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Old 23rd January 2009, 02:44 AM   #1
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Default Transformer question

Am I going crazy?? I have a 36 volt center transformer,. So I should had 2 18 volt ac output right?

But for some reason I measure 21 volt ac on each output?
Can some shed some light this for me, and by the way this the 3 one I have tried 2 of them were used.

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Old 23rd January 2009, 04:10 AM   #2
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Can you check and see what your input voltage (wall) is. A lot of transformers are spec'd at 115 VAC whereas many houses have 120 volts at the wall. You are going to use this transformer on a regulated power supply I presume, so an extra few volts on the input side is likely to be of no consequence. It is the DC output of the power supply that is more important.
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Old 23rd January 2009, 08:37 AM   #3
djk is offline djk
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Small transformers have very poor regulation, the off-load voltage will be as much as 20% higher.

http://www.amveco.com/pdf/Amveco_Catalog.pdf#page=25
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Old 23rd January 2009, 11:12 AM   #4
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Exactly. The nominal voltage is given at nominal load.
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Old 23rd January 2009, 02:57 PM   #5
AndrewT is offline AndrewT  Scotland
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the no load secondary voltage will be =
mains voltage / rated input voltage * rated secondary voltage * transformer regulation.

eg mains is 121Vac.
rated voltage is 115:18
regulation is 15%

no load output voltage = 21.78Vac.

Now look at the range of mains voltage that your supplier can send to you.
In the UK, this is between 216Vac and 254Vac.
Your design MUST take account of these variations.
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Old 18th February 2009, 10:03 AM   #6
sasmit is offline sasmit  India
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I am building a 2 * 100 W amp, there is one LM4780 per channel running in paralell mode. I plan to run them from a single 330VA toroid. Is this sufficient ?
Andrew I read one of ur posts where u arrived at a minimum required trafo capacity of 260 VA ( don't remember the exact value )
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Old 18th February 2009, 10:17 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
the common range of recommendation for ClassAB power amps is 1* to 2* the total maximum output power.
If you plan 100W+100W then transformers from 200VA to 400VA would work.
I would try 300VA.

But why use 4 chipamps when two discrete 100W amps can do the job better?
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Old 18th February 2009, 10:22 AM   #8
sasmit is offline sasmit  India
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it's actually 2 * 4780 ..i understand that internally each is 2*LM3886...I wanted to try this extremely simple 100W amp. you would agree paralelling a 4780 is relatively easy compared to a discrete. I have another design in progress using a LME..series driver chip..but i wanted to try out paralelled chip amp first
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Old 18th February 2009, 11:14 AM   #9
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With a 2*18 V or 36 V CT transformer you can expect ~22 W output power into 8 Ohm and ~42 W into 4 Ohm, no matter, if you use a single channel or two parallel channels.

For 100 W into 4 Ohm you need two parallel channels and a 2*26 V or 52 V CT transformer.
You can get 100 W into 8 Ohm with two bridged channels and a 2*20 V or 40 V CT transformer.

If you use the two channels of an LM4780 parallel or bridged for that amount of output power, you will need an incredibly big heatsink or, better still, fans on top of a still big heatsink.
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Old 18th February 2009, 11:21 AM   #10
sasmit is offline sasmit  India
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actually the national suggested heatsink for PA100 is not that big....I have two of them made to order, it's quite cheap here in India ...around 30 euros. Regarding the transformer I was more concerned about the capacity ( VA ). National data sheet suggests around +- 32 volts for ~100W into 8 ohm.
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