Power Amp Doubling Output at 4 Ohm

Hi, all,

Another random musing. From time to time, I see ads that mention such and such power amp can double its output from 8 ohm to 4 ohm (and even double again from 4 ohm to 2 ohm). To me, this does seem to be a highly desirable, quantifiable, quality. I don't know anything about the engineering behind equipment design, so here is my question. Is this an important parameter to you? If not, why not?

Thanks for your inputs.
It is important in that it's an indication of a robust power supply (and PS are an expensive part of the amp).

However, please note that doubling is not really possible, for a number of tech reasons. This does NOT mean that manufacturers quoting doubling are fraudulent!

What you do is, officially spec your amp at 8ohms for less than max output, which will allow the arithmetic to hold true to specs. Please note that amps capable of dealing with difficult loads (i.e. low ohm loads) are considered good designs and trustworthy.
Is this an important parameter to you? If not, why not?
It only became important to me when I recently bought my first pair of inefficient speakers that are a difficult load. (4 ohm 82-87db). Before these speakers, I never paid much attention to amps that double power.
1/ Realise that this concept ONLY applies to solid state amps - a tube amp delivers it's maximum power into the rated impedance, and less as the impedance climbs or drops.

2/ If the SS amp is well designed, with a decent sized powersupply and enough output devices, it SHOULD double it's 8 ohm power into 4 ohms.

Regards, Allen (Vacuum State)
Sometimes, it just means large amounts of stabilizing feedback.
A perfect transistor amp would double every time you halve the impedance. Ones with big enough power supplies more or less do this. If you have inefficient speakers that have low impedance this is an important consideration. But it is not the only one. Many class a amps will not do this and if you have efficient high impedance speakers you can look for amps that are designed to maximize other parameters of performance. All design is compromise and a powerhouse large amp MAY give up speed and transparency to amps that do not have the same power but which can react faster. There is no real rule to guide you here, every amp has to be considered individually in terms of how it meets your needs. A general principal is to have plenty of power but to go with the smaller amp if you are choosing between two that meets your needs as it will PROBABLY have a simpler circuit [need fewer power transistors] than the bigger one. All the above may not hold in individual cases and this complexity explains the theoretical advantages of active speakers in which the amp is tailored to the individual speaker. Again some designs execute this better than others.
It wasn't until I installed an amp that doubles output as impedance is halved that I heard good tonal balance on my speakers (which do not have a flat impedance curve). Before that, the speakers sounded tipped up, or "bright", because the high frequencies were louder than the bass frequencies.

So, for me, doubling power output as impedance is halved has become important. IMO, it allows one to use a wider variety of speakers.
The issue is really how important this is to with the way we perceive sound! Being able to double power as impedance is cut in half (or cut power to 1/2 as impedance is doubled) will allow flat frequency response with certain types of speaker but definitely not **all** speakers (IOW the speaker has to be designed to expect this behavior out of the amp and not all are- some are designed for amps that **won't** double power). There are also more important things to the human ear that can supersede the importance of flat frequency response. Sensing spatiality (soundstage) and the presence of odd-ordered harmonics (5th, 7th and 9th) are two examples of that.


for more information.
As noted it's an indication of the budget spent on the power supply, but it's not important at all. The only thing that matters is that enough power is available to drive your speakers to the level you desire.

If your speakers have a minimum impedance of 4 ohms and 300 watts are necessary at that impedance to produce the level you desire, it doesn't matter whether you use a 150 watts into 8 ohm (300 watts into 4 ohm) amp or a 200 watts into 8 ohm (300 watts into 4 ohm) amp.

A Parasound JC-1 is spec'd for 400 watts into 8 ohm (800 watts into 4 ohms, 1200 watts into 2 ohms). A Bryston 7B SST is spec'd for 600 watts into 8 ohms (900 watts into 4 ohms). A 7B SST can't handle a 2 ohm load, but depending on the speaker and room, the extra power at higher impedances may make it a better choice than a JC-1.
I'm in the same boat as Foster and Tvad with my current "difficult load" speakers (OHMs and Dynaudios)...it's important.
Simply put, if everything else is equal an amplifier that can double power into 4 ohms compared to 8 ohms can supply more current to the speaker than one that cannot double power. Lower speaker impedances require more current, everything else being equal.

Of course, along the lines of Stan's comment, everything else is rarely equal.

On the question of maintaining tonal balance with "difficult" speaker loads, see my post dated yesterday (Sept. 8) in this thread.

-- Al
As an observation, I once demo'ed NuForce Reference 9V2 SE, which do not double power as impedance is halved. It has a damping factor of >4000. Using your calculation, this translates to an output impedance of .0002.

The Pass Labs XA-60.5 I own have a damping factor of 150, which translates to an output impedance of .05.

The Pass Labs, which doubles power as impedance is halved produces better tonal balance and bass control, than did the NuForce amp, which has a significantly lower output impedance.
The Pass Labs, which doubles power as impedance is halved produces better tonal balance and bass control, than did the NuForce amp, which has a significantly lower output impedance.

TVAD -- I'm sure that your observations were accurate, at least with the speakers you were using.

But once the output impedance gets below an order of magnitude less than the minimum impedance of the speaker (perhaps by a factor of 10 or so), it stands to reason that further improvements in that parameter would be far outweighed by other differences between the amplifiers. And in this case you are comparing a pure Class A amplifier (weighing 75 pounds per channel) with a Class D amplifier (weighing 8 pounds per channel), so there are certainly a great many other differences.

Also, keep in mind that once the output impedance gets below a certain point, it will be overshadowed by the resistance and/or inductance of the speaker cables themselves.

A minor correction, btw: Damping factor 4000 = Output impedance 0.002 ohms, not 0.0002.

-- Al
Yeah, a typo. Damn fingers. I even used a calculator.

To be honest, of all the SS amps (or digital and/or switching amps) I have owned, all of them have had damping factors above 40, and I will venture to say they have all been substantially higher (Bryston 4B SST, Bryston 14B SST, Odyssey Stratos Extreme, Moscode 401HR, Belles 350A Reference, Belles 150A Reference, McCormack DNA-2, Bel Canto Ref1000).

I'm going to stick with my flawed but repeatable (in my system) concept of the superiority of amps that double power output as impedance is halved. In my system, the amps that double power have produced better bass and better tonal balance (on Von Schweikert VR4 Gen III HSE, Silverline Sonata III, and Castle Howard II speakers).