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Old 13th October 2006, 10:51 PM   #1
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Default Toroidal for LM3886

Hi there folks!

I got that crazy idea of making a 8 channel LM3886 amp in one chassis. I got a torroidal tranformer for the 2 primary channels wich specs are:

Prim: 0-220-240V
Sec: 22V @ 4A
28V @ 7.5A
19V @ 2.6A

but the three other toroidal tranfos (wich will drive the 6 remaining channels) are much smaller Hammond units wich specs are:

Dual 117/234 VAC primary, 50/60 Hz. operation. Secondary VAC (RMS): series: 20V + 20V @ 4A, parallel: 8A


I'm a total newbie hoping to get clear answers so I don't get to much mixed up. I know the big tranny will do it for the main channels but do you guys think the small one are big enough?

Thanks a lot!
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Old 14th October 2006, 05:43 AM   #2
Leolabs is offline Leolabs  Malaysia
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Seem that you got yourself a bunch of different voltage rating toroid.
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Old 14th October 2006, 07:49 AM   #3
Nordic is offline Nordic  South Africa
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+-20V recitfied i.e. 28V is more LM3875 + 8 ohm speaker territory...

into 8 ohms; 28/8 = 3.5A
into 4 ohms; 28/4 = 7.0A
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Old 14th October 2006, 08:26 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
the second transformer 20-0-20 looks good for upto 100W of output.

Fit a bridge rectifier and smoothing caps for EACH amplifier module. This way you can run upto 50W into 8 ohm, twice from each transformer. That's 6 channels done.

Your first transformer looks as though it is about 250VA to 330VA depending on how the manufacturer rated the core and primary winding.
But the voltages are no good for your chipamps.

You need to create a dual polarity supply for your chipamps.
For this you require either a centre tapped transformer with a VA rating and voltage to suit your proposed output power, or a twin secondary with matched current and voltage on each winding i.e. 20+20Vac, or even 18+18Vac.

If you are feeling really enthusiatic you could rewind the big transformers and create 4 secondaries on each. One pair of secondaries could drive each rectifier and amplifier. This arrangment is almost as good as monoblocking.
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Old 14th October 2006, 12:12 PM   #5
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Thanks for your answers guys.

Andrew, what do you mean by rewinding the big transfo and making 4 secondary?

Jordan
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Old 14th October 2006, 02:23 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi Jordan,
if you don't know then forget I mentioned it.
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Old 14th October 2006, 09:19 PM   #7
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Ok! so I guess you suggestion would be to find a tranny having dual secondaries a little biger than the three others so I can take advantage of the full 68watts of the lm3886...

something like this:


http://www.partsexpress.com/pe/showd...number=122-620


Am I right?

Jordan
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Old 15th October 2006, 08:28 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
that Parts Express Avel looks OK.
It will support about 170W of output power very well.

But watch the stack up of tolerances and variations.

Tolerance +-3%
Regulation ~6.4%
Mains +?%,-?%. In the UK we have +6%-10%.
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Old 15th October 2006, 12:36 PM   #9
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Thank you for your answers. I'll continue my research and let you know what's up.

Jordan
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Old 27th October 2006, 01:29 AM   #10
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Jordan

Coincidentally, I too have made a 8 channel amp from 3886. Although my current incarmation is using a small (90watt) toroid and does get a little hotter than I'd like, I think most people way over-do it in the transformer dept. This coming from a person who has designed and built many transformers. I recently bought a 400 VA toroid from AnTek, and that I'm sure will not overheat.

If you send me an email (subject = audio, to Doug ), I will send you a WORD document about the amplifier I am building.

Doug
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