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Old 4th October 2008, 10:50 AM   #41
Mooly is online now Mooly  United Kingdom
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Hi Andrew,
I can not see any real problem moving the blue wire to the RCA's. There is no current flow in this ground connection.
I can see that as I have drawn it that it implies the two channels on one PCB, which may be better if more awkward. Even with separate PCB's however the principle is the same, "two blue wires" one to each input ground, and there is still no potential difference between them.
As to the blue wire picking up interference, maybe it isn't very susceptible to pick up as there is no "loop" for an induced current to flow.
Two separate PCB's though, yes there is a loop, so yes in that case beware.
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Old 4th October 2008, 11:19 AM   #42
Bonsai is offline Bonsai  Taiwan
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JCX, I agree with your statement that the combined currents in the + and - supply lines will be linear and reflect the load current. However, when the amp is running class B, only one of the supply leads is carrying current (half cycle) and this is full of harmonics. I think this is what amplifer guru was referring to and trying to minimize the coupling of the garbge into the small signal stages. This is a tradeoff between minimizing the loop area or dealing with the spray harmonics.
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Old 4th October 2008, 11:32 AM   #43
AndrewT is offline AndrewT  Scotland
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Hi,
if only one of the supply lines is carrying that half waveform then some other wire is returning that same current back to the PSU. It must be a loop. That loop will try to generate a field. Minimise the field.

Now, back to the dual Blue Signal Ground.
If the RCAs are not commoned and there are two Signal Ground wires then the loop between these two wires is capable of generating an EMF due to external fields. That input loop area must be minimised to minimise the EMF on the Signal Grounds.
The smallest loop area will be a single wire and that demands a commoned pair of RCAs.

I have just looked again at D.Self's ed2 grounding scheme. It shows the Signal Ground going from RCA through the input screen (missing/bypassing the PCB) then connected to the Audio Ground. Except for using the screen this is topologically the same as Leach's alternative. What is missing from the ed2 diagram are the power ground and Zobel return and PCB Signal Ground connections to the Audio ground. I like to see these as separately connected to Audio Ground. Many PCB designers combine them into one return wire.
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Old 7th October 2008, 07:56 AM   #44
Mooly is online now Mooly  United Kingdom
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Finally I seem to have made some sense of all this. I have been speaking to AndrewT over the grounding scheme of DSelfs amp and we are both in agreement that the official PCB layout and proposed wiring layout certainly appears incorrect.
This has caused me much confusion over the years, I felt it was "inconsistent" to say the least with all that Doug had said on layout etc at the time.
Even as a Monoblock it's not ideal. There are two 0.1 mfd caps that feed into the input ground carrying rail disturbances into the critical input ground.
Having built these amps some 14 years ago I yesterday modified the boards. Cutting the input ground, this point was now directly returned to the star. Also the speaker return was taken off the PCB and returned to the star. For the first time the amp was now silent, no audible hum/buzz etc. All the hours I spent on this at the time as well. The problem of the channel interaction that was again a major puzzle disappeared.
This diagram shows the grounding of the original PCB with the main components laid out as they actually appear.
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Old 7th October 2008, 07:57 AM   #45
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This is how it ended up,
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Old 7th October 2008, 08:09 AM   #46
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I feel the best solution may well be to have the left and right channels on a common PCB. Even with the above diagram can we really say the volt drops caused by the input circuit wiring ie the music signal from the player causing a volt drop in the wiring is zero. In the pursuit of perfection probably not.
Also the now separate grounds from input, back via the pot, then via the cables to the CD player also form a loop of a sorts.
Certainly worth thinking about !
Ultimately I have been pleased to not only find an answer after all these years, but to have increased my understanding of this subject.
Karl
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Old 7th October 2008, 10:10 AM   #47
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Quote:
Originally posted by Mooly
I feel the best solution may well be to have the left and right channels on a common PCB. Even with the above diagram can we really say the volt drops caused by the input circuit wiring ie the music signal from the player causing a volt drop in the wiring is zero. In the pursuit of perfection probably not.
Also the now separate grounds from input, back via the pot, then via the cables to the CD player also form a loop of a sorts.
Certainly worth thinking about !
Ultimately I have been pleased to not only find an answer after all these years, but to have increased my understanding of this subject.
Karl
Your diagram in post 45 looks good to me.
Loops are not important unless the current flow around them causes enough voltage drop to be significant.
I'd recommend keeping the two channels gnds together as far as practical and minimizing the loop area where they are split. You can connect the RCA connectors gnds together at the case panel. Avoid allowing the RCAs to gnd to the case itself at this point - use insulators.
Glad you've banished the buzzing.
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Old 7th October 2008, 12:19 PM   #48
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Hi Traderbam,
A mystery solved, that's for sure. Those 0.1 mfds cause visible distortion on squarewave testing too. Just confirmed by measurement.
Grounding is a subject on it's own isn't it ?
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Old 7th October 2008, 07:39 PM   #49
VivaVee is offline VivaVee  New Zealand
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What you have drawn is a facsimile of Fig 14.1 "Grounding scheme for a typical power amplifier"from Douglas Self's book. 3rd Edition.

The only thing missing is the power transformer and main power supply caps.

What you have noted in respect to moving the grounding point for the 0.1uF caps is what D Self referred to as Distortion number 5: rail decoupling distortion (see Chapter 6). Moving the grounding point of these caps to the star ground predictably provides the benefit you observed.

With regard to reducing the Class B related noise currents in the power supply lines, this was first published by Cherry in 1981. The AES paper is worth digging out as it makes more sense than Self's writing on the topic as I recall.

I am not surprised by the confusion over a suitable remedy. Self describes two solutions with no preference given.

1. tightly twisting + and - and GND wires
2. tightly twisting just + and - wires.

More relevant is his advice to reduce the loop area in the input and feedback circuits of the amplifier. This is standard designing for EMC advice familiar to most engineers.
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Old 7th October 2008, 10:22 PM   #50
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Default I am giving a prize.... a nice double Ice cream covered with Chocolate


to the first one able to perceive differences in sound reproduction with any of those approaches.

All rigth..... Coke can go together.

regards,

Carlos
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