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Old 2nd October 2008, 01:04 AM   #11
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Default Re: 3 stage LIN topology - NFB tappings?

Quote:
Originally posted by AndrewT
. . .or locate the lower leg resistor at the source component (and the source resistor controls the amp gain, =input 3). . .
Hi man!
So sorry that it took such a long time.
I don't have much tools for this.

Each physical model that I've built operates best with the NFB ground farthest from the speaker ground (a noise). All attempt some of your #3 above, except that I always had the additional series resistor on the -input. Its all similar to a balanced amplifier that just happens to have an RCA jack.

In my opinion, the "long wire for - input" concern is a smaller concern. Its unknown if there will be a noise on that line, but it certain that its farther away from a known source of noise.

And, that's the best I can do, until someday there will be a method devised with diodes to make sure that the 0v line is never polluted with AC from the speaker ground.

Cheers!
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Old 2nd October 2008, 01:08 AM   #12
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You should organize a point with the lowest dynamic impedance to all powering voltages, for your star ground, and connect this point to chassis. In such case ground loops caused by external connections will have the lesser possible impact.
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Old 2nd October 2008, 01:21 AM   #13
Eva is offline Eva  Spain
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My suggestion:

RCA ground wired to central ground, then to amplifier module as signal and feedback ground. This prevents any parasitic current to disturb the signal ground reference on the module. No DC should flow through this wire and it shouldn't be used for decoupling either.

Speaker ground wired to central ground, then to amplifier module as power and decoupling ground. This prevents signal ground to be disturbed by the currents flowing through decoupling capacitors and other elements.

Both grounds joined only for RF at the amplifier module through a RC network (like 10n and 15 ohms which becomes effective above 1Mhz).
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Old 2nd October 2008, 07:23 AM   #14
Mooly is online now Mooly  United Kingdom
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Eva, Wavebourne, AndrewT, Daniel and all,
What's your take on this ?
This follows D Selfs work on his blameless amps and is the arrangement Doug recommends. Each amp is discrete, the grounding line on the PCB follows that in my drawing. The speaker ground is as shown on the PCB.
What I can't grasp is this. In isolation ( as a Mono amp )each amp is perfect.
What happens when the the two input grounds connect together at the source.
If the top amp is supplying say 5 amps positive into the load then the ground lead to the star will have a volt drop along it with reference to the star. No problem in itself. The lower amp will be at a different ground potential to the upper, so what happens when those input grounds connect. To me an unwanted volt drop is now developed along the ground track of both amps.
Hope you can follow that. This is something that really bugs me, I can't grasp this.
To me the speaker return should not be on the PCB for a stereo pair on a single PSU.
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Old 2nd October 2008, 07:35 AM   #15
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Wild guess: A stereo (or dynamic) expander, if the amp shown in your picture is inverted?
Otherwise, I have no clue, so I'll leave it up to someone who does.
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Old 2nd October 2008, 08:18 AM   #16
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by Mooly
Eva, Wavebourne, AndrewT, Daniel and all,
What's your take on this ?
This follows D Selfs work on his blameless amps and is the arrangement Doug recommends. Each amp is discrete, the grounding line on the PCB follows that in my drawing. The speaker ground is as shown on the PCB.
What I can't grasp is this. In isolation ( as a Mono amp )each amp is perfect.
What happens when the the two input grounds connect together at the source.
If the top amp is supplying say 5 amps positive into the load then the ground lead to the star will have a volt drop along it with reference to the star. No problem in itself. The lower amp will be at a different ground potential to the upper, so what happens when those input grounds connect. To me an unwanted volt drop is now developed along the ground track of both amps.
Hope you can follow that. This is something that really bugs me, I can't grasp this.
To me the speaker return should not be on the PCB for a stereo pair on a single PSU.
Help
Back in 2000 I modified my Crimsons to match the D.Self grounding layout. It reduced the buzzing I had all those years (Crimson grounding is plain wrong). It did not eliminate the buzzing.

