• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dynamic load line analysis using music and speaker load

Another test I performed on the Complementary Current Mirror amplifier (Tube/MOSFET push-pull) was looking at the dynamic load lines for both the triode and MOSFET. They should be identical and they are.

But in the course of looking at the dynamic load lines I obtained some interesting plots.

Top left shows what I'm measuring. I set up my scope in x-y mode to plot anode voltage on one axis and anode current on the other, and took a picture of a purely resistive textbook RDH4 load line. I then superimposed the anode characteristic curve over the trace to show approximately where the load line belongs on the anode chart.

The rest of this row and the next show the load "line" when driving various sine wave frequencies into a JBL4312 loudspeaker. It becomes elliptical due to the reactance of the speaker and crossover. The impedance (angle) and phase (ellipse ratio) moves all over the place as frequency changes (as expected). 60 Hz seems to be a resonant peak in this box and indeed at lower damping factors it booms around this frequency.

The effect with music on the bottom row is as expected, but different kinds of music result in different shapes, etc. of load curves, from loopy to jagged to bushy. I think the bushy one had a lot of crunchy guitar and the loopy one was a smooth vocal + piano and a lot of bass.

It changes one's idea of the "load line" to look at it this way.

Anyway, I did prove that the current mirror does the "right thing" on the other side of the OPT and made some interesting observations.




  • dynloadlines.pdf
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