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Topic: Rough Estimate of Amplifier Output (Read 3968 times) previous topic - next topic
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Rough Estimate of Amplifier Output

My audio system consists of an Intel NUC wired directly into an old B&K EX4420 amplifier connected to a pair of B&W 685 S2 speakers.  One of those speakers is in the shop for a blown tweeter.  The amp is a 200 watt per channel monster, 21 years old.

When connected to a Kill-A-Watt it draws 75 watts from the wall with no input signal.  With an input signal it continues to draw 75 watts AC until volumes much louder than what I normally listen to are reached.  It is rather loud in a large (5000 cubic foot) room at only 78 watts AC.

Is there some kind of way to estimate how much power the amplifier is putting out based on a typical class AB efficiency level of around 50 to 60% just before the amp starts to draw more than 75 watts from the wall?  How about peak?

I am guessing if the efficiency is 50% average output is around 17 WPC with peak several times that.  What multiplier should I use.

An accurate number is not needed.  It could be off by as much as 20%.  I would like to replace the B&K with a smaller integrated amp that automatically goes into a standby mode and need an estimate of how much power is required.

Another approach is the speakers are 87 db efficient, I tend to listen to jazz and vocals a lot rather than huge symphonic pieces.  House shaking volumes are not needed and as mentioned above the room is 5000 cubic feet and more reflective than average.

So either how much power am using, or how much do I need.  B&W says 25-100 watts into 8 ohms, unclipped program.  Somehow I don't think 25 is enough.

Re: Rough Estimate of Amplifier Output

Reply #1 –
Assuming you mean 87 dB per watt per channel, you are probably using about a watt per channel, give or take.

Re: Rough Estimate of Amplifier Output

Reply #2 –
Quote from: saratoga on
Assuming you mean 87 dB per watt per channel, you are probably using about a watt per channel, give or take.
It's 87db @ 2.87v and 1 meter.  Could you elaborate on how you reach this conclusion.  I consider it possible as if the amp is wasting 75 watts AC doing nothing, and still says 75 watts when running, it isn't likely to be doing much work.

Re: Rough Estimate of Amplifier Output

Reply #3 –
Quote from: eagleray on
Could you elaborate on how you reach this conclusion.

Unless you have a very large room, you're probably listening with the peak around 70-80 dB, so I rounded up to 1 watt.

Quote from: eagleray on
I consider it possible as if the amp is wasting 75 watts AC doing nothing,

You said above that it uses 75w with no input, so yes, that is probably the case. 

Re: Rough Estimate of Amplifier Output

Reply #4 –
May be the amp is running in class A upto ~10 watts and then going into class AB after that !

Re: Rough Estimate of Amplifier Output

Reply #5 –
Quote
When connected to a Kill-A-Watt it draws 75 watts from the wall with no input signal.  With an input signal it continues to draw 75 watts AC until volumes much louder than what I normally listen to are reached.  It is rather loud in a large (5000 cubic foot) room at only 78 watts AC.
The only thing you can conclude is that the amp is putting-out less than 75/78 Watts average (half that into each channel).

Was anything unusual happening when the tweeter blew?   It may have been a fluke/random failure, or the crossover may have a defect (sending low frequencies to the tweeter) or your amp could have a failure, or there could have been some strange failure somewhere else in your system sending a 200W ultrasonic signal to the tweeter.

Quote
The amp is a 200 watt per channel monster, 21 years old...

...So either how much power am using, or how much do I need.  B&W says 25-100 watts into 8 ohms, unclipped program.  Somehow I don't think 25 is enough.
  There's no easy answer, except maybe "bigger" speakers.   The problem is speakers get blow by average power, and tweeters can't handle as much power as the woofer, so if you're blowing the speaker with a 200W amp you might turn-up the 100W amp trying to get the same loudness, the average power will increase (with the peaks clipped at 100W) and you may blow the tweeter again.



Re: Rough Estimate of Amplifier Output

Reply #6 –
As DVDdoug says, you can only really say the amp is delivering less than 37.5W per channel based on a power consumption of 75W at the input. The problem you have is that measuring input power only puts a ceiling on output power, not a floor. However, at lower levels it's likely to be running in class 'A', which is probably not more than 25% efficient. Knock off a few watts for the ancillary circuitry and a few for power supply inefficiency and you end up with somewhere in the 5-7W RMS region output. Peaks aren't relevant to blowing a tweeter at those sorts of figures (assuming no defects are involved).

Of course, all of the above is based on estimates and assumptions, so the figures could be out by a fair margin. Realistically, unless you can obtain an appropriate output power meter (which would need to be fairly sophisticated and therefore quite expensive) you really don't know the output. The best advice is to be guided by the manufacturer's recommendations, which suggests either higher-rated speakers or a lower power amp, combined with some common sense on volume levels.

The other thing you're assuming is that the tweeter blew due to overload and unless you know for sure it was being abused you don't know this to be so. It could have been defective, as could the crossover (as pointed out above).

Re: Rough Estimate of Amplifier Output

Reply #7 –
 Advantages of going to a smaller integrated amp include reduced power consumption, reduced waste heat, improved WAF and convenience.  With an internal DAC it could be electrically isolated from the computer with a toslink connection.  I am looking at a NAD D 3020.  It's cheap, small and has several digital inputs.  I don't listen to vinyl and all my FM is internet based.  Based on the discussion above it should easily have enough power for my listening habits.  As for how it will sound, I will just have to find out.  I don't expect a major difference.

Something unusual, like a power spike, may have been the culprit.  I don't know for sure.

Lots of those 75 AC watts are turning into heat. 

