Good Reading on Ampliifiers

[digitaldj]

I have posted many times that I am not a fan of computer speakers for jukebox use and have also posted on threads how the industry likes to massage specifications on speakers and amplifiers. The typical consumers doesn't understand specifications and a generally are taken advantage of. I ran across many articles on the internet that describe amplifier specifications. I thought I would post the article here for all to read. Though it doesn't go into beating up the industry for these misconception but it tells you that it is done. Another forum I have been watching and posting to has had a set of speakers posted that is a good example of the power outut being inflated. First off the size of the amp is so small that there is no way of it producing 500watts of output power. As the industry is today commercial amplifiers are the only industry manufacturers that state true specifications and in most cases these manufacturers packag true specifiaction testing printouts with the unit.

Enjoy! http://www.techlore.com/article/10309/Understanding-Amplifier--amp--Receiver-Power-Specifications/;jsessionid=75D80D28C40CE50BEA145908128D692B

Kevin

[digitaldj]

If your a Bose lover you may no longer be!

http://www.intellexual.net/bose.html

[richms]

I never have liked bose since the things start to distort to hell and your not even at 85dBA in the room! And there is no bass and all the midrange is attenuated and the upper bass is all coming from the big box they never show in their promotional shots

It does play some instruments quite well tho with all its EQ it applies. So I guess thats why its allways hell freezes over on their demo's .

edit - and after looking at the amp story, since there is no such thing as RMS watts and they are saying that they are the only way to compare amps then really they lost my interest there.

[digitaldj]

I am not getting what your saying in your edit!

There are two ways of expressing power, peak and RMS. Peak power is the amplifier's ability to provide an instantaneous burst of power. However, peak power measurements are somewhat fictitious since they cannot be sustained for long. RMS (Root Mean Square) power ratings are a mathematically correct way to express an amplifier's usable average power, which is a representation of real-world performance.

[richms]

RMS is applied to AC voltages and waveforms to get the equivalent DC voltage into a resistive load. Watts are watts, as in joules per second. You can say that an amp will produce an average of 100 watts when driven with a 1vRMS input, or that it will produce 100 watts peak on music with a 10dB peak to RMS ratio, but the use of the mathematical expression root of the mean of the square which is what RMS means, is meaningless when applied to power. Since it is used on a voltage which is squared in ohms law when calculating power the direct average of it can't be used.

Any electronic or electrical engineer will know this. The common way of getting the "RMS watts" figure is to halve the measureable peak of the amplifier, but this is worthless since most amplifiers (car ones especially) cannot hold the peak power for very long, so even when given a 1kHz sinewave you will be hard pressed to get the quoted average power levels out of the amp without it either shutting down or else getting so hot that it wont have a very long lifespan.

[digitaldj]

RMS is applied to AC voltages and waveforms to get the equivalent DC voltage into a resistive load.

No DC when you dealing with audio, a resistive load doesn't make it DC. i'm not getting you there! Any Electronics Engineer knows when dealing with amplifiers that RMS is the only measurable way to get watts. They do make peak and hold watt meters that are pretty accurate. We have got a little off subject but let me ask you if you believe that the industry inflates specifications on amps and speakers especially computer speakers to appeal to the consumer?

[richms]

They do inflate them to hell, and RMS is one of the ones that is BS because no amplifier designed for audio will be able to produce that power level for any length of time, there are thermal issues which are what kill most mid power rated amplifiers, power supply sag, and nasty non linearitys because they are quoting them at loads of distortion. The ability of an amplifier to hold its rail voltages under maximum current is whats important in the short term, and that it doesnt blow up doing it is the long term goal.

I know there is no DC in audio (unless its an unwanted offset) - RMS is a measurement for voltages when you are trying to figure out the equivilant DC that would cause the same heating from an ac waveform under analysis, an RMS is an averaging that accounts for ohms law, nothing more then that. An amplifier cannot produce or be measured in RMS watts because the unit doesn't exist outside of a marketing department. Its as stupid as measuring the length of a piece of string in gallon meters. RMS is only an analysis tool for ac waveforms to determine their effect on resistive elements.

