150 W 8 ohms speaker on a 300 W 4 ohms channel - have I done my maths right?

I'm building a DIY FrankenAmp. It currently consists of a speaker cab with a 15 inch, 150 W @ 8 ohms celestion truvox red label, which is plugged into one channel of a Skytec PRO-600 stereo power amplifier.

The amplifier delivers 300 W @ 4 ohm to each channel - am I correct in thinking that this means my speaker will get 150 W of power, given that it is 8 ohms, not 4 ohms (and so it won't melt the coil)? If not, what electrical power / impedance does the speaker need to cope with?

Secondly, is there a way doing this that could cause any damage to the amplifier or speaker?

I have given it a quick test and pushed up to halfway, and it was sounding great, but I don't want to risk anything before wiser heads have shared their wisdom :)

-
You might also ask this question on the "Audio/Video Production" stack exchange site: avp.stackexchange.com/questions/tagged/video Good luck! –  WildGeese Aug 14 '12 at 13:43
I think it's probably more suited to electronics.stackexchange.com than avp.se –  naught101 Aug 16 '12 at 13:05
I'd be happy to ask it again on electronics.se but i'm not certain of the rules about posting already answered questions? should I just copypasta it or note that it was answered already, or post as an answered question? –  jammypeach Aug 16 '12 at 15:02

Usually the ohms rating is the minimum the amp is safe with (as when you reduce resistance you increase current) so you are going the safer direction here by using an 8 ohm speaker.

So for a 300W amplifier through a 4ohm speaker, using Power = Current squared X Resistance, you can supply up to around 8A. Using your 8ohm speaker the same equation gives your maximum current draw as just over 4A, so the amplifier and speaker should be fine.

-
ah good, thanks. this tallies with what I've been reading on the issue, I still seem to have a bit of a mental block with actually understanding how it works but the numbers make sense. I've run a test now and slowly pushed it up to full volume, nothing it couldn't handle - thanks :) –  jammypeach Aug 12 '12 at 18:16
In your answer, you do not mention the most important part - WHY the maximum draw is just over 4A using the 8ohm speaker. –  groovingandi Aug 16 '12 at 11:52
using the wonders of ohms law and it's variants :-) Updated wording to clarify slightly. Guessing you rounded the other way to me, but good to see your figures come out very similar to mine (I rounded voltage to 32V for ease of calculation) –  Dr Mayhem Aug 16 '12 at 12:00

If your amp is specified for 300W @ 4 Ohm, it means that your amp was designed for a speaker with 4 Ohm. Assuming a constant load, this would be about 9A @ 35V (rounded).

It's a common misunderstanding that audio amps were impedance matched to the speakers, i.e. that their internal resistance would be the same as the speaker's resistance. Instead, amps usually have a very low internal resistance (around 0.1 Ohm). You can assume them to be a constant voltage source. A speaker with lower resistance would draw more current from the amp at full volume setting, therefore it would probably damage the amp by overheating or blowing a fuse. A speaker with higher resistance will draw less current at the same voltage. In this case, at maximum volume setting, the 35V are the limit of the amp, it cannot provide more due to its design.

At 35V, your speaker with 8 Ohm would draw somewhat above 4A, resulting in a total power of 150W. The amp cannot give more due to the voltage limit, so your speaker is safe.

Note: All was calculated by just using Ohm's law

• resistance = voltage / current
• power = voltage * current
• power = voltage squared / resistance
• power = current squared * resistance
-
that's a great explanation, thanks for posting. future generations of DIY amp butchers like myself will thank you :) –  jammypeach Aug 16 '12 at 15:10