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I want to test an audio amplifier based on TDA7294 and the highest rated speaker I have has 20W @ 4 ohms.

How can I connect it to the output of the amplifier without destroying it? I'm thinking I should use a series resistor, but how to calculate value and power?

I don't need a long term, reliable solution. I just want to test the amplifier at its maximum output power and I don't have the required speaker.

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What "test" do you want to do? If you want to verify its electrical performance, just use a 4-ohm, 100 W resistor as a dummy load, and forget about the speaker. – Dave Tweed May 8 '14 at 14:26
If you follow the advice of those here about using a resistive load, I would use a scope to look at the signal to be sure there is no distortion (which you would have heard using a speaker, although a scope would have been useful then too). – tcrosley May 8 '14 at 16:23
up vote 5 down vote accepted

You don't need a speaker at all. Get a 4 Ω power resistor, or parallel a bunch of higher resistance resistors to get the equivalent of 4 Ω at the required power handling capability. For example, twentyfive 100 Ω resistor in parallel will give you 4 Ω at 25 times the power rating of the individual resistors.

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You could use a resistor instead of the speaker, but it's not really exactly the same thing electrically.

Although the datasheet test circuit shows a speaker, the performance ise specified with a \$R_L\$ of 8\$\Omega\$.

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Your goal is to test the amplifier, but your question is how to calculate the value and power for a resistor in series with a under-dimensioned test speaker. If I were you, and really wanted to test the amplifier, I would so as suggested by others to only use a power resistor. In addition I would thrown in a 0.5 mH inductor as well (rated for the current used). I would probably go for 5 ohms resistor value, because that is most typical. In addition I would throw in a passive RLC network to emulate the phaseshifts you'll find in a speaker with passive crossover networks.

Test with real signals, white noise and single frequencies. Test also with and without the RLC network. Then you'll get fair idea of how the amplifier behaves under different kinds of loads. As always you should do the same with an amplifier you know, because you need some kind of reference in order to judge the results.

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