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Old 20th April 2010, 10:08 PM   #1
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Default Ground Plane vs Star earth

Hey guys.
I was wondering wha other members experience was with a ground plane as advocated by Pete Millett as opposed to the star earth arrangement. I have no doubts at all that Peter knows what he is doing but am worried that when I buid my next amp (6550 set) that it is quite involved to arrange the posts etc for the ground plane and i would hate to have a hum problem which would necessitate a rebuild. Given my basic understanding of some hum being caused by unequal potentials back to earth and given the advocates of star earthing what have other members found when using a ground plane?
Thanks
Nick
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Old 20th April 2010, 10:45 PM   #2
Matt BH is offline Matt BH  United Kingdom
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Good question

I like your thing at the bottom, I spoke too much once and lost my job to redundency. My saying now is "Empty drums make the loudest noise"

Pete Millets method is perfect. There was some articals a few years back in EW that went deeply into ground planes. The gist of it was that unless you are thinking of light type frequencies then the ground plane is ideal(it was a long and heated argument, mostly by people who work mainly in theory)

I have found it hard to get the lovely turrets he uses so have used normal turrets so far. I use a piece of plain FR4 and stick all sockets on it. This allows you to have minimal bolts and junk sticking through the top plate. FR4 is another thing as some people say it is hygroscopic and so there will be leakage currents. Who uses PCBs, well everyone and I have never had or heard of a problem. Even MJ mentions this in the 3rd edition.

I use the normal tree type grounding for each section and then all of them to a star ground somewhere convenient for you and on to the PS.

Cheers Matt.
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Old 20th April 2010, 10:59 PM   #3
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Good question. My answer is: in order to avoid such questions always think about currents through conductors, be they parts of stars, or parts of ground planes.
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Old 20th April 2010, 11:12 PM   #4
Matt BH is offline Matt BH  United Kingdom
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Good point Wavebourne, I guess you mean in a ground plane current(not voltage unless DC) will always take the shortest route?
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Old 20th April 2010, 11:26 PM   #5
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With a star ground, there is exactly one return path for each circuit and exactly one reference point. This is a good thing and works well for audio circuits where the goal is current loop isolation and reduced susceptibility to electromagnetic pickup.

A ground plane has some advantage in potentially lower impedance, but I don't see that as an issue in audio circuits. Ground planes are nice for high speed circuits but still suffer from problems coupling one circuit into another and require careful capacitive decoupling at high frequency. I.e. an output current loop in a ground plane can couple into a nearby input circuit, even if it's not along the "shortest path".

I vote for a star return connection as having a slight advantage in audio.

Michael

Last edited by Michael Koster; 20th April 2010 at 11:28 PM.
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Old 21st April 2010, 12:22 AM   #6
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Ground plane is fine but DON'T run any current through it - ESPECIALLY AC current. The first filter cap after the rectifier has a large AC (ripple) current through it... return THAT current to the transformer WITHOUT passing through the ground plane and 90% of your hum problems have been solved.

Most hum problems can be traced to the creation of that awful GROUND symbol on the schematic. Not all grounds are equal...
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Old 21st April 2010, 12:35 AM   #7
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by duderduderini View Post
Hey guys.
I was wondering wha other members experience was with a ground plane as advocated by Pete Millett as opposed to the star earth arrangement.
I think it's simple. Any time a current flows through a conductor there is a voltage gradient along the conductor. If you build your ground plane wrong with a speaker's return connection and the transformer's center tap on opposite ends of the plane then you have a good size current flowing across the ground plane. With current in the ground plane all it's points are at different voltages.

I think if you design the ground plane correctly it will look a lot like a star ground made with fat wires

Last edited by ChrisA; 21st April 2010 at 12:38 AM.
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Old 21st April 2010, 12:44 AM   #8
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Quote:
Originally Posted by Matt.B.H. View Post
Good point Wavebourne, I guess you mean in a ground plane current(not voltage unless DC) will always take the shortest route?
Why shortest? The whole conductor.
In case of ground plane all things share the same conductor. In case of star ground all of them share conductor that goes from the star to filter capacitor. It is inevitable.
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Old 21st April 2010, 02:00 AM   #9
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Default 6 of one...

Hey guys
So can i assume that no current into the ground plane means the cathode resistor to the power valves as well? It seems that where you drop to the ground plane influences the result. As an example the speaker ground and the tranny centre tap. So where would one ground the centre tap wrt to the ground plane? It seems the potential (pardon the pun) for capactitance and ground loops is more present in a ground plane than star arrangement.
I think i might use the star as I am least competent at that setup. If i had a tenth of the ability of Pete Millett then I would tackle a ground plane but i dont.
Thanks for the opinions
Nick
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Old 21st April 2010, 02:51 AM   #10
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Water flows down hill. So does electricity. If you have a narrow channel with big waves and you direct it to a wide plane the water still flows but the waves are reduced in height.

That is about as far as you can take the open water concept and relate it to star and ground planes. Now if you think in terms of length of pipe and pumps the water idea still holds. It is much easier to show with a diagram, might even do it one day.

The electricity does not take the shortest route across the ground plane. It uses the whole thing, your power supply hum is not fussy. The low level signal you are transferring through the ground plane also uses the whole ground plane. Sometimes we can get away with using it for both if the ground plane is big enough and its resistance is so small there is barely a voltage drop across it.

Think in terms of every copper path as a resistor. A star ground where every current has its own circuit path to a common point no longer acts as a true star if there is enough distance between the individual wires at the common point and the current is very much larger through one path as compared to the other. It all comes down to IR. When you share the same R....
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