Power Amp Doubles Power As Load Impedance Halves?

3659 Views 3 Replies 4 Participants Last post by jackfish, Aug 21, 2009

arnyk

Discussion Starter · Aug 21, 2009

A theoretically ideal power amp would double its power every time the load impedance halves.

There are power amps on the market that either claim this or come close to claiming it.

IME, these amps are the results of specsmanship, not technology.

Here's a real world data point for the controversy about power amps needing to be able to put out twice the power for half the impedance:

I ran some bench tests on what may be one of the most overbuilt power amps in the history of audio - the Threshold SA/4e from the early 1990s.

No visible effort was spared to build a robust power amp, as this article shows:

http://www.thresholdlovers.com/artic...?lng=en&pg=394

The Threshold SA/4e weighs about 120 pounds, according to my calibrated biceps. You really want two people to tote it around, and it facilitates this by having two comfortable handles on each end.

In contrast, in similar tests a Behringer A500 which weighs about 15 pounds puts out about the same amount of power at clipping at 8 ohms, but puts out about 144 watts at both 4 and 2 ohms. This is what I would expect from a typical contemporary mainstream amplifier.

Among other features, the Threshold SA/4e amplifier has extremely soft clipping. In the end I decided to measure its output power at about 0.1% THD @ 1KHz.

The actual measured power is as listed here:

http://cgi.audiogon.com/cgi-bin/auc....82027&auc&3&4&

Namely 112 Watts per channel @ 8 ohms with both channels driven @ 1000 hz, 196 Watts @ 4 ohms, 264 Watts @ 2 ohms.

This is still very far from doubling power as load impedance halves, particularly at 2 ohms.

The emotiva XPA-2 is not expensive and they claim doubles at 4 ohms but there is no spec for 2 ohms.

But, that's not what their own test reports show. They show just over 200 wpc into 8 ohms and 300 wpc into 4 ohms.

Their spec sheet claims 300 watts into 8 ohms and 550 into 4. Close but no cigar on that power doubling claim. They use what I consider to be a bogus means of rating it - power at 1% THD. IMO, power amps should be rated by power at clipping, not power so deep into clipping.

By the same standards that I consider the A500 to be a 100 wpc amp, the XPA-2 is a 200 wpc amp. You can buy 4 A500s for the price of one XPA-2.

As a rule, you should get more watts per buck as the power goes up. Economises of scale and all that.

There's only a 3 dB difference in power, which you'd be hard put to actually hear unless you do a very close comparison.

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Anthony

· #2 · Aug 21, 2009

Well, eventually you are running into current limitations and power supply limitations, so you can't just keep doubling at halved impedances. And remember, that's the theoretical limit.

A good, beefy amp, will almost double going from 8 to 4. 4 to 2 is less likely.

Also, the type of amplifier makes a big difference. I don't think digital switching amplifiers work on the "double down" theory the same way transistor amps do. I could be wrong though, there are plenty of electrical engineer types around here to correct me.

But you are right, a lot regarding amps is specsmanship. In the 80s and 90s designers would run massive feedback in an effort to get absurdly low THD and power output (at one frequency, one channel driven). The amps sounded tinny, almost fake (like the edge enhancement on some HDTVs -- it looks sharper, but it's fake). This led to people referring to "Japanese sound" of amps, although american and euro designers fell victim as well.

The best specs are all channels driven, broadband noise at varying impedances. Those usually give you an idea if the power supply is robust enough and if the amp is well designed. But specs aren't everything, my 8W tube amps with awful distortion figures somehow sound magical in my music rig. I would have dismissed it until I heard it. Who knew?

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tonyvdb

· #3 · Aug 21, 2009

Anthony said:
The best specs are all channels driven, broadband noise at varying impedances. Those usually give you an idea if the power supply is robust enough and if the amp is well designed. But specs aren't everything, my 8W tube amps with awful distortion figures somehow sound magical in my music rig. I would have dismissed it until I heard it. Who knew?

You hit the nail on the head Anthony, it really boils down to the power supply and what kind of class of amp it is. A person can't even go by the specifications that most have in the manual as in the real world speakers can go down into the 2 ohm area at times if driven hard.
For example my Samsung 4 channel amp is bridged into two channels at 240watts per ch. however I am driving speakers that are 4 ohms and the manual clearly states not to drive 4 ohm speakers if bridged. It has been running flawlessly for well over a year like this and sounds fantastic (its a good thing they are fairly efficient speakers @93db)

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jackfish

· #4 · Aug 21, 2009

Thankfully the one thing we all can do is listen.

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