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Old 4th February 2010, 03:44 AM   #1
JozzaG is offline JozzaG  Australia
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Default toroidal transformer

can anyone help me as to how i would go about wiring up a toroidal transformer for my LM1876 power amplifier, and what size toroidal would i need.

if possible can a schematic be supplied and/or a wiring diagram.
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Old 11th February 2010, 07:39 AM   #2
Corax is offline Corax  Germany
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Hello JozzaG,

in case you mean the mains transformer, you must be kidden to wind that kind of transformer all by yourself.
Find a distributor and by one with the appropriate output voltage. Prior to do so, decide first if you wanna run the LM1876 with single ended or symmetrical power supply, to figure out if you need just a single output voltage or a dual output voltage (also known as "center tapped"). Power rating, as a rule of thumb, should be in the range of 50% more power than the total output power of the LM1876. With dual output of 20W on 8 Ohm I would suggest a transformer of approx. 60W. Choose the next and higher available power rating of a transformer if exactly 60W are out of reach. The bigger the more stable the power supply will be, but the size, weight, prize, etc. will rise too. Keep in mind that a rectified AC voltage will be multiplied by a factor of SQRT(2). For a symmetrical power supply of +/-20...24Vdc you need a transformer with two windings with 15...18Vac (or 30...36Vac center tapped). Pay attention to the circumstance that the smaller the transformer the higher the no-load voltage of the secondary output voltage. Consult the datasheet of the selected transformer to figure out the factor to multiply the rated output voltage, which could be (for instance) in the range of 1.1 to 1.2 for a 60W toroidal. This will lead to an output DC voltage (worst case with a 36Vac center tapped transformer) of approx +/-29V which is quite close to the absolute maximum rating of the LM1876 (+/-32V depending on single supply or symmetrical supply voltages). If you consider in addition a fluctuation of the mains voltage rating of +/-10% you might consider using the smaller output voltage of the transformer.

Schematics, by the way, can be found in the datasheet of the LM1876 at Nationals homepage (LM1876 - Overture Audio Power Amplifier Series Dual 20-Watt Audio Power Amplifier with Mute and Standby Modes).
Furthermore I would suggest (for instance) you google for "LM1876" and "schematic" and might come up with some pages showing you the work of other DIYers.

Good luck.
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Old 11th February 2010, 01:39 PM   #3
mikje is offline mikje  United States
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JozzaG,
I don't know if you've seen this, but here's a site with a lot of info about the LM1875. It has a link to a torroidal transformer the will work with the 1875. The transformer has a diagram on it for the proper way to wire it. There are plenty of vendors who can supply a proper transformer, so at least you can see what is needed from this site. Shipping from Parts Express would probably be pretty pricey for you. If you Google "Avel Y236651 250VA 18V+18V Toroidal Transformer" you'll probably find what you need.
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Old 25th February 2010, 06:08 AM   #4
acio is offline acio  United States
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Can anyone give me a hand with a Toroidal Transformer? I am working on my first SS amp project. I have a Toroidal 300VA 25 transformer with dual primaries (2x red, 2x black) and dual primaries (2x blue, 2x green). A total of eight wires. This is a first for me as I have always only had 2 wires for primary and if more than two secondary, different voltages. Do I just cap up the ones I don't need to use or do I for some reason need four wires for primary and four wires for secondary with this type of transformer? Any help on this would be greatly appreciated
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Old 26th February 2010, 08:27 AM   #5
Kinnja is offline Kinnja  United States
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Quote:
Originally Posted by acio View Post
Can anyone give me a hand with a Toroidal Transformer? I am working on my first SS amp project. I have a Toroidal 300VA 25 transformer with dual primaries (2x red, 2x black) and dual primaries (2x blue, 2x green). A total of eight wires. This is a first for me as I have always only had 2 wires for primary and if more than two secondary, different voltages. Do I just cap up the ones I don't need to use or do I for some reason need four wires for primary and four wires for secondary with this type of transformer? Any help on this would be greatly appreciated
For a single rectifier I connect the secondaries in parallel. If you need two rectifiers run one pair of secondaries to each one. I also connect the primaries in parallel.
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Old 26th February 2010, 09:27 AM   #6
tinitus is offline tinitus  Europe
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The two primary wires are connected in paralel if you have 120V mains(US)
Fore EU with 230V they would be connected in series

Fore most SS amps you need double secondary to make a +/- supply
1 red from one secondary to ac on bridge rectifer
1 black from other secondary to ac bridge
Other two red/black are joined and connected to ground

You need to measure with multimeter which wires belong to same secondary

Thats with one bridge
You could also use double bridge
Which is connected slightly different

A few amps are +/0(ground), fore which you would paralel the two secondary
I believe many chipamps are

But it would be a good idea to measure voltage on each secondary before connecting
Just be careful

Best if there would be different colour on all wires, and a specsheet from manufactor
Like this http://www.amplimo.nl/download/770xx.pdf

btw, the thin wires are usually the primary windings, at least on trafos like this one

Might be of better help if you informed about trafo manufactor and amp design
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Old 26th February 2010, 10:18 AM   #7
AndrewT is offline AndrewT  Scotland
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if you are unsure of the polarity of the primary or secondary windings then ALWAYS use a bulb tester to reduce the risk of a serious accident.

If either the primary or the secondary windings are connected out of phase or if there is a short of Live to Neutral, or Live to Safety Earth, the bulb will light up warning that something is wrong and MUST be sorted before proceeding.
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Old 13th March 2010, 07:16 AM   #8
acio is offline acio  United States
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Hey everyone! I kind of got a little impatient and just started trying to figure stuff out on my own. This is the first time I checked back on the question I posted. I totally forgot about it. I really appreciate everyone's help here. I had two of the primaries capped off as I wasn't sure at the time how to do it and I was getting the proper VAC out of the secondaries like that (120V mains). I wonder if that's why things kept frying on the power supply section? I thought perhaps the 300VA 25 transformer was too powerful for the GGG 3886 power amp project. I finally gave up on that and went to a circuit that I know for sure will work with this transformer (LM4780 amp kit).
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