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Old 7th December 2009, 01:12 AM   #1
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Default Grounding a wooden box chipamp

I a bit behind the curve on this board, but thanks to those who have helped me get my chipamp going to this point. I am now staring at a wooden box which will serve as the amp enclosure, and I am wondering if there are any considerations/strategies that are relevant to the grounding of an all wooden chassis design. The amp in question is a briangt stereo kit, with an iec, dp switch, and a CT toroidal transformer. Thanks.
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Old 7th December 2009, 02:09 AM   #2
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You will need to connect the IEC ground to the centre tap of the transformer either directly or through a "disconnect" - basically a 100 ohm, 10 watt resistor bypassed with opposing diodes. This disconnect can stop a ground loop but allows the grounds to be safely connected.
All exposed metal parts should be grounded.
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Old 7th December 2009, 05:14 AM   #3
star882 is offline star882  United States
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If the transformer is double insulated (most are), you don't even need a ground.

And grounding in a wood box is the same as for any nonconductive enclosure. So you can apply the same techniques seen in plastic enclosures,
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Old 8th December 2009, 05:32 PM   #4
Jidis is offline Jidis  United States
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I'm in the same boat as Jasenlee here. I'm trying to do a wall mounted shelf thing with a pair of small 3.5" 4-ohm drivers for TV watching. It's running a pair of TDA2050 amps and I'm trying to get it all in one unit (amp and speaker pair).

I've tested the layout of the amp boards on a scrap of plywood and I seem to be fine on electrical noise there, but I don't want to screw up on the thermal issues. At this point, I'm figuring these amp parts will end up in between a pair of small sealed box sections for the drivers (facing outward- the amp section will be between the two speaker magnets).

I haven't been able to test at full volume with a pair of 4-ohm speakers. I've just run loud for a while with a single 8-ohm cabinet tied to one of the amps. The heatsinks warm up, but don't get hot, and I can't detect much heat from anything else.

Is this something I can trap in a wooden enclosure, or do I likely need to provide some sort of ventilation to be safe? If so, how much? Some additional details: I'm running 50k pots for input trims. Transformer is a dual primary 43VA to two 14V secondaries (I actually measure 16.1VAC there). Outputs of the rectifier board are fused, and I'll likely add a fuse at the mains input to the primary. I also can't easily switch the power to this thing as the TV comes on, so I have to consider the possibility that someone may accidentally leave it on overnight once in a while. Just don't want to burn the house down.

Thanks for reading!

George
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Old 9th December 2009, 10:37 AM   #5
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Either let the heatsinks stick out or drill a few holes into top and bottom of that wooden enclosure. If the TDA's thermal shutdown activates, make the holes bigger and/or add a fan, maybe temperature controlled by a simple thermostat mounted to the heatsink.
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Old 9th December 2009, 10:45 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by star882 View Post
If the transformer is double insulated (most are), you don't even need a ground.

And grounding in a wood box is the same as for any nonconductive enclosure. So you can apply the same techniques seen in plastic enclosures,
this is wrong and dangerous advice.

All exposed conductive parts must be Safety Earthed.

We don't and can't build Double Insulated equipment and we can't guarantee that any builder can test to the standard required by the Double Insulation specification.

Think about an interconnect that has a bare end. Child picks it up and sucks on it --- while earlier the mains lead became detached and touched a part of the internal electronics.
Do I need go further in my hypothetical description?
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Old 9th December 2009, 03:25 PM   #7
Jidis is offline Jidis  United States
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Thanks Pacificblue,

I think I'll be safe and get some vents on it. I let it run for a few hours yesterday and eventually the transformer's outer case got nice and warm (about like the TDA heatsinks), but neither was too hot to touch. This of course was with everything sitting out in the open, so sealing it up probably would get rough on it.

Take Care
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Old 10th December 2009, 11:51 PM   #8
tomchr is offline tomchr  United States
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One could always use a metal chassis inside a wooden box if grounding is needed. Or line the inside of the wooden box with copper foil or screen and ground it. Both these methods work well for screening against electrostatic fields. For safety, though, ground all metal parts as stated above.

~Tom
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