Speaker Impedance vs Amplifier Power

Posted by: whippersnapper

Speaker Impedance vs Amplifier Power - 09/02/12 10:46 PM

Hey all you electrical engineer types:

In my feeble mind, I've always equated higher speaker impedance with higher efficiency. I tended to think of an 8-ohm speaker system as more efficient, requiring less amplifier power to reach a given sound pressure level. As the impedance drops, additional stable amplifier power is necessary to drive the "less efficient" speaker to an equivalent sound pressure level.

Not long ago, I was working on putting a custom stereo in my car. I bought a subwoofer with dual 4-ohm voice coils. The sub came with jumpers to wire the voice coils in series or in parallel, thus presenting either an 8-ohm or 2-ohm load. I bought a dedicated mono class-D amp to drive the sub. It was rated to deliver roughly 200 watts into 8 ohms or 500 watts into 2 ohms.

Now here's where I get confused: Everything I read made it sound desirable to use the 2-ohm setting for the sub (which is rated to handle up to 500 watts), in order to draw the full 500 watts from the amplifier. This would reportedly keep the sub "happier" by providing extra power.

I would have thought I'd be better to drive an 8-0hm load and not require the amplifier to work quite so hard. Can anyone help me understand the pluses and minuses of wiring and driving the sub one way versus the other?

Your puzzled Whippersnapper friend.
Posted by: dakkon

Re: Speaker Impedance vs Amplifier Power - 09/02/12 11:48 PM

It kind of depends on what type of box your planning on using. if your using a sealed box.. For a given output you will want the extra power a 2ohm configuration will provide.... However, if you are going with a ported box.. Then you could probably get away with the 8ohm configuration... Have you bought a 1 farad or so capacitor, if not.. I would buy one...

The only consideration i would point out to you would be the intended location of the amp.. If you are putting your amp in a trunk where it gets hot, you may have problems with the amp shutting down on thermal protection (wait, your in montana, does it even get hot up there?)... This use to happen to my amps when i had a system in one of my previous cars... Texas heat+high power amps in the trunk of a car is not a great combo..... However, it did allow me to verify the thermal protection circuitry was operation.

But, if you're putting the amp in the cab where it will be in the air conditioning.. Then, i say rock the 2 ohm configuration.. It doesn't really matter, as long as you don't over load your alternator which is unlikely...
Posted by: JohnK

Re: Speaker Impedance vs Amplifier Power - 09/03/12 04:01 AM

Whip, there's no necessary connection between speaker impedance and speaker efficiency; other design factors can make a lower impedance speaker either more or less efficient overall. For example, the M80 rated at 4 ohms is slightly more efficient than the M22 rated at 8 ohms and requires less power for a given sound output, so in that sense is easier to drive.

For a given voltage output from an amplifier, lower impedance allows greater current, following Ohm's Law(current equals voltage divided by resistance)and greater power(current times voltage). On the other hand, the factor which might increase the difficulty is that the higher current increases the heating effect on the amplifier, which could cause it to overheat and shut down.

A speaker or sub isn't "happier" if more maximum power is available if it doesn't need that power for the sound levels being used. Unused headroom is simply that: unused.
Posted by: bridgman

Re: Speaker Impedance vs Amplifier Power - 09/03/12 11:10 PM

I think the non-obvious thing is that limits on amplifier output voltage tend to appear at the higher end of the power range, which is why you tend to see lower impedence speakers hanging out in the same space.

As an example, affordable car audio equipment used to be limited by the 13-14v you could get from the electrical system, so lower impedence speakers became the norm in order to get more power from the amp without having to use a DC-AC inverting power supply.

In home audio, I think you start hitting the limits of what the output devices (and the power supply filter caps) can handle without breaking down... you can go to higher voltages by specifying more expensive devices, but again going with a lower impedence for the speaker when possible is a very practical way to get more power.
Posted by: whippersnapper

Re: Speaker Impedance vs Amplifier Power - 09/04/12 09:29 PM

Thanks for the clarifications!

The box is sealed, as I tend to prefer my bass a bit tighter. Dakkon, what's the point of the capacitor?

Anyway, let's say I like to listen to bass-heavy music at around 90 decibels or so. In my example above is there any practical difference between driving the sub @ 8 ohms with the amp providing 200 watts vs driving the same sub @ 2 ohms with the amp producing 500 watts? I've tried it both ways and the amp can handle this load under either configuration (and it doesn't seem to overheat). Is the whole thing a wash, or is there a compelling reason to set it up one way or the other?
Posted by: JohnK

Re: Speaker Impedance vs Amplifier Power - 09/04/12 09:44 PM

If you have adequate volume at 8 ohms the added maximum power theoretically available at 2 ohms is meaningless. If it doesn't overheat, you could use the 2ohm connection, but there's no significant benefit, so yes, either way should be fine.