Questions about amplifier power specs

[PENG]

Your forgetting the point of audable distortion. From all the power curves I've read, the distortion figures raise exponentially at some point. I want all channels tested to the point just before audable distortion arises. That will severly limit current draw as well. Your assuming based on your calculations that an amp can deliver its rated power all channels driven, full bandwidth at distortion levels below audability. I'm saying, thats not the case...espcially if the loads are becoming increasingly reactive. Its one thing for an amp to handle a pure resisitive laod, quite another to handle a complex load such as a loud spaker.

Let me be clear, I did not assume anything (except you did not specify the rated output of the amp to be tested) other than what you specified, that is: All channel driven (I used short form ACD may be that's why you missed), into a 8 ohms, 4 ohms and 2 ohms load (I calculated it for 2 ohms, the most severe case), and a 66.5 degrees phase angle. Regarding the phase angle, you said leading or lagging so that means either inductive or capacitive. Finally, you said test to the point of audible distortion. I did not forget that either, but you will need even more power for the test because by the time you hear distortion, a typical class A/B amp would be putting out more than their rated power. So if my calculations show the need for a 120V 150 A service, you may as well up it to 200A just to be sure.

Since you did not specify the manufacturer's power output spec for the test subject, I arbitrarily assume a 200 WPC amp. If you want to test a more, or less powerful amp, you can just scale the numbers I came up with from my calculations.

Anyway, hopefully I managed to clear things up a little, let me emphasize again that I do understand why you would want an amp that is capable of doing ACD driving a 2 ohm highly inductive load, and still able to deliver its rated power, but please understand the points I was trying to make.

1. If the amp as capable as you are thinking, can deliver the assumed (yes on my part) 200 WPC into 8 ohms, it has to then deliver 800 WPC or 5.6 kW into a 2 ohm load. Conversely, if it can only deliver 200 WPC into a 2 ohm load, then it will likely deliver only 50 WPC into an 8 ohm load and I assume that's not what you have in mind.

2. The inductive load (you specified 66.5 degrees) for the test represents a load power factor of 0.4, so that bumps the kVA (as opposed to kW) up to 5.6/0.4=14,000 or 14 kW.

And I repeat, by the time you factor in voltage drop and efficiency considerations, to do the test you are asking for you will need a 150 to 200 A service at the 120V level. No disagreement with you on the need of such a nice powerful amp (I want one too), just want to make sure you know a 30 or 60A service will not cut it, but you can do those tests properly with a 200 A service.

 

[3db]

I went home looking at some old HT Magazines at teh 2 channel power tests and for an Onkyo, (forget which model now) it was running at aproximately 150 watts before it reached .5% distortion. Taking in the affect of the power factor would pump up the power consumed to even more. The point I was trying to make which I think you missed becuase you didn't address it in your last response is...just because an amp can deliver that kind of power in 2 channels at the rated distortion doesn't it mean it can deliver that times the number of channels the amp has. Your applying this linearly where I'm saying...no, each channel you add into the test is not a linear function. Maybe power amps can deliver that kind of juice at distortion levels below audability and full spectrum and yes, I would totally agree with your arguements. But put a Denon, Yamaha, or Onkyo receiver to that kind of test and you'll soon see that the power transformer simply cannot put forth this kind of power (even if the circuit they had it hooked up too could) while maintaining inaudable distortion with a full bandwidth signal.

Getting back to the car analogy.. if road testers build special test tracks to get find out the top end, handling, etc (which can never be used on teh highways,) why can't people who test audio do the same even though this kind of condtion will never be found at a typical home?

 

[highfigh]

I went home looking at some old HT Magazines at teh 2 channel power tests and for an Onkyo, (forget which model now) it was running at aproximately 150 watts before it reached .5% distortion. Taking in the affect of the power factor would pump up the power consumed to even more. The point I was trying to make which I think you missed becuase you didn't address it in your last response is...just because an amp can deliver that kind of power in 2 channels at the rated distortion doesn't it mean it can deliver that times the number of channels the amp has. Your applying this linearly where I'm saying...no, each channel you add into the test is not a linear function. Maybe power amps can deliver that kind of juice at distortion levels below audability and full spectrum and yes, I would totally agree with your arguements. But put a Denon, Yamaha, or Onkyo receiver to that kind of test and you'll soon see that the power transformer simply cannot put forth this kind of power (even if the circuit they had it hooked up too could) while maintaining inaudable distortion with a full bandwidth signal.

Getting back to the car analogy.. if road testers build special test tracks to get find out the top end, handling, etc (which can never be used on teh highways,) why can't people who test audio do the same even though this kind of condtion will never be found at a typical home?

First, I think it's a good idea to never say never. Think about the home built speakers, switching devices and wiring done by people who don't have a clue as to what they're doing before thinking that an amp will never see some kinds of difficult loads. Ever seen screw-in house fuses on a speaker to protect it? I have and it was scary.

