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Old 9th September 2009, 04:00 AM   #1
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Default grounding cable

hello,

I am working on a separate power supply for a tube headphone amp.
The power supply will have 150vdc and 2 6.3vdc lines. My question is do I run a ground for each DC line in the cable(Aphenol CONNECTOR) or just use one ground and take off from a common point in the amp? I would think with multiple grounds you will have ground loops and with one ground all the B+ current will mingle with the heater current.


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Old 9th September 2009, 06:42 AM   #2
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Default mingle??? Are you insane????

the two currents would never mingle - they barely talk! B+ is a MUCH higher class curent than lowly heater current - the heater current a mere servant to the B+ signal current.

No no no no no, have no fear, the two currents and their associated electrons will divide themselves based on income, accent and breeding, and apart from the occasional Romeo and Juliet-esque situaition, no mingling will occur.
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Old 9th September 2009, 07:26 AM   #3
Gordy is offline Gordy  United Kingdom
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I assume that your power supply contains more than just the transformer, and has rectifier and smoothing capacitors also. So...

First attach the incoming ground from the mains to your power supply chassis as a safety ground. Take an extension from that point through your umbilical to your headphone amp chassis, also as a safety ground. OK, so now both boxes are grounded for safety. You are not going to use this safety ground for any audio signal, so make sure no signals use the chassis for a return path. (For example, that means insulating the input sockets from the amp chassis).

Back to the power supply: from the safety ground point at the power supply chassis make a connection to the power supply capacitor(s) negative terminal. This is your power supply ground reference point. (Some people like to add a small resistor between the chassis safety ground and the power supply ground reference point to help break any loops... I use a 10R 7W resistor). Now run two wires from your power supply to your amp - one for your high tension, and one from the power supply ground reference point. Once inside your amp the wire from your power supply ground reference point magically becomes your audio signal ground reference point. Do not connect it to the chassis. Use it for all audio signals.

Now to the heaters. This will depend on how they are configured. Some heater circuits need to be referenced to a tapping from the high tension to place them at the right potential relative to the cathode, while other heaters can simply be directly grounded. You need to let us know more before we can advise. Best to scan and post a circuit diagram if possible.
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Old 9th September 2009, 04:28 PM   #4
pointy is offline pointy  United Kingdom
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have you tried a rf choke in place of the 10 ohm resistor
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Old 9th September 2009, 05:00 PM   #5
Gordy is offline Gordy  United Kingdom
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Quote:
Originally Posted by pointy View Post
have you tried a rf choke in place of the 10 ohm resistor
No, but it seems a fair idea and I would give it a try. What specification of part would work? Do you have a RS / Farnell / etc. part number that you could recommend? Thanks.
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Old 9th September 2009, 05:15 PM   #6
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by tenderland View Post
hello,

I am working on a separate power supply for a tube headphone amp.
The power supply will have 150vdc and 2 6.3vdc lines. My question is do I run a ground for each DC line in the cable(Aphenol CONNECTOR) or just use one ground and take off from a common point in the amp? I would think with multiple grounds you will have ground loops and with one ground all the B+ current will mingle with the heater current.


signed
drowning in analysis
Each DC supply needs it's own "return line". In other words you make a "loop" with supply and return for each voltage. The return line is NOT a "ground" but inside your power supply there should be a common ground point. These return paths are isolated from ground except at this common point inside the supply.

Then for safety the ground connector from the power cord meeds to connect to the metal chassis and this safety ground should connect to the above "common ground".

Finally this power cable that supplies the positive voltages and the returns for each of them really, really should also contain a large diameter conductor for the safety ground which connects only to the metal chassis. A rule is that this conductors should be at least as large is any other conductor in the cable. (I'd make it one size larger than the wire used for the heaters)

Also your power supply needs to have a fuse on each voltage. The fuse needs to be on the supply end of the cable to protect the wiring in the cable from fire in case of a short in the amp.

If your high voltage power supply is not housed in the same enclosure and the cable runs unprotected, (something like the way RCA cables run in typical consumer hifi rigs) then it really needs to be inside a braided safety ground shield so that if it is ever damaged, bitten by a dog, rat or cat or handled by a small child there is some protection from electrocution. High voltage DC is VERY dangerous, much worse than AC mains power. In my opinion HT voltages needs to remain always inside some kind on grounded conductive enclosure, either a metal chassis, conduit or shielded cable. But not everyone does this.

You add a lot of complexity and safety considerations when you use a remote high voltage power supply, most commercial designs avoid this because of the high cost. But don't skimp. Think about failure modes (a cap explodes and tosses conductive metal fragments all over and then you have 300 volts shorted to a pair of headphones on your head.) It's harmless if the above guidelines are followed, really bad if redundant safety systems were not in place.

Last edited by ChrisA; 9th September 2009 at 05:28 PM.
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Old 9th September 2009, 09:39 PM   #7
pointy is offline pointy  United Kingdom
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Gordy...............................

something like this <http://www.iop.org/EJ/abstract/0950-7671/40/4/425>

in the link they use 39 turns i like to use about 10 turns of the earth (ground) wire through
a 1 inch toroidal (ring) mumetal core.
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Old 10th September 2009, 12:49 AM   #8
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Default excelent stuff

Great reply s , thank you. Looks like there will be 3 DC voltage lines. A B+ =150v
and 2) 6.3V DC lines for the heaters. I f i understand correctly I would ground each filter cap from each DC supply to a common point in the chassis of the supply . These grounds will not connect to chassis ground in the amp. kinda like a star ground in a regular amp. I appreciate the safety advice, when you're dreaming of Dark Side of the Moon on the HD 600's You kinda neglect the fact that you have "300V on you're head"


Thanks again , might be time to buy some Iron
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Old 10th September 2009, 01:40 AM   #9
Gordy is offline Gordy  United Kingdom
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Quote:
Originally Posted by pointy View Post
Gordy...............................

something like this <http://www.iop.org/EJ/abstract/0950-7671/40/4/425>

in the link they use 39 turns i like to use about 10 turns of the earth (ground) wire through
a 1 inch toroidal (ring) mumetal core.
Thanks pointy, I appreciate the advice. I'll be giving it a go in a future project.
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Old 10th September 2009, 01:56 AM   #10
Gordy is offline Gordy  United Kingdom
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Quote:
Originally Posted by tenderland View Post
Great reply s , thank you. Looks like there will be 3 DC voltage lines. A B+ =150v
and 2) 6.3V DC lines for the heaters. I f i understand correctly I would ground each filter cap from each DC supply to a common point in the chassis of the supply . These grounds will not connect to chassis ground in the amp. kinda like a star ground in a regular amp.
The safety points made by ChrisA are very sensible.

Regarding heaters; yes, but only if they are not floating / offset at a high voltage (because of heater - cathode ratings). Even after grounding in the power supply I would still tend to run separate out and return lines for each of the heater supplies. In the amp chassis they would touch nothing else except the tube pins.

So, it looks as if your umbilical will have a grounded overall braid and the following connector to connector wiring:
1. Safety ground from power supply chassis to amp chassis
2. Ground from power supply ground reference point (at filter caps) to audio ground in amp
3. High voltage DC supply
4. Heater circuit 1, 6.3V out
5. Heater circuit 1, return
6. Heater circuit 2, 6.3V out
7. Heater circuit 2, return


Quote:
Originally Posted by tenderland View Post

I appreciate the safety advice, when you're dreaming of Dark Side of the Moon on the HD 600's You kinda neglect the fact that you have "300V on you're head"

That's a very good reason for using a transformer coupled amp!
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