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Old 29th January 2009, 09:06 AM   #11
sangram is offline sangram  India
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Location: India
I found no difference between good custom EIs and made-to-order toroids, so I just use the former.

I use 22V secondaries for all chipamp transformers, this is about right for 6 ohm speakers but with decent heatsinks, can drive 4 ohm loads at moderate volumes.

Transformers (in India) are usually rated for RMS secondary current and either rectified DC or secondary AC voltage - terribly confusing for most. I specify all my transformers for about 5-10A of secondary current depending on the application, so that is 220 or 440VA - 220 for each channel is what I normally try to maintain, per 60 watts of output power. I was advised 300VA for each chip, but I seem to be fine with 'just' 3.5x of output power.

For less stringent applications I buy 20 or 24V (28-36V DC) transformers, usually center-tapped, and cut secondaries if I need dual windings. Again, I find little difference between a dual-tapped supply (dual bridges) and center-tapped supply (single bridge), so I opt for center-tapped, single bridge designs when I build from scratch.

Oh yes, these things cost between 20-35US for custom made 440VA units, down to $7 for real basic units.
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Old 2nd February 2009, 06:43 AM   #12
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Quote:
Am I just looking in the wrong place?

Is $60 to $80 what I should expect to pay?
I would NOT pay near that much for powering a small 3886 amp.


It's easy just to go to RadioShack and get two of their CT 25.2V 2A transformers. They are only about $14 each.

If you want two good-sized voltage rails, just use one transformer for each rail. That will give you plenty of power.

RS transformers are made well, and I've NEVER burned one out, and I've ABUSED them over the years without one failure ever! They also put out MORE than their rated amps when you load them hard and sag the rails a bit. You can't go wrong for the money! You should be able to drive 8, 6, or 4 ohm speakers just fine!

1 trans = 50VA
2 trans = 100VA (and only $28 for your amp)

Have fun with your project!
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Old 10th February 2009, 09:34 PM   #13
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When you're picking the search filters in Digikey, you can hold shift or ctrl to select a range of values. So you can select for example 20 to 100 VA and it will show all transformers within those values. I just found this out the other day and it makes searching so much easier.

For a 3886 amp, the easiest method is to get a single secondary with CT, or a dual secondary with no CT. This will give you the +V - 0 - -V.
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Old 15th February 2009, 03:25 PM   #14
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Default Try Antek

Hi,
I just finished a 3886 2 channel amp using the AN2222 from AnTek, it was $40 including shipping.

However, I also like EWorkshop's solution, $28 for a 100 VA solution isn't bad at all.

The 100 VA, 22-22 tororid from AnTek is the same price, $28 shipped.
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Old 19th February 2009, 03:14 PM   #15
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Default Re: LM3886 Power supply

Quote:
Originally posted by jfnewell1965
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…..
Start with the speaker impedance and the amount of power you want to get out of the amplifier. If you look at the LM3886 datasheet on page 14 lower left there is a graph that specifies the power output for a given supply voltage for different speaker loads. Assume you want to get 60 W with an 8 Ohm speaker, look into the graph and you find something like ±34 to ±35 V.
Now you need a transformer that leads to that rail voltage, when it is loaded.

For that you need to know two things. One is transformer voltages are always given at nominal output power. The other is, how to convert the transformer voltage into the rail voltage.

Many times you will find quoted that the rail voltage is 1,41 times the transformer voltage. Well, nearly, but only as long as the transformer is not loaded. At nominal load even well designed power supplies will sag up to 10 %. So rather use a factor of 1,27 times transformer voltage to find the rail voltage. subtract 0,7 V diode drop, if you use a center-tapped transformer with one recitfier bridge, subtract 1,4 V diode drop, if you use a dual secondary transformer with one bridge per secondary.

In your case you have to reverse those steps. E. g. (35 + 1,4) / 1,27 = 28,67, so you should buy a transformer with 2*28 V, if you want to get 60 W into 8 Ohm.

Back to the no-load situation. The rail voltage will be higher than 35 V, because at no load the rail voltage will be 1,41 times the transformer voltage and the transformer voltage will be higher than at nominal load. This is called regulation. Regulation can be 10 % for a 200 VA transformer, 5 % for a 1000 VA transformer or as bad 30 % for very small transformers. So you have to check if ((nominal voltage * mains tolerance) + regulation) *1,41 is within the voltage rating of all other components.

