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PostPosted: 24 Sep 2017, 13:03 
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Hi, if you are talking about the shields on the input cables, no. It is isolated from the chassis/earth ground. It should connect to the signal circuitry ground.

Good listening
Bruce

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PostPosted: 24 Sep 2017, 15:38 
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I'm using shielded cables inside preamplifier. Where should shields of the signal cables inside amplifier go? I believe it's better to connect them to earth ground (ofcourse only one sided). So we don't have to worry about contaminating signal circuitry/ground. What's your opinion Bruce?


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PostPosted: 24 Sep 2017, 16:49 
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Hi, Inside ones are the same as the outside ones. The shields go to the signal ground. If you think about it...anything attached to the earth ground will be at a slightly different potential from the signal ground. Say the difference is something like 25 millivolts (not an unusual amount of difference BTW) . It is impossible to really tell which ground, signal or earth will actually not be at a zero potential. Likely neither and it doesn't matter....the difference is all important. Then if you run the signal through a shielded wire connected to the earth ground there will be that difference when the signal portion connects to the active circuitry which has the other ground level. The circuitry would see the signal + some fraction of the 25 millivolts and not signal + essentially zero. The shield will tend to radiate into the center conductor. This will result in noise and hum usually. If it is audible depends on what the actual signal strength in the wire is. In high gain circuits it could be disastrous. The mixed signal + noise will be amplified just as if it were the desired signal. In days long gone past they used to connect the signal and chassis together with varying degrees of success. Since then we have better gear and the hum they probably had would now be seriously objectionable. Folks were grateful to have any sound and a little hum didn't bug them. Additionally they only had two wire mains systems. Sometimes the chassis were isolated and sometimes not. Again with varying degrees of success. Now in most countries you must have all external metal parts at earth ground potential of double isolated from the internal circuitry. The latter is hard to do in diy gear. There a many books written on grounding and shielding. It is not as easy as it would seem. The way I recommend works. I can achieve signal to noise levels below -90 dbv in high gain circuits with it. In the power amps I don't find that shielding of the input signal is necessary unless the distance from the input jack is over about 4 inches. No harm in doing it for shorter distances, just I have not found it needed as the signal strength in that circuit is relatively high.

All this is probably more information than you wanted. I have posted additional information on shielding and grounding in the tech section of the web site. I don't claim it covers everything, but it will get you through most situations.

Good listening
Bruce

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PostPosted: 24 Sep 2017, 18:53 
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I forgot to tell which cable I used. I have 3 wire cable. One is for signal, the other is for signal ground and the last one is the shield. If I connect only one side of the shield wire to the earth/chassis ground, will hum or noise occur?


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PostPosted: 26 Sep 2017, 16:58 
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Hi, I attach the shield in such cables to the signal ground. There is some contraversy over which end (source or termination) of the cable's shield to do this with though. I find it quieter in my gear to have the shield on the source side. Others have had the experience being better on the termination end. If it is inside the gear it probably will not matter as the distance is small. Attaching it to the earth ground will likely cause noise or hum pick up because it will be the same situation as I mentioned earlier in that it will be at a different potential from the signal ground.

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Bruce

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PostPosted: 21 Feb 2018, 14:47 
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http://www.rane.com/note151.html

Under the pin 1 problem section

"Any currents induced into the shield modulate the ground where the shield is terminated. This also modulates the signal referenced to that ground. Normally great pains are taken by circuit designers to ensure "clean and quiet" audio signal grounds. It is surprising that the practice of draining noisy shield currents to audio signal ground is so widespread. Amazingly enough, acceptable performance in some systems is achievable, further providing confidence for the manufacturer to continue this improper practice -- unfortunately for the unwitting user."

According to this article, a shield should not be terminated on signal ground. Well, I don't use the shield as signal return. Additionally, I'll connect shield only at one end. But I couldn't decide where to connect it yet. At signal ground or chassis??

My bests,
Ozan


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PostPosted: 22 Feb 2018, 01:45 
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I should also add I'm talking about the shield of a signal cable inside the amp.


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PostPosted: 28 Apr 2018, 03:41 
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For a preamplifier,
- Should both channel input (RCA) grounds be connected together and then to chassis ground? If it should be connected, doesn't it form a ground loop?
- Should channel output grounds be connected to input grounds and then to chassis ground?

My bests,
Ozan


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PostPosted: 28 Apr 2018, 16:22 
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ozapaydin wrote:
For a preamplifier,
- Should both channel input (RCA) grounds be connected together and then to chassis ground? If it should be connected, doesn't it form a ground loop?
- Should channel output grounds be connected to input grounds and then to chassis ground?

My bests,
Ozan

I would recomend both rca to be isolated from chassis, input connects to pcb or ampflier at the point where all grounds return.

If theres two independant grounds for left and right channel which I perfer, then you connect two two diffrent grounds for left and right input output cables.
Normally this is for a large size amplifier or pre amp only. Smaller ones have only one return ground.

These return grounds where all points return to should be directly grounded to the mains ground. Never connect to mains ground via metal case I had massive hum doing this.

The output ground should also return to ampfiler ground where all gnds return to.
Where everything returns should be away from noise sources, like transformers espcioally, away from ac cables when possible.

Thats how I do my amps and I find it to work well.


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PostPosted: 28 Apr 2018, 16:43 
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Hi, Perhaps there is a language problem here

"These return grounds where all points return to should be directly grounded to the mains ground. Never connect to mains ground via metal case I had massive hum doing this."

Nearly all electrical codes require all exposed metal parts to be either directly connected to the AC mains earth ground or be double insulated from it. This is for safety reasons. The best way I have found to handle internal circuit grounds is to connect them to the AC mains one (which would include the chassis) via either a resistor and parallel X2 capacitor or a bridge rectifier. There are numerous examples of how this is done. This allows the chassis to act both as a safety barrier and EMI shield. The input and output connections like RCA jacks need to be insulated from the chassis or as noted you will get hum.

Good listening
Bruce

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