Rated Power vs. Practical Power

[Thai]

Hey all,

While i was reading reviews on various amps and receivers, i came across this paragraph of a review of the Denon 5805 receiver:

"...When driving the AVR-5805 at around 157wpc into 8 ohms, I ran a frequency response sweep against a 4 ohm load under the same test conditions where it happily delivered exactly twice the power (315wpc).

This indicated two things:

1. The Denon amp had a low enough output impedance and enough power supply reserves to act like an ideal voltage source to double down its power delivery as the load impedance halved.

2. While doing so, it maintained excellent frequency response uniformity (falling well within our preferred +-.25dB deviation window) indicating excellent gain product bandwidth and lack of slew induced distortion when driven hard.

Since the AVR-5805 sported a cool "Bi-Amp" mode, I wanted to see if it could deliver the goods in this scenario. I configured the AVR-5805 front channels for biamping and tested for maximum unclipped power. Note, we do not hold the line voltage constant like some publications do, as we believe in testing for real world conditions. Consumers typically don't have the luxury of stabilizing their power and/or providing enough wall current to source max power with all channels driven (a very unrealistic test condition and a very idealistic test load we might add). In any event, I was able to achieve a 170wpc at 0.1% THD with four channels driven but noted that my dedicated 20A line sagged from 124V to 119V under this test. In reality, the AVR-5805 probably could have delivered an extra 10wpc or so in a more sterile lab environment. This test certainly proved the AVR-5805 was a true powerhouse. For the fun of it I reran this test for five channels driven and was able to achieve about 145wpc at 0.1% distortion but again watched the line voltage sag down to 110V during this test. Thus again, this amp is certainly able to deliver the full capabilities of a 15A outlet and then some! Based on the amount of power reserves available in this receiver I would venture to say if you had a stabilized dedicated line to run this receiver, it could probably drive up to 7 channels simultaneously at full bandwidth unclipped at its rated 170wpc into 8 ohms assuming an internal fuse doesn't blown first. Before you actually declare your set-up can achieve this power, consider this: The efficiency of a typical class A/B amp, such as the ones found in this receiver, is around 35-40%. Let's assume 40% in this instance. In order to deliver 170wpc x 7, it would require: 170*7/.4 = 2975 watts! For a 120VAC system your line would be required to deliver 25 amps! Unless you are running 3 phase power, or convince the power company to run 6AWG romex and up the circuit breaker from a standard 15A to a hulking 25A, this simply isn't gonna happen (no, not even with name brand separates). Not to mention the fact that UL certified products such as this one are limited to 15A power consumption at the interface connector for safety reasons. Remember this next time your friend boasts about his dedicated 7CH 200wpc Class A/B amp which sports a single line cord connection.

In reality, the power supply and amp section of the AVR-5805 are not the limiting factors for maximum power delivery for all channels driven, the outlet and power receptacle are.

Food for thought for power freaks that think they will actually use this much power in all channels at the same time."

Actual link to article: http://www.audioholics.com/productreviews/avhardware/DenonAVR5805p13.php

What do you guys think?? Is the outlet truly the limiting factor when talking about all these wattages??

Sorry if this is something obvious...i am still a newbie with all of this HT stuff.

Thai.

 

[ralflar]

Yes and no. Fortunately one does not need full power from all channels all the time. Extreme power is needed for dynamic reserves. It is needed for very short times and not on all channels. In practice I do not think one needs to be too concerned about the wall power unless big screen TV/projector, HT amp, subs all feed from the same circuit.

 

[Rich]

I agree with what Ralflar says. You will never need full power on all channels for any length of time. But you do need power reserves for dynamic peaks, which are usually short in duration and not on all channels at once.

Having said that, however, I think a there are a lot of people that have a room with one 15 amp circuit to which they connect a large television, home theater preamp, multichannel amplifier, dvd player, ML speakers, subwoofer, lighting, etc. and then wonder why their expensive electronics don't sound so good, lacking punch and dynamics. In these situations, the electronics are often much more capable than they are allowed to be because of insufficient power available during dynamic peaks.

I have dealt with this issue in my current house and that has lead me to go overboard with the listening room in the new house we are renovating. I had the electricians install four dedicated 20 amp circuits with isolated grounds. That is more power than I will ever need, but my system will never thirst for juice.

 

[Sambob2]

I'm not an EE (Electrical Engineer) but isn't the transformer responsible for upping the voltage to the power transistors to provide for gain?

I would suspect that the electricity from the outlet is only providing "juice" at idle until the amplifier demands lots of current. For the most part I thought most amplifiers require only a 15 amp circuit, up to 1875 watts and that only the largest require 20 amp service. True the lower load of a 4 ohm speaker will demand more from the amplifier that an eight ohm load...but only if the reserve capacitors and transformer can deal the extra power needed on call. Ampifiers that can "double-down" are true workhorses with huge transformers and reserve capaictors.

As RALFLAR commented these are only short bursts of power demand usually for milli-seconds. I do agree with THAI that the outlet voltage can drop some but I think the amp is only asking for 120 to the input or primary side of the transformer winding. Any comments or education from a true EE?

Sam

 

[Sambob2]

Having said that, however, I think a there are a lot of people that have a room with one 15 amp circuit to which they connect a large television, home theater preamp, multichannel amplifier, dvd player, ML speakers, subwoofer, lighting, etc. and then wonder why their expensive electronics don't sound so good, lacking punch and dynamics. In these situations, the electronics are often much more capable than they are allowed to be because of insufficient power available during dynamic peaks.

Rich,
Yes I agree with that statement also. To much can ruin a good thing for sure. Most of the time all that is required is to look at the rear of the equipment and simply add the amperage. Spread that total amperage across separate circuits and you should be fine. Definetly get your electrician to wire up extra circuits in your listening room.

Also, make sure they use the same polarity for each run...I'm not exactly sure about this but I know there are three rails they can tap into in that breaker box. One is a common, nuetral, and the other two are hot lines for each bank of breakers. I thought I read somewhere that you can avoid noise and hum or at least minimize it by verifying the polarity is the same across the outlet openings. Anyone else have input on this?