How Do You Calculate Watts Per Channel?


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You are bi-amping a speaker with two monoblocs rated at 200 wpc each. Does that speaker have 400 wpc or 200 wpc on available?
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Most likely somewhere in between. But it's not possible to define a specific answer because there are too many variables involved, including the frequency content of the music, which is constantly changing.

If the amps are rated at 200W into 8 ohms, and if the impedance of the speakers is close to 8 ohms at all frequencies (which is unlikely), 200W will be available at frequencies that are substantially below the crossover point, and another 200W will be available at frequencies that are substantially above the crossover point. But since music consists of many frequencies that are present at the same time, the answer will vary depending on the relation between the specific frequency content of the music at any instant of time and the frequency of the crossover point, as well as the slopes of the crossover.

Depending on the voltage swing capability of the amplifiers, the answer may also be affected by whether the biamping is done passively (using the crossover network in the speakers) or actively (using an electronic crossover between the preamp and power amps).

It's even possible that under some circumstances, at some instants of time, more than 400W may be available. To cite an extreme example, if the amps are solid state and are rated at 200W into 8 ohms, and the impedance of the nominally 8 ohm speakers is more like 4 ohms at low frequencies and 12 ohms at high frequencies, the low frequency amplifier may be able to deliver 400W by itself at those low frequencies, and the high frequency amplifier may be able to add to that another 133 watts or thereabouts at high frequencies.

Regards,
-- Al
A 200 watt mono X 2 is the same as a 200wpch stereo amp. Both are 400 watts total.
Peter, while if everything else is equal between the two amps your comment is true, it doesn't address the OP's question. The question is how much total power is available for delivery to a speaker that is biamped, with one 200W mono amplifier or stereo amplifier channel driving the low frequency part of the speaker and another 200W mono amplifier or stereo amplifier channel driving the high frequency part of the speaker. That is the question my post addresses, and the answer is not 400W under most circumstances.

Best regards,
-- Al
Sorry, misunderstood the op. I thought 2 amps total.
If you are bi-amping with 2x200W (@8 Ohms) amps then yes you have 400WPC (@8 Ohms).

As Al said above, the instantaneous power is a function of the impedance of the speaker at a given frequency as well as the power supply of the amplifier.

Utilizing Ohms law, an "ideal" amplifier will double it's power as the impedance is halved.

200W @ 8 Ohms
400W @ 4 Ohms
800W @ 2 Ohms

etc.
08-05-13: Hk_fan
If you are bi-amping with 2x200W (@8 Ohms) amps then yes you have 400WPC (@8 Ohms).

As Al said above, the instantaneous power is a function of the impedance of the speaker at a given frequency as well as the power supply of the amplifier.
However, note that I also said the following:
If the amps are rated at 200W into 8 ohms, and if the impedance of the speakers is close to 8 ohms at all frequencies (which is unlikely), 200W will be available at frequencies that are substantially below the crossover point, and another 200W will be available at frequencies that are substantially above the crossover point. But since music consists of many frequencies that are present at the same time, the answer will vary depending on the relation between the specific frequency content of the music at any instant of time and the frequency of the crossover point, as well as the slopes of the crossover.
Let me cite an extreme example to illustrate this point. Suppose that the crossover point is 5 kHz, and the slopes of the crossover filters are sharp. The high frequency amplifier will handle very little energy below 5 kHz, while the low frequency amplifier will handle very little energy above 5 kHz. On nearly all music at nearly all times the energy that is present above 5 kHz is vastly less than the energy that is present below 5 kHz. Therefore most of the power capability of the high frequency amp will almost never be utilized. Very conceivably no more than perhaps 20 watts of its capability will ever be used (20 watts being only 10 db less than 200 watts). Therefore it is incorrect to say that 400W is "available," which was the OP's question.

My understanding is that most music at most times tends to have a roughly equal amount of energy above and below a frequency of something like 350 Hz.

Regards,
-- Al
I was not trying to state what Wattage was being consumed by the speakers at any given time, merely the theoretical power available which is how I read the OP's question.

I think most people think of this question in terms of a perfect 8Ohm load and not the various impedance swings which we know is reality.

I stand by my assertion that there is 400W on tap... should it be required. :)
Hk_fan, in case it's not clear my last point has nothing to do with impedance swings, and holds true even if the speakers are a perfect 8 ohm load at all frequencies.

To further clarify my last point, consider the situation in which the speakers are single-amped, using either a pair of 400W monoblocks or a 400W per channel stereo amplifier. Let's say that the speaker efficiency and volume setting are such that during peak passages of some recordings the full 400W capability of that amplifier is called upon.

Then let's change the setup to a pair of biamped 200W monoblocks per channel. Will that setup be able to play those same recordings at the same volume levels, without clipping or otherwise running out of steam?

The answer is that it will not, unless the frequency content of those musical peaks happens to be such that their power requirements at frequencies above and below the frequency of the crossover point are equal, which is very unlikely. So it would not be correct to consider the 2 x 200W biamped configuration as having the same power capability as a single-amped 400W configuration.

Regards,
-- Al
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So it would not be correct to consider the 2 x 200W biamped configuration as having the same power capability as a single-amped 400W configuration.
Al you precisely answered the question that I imprecisely asked. Thank you!
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In your first scenario the full 400W are being SHARED among all the drivers.

For sake of simplicity lets say there are 2 drivers, and assume that they have exactly the same impedance and efficiency. Therefore the two drivers will perfectly split the 400W, 200W per driver.

This is no different than individual drivers connected to a single 200W amp each.
Mitch, thanks!
08-06-13: Hk_fan
In your first scenario the full 400W are being SHARED among all the drivers.

For sake of simplicity lets say there are 2 drivers, and assume that they have exactly the same impedance and efficiency. Therefore the two drivers will perfectly split the 400W, 200W per driver.
Somehow my point is not getting across, and I'm not sure how to explain it any more clearly than I already have.

But in the case of your two-driver example with the 400W amplifier, the two drivers will NOT "perfectly split the 400W, 200W per driver."

The 400W will be divided between the two drivers based on the frequency content of the music, at any instant of time. That is what you seem to be overlooking.

If a bass drum beat is loud enough to require 400W, since most of its energy is at low frequencies the low frequency driver will get most of the 400W, assuming that no other loud notes occur at the same time. While if a cymbal crash is loud enough to require 400W the high frequency driver will get most of the 400W (just before it expires :-)), assuming that no other loud notes occur at the same time.
This is no different than individual drivers connected to a single 200W amp each.
It is very different. In the above situation the biamped 200W amplifiers will only be able to supply 200W to the low frequency driver when the bass drum beat occurs, and they will only be able to supply 200W to the high frequency driver when the cymbal crash occurs. Not 400W (or close to it, depending on the harmonic structure of the note), which could be supplied to that one driver by the single-amped 400W amplifier.

Regards,
-- Al
Somehow my point is not getting across, and I'm not sure how to explain it any more clearly than I already have.
Amen, Almarg!!
Your explanation couldn't have been more lucid....
thanks.
It's a ploy, Al. We just love hearing your expertise:)