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Class D Switching Power Amplifiers and Power D/A conversion

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Old 10th January 2012, 12:09 PM   #1
back is offline back  Greece
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Default twisting cables

hi guys.

i was wondering how critical it is to twist the cables in class d amps from

power supply to amp and what`s the best way to do it since they 3 cables

and not 2.

thanks in advance..
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Old 10th January 2012, 12:22 PM   #2
AndrewT is offline AndrewT  Scotland
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What are the cables carrying?

The twisting is an attempt to reduce loop area and to average out the magnetic field.
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Old 10th January 2012, 12:22 PM   #3
Einric is offline Einric  United States
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Get a sizable length of cable and tie one end of the bundle in a knot.
Slip the knot end over something strong.
Chuck the other end into a portable drill.
SPIN BABY SPIN!!!
Just be careful to pull tightly when the spinning is going on (This will keep your twists tight).
Be Careful when you loosen the chuck on the drill, keep control of the wire bundle with your second hand and control the spinout.
If you just let it go you run the risk of your nice twisted wire turning into a bundle of crap.
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Old 10th January 2012, 12:26 PM   #4
AndrewT is offline AndrewT  Scotland
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No !

The individual wires/cables must be contra-rotated to allow the twisting of the set.

Or do it by hand. You can feel the rotation necessary to allow the twisting to happen.
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Old 10th January 2012, 01:44 PM   #5
gmarsh is offline gmarsh  Canada
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The drill method is fine - manufacturers of twisted pair cable for ethernet cables, etc. effectively do it that way. It does impart a twist on the individual conductors, but unless you're doing a ridiculously high twist density, the conductors can handle it.

For the short runs you're talking about, just do it by hand. Cut the three conductors to length (give yourself extra length, as twisting will shorten the cable), zip tie one end together. Sit in a chair, one wire between your legs and one wire on each side of you, and start twisting the zip tied end, guiding everything together with your other hand. The three wires on the floor will spin a bit when you do this, make sure that they don't get tangled up in each other.
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Old 10th January 2012, 02:06 PM   #6
back is offline back  Greece
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the cables are dc + - and ground.

my question was because of the three cables.

should i twist + with - and then on top of the twisted pear

the ground?
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Old 10th January 2012, 02:10 PM   #7
back is offline back  Greece
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Quote:
Originally Posted by gmarsh View Post

For the short runs you're talking about, just do it by hand. Cut the three conductors to length (give yourself extra length, as twisting will shorten the cable), zip tie one end together. Sit in a chair, one wire between your legs and one wire on each side of you, and start twisting the zip tied end, guiding everything together with your other hand. The three wires on the floor will spin a bit when you do this, make sure that they don't get tangled up in each other.
now i saw that.

if i follow your advise i could use the ground for mid and twist on it the other two?
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Old 10th January 2012, 02:31 PM   #8
gmarsh is offline gmarsh  Canada
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Just twist the three together - form the three wires into a "triangle" of sorts and twist them that way. Don't try to create an inner conductor or anything.
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Old 10th January 2012, 02:40 PM   #9
Einric is offline Einric  United States
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Here is the twist I use for three wires.
I don't see the problem/flaw in my process.
Attached Images
File Type: jpg Twisted Wire.JPG (339.7 KB, 194 views)
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Old 10th January 2012, 02:58 PM   #10
AndrewT is offline AndrewT  Scotland
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Where are the currents flowing in the +ve and -ve and ground?

An amplifier that has exactly equal +ve rail current and -ve rail current returns exactly zero current back up the ground wire. (please call it PSU Zero Volts to avoid confusing all those that think all grounds are the same).

If the amplifier is outputting some noise, that noise returns through the speaker return and that noise current passes back through the PSU Zero Volts to the PSU. Now when you measure the currents in the +ve and -ve supplies you will find they are very slightly different. So little you can ignore the noise current if the SNR is >100dB.

Now jump to an extreme case of a ClassAB amplifier pushing full power into the speaker load.
The +ve rail current returns via the speaker return to audio Ground and from there the speaker current returns via PSU Zero Volts to the PSU. Half a wave cycle later the speaker is moving the other way and it is drawing current out of the Speaker Return and sending it back to the -ve supply rail.

This weird operation of two different wires carrying the speaker current back and forth to the PSU is fundamental to ClassAB operation. It also, but to a more limited extent, applies to ClassA push pull amplifiers.

The three wires +ve, -ve & Zero Volts must be twisted as a triplet to allow the to and fro currents to cancel their fields between the alternating different pairs.
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