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Old 27th January 2009, 02:39 AM   #1
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Default LM3886 Power supply

Hi,

Here comes a stupid question/comment/begging...

I have seen literally (almost) thousands of schematics for LM3886 based amplifier circuits.

Along with these schematics, I have seen hundreds of posts discussing every minute detail including caps, layouts, wires, …..everthing! Discussed for days, week, months…people have fights over their preferences. And then….

At the very end of these incredibly detailed and technical discussions/fights/debates, the subject of transformers comes up, and invariably there is a comment along the lines of “Yep, I throw on a “standard” transformer and it works.” What!?

I have been digging for weeks, and I can’t find a recommendation for a transformer that I am able to order. People talk about 24v this, dual - center tap that, and while it all seems to make sense to other people who join in the discussion, I am finding it impossible to take any of what I have read, and order anything from a site like Digi-key, Mouser, Newark, etc…..

All the circuits make sense…..all the enclosures make sense….the power supply cicuits NOT including the transformer make sense……I just can’t seem to grasp the transformer part itself.

Can any one help me out…point me in a direction…I would even take an insult along with a part number, as in “It’s Mouser part number 123-ABC you moron!”

Anything, please.

Jim
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Old 27th January 2009, 06:54 AM   #2
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Well I'm no pro at this, but before anyone can help a lot we need some info.

Like what impedance of speakers are you planning to run?( 4 or 8 ohm)
How many amplifiers per transformer? (1 amplifier per transformer or several per transformer)
Regulation of the transformer? (usually the bigger the VA the better the regulation)

For example (assuming 1 amplifier at 8 ohm) this transformer from digikey HM555-ND would work for 8 ohm load but would have very poor regulation. As where this one HM557-ND would be the same but have better regulation.( I would use this if the price wasn't so scary)


These are some questions that will come up(and I'm sure theres more) while trying to pick out a transformer.
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Old 27th January 2009, 08:31 AM   #3
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Beware!

Those 25VAC center tapped transformers mentioned above have too low secondary voltage.
They will give you approximately plus and minus 20VDC using a full rectifier bridge and your amp will put out less than 20 Watt.

The LM3886 can handle something like plus and minus 35VDC.
What you need is perhaps 48VAC center tapped or two 24VAC windings.
I cannot tell you how much VA you need. I know that 120VA for a stereo amp was enough for me for many years.

Cheers.
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Old 27th January 2009, 03:37 PM   #4
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Default More on my confusion

Here, as an example of why I am confused, is what Digi-key asks you when you are narrowing down what type of transformer you want...

Maximum Power (Asking VA)

Parallel Output Voltage @ Current (Asking VAC @ A)

Primary Windings (Dual, One with multiple taps, Single)

Secondary Winding(s) (Dual-Center Tap, Dual - No Center Tap, Single-Center Tap, Single - No Center Tap)

Series Output Voltage @ Current (Asking VAC @ A)

Type (Dual Primary, Quad Primary, Semi Toroidal, Single Primary, Single Secondary, Standard)
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Old 27th January 2009, 03:37 PM   #5
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...More info, I can do that....

8 ohm speaker(s)
1 amplifier per transformer
PSU is CarlosFM circuit

Just to complicate matters, what variation in selection would I need to make if I was driving 4 ohm speakers?
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Old 28th January 2009, 09:18 AM   #6
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Hi,
I would choose AA51402-024 from digikey. 2X24V 140VA total.
If you have 110 or 120V connect the primary windings parallel, for 230V connect them in series.
Don't forget the primary fuse. If it blows instantly you have probably connected them the wrong way.

Cheers!
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Old 28th January 2009, 10:47 AM   #7
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Quote:
Originally posted by Agent327
Beware!

Those 25VAC center tapped transformers mentioned above have too low secondary voltage.
They will give you approximately plus and minus 20VDC using a full rectifier bridge and your amp will put out less than 20 Watt.
Cheers.
Ops, my mistake, I was thinking 25VAC-0-25VAC not 12.5VAC-0-12.5VAC.

Tanks for catching that Agent327.
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Old 28th January 2009, 11:35 PM   #8
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OK...now we are getting closer.....

My next question has to do with price....

The transformers seem expensive! Or.....

Am I just looking in the wrong place?

Is $60 to $80 what I should expect to pay?
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Old 29th January 2009, 07:22 AM   #9
Atilla is offline Atilla  Norway
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Quote:
Originally posted by Agent327
Hi,
I would choose AA51402-024 from digikey. 2X24V 140VA total.
If you have 110 or 120V connect the primary windings parallel, for 230V connect them in series.
Pretty much exactly what I got for a chipamp.com board. I have this - toroidal transformer with a single primary for 230Vac. It has two secondary windings, without center tap. Each secondary winding goes into each rectifier bridge on the PSU board. 2x24Vac output is perfect, it reaches 34-35Vdc when rectified, so it's quite a solid voltage for running 8ohm speakers. It's rated 120VA, but I've two of them, one for each channel.

Yes, transformers can be sometimes pretty expensive, perhaps the most expensive part of the chipamp, so it's a good thing to look around, see if you can find any sales, surplus stock or ust anything that sells them cheaper. Mine probably cost me a small fortune, but I'd rather not dig out how much.
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Old 29th January 2009, 08:23 AM   #10
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If you want to save money use just one transformer.
Two are not necessary if they are in the same enclosure.
Some very good amplifiers have been made using "only" one. Yes, just get the grounding scheme right.
Find out the difference in price between two 100VA transformers and one 200VA.
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