So what is the difference from a center-tapped transformer and a dual secondary type? A center-tapped transformer has one secondary winding with an additional tap at its center. The expression dual secondaries explains itself. You should use two rectifier bridges. Each has its advantages. Center-tapped is cheaper and has half the voltage drop, because you use it with a single bridge rectifier. Dual secondaries is slightly better in theory. Although it is better to use with two rectifier bridges, which leads to double the diode drop, you don't loose efficiency, because the full-wave rectification improves the transformers efficiency. It is also less prone to humming. The practical differences are too small to make a fuss about it. Your PSU layout will have a bigger influence.

Quote:
Originally posted by sangram
I found no difference between good custom EIs and made-to-order toroids, so I just use the former.
Each has its advantages. EIs usually cope better with overload. Toroids are more efficient and have a smaller (but stronger) electromagnetic field around them.
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Old 19th February 2009, 03:20 PM   #16
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With great power,comes with great sound quality.

It's much better if each ch has it's own power supply system and at least 150VA.
If you prefer quick response,the filter capacitors should not be too high,20000uF is large enough.
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Old 26th February 2009, 07:36 AM   #17
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Default Speedy Caps

Quote:
Originally posted by audio.dream
With great power,comes with great sound quality.

It's much better if each ch has it's own power supply system and at least 150VA.
If you prefer quick response,the filter capacitors should not be too high,20000uF is large enough.
Hi all!
For quicker response try placing more smaller caps in parallel, equalling the capacitance you want. This will lessen the ESR and ESL (equivalent series resistance and equivalent series inductance, the main cause of slow charge/discharge times)

Resistors and inductors in parralell decreace, caps in parallel increace, win win situation. (You will lose a bit of pcb space though)

If you can spend a bit extra you can get low ESR caps.
Hope thats helpful,
Cheers,
Carl
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Old 28th February 2009, 05:29 PM   #18
Minion is offline Minion  Canada
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Transformers are super expensive , especially if you buy new from mouser or digikey..I would suggest maybe looking on e-bay as I have seen a lot of good torroids on there for reasonable prices....

I personally have never had to pay for a transformer , I usually salvage all of my Power transformers from old equipment that I find in the Garbage or at the trift store for super cheap prices...
I recently bought a Logitech 5.1 amp/Subwoofer at the salvation army for $8 that has a 400vA 2x24v Torroid and 7 TDA7293 100W chips in it and a lot of big 10,000uF caps in it, I also found another 5.1 system in the Trash at the brick furniture store that has 5 TDA2030 chips and 2 TDA2050 chips and a 200vA 2x14 Torroid in it....

Maybe you should look arround and see if you can find an old stereo or 5.1 system and see if you can find a suitable transformer in one....


Cheers
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Old 28th February 2009, 05:54 PM   #19
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My recommendation: For 4 ohm loads use no more than 18 to 20 volt dual secondaries, or 36 to 40 secondary with center tap. For 8 ohm loads I use 24/25 volt dual secondary, or 48 volt with center tap.

You should take a look at these: http://www.national.com/appinfo/audi...gn_Guide15.xls
http://www.national.com/appinfo/audi...sign_Guide.pdf

Also these transformers look good at a reasonable price: http://www.antekinc.com/

Good luck and have fun!
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Old 6th March 2009, 02:47 AM   #20
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You might wanna have a look at what PartsExpress has to offer. They tend to have a little better selection of power transformers than Digikey. It seems they cater specifically to audio guys and hobbyists, whereas Digikey's clientele is far more diversified.

You mentioned that you want to REGULATE the PS using CarlosFM's LM338 circuit, so I'd like to remind you that you'll need a few extra volts (and amps) from your transformer to keep the regulator from dropping out.
Personally, I don't find a lot of merit in that idea. Why waste money having to buy a bigger transformer just to put that extra energy into a heatsink when you can turn it into decibels instead?
Keep in mind that these National chips offer excellent PSRR which is part of it's claim to fame. They are designed to give excellent performance while being SIMPLE in their application.
The original GainCard, that started this whole trend, was probably meant to be a practical joke. It was about as minimalistic as minimal could possibly be, or even less. Underneath all the smoke and mirrors, it looked like something Gilligan and the Professor hodged together out of coconut shavings and the support wires from Maryann's under-garments... but that joke backfired and we hobbyists get to reap the benefits of sharing in that last laugh. It's become a cult classic almost as popular as the frisbee.
Who makes this stuff up!!! It's hillarious when you think about it.

Good luck with your project. Keep it simple and you'll have less to worry about... and more time and money to spend on your next project.

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