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Old 26th June 2012, 08:14 AM   #1
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Default How much power do I need?

A quick question.

Say I am bi amping a typical off the shelf inefficient speaker, rated something like 88dB/W. Listening position is say 2m/6' away from the speaker. How much power should I want for the mid/high so that I have enough headroom?
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Old 26th June 2012, 09:14 AM   #2
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Hi!

This is a question which is often asked, unfortunately this is something which nobody can really sensibly answer for antoher person. It depends on so many factors:

speaker efficiency: There is a big difference in the way speaker manufactureres state their efficiency. Often this is just measured at the loufest peak of the frequency response. I've seen speakers which claimed to have 98dB but really were something like 92dB

listening habits, room size: People have very different listening habits, If for one person 5W are enough for a specific speaker, another one might need 50W for the same speaker.

Best method to check yourself: Set the speaker at the loudest level you usually listen to and measure the peak power. Add some margin to that.

Thomas
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Old 26th June 2012, 09:19 AM   #3
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As Vinylsavor has discussed, this is an almost impossible qustion to answer.

For the average Jo, 50W is normally more than sufficient. Bearing in mind that most amps are run at about the 12 o'clock position and may only be developing 5W which is still quite loud with reasonably efficient speakers.

To get twice the volume of a 50W set-up you need 500W.
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Old 26th June 2012, 12:25 PM   #4
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Perhaps a better way phrase my question is, how much less power can I get away if I am powering the woofer with another amp?

I found this:

BiAmp (Bi-Amplification - Not Quite Magic, But Close) - Part 1

My speaker has a crossover frequency of 150Hz, if the information in the link is accurate, I would need something like 70-80% of the wattage (of a hypothetically "enough" wattage for both low and mid/high) for the mid high driver. That seems like a lot to me.

The article also mentioned that at the "equal power frequency", two 100W amp (one for low and one for mid/high) can push 400W worth of power. In other words at that crosover frequency I need only about a quarter of wattage for the mid/high to match the power of having a single amp to power the low and mid/high.
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Old 26th June 2012, 12:49 PM   #5
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Well, the fact of the matter is that most of us do listen to amps that get clipped on peaks. Instantaneous clipping on peaks, hard to hear for the most part.

One of the problems is that if you decide to use amps that will not clip on peaks at all then you probably will have to use amps with voltage swings that exceed the power rating of the speakers. This is OK as long as there are no "accidents" and one does not try to play the system too loudly. Otherwise you will fry the speakers.

It's not hard to see why this is, since one would like to have 20dB headroom over the average listening level. So, if your average listening level works out to be something like 4 watts, which seems modest, one notices that 20dB up from there is about 500 watts! Now you don't really need that much current in most speakers, but you do need the voltage swing, and the two are linked in a typical power amplifier...

Sorry, but I did not read the article (yet) but I don't see how two 100 watt amps in a biamp situation are equal to a single 400w amp. You still have 6dB less in terms of voltage to swing...

What you can get is an amp tailored to the sound and DF needs of the woofer, which can be very different than what you would prefer sonically and DF wise for the mids and highs - such as a tube amp for them...

How much you need for the mids and highs depends only on the reference sensitivity of the pair AND the relative power WRT spectrum in typical music program... so if there is less energy in the mids and highs for typical recordings (this can be looked up, it has been investigated) , then yes you can have less available power to run the speakers - but with the caveat that one still needs to look at the peak requirements! (to do it "right")

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Old 26th June 2012, 01:10 PM   #6
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Ah, Rod Elliot is saying that with the two amps where the LF and HF sections are crossed over, that the HF signal "rides" on the LF signal like modulation on a carrier, causing the HF signal to be able to increase the level above the maximum of the LF... effectively increasing the total peak swing. But he says that this only works to the tune of 1-2dB in practice with most program material...

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Old 26th June 2012, 01:39 PM   #7
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Hi!

I think the terms Bi-amping and active are beeing mixed up here. As far as I'm aware, bi-amping refers to the use of separate amplifiers of the high and low drivers, but still with the associated high and low path filters after the amps (but the filters being electrically separated from each other).

An active set up would have the crossover between preamp and power amp. In that case you can normally get away with less power for the tweeter. and eliminate the crossover in the speaker. Usually tweeters are more sensitive than the woofers and their level is adapted with some sort of voltage divider in the crossover.

Going active enables you to work without voltage divider by adapting the volume to it's amp or the gain of the different amps.

How much less you need depends on the sensitivity of the drivers.

Again measuring would give you an idea about the power range you need.

However for such a modification you should know the parameters of your drivers and adjust the levels of the different frequency ranges appropriately. Otherwise you will get a random result.

Best regards

Thomas
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Old 26th June 2012, 02:02 PM   #8
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Quote:
Originally Posted by bear View Post
One of the problems is that if you decide to use amps that will not clip on peaks at all then you probably will have to use amps with voltage swings that exceed the power rating of the speakers. This is OK as long as there are no "accidents" and one does not try to play the system too loudly. Otherwise you will fry the speakers.
It is actually quite a lot easier to fry tweeters with a lower powered amp. The fact is that musical program material has little (average) power in the upper two or so octaves. But if you clip your amp, the high frequency harmonics created will have a much higher average power than the music has, and will get diverted by the crossover right into the tweeter. So it is usually much safer to have a high powered actively crossed system than an underpowered passively crossed one.

With woofers, the opposite is true; they are unlikely to be damaged by a clipping low-powered amp, and very likely to be damaged by a high powered amp. The high powered amp can cause the woofer to exceed its excursion limits, which can cause permanent damage in several ways, either instantaneously, or over time. Tweeters are much less likely to be damaged by over excursion (just calculate the power required to exceed its Xmax at 10000 Hz...) but likely to be damaged by heat due to very small voice coils. Most tweeters damaged by excursion were probably subjected to signals below their designed cut-off frequency (including DC).
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Old 26th June 2012, 02:45 PM   #9
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Have we determined which version of Bi-amping we are referring to?
Does Navy know the difference?

In the UK, active speakers meant the power amplifier connected directly to the speaker driver with just a cable pair between the two. The crossover is all done "actively" before the Power Amplifiers.

In the USA these "active speakers" seem to get called bi-amped.

IN the UK Bi-amped means taking a passive crossover speaker and feeding a Power Amplifier to each half of the passive crossover. This only works with a parallel type crossover. The series type crossover cannot be split, but this type is relatively rare.
If the speaker has two sets of terminals for "bi-wiring" then this speaker is also ready for bi-amping.

Navy,
tell us what you are asking about.
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Old 26th June 2012, 03:08 PM   #10
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Hi guys,

Thanks for your inputs, they are greatly appreciated.

I will need a bit of time to digest these info.

My speakers are bi-amp-able/bi-wire-able.

I don't really have a concrete plan, just exploring options at the moment.

Reconstructing the passive crossover would be a problem as Thomas pointed out as I don't have information on the drivers, but it seems like a more efficient way.

If this is impossible, I am thinking of having some kind of crossover before the power amp. As for what kind, I have yet to figure out.
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