Axiom Home Page Forums General Discussion Technical Questions Ohms--matching amplifiers and speakers

OK, the message board may have given me "Frequent Flier" status, but I'm really still a Newbie...........So, can anyone help me understand the concept of matching amplifiers to speakers by virtue of how many Ohms they can "handle"?

Thanks!

This will probably be far more info than you need but that's what happens when you ask an EE about Ohm's law.

The "Ohm" rating you see is a reference to the resistive load of the speaker. The lower the rating, the less resistance and the more power delivered, assuming current and voltage remain the same. The basic equation that governs this is Ohms Law:

V = I * R

Where V refers to the voltage potential between the two terminals, I refers to the amount of current flowing through the load and R is the resistance. Power is defined as such:

P = I * V

Plug the first into the second and you get:

P = I^2/R

As such, the lower your resistance, the more power you'll draw from the system. The electronics in your receiver have a set ability to deliver power. If you try and pull too much, you'll fry the components. Thankfully, all receivers (at least all of which I'm familiar) have internal circuitry to prevent the receiver from trying to output too much power. As such, most will simply turn off if you attempt to pull too much power from it.

That's why it's a good idea to look for a receiver that's rated to supply to such a small load. However, most receivers out there don't bother to spec it in their documentation but will provide to a load of this size with ease. When in doubt, ask.

Regards,
Josh

Thanks, Josh--I knew it was just a matter of dusting off that pre-med physics. It really makes perfect sense.

Jon

My pleasure. Any excuse to type out equations!

In reply to:

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It really makes perfect sense.

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Except that: P = V^2 / R (or P = I^2 * R)
Sorry to pick nits, Semi...

At any rate, the max power output of a solidstate amp is basically limited by its swing voltage (rather than the current) that can never go beyond the power-supply rail voltage. So, for a given max swing voltage (which is higher in a more powerful amp), the lower the load ohms (impedance), the higher the max power. This is as long as the amp can tolerate supplying the larger and larger output current into the speaker.

As shown in the equation, if the output current is truly a non-issue for an amp, then the max power at 4-ohm and 2-ohm load should be twice and four times that at 8-ohm load, respectively. Some expensive power amps, such as the Krell KAV series, actually meet this relationship, indicating that they use a very well-regulated power supply.

In reply to:

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Except that: P = V^2 / R (or P = I^2 * R)
Sorry to pick nits, Semi...

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Mother fuc...

Quite right. That'll teach me to blaze through the equations without actually thinking about what I was saying.

Yeah, P=I²R. Don't want to get Ohm hot at us.

You know it's a party when people start making engineering puns.

It's like being in college again.

Coming back to Doc's original question, if you solve the same equation for I, you get:

I = sqrt[P] / sqrt[R]

This means that the lower the load ohms, the more current the amp has to pump out for a given power output. More current means more heat&stress born by the power transistors in the amp. That is why the amp may eventually engage its thermal or overcurrent shutdown circuit when driving low ohm speakers very hard. Some speakers actually have impedance values dip drastically below its rated ohms, at certain frequencies. Those speakers, especially if they are less efficient and thus gobbling up lots of power, can cause shutdown problems on less tolerant amps.

Luckily, Axioms' impedance curves behave pretty well in general (no huge dips in the power-hungry bass frequencies), according to the NRC Canada measurements available online. Plus, the Axioms are very efficient speakers, leading to the claim that they are "very easy to drive'!


Impedance curve for the M22 (rated 8 ohm):


Impedance curve for the M80 (rated 4 ohm):


Source: www.soundstagemagazine.com