[digitaldj]

Yes I agree there is alot of factors involved and typically the consumer is mislead to believe something that is not. A 100watt amplifier should never be considered to produce 100watts for extended periods of time, to me this is logical. If you want to run 100watts continous then you need the head room to do so and should require doubling the power to 200watts.

The amplitude of an AC waveform is its height as depicted on a graph over time. An amplitude measurement can take the form of peak, peak-to-peak, average, or RMS quantity.
Peak amplitude is the height of an AC waveform as measured from the zero mark to the highest positive or lowest negative point on a graph. Also known as the crest amplitude of a wave.
Peak-to-peak amplitude is the total height of an AC waveform as measured from maximum positive to maximum negative peaks on a graph. Often abbreviated as "P-P".
Average amplitude is the mathematical "mean" of all a waveform's points over the period of one cycle. Technically, the average amplitude of any waveform with equal-area portions above and below the "zero" line on a graph is zero. However, as a practical measure of amplitude, a waveform's average value is often calculated as the mathematical mean of all the points' absolute values (taking all the negative values and considering them as positive). For a sine wave, the average value so calculated is approximately 0.637 of its peak value.
"RMS" stands for Root Mean Square, and is a way of expressing an AC quantity of voltage or current in terms functionally equivalent to DC. For example, 10 volts AC RMS is the amount of voltage that would produce the same amount of heat dissipation across a resistor of given value as a 10 volt DC power supply. Also known as the "equivalent" or "DC equivalent" value of an AC voltage or current. For a sine wave, the RMS value is approximately 0.707 of its peak value.
The crest factor of an AC waveform is the ratio of its peak (crest) to its RMS value.
The form factor of an AC waveform is the ratio of its peak (crest) value to its average value.

What is really interesting is the .707 factor is also a factor used in trig.

As long as RMS is calculated correctly it is a viable way of showing a true RMS watts. This is more accurate then what manufacturers try to get the consumer to believe.

Tell me what your procedure would be for checking true power output?

I have a friend that is a disc jockey and is totally nuts with checking a new amplifier. He goes through the whole procedure and if it didn't match up to it's specs it goes back.

[digitaldj]

The other factor that gets overlooked is a speaker is a inductive load not a resistive load so frequency of the waveform and inductive load is calculated different then if it was a resistor as a load for the amp.

[richms]

As long as RMS is calculated correctly it is a viable way of showing a true RMS watts. This is more accurate then what manufacturers try to get the consumer to believe.

Tell me what your procedure would be for checking true power output?

Peak power, which can be given as a wattage on an impedance, or else as V full scale and the amplifiers design current. The peaks are what get clipped, so they are what you worry about. For powering the amplifier you need to take into account the average levels of the program material being played vs their peaks, so you would nees a much higer peak amp power to get the same average power and percieved loudness from some nicely mastered jazz vs some overcompressed pop material. The RMS is only usable as a comparison between amps if they have calculated it the same way.

The "RMS wattage" is calculated _usually_ on a 1kHz sine wave, which will make the amplifier allow for more power to be drawn then under any normal operating conditions giving a bigger number. All it is, is the average power delivered (since there is no RMS watts) on that test tone. It does nothing to give you the real world performance of 2 amplifiers for comparison. Allowable distortion on the signal varies between who is measuring it, but really there is stuff all difference between a 0.5% and 0.005 amp since the clipping is pretty much brick wall on most amps.

You cannot calculate "RMS watts" because they dont exist. If you do the RMS calculation on the test waveform and then average it you get the average power which is what they are telling you. Otherwise on a sine wave its 1/2 because of the 0.707 relationship between the RMS voltage and the peak voltage.