omh ratings

[cjh80126]

I need some help with my home theater setup. I purchased the Klipsch RCW-5 and to fill in the voids RCR-5 for my rear effects.

I know they are 8 ohm rated. The amp I am looking state it is stable through 2 ohm with 80 watts per channel.

My basic question is this a good thing or a bad thing?

Thanks for your help,

Carl

[Ray Garrison]

It is a good thing.

The higher the speaker's ohm rating, within reason, the easier it is to drive. An 8 ohm speaker is easier for an amplifier to drive than a 4 ohm or 2 ohm speaker.

An amplifier that says it is "stable into 2 ohms" means that the amplifier won't break if you use it to drive a difficult speaker. It will easily drive a speaker with a higher rating, like the 8 ohm Klipsch.

This is a very terse answer to what is actually a fairly intricate question, but I didn't want to go into gory details unless you asked.

------------------

Music is art

Audio is engineering

[Tron]

I'm game for the gory details on ohm ratings... Anyone else?

[WMcD]

I'll take a first shot at this. It is called picking the low hanging fruit.

It repeats some of my earlier posts.

Impedance is a widely used term.

In a load, like a speaker, it describes the relation between the voltage (V) applied to it by an amplifier and the amount of current (I) it draws.

By Ohm's Law, Voltage / Current = Impedance.

If there is an 8 ohm load, and you apply 8 volts, 1 amp of current passes throught the speaker. If 16 volts, 2 amps. If 0.8 volts, 0.1 amps.

If there is a 4 ohm load, and you apply 8 volts, 2 amps flow.

The above makes one very big assumption. That is that the amp can actually supply the required current.

Consider that if there is a 1 ohm load and you apply 8 volts, the amp must supply 8 amps. Maybe it can't. The voltage output should sag to match the limit of the current in accord with Ohm's law.

With a well designed amp, the current sensing circuit will cut out if current demand to too great.

We can infer somethings about your amp.

One equation for power is P = I x I x R.

So 80 = I x I x 2 ohms.

I = the square root of 40 = 6.3 amps.

Another is P = V x V / R

Or 80 = V x V / 2 ohms

V = square root of 160 = 12.6 volts

We can check the results with the third equation for power.

P = V x I

12.6 x 6.3 = 79.3 watts. The error is my round off error.

So, we conclude that if there was a 2 ohm load attached the amplifier, at full power could develop 12.6 volts and 6.3 amps of current.

That the amp can handle a 2 ohm means that it will have enough current capability to handle a 4 ohm load.

The above is the easy part of speaker impedance.

1) In truth, the impedance of the speaker is not uniform across the range of frequencies. Ususally there is a dip to the lowest impedance at the bass frequencies.

2) Impedance is said to be "complex" when the sinusoidal current drawn is not in phase with the sinusoidal voltage applied. In music, we always are working with these sine wave like signals. Just about all speakers have a somewhat complex impedance.

Suffice to say, if the current and voltage get too far out of phase, and this is because of the nature of the speaker, it can cause amplifier instability. The amp could shut down or it could go into oscillation. The latter can cause damage to the amp.

These "diffiult loads" are typically only seen in very exotic speakers.

Overall, I think you have nothing to worry about and you've bought a good amplifier.

Gil