After joining this Forum I discovered that the speaker return to star ground was an option, voila!
This is closer to the Leach layout with RCA terminal being the signal ground and the main star ground (Audio Ground) where all come together.

It was this experimentation (I have tried with literally hundreds) that prompted the original question in this thread.
Where best to take the signal ground?

I have not tried input3 since it means modifying my preamps/sources to similar to pseudo balanced. But if it works, universally, then it's something I could and would do.
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Old 2nd October 2008, 11:12 AM   #17
Mooly is online now Mooly  United Kingdom
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Hi Andrew,
I tried everything I could think of on Dougs amp. I always got a small buzz when those inputs were all shorted together.
The only answer ( but is it the right one ) to me is too connect the speaker grounds back to the star.
Doug talks of distortion figures in the 0.00xx region. This problem must absolutely dwarf that. I don't get it !!
If you look at my drawing and imagine the top amp is belting out 100 watts into 8 ohms. That puts a very measurable signal between the two (as yet not connected ) grounds of the two amps. If just the input of the lower amp is connected to the ground of the upper amp a very recognisable signal will now be fed into the lower amp.
Question ? If you connect the grounds together does or does not a "circulating" current flow in the loop that has now been created and this current is related to the audio. If you isolate the grounds with lift resistors doesn't this in itself cause the input to the amp to be modified slightly.
This is the "Final Frontier" for me. I have got to understand this
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Old 2nd October 2008, 11:26 AM   #18
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back to the original question, the reason that nothing is said about where the feedback ground returns to is, it'd not as important as where the feedback taps off from. there are significant currents and voltage drops present in the output stage wiring, which can be sources of error in the feedback. the input ground does not have these large currents circulating through it and so does not present a significant source of error.
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Old 2nd October 2008, 11:46 AM   #19
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by unclejed613
the input ground does not have these large currents circulating through it and so does not present a significant source of error.
but where the signal ground terminates does make a difference to the final output signal.
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Old 2nd October 2008, 12:04 PM   #20
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Quote:
Originally posted by Mooly
Hi Andrew,
I tried everything I could think of on Dougs amp. I always got a small buzz when those inputs were all shorted together.
The only answer ( but is it the right one ) to me is too connect the speaker grounds back to the star.
Doug talks of distortion figures in the 0.00xx region. This problem must absolutely dwarf that. I don't get it !!
If you look at my drawing and imagine the top amp is belting out 100 watts into 8 ohms. That puts a very measurable signal between the two (as yet not connected ) grounds of the two amps. If just the input of the lower amp is connected to the ground of the upper amp a very recognisable signal will now be fed into the lower amp.
Question ? If you connect the grounds together does or does not a "circulating" current flow in the loop that has now been created and this current is related to the audio. If you isolate the grounds with lift resistors doesn't this in itself cause the input to the amp to be modified slightly.
This is the "Final Frontier" for me. I have got to understand this
You are most wise if this is your final frontier! Your questions show you have a brain and aren't afraid to use it. Good.
Let me remind folks of the basics: voltages are relative and currents flow in circuits (loops) - at least at the frequencies we are concerned with here. It is really very simple, but often overlooked on the erroneous assumption that wires have negligible resistance.

Of course, if the input gnds of the two amps are connected via wires of finite resistance carrying high currents, the input gnds will be at different voltages as a result. And if you connect the two inputs to the same source component, you'll cause ground currents to flow to and from the source component through the interconnect shield. This is not good. One solution is to connect the input gnds to the star gnd directly using their own, dedicated wires. With some pcb layouts you'll need to cut the connection between input gnd and spkr gnd (or other grounds). You also have to be careful how you connect things at the star ground - for it is not a zero-resistance point, it is usually a bolt thread - the sequence of eyelets on the bolt matters too.

Keep questioning everything and thinking for yourself. In the audio world, notoriety and amount of publishing is no guarantee of competence.
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