in the direction of what I mean if we look at how pin 1 of the XLR connection is normally grounded. In most cases it’s connected to audio ground on the  PCB instead of  being connected first  to the chassis. Circulating  currents could easily come in through the shielding  into the audio circuit and it could create hum.  And this would be "activated" in a DAC for example  by connecting it to a preamp which would really look like a typical ground loop.

You need a circuit like shown in the attachment  because with a regular voltage meter you cannot measure very high impedance leaks.  The diode in this circuit is not reliable to indicate very small leak voltages but a regular voltage meter will give you very precise readings under all conditions. The diode flashing  means less leak current/correct polarity. But better only rely on the readings.

The amp/DAC etc needs to be disconnected from mains earth and the rest of the equipment. If you use a cheater plug for the test you can turn the power cable 180 degrees and find the better position for lowest voltage readings . One test lead needs to be connected to the audio ground (RCA, speaker) the other to mains earth. Before testing you always have to press the button (see schematics).

I cannot guarantee that it will solve your problem with the hum if you build this circuit and do the testing.  You do it at your own risk. But you have to start somewhere.



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Reply with quote  #40 
I forgot the switch for the 9V battery in the drawing.
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