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Old 1st November 2007, 04:50 PM   #1
Jamh is offline Jamh  United States
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Default ground loop

I need your opinion.

I am using a typical gainclone (that I built among others) with a Cambridge Holman preamp. The preamp has a number of things plugged into it, tapes, cable tv, computer, all sorts of unnatural things. It only has a regular 2 prong power chord (without the ground). It is a wonderful preamp, if you know it.

I am using one of those ground-breaking audio cables from radioshack between the two, and the sound is really really good, no complaints at all. But if I were to use a regular cable instead, there would be oozes of ground loop noise, like you're listening to raw AC.

I know the amp is flawless without the preamp and his setup has worked for a year now, but the other day I tried removing the cable and was appalled at how much noise there is. I am worried about it, and was wondering what can be done. Some possibilities:

1. Keep using the ground-breaker. Can it fail and damage the amp?.

2. Somehow install a ground connection to the preamp.

3. Loose the ground connection of the gainclone. Can this be done? It is using a torroidal tranny.
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Old 1st November 2007, 05:22 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
if the ungrounded pre is double insulated you are NOT allowed to modify it by adding a grounded safety earth.

If you try to remove the safety earth from the grounded power amp you are putting all users at serious risk of electrocution if the mains side develops a fault.

You should investigate other ways of removing/minimising the hum loop symptoms.

An audio transformer (loop breaker) is an established method of achieving this. The slight disadvantage is that it can require multiple breakers costing a fair proportion of your budget and can reduce the audio performance of the system.
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Old 1st November 2007, 05:26 PM   #3
Sherman is offline Sherman  United States
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1- Keep using the cable. If it fails things get noisy but no damage.

2- It is easy enough to remove the 2 wire cord and replace it with a 3 wire mains cord and to connect the ground to the chassis. However you should only do this for equipment that is not "double-insulated". Otherwise part of the chassis could be "hot" and you will be providing a return path creating a risk of electrocution.

However, that may not fix the problem. A ground loop occurs when there is a difference in potential (voltage) between two "ground" points that are both supposed to be at the same potential (usually 0 volts). Simply adding another ground point may not change the difference potential that exists somewhere in your chain.

3- You can try disconnecting the "chassis ground" connection in the gainclone if you have one. The amp should operate just fine without it. I've built a few this way, using an external power supply with no earth connection between the PS and the amp or between the amp boards and amp chassis. Only the V+, PG+, V- and PG- (to use the Brian GT board terminolgy) wire run from the PS to the amp.
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Old 1st November 2007, 05:43 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Sherman
........ I've built a few this way, using an external power supply with no earth connection between the PS and the amp or between the amp boards and amp chassis. Only the V+, PG+, V- and PG- (to use the Brian GT board terminolgy) wire run from the PS to the amp.
and if the mains live breaks and touches the PSU sending Live down the umbilical?????
or
if another component similarly fails and sends Live down the screen of the interconnect.

If chassis is not Safety Earthed then the mains fuse cannot rupture and the chassis can remain Live just waiting for someone to touch it!!!!!!

You are playing with fire.

All exposed conductive parts MUST be connected to Safety Earth.
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Old 1st November 2007, 07:03 PM   #5
jaycee is offline jaycee  United Kingdom
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It is probably one of the sources connected to the preamp causing it. Ten to one, it's your cable tv connection.

Try just the preamp and amp, no sources.
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Old 1st November 2007, 07:28 PM   #6
Jeb-D. is offline Jeb-D.  United States
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Soundcards in desktop computers are almost always grounded sources. If your computer is a desktop check it out.
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Old 1st November 2007, 11:29 PM   #7
MartyM is offline MartyM  United States
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I'm not familiar with your preamp, as I am mainly interested in removing noise in the car environment, which is a b1tch to do.

I found a great solution for my 12AX7 tube preamps modified for car use: I use an isolated 9-18V DC: 12V DC regulated power supply, thus removing the ground loop back to the car's chassis, or in this case, my system's star ground. Noise is gone & no harm to my input audio signal fidelity.

Works GREAT, and my main goal was to avoid the use of RCA isolation transformers because of the harm it does to the sound quality.

Paid about $25 for this 1.25A DC-DC supply, and l'm loving it! Small, too.

You can get them with just about any output voltages you need, both positive only or +/-, if that helps....
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Old 2nd November 2007, 04:34 AM   #8
Jamh is offline Jamh  United States
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ok, here is the result of some tests:

1. No hum with just the preamp and amp (no other inputs attached).

2. No hum with the turntable added.

3. Moderately unacceptable hum from the dac (connected to the computer via digital rca). I'm surprised at this. My home-built dac is super quiet in other settings.

4. Lots of hum from the cable set top box.

5. More hum added when the audio out to the computer's audio in is attached.

All the noises are additive, and present at the same level even when the volume is turned all the way down, or even if that input is not selected.
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Old 2nd November 2007, 10:50 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
try inserting the loop breaker into the route from cable set top to pre-amp.
Then move the loop breaker to the computer's audio in/out.
If this cures these two worst problems then buying sufficient loop breakers for these alone is a simple solution and if there is any SQ degradation it affects only those two sources.
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Old 2nd November 2007, 10:56 AM   #10
jaycee is offline jaycee  United Kingdom
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Quote:
Originally posted by Jamh

3. Moderately unacceptable hum from the dac (connected to the computer via digital rca). I'm surprised at this. My home-built dac is super quiet in other settings.

5. More hum added when the audio out to the computer's audio in is attached.
Check the case is grounded correctly - don't use 2-prong plugs on either the case, or your monitor (if it's a CRT type).

It's a bit of a surprise that the hum comes through your DAC too. What are the connections ? If you're using "coax" SPDIF, does your dac use a transformer for this?

Quote:
Originally posted by Jamh
4. Lots of hum from the cable set top box.
No surprise there! CATV systems suck. Over here in the UK, they use galvanic isolators on each persons property to avoid issues like this. Try the cable box alone - ie, disconnect it's CATV feed. I bet the hum disappears. If this is the case, you can either continue using your loop breaker cable, or better still investigate some kind of galvanic isolator for the cable feed.
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