Controversy regarding Bridged Amplifier power output - Page 4 - diyAudio
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Old 20th January 2011, 06:53 PM   #31
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Originally Posted by DF96 View Post
I am surprised that such a long argument can arise over such a trivial issue. There is no controversy, just some confusion. Twice the voltage, twice the impedance, so twice the power (as has been said).
What do you mean by "twice the impedance"?

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Old 20th January 2011, 06:55 PM   #32
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Your first time to a Forum?
Certainly not his first time to a forum. So clearly he's an optimist.

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Old 20th January 2011, 07:07 PM   #33
DF96 is offline DF96  England
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Possibly my first time to this forum. I'm usually to be found in Tubes/Valves, and a few others.

By "twice the impedance" I mean twice the impedance e.g. if the amp is meant to drive 8R, then 16R. This means that the current does not change.
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Old 20th January 2011, 07:51 PM   #34
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By "twice the impedance" I mean twice the impedance e.g. if the amp is meant to drive 8R, then 16R. This means that the current does not change.
Why would you go from 8 to 16 ohms? The point of bridging is to gain more power into a given load.

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Old 20th January 2011, 09:06 PM   #35
DF96 is offline DF96  England
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Bridging gives you twice the power into twice the load. If you want to know what you would get into the same load, you first have to discover what one amp will give you into half the load (then double the power for bridging). If you want to get the right answer, you have to ask the right question.
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Old 20th January 2011, 09:16 PM   #36
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Originally Posted by DF96 View Post
Bridging gives you twice the power into twice the load.
Sure, but what sense does that make?

You've got 8 ohm speakers. So you bridge your amp and then go out and buy 16 ohm speakers?

Sorry, but I just don't get the point of the phrase "bridging gives you twice the power into twice the load."

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If you want to know what you would get into the same load, you first have to discover what one amp will give you into half the load (then double the power for bridging).
Assuming it can deliver the current and dissipate the power, and also depending on how the input signal is handled, bridging essentially gives you four times the power into the same load.

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If you want to get the right answer, you have to ask the right question.
Ok. Don't recall seeing anyone asking how much power you get from bridging into twice the load.

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Old 20th January 2011, 09:30 PM   #37
DF96 is offline DF96  England
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If the amp can deliver the current without overheating or PSU droop then you can get four times the power. The issue is the 'if' at the start of the sentence. The factor of 4 comes from two factors of 2:
2 comes from bridging, and is always equal to 2
2 comes from halving the impedance seen by each amp, and can be less than 2

So the question boils down to what the amp does with half the load impedance. This is the question to ask. Ask the right question; get the right answer.
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Old 20th January 2011, 09:41 PM   #38
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Originally Posted by DF96 View Post
If the amp can deliver the current without overheating or PSU droop then you can get four times the power. The issue is the 'if' at the start of the sentence. The factor of 4 comes from two factors of 2:
2 comes from bridging, and is always equal to 2
Not always. It depends on how the input signal is handled in bridged mode. If you're simply splitting the signal to get an inverted signal to feed to the second channel, then you end up with a factor of 1, not 2.

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2 comes from halving the impedance seen by each amp, and can be less than 2
The impedance isn't halved. The voltage across the load is doubled, again depending on how the input signal is handled.

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So the question boils down to what the amp does with half the load impedance.
Well, if you're using 8 ohm speakers, and if the amp is capable of driving 4 ohm speakers, then just double the four ohm power rating, which for a decent amp will be roughly four times the 8 ohm rating when bridged.

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Old 20th January 2011, 09:58 PM   #39
DF96 is offline DF96  England
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Now you are confusing gain with power delivery. In the unlikely event of splitting the signal between the amps you get reduced gain, but the same maximum power output as I have been saying.

I will say it once more: to discover what power output you can get from N ohms in bridge mode, first find what you get for N/2 ohms with one amp then double it. That's all. Anything else is pure confusion. The reason this works is that each amp sees half the load; it can't distinguish between N/2 ohms connected to ground, and N ohms connected to an antiphase signal. Think of it as the opposite of bootstrapping, because that is exactly what it is.
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Old 20th January 2011, 10:30 PM   #40
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Now you are confusing gain with power delivery.
No, I'm not.

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In the unlikely event of splitting the signal between the amps you get reduced gain, but the same maximum power output as I have been saying.
If you simply split the input signal, say with a simple resistor divider or a 1:1 input transformer, the gain would remain the same as if you were driving just a single channel.

Each channel would get half the signal so when they're bridged, you're back to square one as far as gain goes.

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I will say it once more: to discover what power output you can get from N ohms in bridge mode, first find what you get for N/2 ohms with one amp then double it.
Which, if it's a decent amp, would be roughly eight times the power into N.

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The reason this works is that each amp sees half the load; it can't distinguish between N/2 ohms connected to ground, and N ohms connected to an antiphase signal.
This "sees half the load" is a source of confusion as well.

They don't see half the load. The reason the current doubles is because the voltage across the load has doubled. Which may or may not actually be the case depending on how the input signal is split.

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