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Old 20th April 2012, 11:15 PM   #1
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Default General basic questions

Hello everyone,

I'm trying to build a basic LM1875 amp. Can someone explain to me a few things:

* The datasheet (Speaker Amplifier and Modulator - Mid/High-Power Amplifier - LM1875 - TI.com) says, given +/-25V, it delivers 20W into a 4 or 8 ohm load. How can it deliver the same amount of power with those different loads?

* How do I figure out how to size the transformer for current? Can I use Ohm's law? If so, do I use 25V or 50V to calculate I? If not, what calculation do I use?

* Does this amp use the same amount of power whether or not there is a signal going into it?

Thanks!
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Old 21st April 2012, 11:28 AM   #2
AndrewT is offline AndrewT  Scotland
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The datasheet should specify the supply voltage for each of the 4r0 and 8r0 loads. They will be different.

Do not try to work out the current requirements of your transformer nor PSU.

Instead use the generally accepted rule.

Transformer VA ~= one times to two times the total maximum audio Power output.

If you build a 20W+20W stereo power amplifier then consider using a 40VA to 80VA transformer.


BUT when working with transformers that are so small you will find that they all have a very high regulation. That high regulation results in big changes in PSU voltage from no load, to quiescent load, to audio listening type load, to full power testing type load.

This massive change in PSU voltage can be substantially avoided by adopting a 160VA transformer as suitable for all small power amplifiers up to a total maximum power of 40W+40W.

Yes, the current draw of a chip amp changes when signal is present compared to no signal being present.
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Old 21st April 2012, 02:42 PM   #3
sreten is offline sreten  United Kingdom
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Location: Brighton UK
Quote:
Originally Posted by mossen View Post
says, given +/-25V, it delivers 20W into a 4 or 8 ohm load.
How can it deliver the same amount of power with those different loads?
Thanks!
Hi,

It does that by the combination of output current and output voltage allowed.
The device has a current limit which presumably limits the 4ohm power.
The power into 8 ohms is presumably limited by the supply voltage swing.
They happen to be the same power at +/- 25V (presumably by design).

With +/= 30V you get more voltage swing and 50% more power into
8 ohms, but due to the current limiting no more power into 4 ohms,
somewhat less if the thermal protection kicks in, due to more losses.

rgds, sreten.
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