#### pancakeplease

##### 100+ Head-Fier

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- Nov 22, 2011

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**Question 1:**I never took electrical engineering in college... just a bunch of biology so this is all new to me.

From what I understand, Impedance is measured in the unit "Ohms", which when it comes to audio equipment, refers to how much resistance the speaker requires?/puts out? (i think it's puts out but I'm not sure). And the amplifier/receiver needs to match the Ohms for it to be able to power it? All I know is that 8 ohm is the most common and "weakest" or "least demanding" of an amplifier, correct? And when you get down to 4ohms, it requires much more from an amp. But requires much more

*WHAT*? Does it require more watts to power speakers with LOWER impedance? How do watts factor into this?

In my real life scenario, my bookshelf speakers are 8 ohms. My receiver/amplifier is rated at 85w PER channel equally distributed to all 5 channels and 1 LFE/Subwoofer channel for a total of 4XX watts. The 85w is rated at 8 ohms. Here is an excerpt from my amp:

**Power Amplifier Section**

Rated output *THD figures are power amp stage values.

Front...........................................85 W + 85 W (8 ohms, 20Hz - 20kHz, 0.05% THD)

Center........................................ 85 W (8 ohms, 20Hz - 20kHz, 0.05% THD)

Surround ................................... 85 W + 85 W (8 ohms, 20Hz - 20kHz, 0.05% THD)

What I don't understand is why does the watts go UP when the impedance is lower? Meaning why does it go up to 120w for 4 ohms? Shouldn't it be lower? The amplifier is able to put out 120w for 4 ohms but only puts out 85w per channel for 8 ohms? Doesn't make any sense....

If one were you use lower impedance (6 ohm or 4 ohm) speakers with the above rated power specs, how would one calculate whether or not it would be safe to do so?

**Question 2:**I have logitech x-530 5.1 computer speakers. These are the power ratings:

- Total FTC power: 70 watts RMS

Satellites:

Left/right: 7.4 watts RMS x 2 (into 4 ohms, @ 1 kHz, @ 10% THD)

Center: 15.5 watts RMS (into 4 ohms, @ 1 kHz, @ 10% THD)

Rear left/right: 7.4 watts RMS x 2 (into 4 ohms, @ 1 kHz, @ 10% THD)

Subwoofer: 25 watts RMS (into 4 ohms, @ 100 Hz, @ 10% THD) - Total peak power: 140 watts

If I were to plug these into my receiver at the same time as my bookshelf speakers, how will I know if it's too much? I tried it and it works fine but I'm not sure if it's safe electrically speaking. Don't want to blow anything. 70w @ 4ohms.. should be a LOWER # of watts @ 8 ohms?

Cliff notes:

Why do Watts go up when Ohms are lower (I.E. 85w @ 8 ohms vs. 120w @ 4 ohms) when lower Ohms indicates more resistance and requires more power from the amplifier?