Yes, the problem here is with the measurement of the input power.
There is a very simple, cheap, readily accessible (no scope required) and accurate method of measuring Pin; feed the ac source through a DC-powered amplifier, and measure the DC power feeding the amplifier. This method has its challenges too, mainly that in order to be capable of supplying 100's of mA to the DUT, the idle current of the amplifier might swamp out the flea power Pin's and Pout's you are dealing with here. I have illustrated the method below in case anyone cares to investigate its merit. Steps to its use: 1) Determine the idle power for the power amplifier (PA). Measure Vavg and Iavg with no signal at the input and no connection to DUT. This is Pidle. 2) Determine the total power used (Ptot) with an input signal, and connection to DUT using Vavg and Iavg (Ptot=Vavg x Iavg). 3) Compute Pin via Pin = Ptot - Pidle Rather than building up a circuit to perform this function, one could also crack open their FG and install banana jacks for the insertion of a CVR and measurement of V+ for the output stage (similar to what is shown here). The extra wiring involved is not a concern because any resultant fluctuations due to parasitic inductance is averaged out by the meters. The only critical requirement is to place the DMM probes as close to the CVR body as possible.
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