I would guess there would be very few amps that would somehow 'fail' a reactive load test. Wouldn't the result of a reactive test almost always show more power than the resistive load test?
Ok, let me explain what a reactive load is and how amplifiers work into reactive loads before everybody gets confused by misunderstandings again...
1. A
reactive load is any load where the voltage and current are not in phase according to Ohms Law equation V=I*Z.
2. A
resistive load is any load where the voltage and current are in phase according to Ohms Law equation V=I*Z. In this condition, Z = R
3. The power delivered to a reactive load where V and I are defined at the same magnitude is P = V * I * cos phase angle,
will ALWAYS be less than the resistive power except at the condition where the phase angle is equal to zero which is the very definition of a resistive load and cos(0) = 1.
4. When measuring and explaining reactive power, the math can become complicated because the voltage and the current each have a phase angle associated with them in addition to the usual magnatude that is present for resistive measurements. This math is called complex math and the numbers have a real part (magnitude) and an imaginary part (vector direction) There are numerical and graphical ways to solve these equations to explain what is happening.
5. The reason why reactive power is important when designing an amplifier is because all of the device design safety margins must include calculations that consider the enegry stored in the imaginary portion of the measureements. Here's something that generally blows people's mind regarding reactive power and Ohm's law. It is absolutely possible to have current flow with zero voltage. Since 99.99% of folks are only familiar with the simplified Ohm's Law, I=V/R... then how is it possible for I to be a finite value with V = 0. It's due to the energy stored in the reactive elements of the load that are released under these conditions. This bit of information is so essential to amplifier design that it's shocking how few musical instrument amps consider the importance in their designs.
6. To design for these reactive conditions, a reactive load line, or adjustments to the resistive calculations must be used to insure adequate safety margin between operational conditions and the transition into failure conditions (called the SOAR or safe operating area region on power devices). This requires additional output silicon as well as the drive capacity to drive them. Most manufacturers use a circuit called a VI limiter that limits the current to a safe (non-destructive) value ifthis current is exceeded and is referenced indirectly to the voltage across the device. SO, it is critical to understand the VI relationship of the complex math involved. Othewise the amplifier will prematurely limit... and there are plenty that do just this.
7. Reactive conditions are such a serious issue that all amplifiers contain protective backswing diodes so that if there is an intermittent connection, or activation of the VI limiter, the stored enegry in the speaker voice coil (immersed in a magnetic field) does not discharge back into the power supply and cause a voltage rise above Vcc and destroy the output transistors from exceeding Vce.
8. The difficulty faced with developing a test for reactive power is determining what reactive load to use. This must have a resistive component (typically equal to the DC resistance of a voice coil) and a reactive component that would be the high Q inductance component of the voice coil, plus whatever crossover and tweeter components present. Then the resulting waveform must be monitored for voltage, current and phase angle AT VARIOUS FREQUENCIES in order to develop a reactive power curve.
Frankly, anybody who has followed and understood this discussion would already understand how much work would be involved and in the big picture how meaningless to the average player this information really is. For those who did not understand this would be more confused than ever with yet another complex rating method. The rating method needs to be simple to understand in order to be useful to most folks.
I haven't solved complex math equations for probably 25 years, but I understand in excrutiating detail how the fundamental principles work. That's what my engineering degree taught me. I also learned that for each person that understands this, there are 1000 people that don't give a rat's ass about it, and rightly so. We must cater to the 1000 people, not the one for a general bass player oriented magazine IMO.
In my next post, I will discuss an alternate testing methodology that is probably the most useful in that it gives a lot of information is a relatively simple, easy to understand test. It gives a lot more information than a reactive test, a non-regulated voltage test, a power versus frequency test (that is just so flawed as to be incredulous), etc.
