I am currently building a lm3886 amp mostly out of kit part etc and am looking at wood as a chassis material. I have used it in the past with any issue’s however that was just stop gap measure. I want to make a nice case for this one am wondering about a few things.
Firstly RF shielding. I used to use aluminium foil glued to the inside. Is this an acceptable method of shielding. In am planning to glue aluminium on the inside along with a aluminium plate for the start ground and transformer and an aluminium back plate. Will this do the job?
The other thing is fire risk. Is there fire risk with wooden chassis? The chip has its own internal thermal and short circuit protection and I am using circuit breaker for the mains. Are there any other safety measures I need to take?
Firstly RF shielding. I used to use aluminium foil glued to the inside. Is this an acceptable method of shielding. In am planning to glue aluminium on the inside along with a aluminium plate for the start ground and transformer and an aluminium back plate. Will this do the job?
The other thing is fire risk. Is there fire risk with wooden chassis? The chip has its own internal thermal and short circuit protection and I am using circuit breaker for the mains. Are there any other safety measures I need to take?
Hammond used to use aluminum foil covered cardboard in the parts of their chassis that were walnut covered and not metal. I never had my Hammond pick up any CB radio transmissions or lamp dimmer howl. By contrast, my disco mixer with wooden end plates did have a real problem with the CB'er driving by emitting dixie beeps when he wasn't talking, until I put toroid 20 turn chokes in series with the power supply from the wall transformer.
Fire is up to your evaluation engineering department, but having mains overcurrent protection is a start. I used to work in an appliance plant that had an evaluation engineeering department that would sit around asking "what if?" all day. their checklist had more than one entry on it.
Fire is up to your evaluation engineering department, but having mains overcurrent protection is a start. I used to work in an appliance plant that had an evaluation engineeering department that would sit around asking "what if?" all day. their checklist had more than one entry on it.
As you say, there may be no issues with using foil lining as a shield for wood or MDF construction as the caveats and risks are well known. You should fit a mains rated thermal switch of 60C in the mains supply as a precaution against leaving the amplifier on and unattended. If you provide plenty of extra ventilation slots, holes etc. and adequate heatsinking because of the increased insulation and drying out + slow breakdown of wood, the results could be as good as a metal case, if you have reliable electrical contact between mains ground and the shield foil, all round.
If you are going to use aluminium plate for the grounding connection and backplate, why not go a little further and make an aluminium pan or "U" shape of the front, bottom and rear panel, using channel or shallow "U" section and sheet aluminium offcuts, assembled with small self-tapping screws or "pop" rivets? Then you only need to make a simple cover or sleeve from wood and foil lining. The Chip amps can mount on the rear panel, perhaps without extra heatsinks and this may even provide sufficient cooling if large and thick enough.
If you are going to use aluminium plate for the grounding connection and backplate, why not go a little further and make an aluminium pan or "U" shape of the front, bottom and rear panel, using channel or shallow "U" section and sheet aluminium offcuts, assembled with small self-tapping screws or "pop" rivets? Then you only need to make a simple cover or sleeve from wood and foil lining. The Chip amps can mount on the rear panel, perhaps without extra heatsinks and this may even provide sufficient cooling if large and thick enough.
I have built several chassis, but the main problem is cooling...
I needed fan to move air thru the small vents but I hate fans.
Id rather use metals, steel is the best.
Just if you dont know how to get clean and straight cuts...
Use andlegrinder and 2mm or 3mm blades. 1mm and 1,5 ones somehow want to bend...
If you "dont" have materials... They lay around you.
If you dont want to use metal, then many noises come from mains... switching and such.
Yes indjano, the ferrite mains crap filter is great add-on, smoothens the startup too.
Heres mine, almost finished but waiting for heatsink...
No large images, 🙂
I needed fan to move air thru the small vents but I hate fans.
Id rather use metals, steel is the best.
Just if you dont know how to get clean and straight cuts...
Use andlegrinder and 2mm or 3mm blades. 1mm and 1,5 ones somehow want to bend...
If you "dont" have materials... They lay around you.
If you dont want to use metal, then many noises come from mains... switching and such.
Yes indjano, the ferrite mains crap filter is great add-on, smoothens the startup too.
Heres mine, almost finished but waiting for heatsink...
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
No large images, 🙂
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If your home has few or no radio transmitter devices, then you can probably get away with no RF shielding of your Power Amplifier.
Take care to satisfy the mandatory requirement:
The Mains Earth must be connected to Chassis.
And ensure compliance with:
All exposed conductive parts must be connected to the (protected) Chassis.
Take care to satisfy the mandatory requirement:
The Mains Earth must be connected to Chassis.
And ensure compliance with:
All exposed conductive parts must be connected to the (protected) Chassis.
I built a 6 channel LM4780 amp with half wooden panels which doesn't seem to have issues . Having said that, I didn't do any sort of test other than listening. We have all the modern appliances in house.
Good luck.
Dean
Good luck.
Dean
BTW,
seams in the cladding just like seams in a chassis let in RF. The LENGTH of the seam determines the amount of RF that leaks past the screen.
The seam must be electrically connected to the adjacent section of cladding. The LENGTH of the gaps between the electrical connections now determine the amount of RF that leaks past the screen.
Aluminium foil cannot easily be electrically connected. With time and air and moisture the surfaces of the foil become oxidised and that oxidation layer is a very good electrical insulator.
Copper foil can be soldered. Tin plate (tin plated steel sheet) can be soldered.
seams in the cladding just like seams in a chassis let in RF. The LENGTH of the seam determines the amount of RF that leaks past the screen.
The seam must be electrically connected to the adjacent section of cladding. The LENGTH of the gaps between the electrical connections now determine the amount of RF that leaks past the screen.
Aluminium foil cannot easily be electrically connected. With time and air and moisture the surfaces of the foil become oxidised and that oxidation layer is a very good electrical insulator.
Copper foil can be soldered. Tin plate (tin plated steel sheet) can be soldered.
But now almost every electrical product that you buy has a small radio transmitter in it. All the new light bulbs have a SMPS. The little wall-wart have a SMPS. The only product that I can of that does not is a room heater.
Three things you can go to jail for: 1) retail markup (tax) evasion, 2) rewinding a transformer without a license, and 3) using a wooden chassis to build an audio amplifier.
I won't tell the RFI police if you don't. I've even built a PC into a wooden chassis. And you're REALLY not supposed to do that.
I won't tell the RFI police if you don't. I've even built a PC into a wooden chassis. And you're REALLY not supposed to do that.
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