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Old 21st March 2005, 04:12 AM   #1
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Default Need help on 3875 Rev 3 PCB placement in enclosure

Help!!!

Need guidance on building Brian's LM3875 Rev C into small enclosure. Using single Avel 330va +-25vac xfrmr, stereo version so two LM3875 boards and one power board.

My understanding is we only use one of the two power boards supplied.

Question:
Is it ok to keep power board attached to one of the LM3875 boards? This way, there is one LM3875 board, and one LM3875/power board combo. Or is it necessary to have more distance from the power board to ensure no hum?
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Old 21st March 2005, 04:19 AM   #2
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Default How close can xfmr be to LM3875 board?

Can the LM3875 pcb be next to (not touching) the 330va transformer?

Can the enclosure be made minimal in size, all parts cramped close together as long as you can dissipate the heat?

Are there hum or feedback issues with this?
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Old 21st March 2005, 05:35 AM   #3
BrianGT is offline BrianGT  United States
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Default Re: How close can xfmr be to LM3875 board?

Quote:
Originally posted by ransom peek
Can the LM3875 pcb be next to (not touching) the 330va transformer?

Can the enclosure be made minimal in size, all parts cramped close together as long as you can dissipate the heat?

Are there hum or feedback issues with this?
I would put the power supply board next to the transformer, and run wires to each channel from that.

As for the size, there shouldn't be any hum/feedback issues if you run your wire correctly.

The input wiring should be kept as far away from the power and output wiring as possible.

Good luck with your project,

--
Brian
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Old 21st March 2005, 08:57 AM   #4
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Default like this??

Click the image to open in full size.

Works a treat for me!


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Old 21st March 2005, 04:16 PM   #5
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Default Blackdog successfully violates protocol

OK, now this first chipamp builder is confused. Brian GT, the chipamp guru advises power supply pcb should be next to trnsfmr, and input wiring far from power and output wiring.

But I like the compact design by blackdog, and the idea of keeping the power board attached to one of the audio boards. But blackdog violates all portocaol, yet he claims he has no problems!

Any others with comments?

By the way, for the Rev CLM3875 kits, where to go for construction manual, building advice?
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Old 21st March 2005, 05:53 PM   #6
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Default Re: Blackdog successfully violates protocol

Quote:
portocaol
?
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Old 21st March 2005, 06:01 PM   #7
BrianGT is offline BrianGT  United States
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Default Re: Blackdog successfully violates protocol

Quote:
Originally posted by ransom peek
OK, now this first chipamp builder is confused. Brian GT, the chipamp guru advises power supply pcb should be next to trnsfmr, and input wiring far from power and output wiring.

But I like the compact design by blackdog, and the idea of keeping the power board attached to one of the audio boards. But blackdog violates all portocaol, yet he claims he has no problems!

Any others with comments?

By the way, for the Rev CLM3875 kits, where to go for construction manual, building advice?
If blackdog's amp is quiet, than it is alright. The idea is to keep the input wires as far apart as the chassis design allows. I wouldn't say that he is violating any protocol (I don't know if any protocols exist ). He does well twisting his wires together.

I would just try to keep the wiring neat and as short as possible,. I have seen that in some amps, if long wires are used, mixing the input wiring together with the output wire in one big mess, problems can occur.

Best of luck with your project. You can always try it out as you are currently planning, and you have issues, reconfigure it.

--
Brian
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Old 21st March 2005, 09:19 PM   #8
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Default I guess it depends upon your design criteria..

When I built this amp, I was mainly using leftovers. The case I bought new, but I wanted a small amp for the limted space in my workshop, and that case was dirt cheap, and about the right size.

The transformer, PCBs, and chips I already had. So all I had to do was come up with a way of fitting them all in, and putting them in 'sensible' places. - So although there isn't acres of spaces between my input and out put wiring, you'll see that the i/p wires are slightly separated. It wasn't accidental.

A lot of commercial amps do much worse than this without having problems!

If you are going for this arrangement, make sure your caps aren't too tall for the space between the two amps PCBs...I had to use 1000uF instead of the preferred 1500uF.

Some more interesting facts. - there's no heatsink, just the bottom of the (rather thin) chassis. The speakers are JVC's nicked from my wife's mini system, and sound quite nice for their size. Oh yes, they're 4 ohm too. The amp still hardly gets warm, even when playing what I perceive to be loud!

So the bottom line is, there are "rules", but I woulod suggest they are more like guidlines. Sometimes you can break the rules and get away with it. If in doubt give it a try. I can gaurantee that most members on here would say I have not enough heatsinking on this amp, but it's been in daily use for around 4 months, with absolutely no problems.

Steve
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