In Audio Amplifier, there is a debatable parameter that holds the real identity of A Power Amplifier – its strength and Capability measured in WATTS to drive the speaker. In my Audio Amplifier Hobby, the only known power I recognized is the Average Power (Pave) which is the product of the voltage and current, both in RMS (Root Mean Square). Some guys called it RMS Power or Power(rms) – including myself, like P = 100W(rms). Why?
RMS – what is that?
The RMS is an acronym for Root Mean Square. It simply means, it is the Alternating Voltage measured by your true RMS voltmeter. For a sinusoidal waveform (sine wave) as shown below with a peak value of “Em”, I able to derived its RMS value formula in this post, see the mathematical and graphical demo,
The Average Power Formula (Pave)
The Average Power is P(ave) = E X I x PF, where
- P(ave) = average power
- E = voltage in RMS
- I = Current in RMS
- PF= Power Factor
To simplify the formula, the PF (Power Factor) is 1, hence the P(ave) – ExI in Watts. Why? This indicates the the load is pure resistance like a Resisitve Load, there is NO Inductor or Capacitor in the load,
The P(ave) = E x I can be expressed in terms of the load resistance by using Ohms Law with the popular formula of E = IR, where :
- E = voltage in RMS
- I = Current in RMS
- R= Resistance in Ohms
By manipultaing the Ohms Law, we can see that I = E/R, and from P(ave) = ExI substititu the formuila of I, P(ave) = E x (E/R), which then became P(ave) = ExE/R, when the condition is true, it can be writen as P=ExE/R, abbreviated as PEER, pronounced as “pir”.
Note! You need to understand the RMS value, in our case of a sinusoidal waveform (Sine Wave) to fully understand the Power. Again, this is only true when we are dealing with a Sine Wave signal and that the Load do not have Inductor and Capacitor, it is a purely Resistive.
Here is an example to compute the Average power of an Audio Amplifier :
- E = voltage Swing at the Load, measured a True RMS voltmeter, 32Vrms
- R = the load Resistance, normally 8 Ohms
- Signal = 1 KHz
- Pave = average Power
- P(ave) = E x E / R
- P(ave) = 32 x 32 / 8
- P(ave) = 1024 / 8
- P(ave) = 128 Watts
Some guys wrote it simply as P = 128 Watts(rms) or P(rms) = 128 Watts or simply P(rms) = 128W. Most of the hobbyist criticized this notation – its a wrong practice and a bad engineering idea ………………….. but in my case, my view is “just fine” and acceptable for them to indicates that the Power was derived from RMS values, which signifies a true power being measured at the load, and not other power computation like the PMPO (peak music power output), which is misleading. Pave or P(rms) or anything that adds the acronym “rms” is simply to indicate that the power was computed from RMS values. If adding the rms or RMS notation in any power reading with an idea to clearly emphasized a true power, that is even encouraging, but when it causes safety and human lives, that is totally unacceptable and should never put into practice!
If there is only one way of specifying Power in the Audio Industry, then we do not go into this trouble, but, there were measurement specification in PMPO by those Marketing guys (probably from sales engineers — at least we assumed from sales engineers with engineering background hehehehehe!) which if we believe it will lead us into frustration, because of the fact that PMPO numerically denotes very high power in Watts as compared to the Average Power.
PMPO
Its an acronym of Peak Music Power Output of an Audio Amplifier it it simply denoted by P = 10000 W(pmpo), this kind of notation is from an unknown commercial amplifier manufacturer. what they wanted to emphasized is that, their product can produced a high power output music.
I do not remember any hobbyist to boostfully tell that thei Audio Amplifier is 10000 W(pmpo), knowing that, it is a disgrace to bosstfully tell anyone of a 10KW output power in PMPO.
But I do know that several hobbyist can boostfully tell their DIY Audio Amplifier can deliver 1KW of average Power while their power supply is just 400 Watts. This is another issue that these audio enthusiast are proving to break the law of physic or maybe thermodynamic.
Back to the PMPO, I do not seems to have a good mathematical computation on how they derived such power, if you do know, please have a comment below to let me (us) know about it. I really do not use it, so, I am not so knowlegeable on this. SALAMAT Po! (Thank You!).
