4 ohm load vs 8 ohm load? What is prefered?

>01-12-10: Rsfphil
The way I understand it is given the same sensitivity the amp would have to double down into 4 ohms for the 4 ohm speaker to play as loud as the 8 ohm speaker. The MA6500 almost gets there so it would be very close. I doubt if anyone would even notice.

If the speakers had the same _efficiency_ that would be the case although most speakers are rated using _sensitivity_.

A 90dB/2.83V/1 meter speaker will play 90dB measured a meter away with a 2.83V RMS (4V peak sine wave) input regardless of its load impedance.

That will take 1W into 8 Ohms or 2W into 4 Ohms.

A 90dB/1W/1 meter efficient speaker has 90dB/2.83V/1 meter sensitivity with an 8 Ohm nominal impedance and 87dB/2.83V/1 meter sensitivity when the impedance is 4 Ohms.

Sensitivity is the relevant number unless output transformers with different taps get involved, since music which doesn't make your ears hurt from the volume has average power so far below the peaks you're just looking to keep those from clipping.

01-13-10: Drew_eckhardt
>When you plan to use the amplifier for music instead of science experiments you should get peaks 3dB higher into a 4Ohm load than an 8Ohm load.

Assuming equal efficiency.

I think the big thing is to not worry as long as you don't drop below the amplifer's rated impedance (some can become unstable and oscillate).

With competent design and high phase angle exceeding the limits might get you thermal protection (wait for the amp to cool off) or blow rail fuses (a bit of a pain when there are no external fuse holders and there are a dozen screws holding the amp cover on).

"should get peaks 3dB higher into a 4ohm load than an 8ohm load."

- 15.7% louder is easier to imagine than 3dB.

"The damping factor is only relevant when you solder the output devices to the speaker terminals and use active cross overs"

- I don't know where you getting this from.

"Of course, with an odd order passive Butterworth electrical filter the damping factor will be just 2.4 at the cross-over point and even order Linkwitz-Riley filters will have a damping factor of 1"

- we are not talking about filter's "alpha" but about damping factor defined as ratio of speaker impedance to amplifier's output impedance and yes it is important up to some point. I'm trying to show that it is limited for woofer by resistance of inductor in series to about 100 and would be nice not to lower it even more.