Re: Rough Estimate of Amplifier Output

Reply #8 –
Quote
Lots of those 75 AC watts are turning into heat. 
With no signal it's ALL heat from the amplifier.    ...The power dissipated by the speakers is also converted to heat.   ...Even the small percentage of power that's converted to sound is converted to heat when the sound waves are absorbed by objects or by the air.  ;)

In the winter the heat isn't wasted because you're going to heat your home somehow anyway (although your normal heat source may be cheaper than electricity).    But, in the summer you may be paying for heat you don't want and if you have air conditioning, you are paying for the heat plus paying again to remove the heat.

Re: Rough Estimate of Amplifier Output

Reply #9 –
Quote from: DVDdoug on
Quote
Lots of those 75 AC watts are turning into heat. 
With no signal it's ALL heat from the amplifier.    ...The power dissipated by the speakers is also converted to heat.   ...Even the small percentage of power that's converted to sound is converted to heat when the sound waves are absorbed by objects or by the air.  ;)

In the winter the heat isn't wasted because you're going to heat your home somehow anyway (although your normal heat source may be cheaper than electricity).    But, in the summer you may be paying for heat you don't want and if you have air conditioning, you are paying for the heat plus paying again to remove the heat.

I live in a warm place.  Heat is from cheap natural gas.  AC runs April through October, almost nonstop June through August.  Anyway, I am thinking that connecting a computer straight into a high powered amp makes accidents more likely to happen.  Electricity here is relatively cheap.  It would take a long time to recover even the modest cost of a D 3020.  I wonder if there is another answer.

Re: Rough Estimate of Amplifier Output

Reply #10 –
Quote from: eagleray on
My audio system consists of an Intel NUC wired directly into an old B&K EX4420 amplifier connected to a pair of B&W 685 S2 speakers.  One of those speakers is in the shop for a blown tweeter.  The amp is a 200 watt per channel monster, 21 years old.

The speaker's specs are here:

http://www.bowers-wilkins.com/Speakers/Home_Audio/600_Series/685.html

Relevant specs are that B&W  685 II has 87 dB/watt sensitivity (about average) and is rated for use with amplifiers up to 100 wpc.

Quote
When connected to a Kill-A-Watt it draws 75 watts from the wall with no input signal.  With an input signal it continues to draw 75 watts AC until volumes much louder than what I normally listen to are reached.

I would expect the Kill-a-watt to be pretty useless for the purpose at hand, and it appears to be turning out that way.

Audio gear has a certain minimal amount of power that it draws even it if is doing nothing, , and it appears that you now have a good estimate of that.  This power is used by peripheral circuits such as indicator lamps, low power signal processing, and idling current or bias for the output stage.  None of this would appear to have anything to do with your questions.

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It is rather loud in a large (5000 cubic foot) room at only 78 watts AC.

"Rather Loud" is your subjective evaluation and can mean very little to the rest of us.  Your loud might be my quiet or vice versa.

There's a reason why objective measures such as watts and amps were invented  over a 100 of years ago, and that is to bypass subjectivity and provide stable data that can be compared sensibly.

Quote
Is there some kind of way to estimate how much power the amplifier is putting out based on a typical class AB efficiency level of around 50 to 60% just before the amp starts to draw more than 75 watts from the wall?  How about peak?

AB operation is actually very vaguely defined. It means that at low levels both halves of the output stage are passing current, but at some undisclosed power level only half the output stage is passing significant current. The point where the transition takes place is the results of someone's personal judgement which is, you guessed it, subjective.  A critical parameter is bias level, and that might be given in a service manual as part of component setup, but its not in the user specs.

The fact that the amp draws 75 watts over much of its operational range suggests that the amp is more heavily biased than many, but again we are speculating and not dealing in any reliable facts.

Quote
I am guessing if the efficiency is 50% average output is around 17 WPC with peak several times that.  What multiplier should I use.

The multiplier varies considerably over the operational range of the amp and therefore it can't be predicted reliably, and is pretty useless.

Quote
An accurate number is not needed.  It could be off by as much as 20%.

If this multipler were known within 20% it would be pretty unusual. Your measurements suggest that it ranges from zero (amp draws significant power when it is doing nothing) to maybe 3 or 4.  Lotsa luck with that!

Quote
I would like to replace the B&K with a smaller integrated amp that automatically goes into a standby mode and need an estimate of how much power is required.

Lotsa luck with that, too.  You are obviously seeking a level of transparency of specification and design that is generally not given.

Quote
Another approach is the speakers are 87 db efficient, I tend to listen to jazz and vocals a lot rather than huge symphonic pieces.  House shaking volumes are not needed and as mentioned above the room is 5000 cubic feet and more reflective than average.

So either how much power am using, or how much do I need.  B&W says 25-100 watts into 8 ohms, unclipped program.  Somehow I don't think 25 is enough.

The best way to estimate power levels is probably  to obtain a SPL meter and measure peak SPLs at your listening position.

There are calculators on the web that take estimates of speaker sensitivity and listening location into account and come up with a crude estimate of power levels.

Here's a technnical discussion of some of the issues: http://www.crownaudio.com/how-much-amplifier-power

Here's some power level calculators:

http://www.crownaudio.com/en/tools/calculators

http://myhometheater.homestead.com/splcalculator.html

Google will provide you with others.

They are all rough approximations, and probably not consistent within the 20% that you mentioned, but they do beat the $#!! out of nothing.

You can find audio SPL meters on the web at prices ranging from $20 to $2,000 and up. You can find apps for smartphones that may do a good job for next to $$ nothing.  Probably something for < $60 or so will be some needed help.
 

Re: Rough Estimate of Amplifier Output

Reply #11 –
You could measure the power going to the speakers by paralleling them with a pair of power resistors, placing the resistors in a container of water, and measuring the rise in temperature or the amount that has boiled away. Either that or place the resistors in an ice/water mixture and measure the amount of ice that melts.

Or you could just follow the above suggestions.