Controversy regarding Bridged Amplifier power output

[Nigel Goodwin]

I'm not sure why you think I have that opinion? Sorry to mislead.

Certainly, a class AB amp uses a lot more power to provide a specific output, than a class D amp.

Whether that is considered 'wasted' power???

It's wasted in that it uses more electricity to produce the same output, which if you're running off batteries is a VERY important consideration.

But you don't get more power from a class-D amp off the same supply voltage, in fact you probably get slightly less.

 

[audioguru]

You talked about "power capacity" and and said that a class-D amplifier has more than a class-AB amplifier. Before I thought you meant amplifier output power. But now I think you meant the power capacity of the power supply.

If the 100W class-D amplifier is 90% efficient then it heats with 11.1W and its power supply uses 111.1W.
If the 100W class-AB amplifier is 70% efficient then it heats with 42.8W and its power supply uses 142.8W.
Not a big difference.

At half of max power the efficiency of a class-AB amplifier is only 48% and the wasted heating power is a little more than at max output power. But the total power from the power supply is less.

 

[rogs]

If the 100W class-D amplifier is 90% efficient then it heats with 11.1W and its power supply uses 111.1W.
If the 100W class-AB amplifier is 70% efficient then it heats with 42.8W and its power supply uses 142.8W.
Not a big difference.

A Nigel Goodwin says in post #41, the extra power can be a big consideration when using battery supplies. Which is exactly what I need to consider in my field. When you have a large emergency public address sytem in a multi story building, which may need to provide kilowatts of audio power from a bank of battery supported power amplifers, then the size and cost of those batteries become a significant factor, and amplifier efficiency can play a big part in cost savings.

 

[audioguru]

Amplifiers are hardly ever used at full blast all the time (acid rock music?).
I have also done many PA systems in multi-story buildings and airports but they used diesel generators for electricity backup, not batteries.
The diesel generators had plenty of power.
Maybe the fire alarm bells and stairwell lights used batteries for backup.

 

[rogs]

The emergency public address systems I am referring to are 'voice alarm' systems, which here in the UK at any rate, are increasingly used instead of the more traditional fire bells -especially in larger venues. As such, they are required to meet the appropriate British Standard - BS5839 Pt.8 - which specifies battery support as a secondary power supply. In many cases, these large building do have back up generators as well, so the battery support is often unnecessary, strictly speaking.
But that's what the standard says, and that's what the insurers insist upon!

It's not only amplifier efficiency that is important in helping to keep these support supplies to a reasonable size. The same standard demands that the whole system has to run, on batteries, fully fault monitored, for 24 hours (sometimes for 72 hours!) followed by a further hour at full power, at the end of that period.
So the quiescent current drain of the amplification system also has to be kept to a minimum as well.
But that's another story........

 

[MrAl]

Hello there, It's not that hard to understand the audio amp output stage and power delivered to the load with a change in configuration. We can do a DC equivalent and get very good information that makes it a lot easier. Here's the long and the short of it... For a linear amp dissipating A watts internally and external load that dissipates B watts and conducts current I, double the voltage and add another linear amp of the same type in series with the first (bridge config now). Now the two amps each dissipate almost 4 times the power (I doubles so P=I^2*R as usual) and the load also 4 times the power. Next, double the load resistance for the same bridge config. We get the amp back to A watts (the original wattage not increased except there are two amps now and they both dissipate A watts each). The load goes to about 2 times the original or 2*B watts. The current, interestingly enough, goes back to the original I current. In this way we can get 2 times the power output without causing a single amp stage to dissipate more than the original power. There's almost no way we can get near 4 times the output power because the linear amps would have to dissipate almost 4 times their rated power. In theory two ideal amps could do 4x the power, but in practice this is not going to happen, so we double the load resistance and be happy with 2 times the output power.

 

[Nigel Goodwin]

There's almost no way we can get near 4 times the output power because the linear amps would have to dissipate almost 4 times their rated power.

You seem to be confused and perpetuating the silly 'myth' about bridged amplifiers, my original post in this thread explained your error.

But basically in a bridged amplifier both amplifiers never exceed their rated output, but provide four times the power in to the same impedance - an impedance which caused the amplifier to only proved HALF it's rated power as a single amp.

EDIT:

For a practical example, the Class-D PA amplifier I'm sat about three feet away from is rated at 400W per channel in to 4 ohms.

If I connect a single 8 ohm speaker to it in single ended mode, it will only provide 200W.

If I switch the amp to bridge mode, it will provide 800W to the same 8 ohm speaker.

That's all bridging is about.

 

[MrAl]

Hello,

Im not sure i know what you are talking about here. I know that in theory a bridged amp can provide 4 times the output power, but in practice something has got to give. If we double the current through the amp it dissipates more power at the very least by the I^2 *R law. Did i misunderstand you somewhere?

 

[Nigel Goodwin]

Hello,

Im not sure i know what you are talking about here. I know that in theory a bridged amp can provide 4 times the output power, but in practice something has got to give. If we double the current through the amp it dissipates more power at the very least by the I^2 *R law. Did i misunderstand you somewhere?

Obviously so - I've explained it repeatedly - I'll do it yet again!!.

It provides four times the power in to 8 ohms that a single amplifier would provide in to the same 8 ohms.

Each amplifier MUST be capable of feeding 4 ohms for this to work.

You don't get any more power out of the amplifiers, just the same amount in to a higher impedance, so the I^2 *R law isn't affected in any way.