The Real Wold of Speaker & Wires
I have a discussion of the Boucherot or Zobel nework in relation to the real world Speaker s shown , in the below schematic diagram, the SPK composed of LB and RB
- LB – represend the inductive reactance of the speaking coil
- RA – represend the Resistance arsing from the inductor winding (8 Ohms for example, but it can be 4 ohms , 6 ohms or even 16 ohms or 32 ohms)
- RA nd CA are called Boucherot or Zobel Networ
In this case, our P(ave) assumption of PF =1 is no longer correct. In our computation above, the P(ave) = 128W may not be correct anymore. What is more complicated is when we included the capacitive part when used long wire, which now become complicated, read the Speaker Wire Capacitive Killer
Simple Rule to Determine R&R of DIY Audio Amplifier
To remove those complexity of Inductive Reactance offered , the P(ave) = E X E / R hold true where the PF = 1, when consider only the Resistive part – this is an assumption and it is acceptable measurement in DIY and the Audio Industry. But it needs to be accurate information iike :
- Maximum Power Output = 128W or 128W(rms) both Channel Driven
- Test Signal 1 KHz
- Input Sensitivity
- 10% THD
Maximum Power Output = 128W or 128W(rms) both Channel Driven – this needs to be done in Stereo or 4 Channels Amplifiers, so we will know that the Power Supply can able to delivered all the needed power when all the Amplifier at maximum output.
Test Signal of 1 KHz – specify the test signal, normally it is 1 KiloHertz (1000 Hz) , this makes it clear of the condition.
Input Sensitivity of 900mV(rms) – its better to specify also the input sensitivity to caused maximum power output. Forexample, a 900mVrms or 0.9Vrms can output 128W, some amplifier needed 1.2Vrms, while may need 2.2Vrms. With a specified input amplitude to have maximum output makes things clearer.
THD or Distortion – it is even clearer when specify the distortion when read the maximum voltage swing at the load, as this has something to do with the fidelity of the music. A Total Harmonic Distortion (THD) of 1% at output power of 128W is entirely different from 10% at an output power of 128W. Well, this is not simple since not all DIYer’s have a distortion Meter. but we can approximate (again approximate ) by observing the voltage swing displayed on the oscilloscope.
- 1% THD @ 1 KHz – unclipped power
- 10% THD @ 1KHz – clipped power
Note! The above observation from an oscilloscope will not always be true as some amplifier will distorted much before showing an amplitude distortion, but again, it is approximate, so it still be acceptable (to some degree).
At least, we are not in the battle of the best of the best amplifier, we only simply know how our DIY Audio amplifier performed.
So, what is R&R? It means Reproducibility and Repeatability. By using a sinewave signal as the input waveform, and a 1 KHZ test frequency, you can always expect “Repeatable” result if you wan to to test it for 10 times, you will always get 128W (for example) and you always “Reproduce” it in a test for 10 times. So, this is the essence of using such Test Frequency of 1KHz and the use of sinewave.
Here is my procedure of evaluating a DIY made Class-D Amplifier, I restricted to say, this is 1diySHARE procedure : Amplifier Parametric Test DR700W mini Class-D
FTC (Federal Trade Commission)
The FTC specifically to Part 432 defined Home Audio Products Power Amplifier specifications. The Amplifier Rule establishes uniform test standards and disclosures so that consumers can make more meaningful comparisons of amplifier equipment performance attributes.
This is a good reference, and some DIYer’s used the FTC Part 432.3 to evaluate DIY made amplifier (you have to specify the Part 432 when refering to FTC as FTC is broad, Part 432 is the POWER OUTPUT CLAIMS FOR AMPLIFIERS UTILIZED IN HOME ENTERTAINMENT PRODUCTS)
Here, in 1diySHARE, we picked up the good stuffs and made my own way of testing DIY Amplifier, see again the Amplifier Parametric Test DR700W mini Class-D , this is the beauty of DIY, Do-It-Yourself, knows no limit!
Conclusion
In the DIY world, the only recognized power of a DIY made amplifier is the Average Power, the true power derived from a Voltage Swing at the load in RMS value.
To make readable and specific, the use of P(ave) is the best choice of notation, but it could be possible also to use P(rms) or Watt(rms)for the sake of clarity deviating the power from PMPO. Cheers everyone!
If you spotted something wrong, kindly inform me . SALAMAT Po!
Very clear indeed, understand every details of the explanation. Thanks prop. Dandy