# Hum



## alan monro (May 9, 2006)

You know when you set up your million dollar HT and to your horror there is a hum that you can not get rid off:hissyfit::hissyfit::hissyfit: Here is the fix . Go out , leave the room and don't go back .:wave: If you have to go back this is what to do. Cut your earth wire in each plug and solder 2x4amp diodes back to back, ie, anode to cathode in//L . insert your two back to front diodes (that are wired in parallel )in line with your earth . How it works is that most HUM voltages are less than .6 volts But any voltages above .6v go to earth .SO , you still have a safe earth and the hum has gone:hail:Kind regards Alan .:T


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## brucek (Apr 11, 2006)

*Re: Hummmmmmmmmmmmmmmmmmmmmmmmm*

This is the standard galvanic isolation, but there's a problem Alan.

What happens when the safety ground is required as a result of a fault and you require the breaker to trip for your protection. Your 4 amp diodes burn up before the slow reaction breaker trips off and the protection is lost. A bit dangerous......

brucek


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## F1 fan (Jul 6, 2006)

It is not wise to cut the safety ground.If you want to do this mod it is best done internally in the unit by leaving the safety ground wire attached to the chassis but the diodes can be placed between circuit ground and chassis.Another approach Ive seen used is to place a thermistor between circuit and chassis ground.


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## alan monro (May 9, 2006)

*Re: Hummmmmmmmmmmmmmmmmmmmmmmmm*

Brucek . I see your point. Maybe use heavy power diodes???? alan


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## alan monro (May 9, 2006)

Fred You are saying have the diodes in series with the thermister?? alan


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## F1 fan (Jul 6, 2006)

alan monro said:


> Fred You are saying have the diodes in series with the thermister?? alan


No Alan just the thermistor alone.


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## F1 fan (Jul 6, 2006)

*Re: Hummmmmmmmmmmmmmmmmmmmmmmmm*



alan monro said:


> Brucek . I see your point. Maybe use heavy power diodes???? alan


Bryston and PASS labs use a high current bridge rectifier for this purpose.Here is a schematic for the Bryston 3B SST.You can see the bridge rec at the top left. http://www.bryston.ca/BrystonSite05/pdfs/SSTAmplifiers/3B-SST-SCH-1C(Oct02).pdf


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## brucek (Apr 11, 2006)

> If you want to do this mod it is best done internally in the unit by leaving the safety ground wire attached to the chassis but the diodes can be placed between circuit ground and chassis.


Unfortunately, there is more work to carrying out that mod than it may be worth. 

Separating the AC and DC ground in a device requires that all connections to the 'outside world' be isolated. For example, all the RCA connectors must be isolated from the chassis since the RCA connectors outer case is at DC ground, you can't allow them to contact the chassis (which is AC grounded). Any manufacturers that provide the "ground lift" feature take precaution to do this extensive isolation. It ain't cheap.

Actually, once you've created the isolation you only need a small resistor between AC and DC ground to provide the ground lift. All my ST Bryston amps use the resistor technique with a selectable switch unlike the newer SST model that you've shown.



> Maybe use heavy power diodes????


Well, I guess. They would have to be mounted to a metal heat sink to dissipate the heat. You would need to test that the circuit would trip before they overheated in a dead short situation. I would advise finding the cause of the hum as a better solution. Actually, this device uses the parallel diode technique.

brucek


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## F1 fan (Jul 6, 2006)

brucek said:


> Unfortunately, there is more work to carrying out that mod than it may be worth.
> 
> Separating the AC and DC ground in a device requires that all connections to the 'outside world' be isolated. For example, all the RCA connectors must be isolated from the chassis since the RCA connectors outer case is at DC ground, you can't allow them to contact the chassis (which is AC grounded). Any manufacturers that provide the "ground lift" feature take precaution to do this extensive isolation. It ain't cheap.
> 
> Actually, once you've created the isolation you only need a small resistor between AC and DC ground to provide the ground lift. All my ST Bryston amps use the resistor technique with a selectable switch unlike the newer SST model that you've shown.


Good point Bruce, yes it would be impractical to do on some units as they are connected to the chassis at several points.However most of the better "Audiophile" grade equipment Im aware of do have their inputs isolated and chassis connection is done at one point so doing the mod is possible. But this should only be attempted if you are confident you know what you are doing and as a last resort.

Yes your ST's and all previous generations use a 10ohm 5watt resistor for ground lift. I believe that tighter standards put forth by the CSA and other standards agentsies will no longer approve of this method.


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## alan monro (May 9, 2006)

Fred How would the thermistor work to remove the hum . puzzled . Alan


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## alan monro (May 9, 2006)

Brucek . In Australia we have earth leakage detectors on our power meter board .They trip I think at 20 ma. and cut off power in milliseconds so there is no shock even if one puts ones hand in a toaster I personally haven't tested the toaster thing yet , and don't think i will . So say using 10 amp power diodes would be sufficient . ?????? Alan


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## F1 fan (Jul 6, 2006)

alan monro said:


> Fred How would the thermistor work to remove the hum . puzzled . Alan


Alan it does so by adding some resistance.Here is a link to a project that uses one.The schematic is on page 2 and a short description is on page 3. http://www.passdiy.com/pdf/balzenpre.pdf


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## lcaillo (May 2, 2006)

Why not locate the source of the ground loop and correct the grounding path that creates the problem, or insert an isolation device in the signal line that solves it if the grounds are OK.


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## alan monro (May 9, 2006)

Fred . The circuit you provided would work with hum picked up from a analogue input line .But , we are talking about hum introduced to the amp due to connections from the other equipment not influenced by the amp volume control . alan


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## brucek (Apr 11, 2006)

Alan, do you have a hum in your system that you could tell us about. Perhaps we can help you get rid of it....


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## alan monro (May 9, 2006)

Bruek . Automotive alternator diodes would be ideal for the job . I tend to think the Ebteck devise would have something similar in its guts . And to my observation over the years when a diode fails it shorts out . alan


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## alan monro (May 9, 2006)

brucek said:


> Alan, do you have a hum in your system that you could tell us about. Perhaps we can help you get rid of it....


Thanks Brucek . I did have hum , it wasn't influenced by the volume control (optical input) so i knew it was a earth problem . The only piece of equipment that had an active earth was my sub amp so i scientifically snapped it off with pliers . It really didn't need a earth as i built it double insulated . wooden frame and plastic pot shaft etc. Then i thought How could i connect an active earth , cure the hum problem , and do it cheaply and safe and so the Thread . alan


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## alan monro (May 9, 2006)

lcaillo said:


> Why not locate the source of the ground loop and correct the grounding path that creates the problem, or insert an isolation device in the signal line that solves it if the grounds are OK.


Icaillo , In my case I found the source of the ground loop and fixed the problem . My only line in is from my dvd and that is optical . I disconnected it and it did not make any difference at all . alan .


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