Zaph Dayton RSS315HF Sub: 4-ohm vs 8-ohm

[tomchr]

Folks,

I'm planning to build John Krutke's (Zaph Audio) sub using the Dayton RSS315HF driver. The design can be found here (look about 1/3 down the page).

I noticed that the driver comes in an 8-ohm version as well as the 4-ohm version that everybody seems to be using.

Higher impedance drivers tend to be easier on the power amps, resulting in less distortion and cable loss. In light of this, is there a particular reason everybody and their dog seems to be going with the 4-ohm version? What am I missing?

Thanks,

~Tom

[kipman725]

Typicaly amplifiers rated for 4ohm loads are capable of a greater power output into 4ohms as they are limited by there rail voltages rather than there thermal disipation, therfore a 4ohm sub will have a greater maximum SPL with a typical amplifier.

[tomchr]

I am planning to drive the sub by two 125 W (into 8 ohm) amps bridged. That should give me 500 W into 8 ohms - plenty for the 8-ohm driver. But of course, I could get 250 W into 4 ohms from just one amp channel...

The data sheets for the 8-ohm version and the 4-ohm version show some subtle differences between the two. Among other things, the 4-ohm has a slightly larger excursion.

~Tom

[kipman725]

Hmm the 4ohm one looks like it can play slightly lower. Although tbh the specs aren't different enough for them to be on purpose different and one suspects that this may just be one sample of each type that are actualy part of the normal distribution of the subwoofer parameters.

[tomchr]

I tossed Parts Express an email. After all, they do design their own kits, so I figured they'd be knowledgeable on the subject as well. Here's their response:

The primary reason that 4 ohm is more common is because of the amplifiers themselves. The power on sub amps is rated into a 4 ohm load on that style of amplifier and the bigger numbers sell better. The higher impedance is better in terms of distortion and heat though so I really do not have a problem with recommending the 8 ohm version at all.

So I guess my hunch was correct - or at least I've found several people with similar hunches... The 4 ohm is mostly a matter of getting more power from the amp and being able to post higher numbers in the marketing brochures.

~Tom