The thing is, a mass marketing manufacturer will build their equipment for the lowest cost possible to achieve a nominal set of specs and if you really want something that's drastically better (more robust), you can't buy their models because this kind of design falls in the high end/custom range. Also, many amplifiers that came to market over the years were existing designs, with minor changes made to avoid having to pay licensing fees. Look at an old guitar amp and it will probably have a reference to Western Electric somewhere.

I saw a Perreaux amplifier that rated its power output as "adequate". IIRC, the actual numbers were 60V @20A.

The sad fact is that the guidelines for rating an amplifier that will be sold in the US are lacking teeth. Used to be (pre-1973) that a manufacturer could basically say anything and get away with it. Then, RMS, both channels driven, 20Hz-20KHz, no more than .xxx% THD came along and it made some manufacturers step up to the line with better designs. A few years later, we saw 200+W/ch receivers that weighed close to 100lb and they weren't really all that great when compared to decent separates with less output. A couple of specifications races later and CDs came out, which revealed the need for dynamic headroom, which became a new spec that could be used by the marketing departments to show that their stuff was oh-so-much better than someone else' for only that spec, but it didn't necessarily sound very good. Also, if a piece of equipment did it once, they assume that all of them can do it.

Marketing and MBAs are what drive most industries now, not the demand or requests for better goods. We can want better equipment more than life itself but if the focus groups don't see it as viable, it ain't gonna happen.

 

[3db]

First, I think it's a good idea to never say never. Think about the home built speakers, switching devices and wiring done by people who don't have a clue as to what they're doing before thinking that an amp will never see some kinds of difficult loads. Ever seen screw-in house fuses on a speaker to protect it? I have and it was scary.

The thing is, a mass marketing manufacturer will build their equipment for the lowest cost possible to achieve a nominal set of specs and if you really want something that's drastically better (more robust), you can't buy their models because this kind of design falls in the high end/custom range. Also, many amplifiers that came to market over the years were existing designs, with minor changes made to avoid having to pay licensing fees. Look at an old guitar amp and it will probably have a reference to Western Electric somewhere.

I saw a Perreaux amplifier that rated its power output as "adequate". IIRC, the actual numbers were 60V @20A.

The sad fact is that the guidelines for rating an amplifier that will be sold in the US are lacking teeth. Used to be (pre-1973) that a manufacturer could basically say anything and get away with it. Then, RMS, both channels driven, 20Hz-20KHz, no more than .xxx% THD came along and it made some manufacturers step up to the line with better designs. A few years later, we saw 200+W/ch receivers that weighed close to 100lb and they weren't really all that great when compared to decent separates with less output. A couple of specifications races later and CDs came out, which revealed the need for dynamic headroom, which became a new spec that could be used by the marketing departments to show that their stuff was oh-so-much better than someone else' for only that spec, but it didn't necessarily sound very good. Also, if a piece of equipment did it once, they assume that all of them can do it.

Marketing and MBAs are what drive most industries now, not the demand or requests for better goods. We can want better equipment more than life itself but if the focus groups don't see it as viable, it ain't gonna happen.

Unfortunately its the sad trut and companies like Bose are masterminds of marketing.

 

[PENG]

The point I was trying to make which I think you missed becuase you didn't address it in your last response is...just because an amp can deliver that kind of power in 2 channels at the rated distortion doesn't it mean it can deliver that times the number of channels the amp has. Your applying this linearly where I'm saying...no, each channel you add into the test is not a linear function.

Okay, now I see where the misunderstanding was. I did not address that point because I was addressing your call for the "TEST" methodology, and specifically how much power we need to conduct such tests. I was not addressing the test subject amp or receiver at all. When we do such a test we must allow for the worst case scenario, meaning the test subject will in fact pass the ACD test with a highly inductive 2 ohm load. So I have to assume the subject (not knowing what it is) amp/receiver has a gigantic power supply and high current amps in it to deliver the goods.

I fully agree with you that Denon receivers, not even the 97 lbs 5805 will pass the test you have in mind. However, some amps (e.g. Boulder, Threshold, Krell) may be able to double down all the way to 2 ohms at power factors of 0.4 (66.5 degree) or lower. So if one of the subject amps is being tested under your specifications, you are going need that 200 service to give them the juice.

You are right though, if the subject amp is a Denon or even HK or NAD receiver your dedicated 30A circuit may just do it. Receiver's protection circuit will trip long before the load drops below 4 ohms in an ACD test and that means long before a 30A circuit breaker trips. To me there is no point to apply the tests you talked about to any receivers and lesser amps because there are enough of less vigorous tests done by AV magazines in the public domain. I posted links to a few reviews, including one for the HKAVR330, in the past. In that test the HK receiver activated the protection mode in a multichannel test after 0.5 seconds when it was asked to produce about 25 watts, and that was not even an inductive load! In that case even a 5A circuit could have done the test.

 

[PENG]

Marketing and MBAs are what drive most industries now, not the demand or requests for better goods. We can want better equipment more than life itself but if the focus groups don't see it as viable, it ain't gonna happen.

Agree, and no wonder why so many engineers quit their jobs to pursuit the MBA degree.