Watts the question ?????

[Captain Scary]

Okay,I picked up some new speakers that are 6 ohm.

My Marantz 2385 manual says it puts out
185 wpc @ 8 ohms
and 240 wpc @ 4 ohms.

So.......what is the effective wpc @ 6 ohms?

How does that work.......

 

[whoaru99]

6 ohms is 1/2 way between, so power will be 1/2 way between.

 

[goat67]

If you are asking how can you get an exact value well you can not.
But if it does
185 into 8
240 into 4
then
how about (240 - 185) / 2 + 185 = 212.5 into 6
Best guess

 

[Captain Scary]

Seriously?
That was my first thought but figured
it wasn't as simple as that.
Well hell,I'm gonna pat myself on the back and
take the rest of the day off,quit with that mental
victory.
Must be something fun to do on a sunny Friday.............

Thanks

 

[whoaru99]

Yup...just that simple. It may or may not 100% spot on (insert lengthy techie talk here), but, if not, it'll be very close.

 

[Jon_Logan]

Output voltage for 185W into 8 ohms is 38.47 Vrms
Output voltage for 240W into 4 ohms is 30.98 Vrms


If the reduction in output voltage (sag) is linear with load and not related to current limiting circuitry, then the expected maximum voltage across a 6 ohm load would be 34.72 Vrms.

This would calculate to an output power of 201 Watts.

 

[Captain Scary]

Thanks JL.
I can relate to that explanation.......

 

[Fred Longworth]

Drawing a load line, you take the 30.98 Vrms value at 4 ohms and the 38.47 Vrms value at 8 ohms and split the difference. This comes to approximately 34.73 V rms at 6 ohms. If you square this and divide by 6, you get 201
Wrms.

This is the correct value. The power is NOT halfway between.

Fred

 

[whoaru99]

All that's sort of what I meant by, "It may or may not 100% spot on (insert lengthy techie talk here), but, if not, it'll be very close."

The difference it makes is 0.2dB, which is about 10-15 times LESS than the generally recognized difference in relative volume perception.

But, yes, from the "techie" perspective, I agree, the power isn't exactly 1/2 way.

 

[Markw]

As you surmised, it's not as simple as that.

No speaker presents a constant impedance. It's got peaks and valleys all over it's frequency range so the (theoretical) power availabe from the amp varies from moment to moment.

 

[Jon_Logan]

True, true, impedance curves will change what power is dissipated across the speaker. Standard resistors are used to level the playing field so that comparisons can be made.

If you were bored out of your mind and had a lot of time on your hands, you could plot max power vs. frequency on your speakers and see that the max power would be an inverse of your impedance curve.
Actually, your impedance curve would change suddenly as you blew your midrange and/or tweeter. The woofer might not survive such a test either.
A better way to do this would be to mimick the impedance curve by adjusting a power resistor for the frequency points on a impedance plot. Wow, that sounds like such a waste of time.

Keep in mind that the voltage output on a feedback design amplifier is the same (constant) for 4,6,or 8 ohm speakers. It's just the maximum output voltage will be different. Lower ohm loads cause more drop on wiring, suck more current out of the supply, cause a larger voltage drop across power transistors, etc. VI limiting circuits will kick in too, hoping to guard the output stage agianst dangerous conditions. This reduces the maximum voltage level at clipping. If your not going to run at clipping all day long, I don't see the need to compare output power ratings vs. load impedance. It's more important that the amp have an adequate thermal system and enough output transistors to share the output currents.