4 ohm and 8 ohm Speakers

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Overloading the amp is highly dependent on the specific amplifier and how loud it's played. A modern powerhouse amp played at moderate volumes will have little trouble delivering current into a 2.7 Ohm load, but it can't be run at anywhere near the volume it would support with a correct load.

Another, significant problem is that the 4 Ohm speaker will absorb twice the power than the 8 Ohm speaker making the 4 Ohm ones 3dB louder. For the same voltage, current will double at 4 Ohms compared to 8 (since power equals I squared R, I will be 4 times but R is 2 times).

For equal volume the 4 Ohm speaker would need an SPL rating that is 3 dB less than the 8 Ohm speaker.

Also please consider that a 4 ohm rating for a modern speaker with crossover is sort of an average, there can easily be dips below 4 Ohms.
 
hermanv said:
Another, significant problem is that the 4 Ohm speaker will absorb twice the power than the 8 Ohm speaker making the 4 Ohm ones 3dB louder. For the same voltage, current will double at 4 Ohms compared to 8 (since power equals I squared R, I will be 4 times but R is 2 times).

That's what I meant: If you install two 20 Watts 8 ohms in parallel, you get 40 watts, 4 ohms . This assembly can be connected in series with a 40 watts 4 ohm speaker for a total of 80 watts @ 8 ohms.
There are many possible combinations.
 
You cannot run non-identical speakers in series, it will change the sound of both pairs of speakers.

A really good pair of autotransformers would do the job, but you could buy a small used amplifiwer to run the second pair of speakers for about the same price.

A pair of these would be OK:

http://www.edcorusa.com/products/transformers/em3000/em3300.html

The 8R speaker would hook to the 8R tap, the 4R speaker to the 4R tap, and the amplifier to the 16R tap. This would present an 8R load to the amplifier. If you put 100W in, both speakers will now receive 50W each.
 
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