2. amps-preamps
  3. 1252471523

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.
millicurie999
tvad
9,467 posts
 
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.
almarg's avatar
almarg
8,673 posts
 
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.

Best,
-- Al
tvad
9,467 posts
 
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).
More to discover