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Old 31st August 2018, 06:13 PM   #1251
oreo382 is online now oreo382  Canada
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Quote:
Originally Posted by Cortez View Post
So there is a ground loop with the left version, right?
And in this case the same is true for this F7 layout (link) as well?
Because the RCA GNDs are connected via the green Power GND wires
through the PS PCB, as attached and marked with the orange loop, right?
Shouldn't this layout be avoided somehow?
The rca connectors + and - are isolated from the chassis in my build and I think most of them (bought mine from digikey ).The - of those rca's goes to the common star point of the +/- supply (common point of the last capacitors in the supply,NOT the first capacitors after the rectifiers),this is the same point for all your boards "grounds".I use quotation marks as this is not the true ac power ground that is connected to your power cord.This common point is then brought back to the true ac ground thru a cl60 thermistor in my build.Mr.Pass has a good schematic diagram of the power supply arrangement in one of his first watt website articles.
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Old 31st August 2018, 06:32 PM   #1252
Cortez is offline Cortez  Hungary
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Again lets put the safety earth and the metal case completely aside.
Let's assume an open desk version on a wood plate. ;-)
The input RCA GNDs are already tied together at the source (preamp),
and then secondly inside the amplifier at the PS GND and so there is a GND loop as I see.
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Old 31st August 2018, 11:26 PM   #1253
Itsmee is offline Itsmee  England
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The L & R input 0V's are in parallel (in most circumstances), hence your loop, isn't that effectively the same as a Litz or an oxidised multi-strand cable?

If it's not causing an issue, don't worry.

Last edited by Itsmee; 31st August 2018 at 11:29 PM.
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Old 1st September 2018, 05:27 AM   #1254
Cortez is offline Cortez  Hungary
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(For me) there are 2 different types of joy regarding electronics:
1) got it working
2) understand why it works... :-)

I draw a picture trying to explain my current level of knowledge about this layout.
Am I missing something? If not, why this isn't humming by default?
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File Type: png F7-gnd-layout.png (36.6 KB, 858 views)
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Old 1st September 2018, 08:39 AM   #1255
Itsmee is offline Itsmee  England
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Papa already gave the answer, there is more to it than the loop (the rabbit hole is quite deep).
You already have the knowledge to answer your own question.
If you powered the circuit from a battery, would you expect hum - no, there are no AC fields to pick up.
How does a transformer work, wire wrapped around a lump of iron, placing wire (// to the winding) close to the transformer will induce a current, rotate the wire to to cut the magnetic field at exactly 90 the pickup will reduce to 0.
The wiring is therefore susceptible to pickup, this can be negated to a degree by proper layout, run +/_ supplies together, using twisted wires on the input; If there is pickup on one wire, it will be equally induced onto the other wire - effectively cancelling and keeping the differential the same.

Bearing the above in mind, placing wiring in an inadvertent loop around the transformer is asking for trouble, move one of the signal lines to the other side to run // with the other channel will collapse the loop and reduce pickup.
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Old 1st September 2018, 01:51 PM   #1256
Cortez is offline Cortez  Hungary
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GND loop is not only causing trouble when winded to the trafo directly.
In this position with this area it usually causes trouble.
Cancelling is also working on balaned inputs with symetrical wires but
now the impedances for the GND and "hot" signals are different.
So the question still stands for me...
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Old 3rd September 2018, 07:26 AM   #1257
Cortez is offline Cortez  Hungary
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So no one could explain me why this GND loop doesn't cause any hum in an F7?
Because in theory this loop should be breaked as explained here on the 33rd page?
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Old 13th September 2018, 05:02 PM   #1258
Cortez is offline Cortez  Hungary
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To be honest I'm still confused about this "looped" common RCA + PS
GND layout regarding both hum + common impedance coupling.

I just draw a 2nd image with the same layout showing a possible current path (RED line) via the signal GND loop:
  • let's assume there is a large output current impulse on channel #1
  • it goes out to the LS and comes back via the LS #1 GND "port"
  • then most of it goes of course to the common PS GND via the CH#1 P-GND wire
  • but because of the impedance of this wire ("loaded" with the actual output current)
  • if there is no current at the moment on channel #2
  • a certain current can flow through the signal GND loop the other channels GND
  • in this case this current will modulate/distort the original signal
  • and in a special way because it's amplitude depends on the difference of the actual output currents of the 2 channels

Any thoughts on that..?
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File Type: png F7-gnd-layout-2.png (38.4 KB, 569 views)

Last edited by Cortez; 13th September 2018 at 05:05 PM.
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Old 14th September 2018, 03:53 PM   #1259
Cortez is offline Cortez  Hungary
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(If anyone is interested I continued my quest for answer here...)
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Old 18th September 2018, 04:49 PM   #1260
Cortez is offline Cortez  Hungary
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Updated simulations: here...
(Still no worries about this GND layout..?)
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