AudiocRavings: 7 - Another Way of Looking at Amp Power Specs - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 6 Old 10-14-16, 03:19 PM Thread Starter
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AudiocRavings: 7 - Another Way of Looking at Amp Power Specs

AudiocRavings, by Wayne Myers:
7 - Another Way of Looking at Amp Power Specs



To Double-Down

It is commonly accepted in the world of high-end audio that a good characteristic of a power amplifier is its ability to Double-Down in output power for every halving of load impedance. In other words, an amplifier capable of providing
  • 200 W rms into 8 Ohms
  • 400 W rms into 4 Ohms
  • 800 W rms into 2 Ohms
Would be said to have that Double-Down capability.

Two questions: (1) Is this really so important? and (2) How hard is it to find?

Beginning with (2), this is not a super easy characteristic to find. It tends to be found in more expensive amplifiers. But it can be found. At RMAF recently, I spent some time listening to a Moon Audio amplifier that can Double-Down all the way to 2 Ohms, and is stable with loads all the way down to a 1 Ohm load. Not bad.

Now back to (1): With a purely resistive speaker load, one whose impedance does NOT vary with frequency, that capability would be of little or no importance. But most speakers are NOT purely resistive, and have loads that can vary quite a bit over their frequency response range. This Double-Down characteristic can be especially important with a capacitive load, such as an electrostatic panel, where the impedance can drop as frequency increases. My MartinLogan ESL has a nominal impedance of 6 Ohms, but drops to a minimum of 1.6 Ohm at high frequencies. While not a guarantee, the Double-Down capacity is a very good indication that the amplifier will be able to handle different types of speaker loads with gracefulness, behaving properly - without oscillations or unexpected ringing or frequency response peaks - and keeping distortion low.


Or NOT To Double-Down

Here is an example of an amplifier whose characteristics appear NOT to Double-Down.
  • 400 W rms into 8 Ohms
  • 600 W rms into 4 Ohms
  • 800 W rms into 2 Ohms
Not as good an amplifier, right? Not so fast! Specs and Capabilities are not necessarily the same thing. Let’s look at it another way.


Different Math

Looking at the previous example:
  • An amp that can drive 400 W rms into 8 Ohms can certainly drive 200 W rms into 8 Ohms.
  • And an amp that can drive 600 W rms into 4 Ohms can certainly drive 400 W rms into 4 Ohms.
  • That ability to drive 800 W rms into 2 Ohms appears to be the most important single number for this particular discussion.
So, our amp that could NOT Double-Down when we were thinking about it as a 400 W amp into an 8 Ohm load suddenly is perfectly capable of Doubling-Down if we think of it as a 200 W amp into an 8 Ohm load. Surprise!


Is This Fair?

Of course those who go to great lengths to design and build amplifiers that can Double-Down with specified power out levels might cry, “Not fair, you are comparing apples and oranges.” My response to their outcry is, “Sure it is fair, it is another way of comparing the weight of a big apple pile to the weight of a smaller orange pile by ignoring some of the apples. Now suddenly there are a lot more Double-Down capable amps to consider for a given application.


A Perspective

In summary, the Double-Down capability of an amp might not always be the single most important factor to look at when considering amplifier output capability. My alternate perspective is NOT intended to minimize the importance of the Double-Down perspective or to diss those amps that are designed to have that capacity. It is simply meant as a perspective that opens up a range of possibilities when looking at amp specs to arrive at the same Double-Down performance level with a bigger amplifier - with 8 Ohm loading. So, if you are willing to ignore some drive capability at 8 Ohms and 4 Ohms, and the amp has the capability of remaining stable while driving 2 Ohms, that amp just might be all you need.


Welcome to AudiocRavings, my blog of audio-related thoughts, musings, ideas, discoveries, suggestions, rants, and ramblings. With luck, a portion will be somewhat useful to someone somewhere somehow.

Wayne Myers
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post #2 of 6 Old 10-14-16, 03:37 PM
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Re: AudiocRavings: 7 - Another Way of Looking at Amp Power Specs

Thanks Wayne for clearing that up. The obvious fact here is that if it were truly important, there would not be so many great amps that do not double down.

While I do look at it when reading about an amps specs, I don't think I have ever noticed it with tube amps. I'll have to look back & check. Have you ever noticed it listed with tubes?
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post #3 of 6 Old 10-14-16, 10:53 PM Thread Starter
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Re: AudiocRavings: 7 - Another Way of Looking at Amp Power Specs

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Thanks Wayne for clearing that up. The obvious fact here is that if it were truly important, there would not be so many great amps that do not double down.

While I do look at it when reading about an amps specs, I don't think I have ever noticed it with tube amps. I'll have to look back & check. Have you ever noticed it listed with tubes?
No I have not. It really only applies to solid state amps with low output impedance and high damping factor. It is not an "absolute must" factor or even a "really good idea" factor, but more of a "this one is special" factor that some people put a lot of stock into when making an amp decision. The real intent here is to demystify the idea a bit, like maybe it is not quite such a "special quality" after all, or that it is a special quality but there are more amps that meet the criteria if you look at the specs in the right way.

It is also not a factor to be considered in isolation.

My reason for pointing this out is that as I was thinking about it a few days ago, it hit me that there was this other perspective. Just thought I would share.


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post #4 of 6 Old 10-15-16, 01:07 PM
 
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Re: AudiocRavings: 7 - Another Way of Looking at Amp Power Specs

Thanks for the info... I think I have always looked at it as a better quality amp build vs a necessity for purchase.

Ron

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post #5 of 6 Old 10-15-16, 01:51 PM Thread Starter
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Re: AudiocRavings: 7 - Another Way of Looking at Amp Power Specs

Agree, that is a good way of thinking about it.


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post #6 of 6 Old 10-17-16, 01:56 PM
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Re: AudiocRavings: 7 - Another Way of Looking at Amp Power Specs

+1, I would never let that dissuade me from buying an amp.
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