Question about Amplifier wattages and efficiency

[ronster]

I have a somewhat different sort of question regarding the efficiency of class AB amplifiers.

This is not about calculating the efficiency of a typical class AB - that can be found in any good
electronics textbook. What I'd like to do is estimate how much more power might be
consumed by a 200 watt amplifier relative to a 50 watt amp, if they were both generating
the same output wattage (such as 10 watts).

Before posing the question, imagine the following hypothetical scenario:

You have two solid state Class AB amplifiers of a similar design. Both use bipolar junction transistors (BJT)
in the output drive stages.

Amplifier #1 is rated at 50 watts rms

Amplifier #2 is rated at 200 watts rms.

Now imagine that both amplifiers are delivering an identical 10 watts rms to their own speaker.

QUESTION: Will the 200 watt amplifier consume significantly more energy than the 50 watt
amplifier even though they are generating equal output (10 watts rms)?

Worded another way, will the 200 watt amplifier dissipate (waste) significantly more
energy as heat (via its power transistors) compared to the 50 watt amp?

(My gut instinct is that there would be much more power consumed by the
200 watt amp since it would have a higher rail voltage power supply.)


I eventually plan to build or purchase a new amplifier. My understanding is that a well designed
higher wattage amplifier should provide more headroom, better dynamics and lower distortion compared
to an equivalent lower wattage amp. On those merits I would prefer 200 watts over 50. My main concern
is for best quality of sound, but am also thinking about long term energy efficiency too (being more green
for mother Earth).

I suspect that at my moderate rock and pop music listening levels, speaker efficiency and relatively
small room size, my stereo amp will likely output about 5 to 10 watts rms to each speaker (not
counting transient peaks) during typical listening.

Part of me would prefer the higher wattage amp for its better headroom etc., but before making any
decisions I am curious as to how much additional power might be needlessly wasted (as heat)
if choosing the higher wattage amplifier?

If the differences in energy use between a 50 watt amp and 200 watt amp at lower output levels (such
as 10 watts rms) is not that significant, then of course I'd choose the higher wattage.

I would appreciate any insights or advice that forum members might have.

Best regards.

Ron (New diyAudio member)

[tinitus]

Originally Posted by ronster

(My gut instinct is that there would be much more power consumed by the
200 watt amp since it would have a higher rail voltage power supply.)

Yes, together with the needed bias, but maybe not that much

My understanding is that a well designed
higher wattage amplifier should provide more headroom, better dynamics and lower distortion compared
to an equivalent lower wattage amp.

Not really, but depends on how loud you listen

On those merits I would prefer 200 watts over 50. My main concern
is for best quality of sound, but am also thinking about long term energy efficiency too (being more green
for mother Earth).

Im happy with my 30-40watt monos
100watt should be plenty

I suspect that at my moderate rock and pop music listening levels, speaker efficiency and relatively
small room size, my stereo amp will likely output about 5 to 10 watts rms to each speaker (not
counting transient peaks) during typical listening.

Part of me would prefer the higher wattage amp for its better headroom etc., but before making any
decisions I am curious as to how much additional power might be needlessly wasted (as heat)
if choosing the higher wattage amplifier?

Go fore ampd design with single pair output devices(transistors)

If the differences in energy use between a 50 watt amp and 200 watt amp at lower output levels (such
as 10 watts rms) is not that significant, then of course I'd choose the higher wattage.

My 40watt monos with rather big heatsinks beats a heavy 200watt Nakamichi, in every way except very loud SPL

I would appreciate any insights or advice that forum members might have.

I would recommend no more than 100watt, max

[Rafael L]

Originally Posted by ronster

I eventually plan to build or purchase a new amplifier. My understanding is that a well designed
higher wattage amplifier should provide more headroom, better dynamics and lower distortion compared
to an equivalent lower wattage amp. On those merits I would prefer 200 watts over 50. My main concern
is for best quality of sound, but am also thinking about long term energy efficiency too (being more green
for mother Earth).

I suspect that at my moderate rock and pop music listening levels, speaker efficiency and relatively
small room size, my stereo amp will likely output about 5 to 10 watts rms to each speaker (not
counting transient peaks) during typical listening.

If you want to save energy or decrease consumption, you should use larger diameter speakers, in general the sensitivity is greater.
That was formerly used when the amps were not very powerful

[SQLGuy]

Hmmm... not so sure.

For example, I've got a 50W Bryston and a 200W PS Audio. The Bryston has 2 TO-3's per channel, the PS Audio has 6. Assuming they both have the same idle current at about 100mA per device, the Bryston's going to idle at about 14W, compared to the 100W or so for the PS Audio.

Now, at low power, it get's trickier. A pure class A amp will actually run cooler when driven hard, and will draw about the same power regardless of whether it's at idle or full out. A pure class B amp will draw nothing at idle; it's draw will be based entirely on load. For AB, it depends on how much bias and how much load.

For the PS Audio, at 100mA per device, we can provide about 1/3W into an 8 Ohm load before going out of class A (if I did my math right). The Bryston would only get about .04W (again, assuming correct math, and that the per-device bias is the same). Either way, this is a small bit of the 10W given, so I think we can tack the 10W onto the idle power, which would give about 120W (stereo) for the PS Audio driving 10W, or 34W for the Bryston. The Bryston is clearly cheaper here as a source of 10W class AB.

On the other hand, you didn't say what speakers, what impedance, or anything. I'm running about 750W per speaker, biamped, because Magnepan's (+/- 82 dB 2W/1m) do need that kind of power, at least in a largeish room. If I had something like Klipsch La Scala's (+/- 102dB 1W/1m), I could be happy driving them directly with an iPod (more or less) ;-)

[djk]

"What I'd like to do is estimate how much more power might be
consumed by a 200 watt amplifier relative to a 50 watt amp, if they were both generating
the same output wattage (such as 10 watts). "

Easy.

Say we are running 8W into 8R, or 1A of current.

The 50W amp runs on about ±42V, the 200W amp runs on about ±80V. The 50W amp has to drop about 32V at 1A vs the 200W amp dropping 70V at the same 1A, so the 200W amp uses over twice the power to produce the 8W.

The power factor stinks on audio amps with large filter caps, so both amps will actually use about twice the power listed above.

Close enough for rock-n-roll.

[djk]

http://www.crestaudio.com/media/pdf/v450_5-20-97.pdf

Rated at 2x225W/4R, draws 3.5A at 1/8 power (music). Thats 420W in for about 56W out.

[SQLGuy]

On the other hand, if you had a SUMO The Nine Plus (Class A, 55W), you'd be running 40V rails and dropping about 4A at idle. So, about 320W (per channel) at idle. If you crank it up, to drive 10W into your 8 Ohm speaker, the only difference is that some of that power that would have gone out the heat sinks is now being routed to the speaker... power draw is STILL about 320W per channel. That's why I'm saying that Class of output is a big factor here.

[ronster]

You've confirmed my instincts.

FYI, my current speakers are 8 ohm with 86 dB sensitivity (one of you noted that
I had forgotten to list the load resistance - good point). Future speakers could end
up being 4 ohm, probably with higher sensitivity. In that case, the estimated power
numbers probably double.

Gives me food for thought.

I thank each of you for taking the time to read my post and respond.

Ron