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Old 16th February 2009, 12:44 AM   #1
krips is offline krips  Canada
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Default BrianGT 680W LM3886 Amp

This is meant as a flexible, good sounding amp for my home theater. I've almost completed building the 10 mono kits, and have my 800W 20V toroid already. I plan on making a MDF enclosure, and have the top vented using perforated aluminum sheet:
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For cooling, I was planning on using an extruded aluminum square bar, measuring 14.5"x1.5"x1.5", then have aluminum plate on top of that allowing me to add heatsinks that I already have.
Here are the pictures of my progress thus far.
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Comments/tips/questions would be appreciated. Thanks .
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Old 16th February 2009, 02:09 AM   #2
krips is offline krips  Canada
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Can I wire my transformer like this? I have dual primaries and quad secondaries.
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Old 16th February 2009, 02:30 AM   #3
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Primary wiring looks OK. As long as each secondary has the same voltage, you'll be fine with that.
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Old 16th February 2009, 04:32 AM   #4
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Old 16th February 2009, 07:49 AM   #5
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Are you aware that a 2*20 V transformer will not turn the LM3886 into a 68 W amplifier?

Expect ~22 W into 8 Ohm or ~43 W into 4 Ohm per channel.
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Old 16th February 2009, 02:32 PM   #6
Moose is offline Moose  Canada
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If you are going to be running all 10 boards off the one transformer, why bother paralleling the secondaries? Why not run 5 boards off each pair of secondaries?
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Old 16th February 2009, 03:17 PM   #7
krips is offline krips  Canada
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Pacificblue, no I was not aware of that. This was not explaned in brianGTs guide if I read it correctly. Why is it that I will not get 68w?
Moose, I have 4 pair of secondaries, so I'm having a little bit of a time figuring out how to wire them.
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Old 16th February 2009, 03:54 PM   #8
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Quote:
Originally posted by krips
Moose, I have 4 pair of secondaries, so I'm having a little bit of a time figuring out how to wire them.
Hi Krips,
You have 4 secondaries, or 2 pair's of secondaries.

Each rectifier board needs 2 secondaries.. Blue/Green AC1 and another Blue/Green AC2.
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Old 16th February 2009, 03:57 PM   #9
Moose is offline Moose  Canada
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Hi Krips,

Take a look at the datasheet for the LM3886 at:

LM3886 Datasheet

On page 14 in the bottom left is a graph entitled "Output power vs Supply Voltage". There are three plots for 4ohm, 6ohm and 8ohm loads (left to right). To get 68W on a 4ohm load, you need +/- 27V DC, and to get 68W on an 8ohm load you need +/- 36V DC.

20V AC gives you approx 28V DC after rectification and filtering, which will give you about 40W into 8ohms. 20V DC (14V AC) gives you the 22W pacificblue was talking about. Recommended transformers for the 3886 into 8 ohms are 20-25V AC (28-36V DC) which give 40-68W into those speakers, so you are fine as long as you are not expecting 68W per channel.

As far as the secondary wiring goes, there are lots of ways to wire to it to make it work, and each one has its advantages and disadvantages. I would read around the forums, as there are lots of people who have build multi channel amps. Once you have done your research post a diagram of what you think you want to build here. You will get lots of good feedback, and some back feedback and flames too (which should be ignored). The key is there is more than one correct way to do it, but there are lots of ways to do it wrong which could damage your parts, your speakers, or yourself!
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Old 16th February 2009, 04:00 PM   #10
Moose is offline Moose  Canada
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Quote:
Originally posted by audio1st


Hi Krips,
You have 4 secondaries, or 2 pair's of secondaries.

Each rectifier board needs 2 secondaries.. Blue/Green AC1 and another Blue/Green AC2.
From your picture above, it looks like you only have 4 secondaries, not 4 pairs of secondaries.
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