Keep in mind that the AMPs AC source Load is not a pure resistive or even a simple impedance load... The Rectifier Diodes are Semiconductor devices that require the anode voltage ( secondary transformer Voltage ) to be .7Vs higher than the Cathode before it will turn on......Until they turn on the amp is disconnected from the Transformer.....
Well....RMS is method to approximate the DC equivalent of a sine wave of a given peak voltage....or the area under the curve. But when using it to calculate power consumption and voltage drops you should use it for a resistive or reactive load that is conducting for the full 360* of the signal... In our case it is the Sine Wave of the Power Company.
Meaning the load is presented to the voltage source for the entire 360*.... In the case of a power amp with a classic Transformer/Rectifier/Filter Cap power supply the Load is only being presented to the Voltage source for approximately 100* of the total 360* sine wave..... for the other 260* the rectifier diodes are not conducting current.... because the Anode voltage is lower than the Cathode.
For those 260* there is a no load condition....Zero Current for those 260* because the load is intentionally disconnected from the transformer. Therefore continuous RMS calculations do not apply..... RMS meters are considering those 260* as being as important as the 100* the Diodes are conducting .... Those 260* of off time are meaning less....and need to be ignored.....
For purposes of calculating voltage drop and current.... you are better off using peak values because the TOTAL OUTPUT POWER CONSUMPTION is jammed into the 100* of the +/- peaks of the Voltage source.... The current is much high and so are the voltage drops....
Remember, RMS Voltage is an approximation to DC assuming the load is connect for 360*..... You can not use RMS currents to calculate V-Drops for a current Signal that spend 260* at Zero amps because it is shut off by design ....
RMS miss calculates the Voltage Drop during the ON TIME....by averaging in the time being spend at zero diode current flow.
The only time we should consider for the Voltage Drop calculations is ~100* of the of 360*..... Not the time that Rectifier Diodes are Shut off.....
For the 260* of the 60 Hz Power Co Sine wave the AMP is running purely off the Main Filter caps.... For 100* it is SUCKING IN CURRENT LIKE MAD.....it is what is happening during those 100* that is significant to the calculations.
Standard RMS Meters are using the entire 360* for "integration" to resolve the RMS voltage..... Only way to get the exact current value during the Diode on time is with a Scope....or approximate backwards by assuming 100% the power is being consumed in the 100* not the 360*.
We simply don't care about the long period of time during the Sine wave that the Rectifier DIODES are not conducting...IT NEEDS TO BE REMOVED FROM THE CALCULATIONS. Is there some way to tell a meter to ignore that 260* ? No.....that is why a scope is needed.
Vdc Rail ~ V(Peak) - 1/(4CF)
C is the Main Filters and F is 6 Hz....
Volts RMS is not used. Volts Peak are used to predict supply Rail Voltage no load. I Peak ( AC supply current peak) should be used to determine the affects of AC line voltage drop due to load. Which in turn causes power supply rail sag.
If you replaced the diodes and MAIN CAPs with a simple resister that was dissipating power for the Full 360* of the Sine Wave, then RMS calculations from a meter on the primary side would work....
Below is a pic of the Current Signal on the 240Vac supply for a 30hz 100WPC Test signal in to 8 Ohms. The Current Signal is far from being a Sine wave....It still is a 60hz waveform, but the CURRENT Spikes reach 5.5amps in both directions vs a measured 2.2 RMS amps. The power is being consumed in very high narrow spikes that correspond to the Peaks and valley of the AC Supply... The spikes are the main Rectifier Diodes turning on and starting to re-charge the Main Filter Caps....
Those spikes create more voltage drop then if using an estimated RMS current.... These spikes are concentrated right when they can adversely affect the Rail Voltage most by creating higher % voltage drops then you would expect with RMS calculations...
THIS ONLY GETS WORSE WITH 120V.... Point is....over size the AC Circuits for large power hungry amps.....the Voltage Drops should be calculated for those Current spikes not RMS values.
Yes the Amp is functioning correctly and the current Spikes are normal.
Think of it, this was only 100WPC.... this is 1/3 power for this amp in 8 ohms....and 1/6 the power into 4 ohms....
600WPC into 4 ohms would produce current spikes in the range of 30amps in a 240 Vac circuit..... But the RMS value would be in the range of 10amps.
johnk
