The graph below shows the THD+N (total harmonic distortion + noise) at 1kHz and 6kHz, the measurement bandwidth is limited to 20kHz. Clearly the noise, at 30uV, dominates here. Only at 30W at 6kHz the distortion starts to rise above the noise floor. What is also clear is that fact that although we set the bias current to allow for 100W/8-ohm, the transformer used for this test left us twenty watts shy of the design goal.
Fig 8. THD+N graph at 1kHz and 6kHz with 20kHz measurement bandwidth.
The next figure shows the THD and THD+N at 40W and 20W, the measurement bandwidth is now limited to 80kHz. The top two graphs clearly show that by using a larger measurement bandwidth the noise now fully dominates, rendering the plots rather meaningless. To be able to make a proper THD measurement we have used a function of the AP measurement system that uses band filters to measure only around the harmonics.
Fig 9. THD and THD+N at 40W and 20W, 80kHz measurement bandwidth.
Next up is the residual THD at 40W/1kHz. The 2nd harmonic is due to the fact we have not used Hfe matched transistors throughout. What can be read from this figure is that at 40W the true THD is 0.00008% (-122dB). A figure you will be hard pressed to find with any commercial amplifier. The nature of class-A distortion and its harmonic distribution allows us to extrapolate down to lower signal levels. At 1W the distortion would be around 0.000002% (-154dB) something which can’t be measured directly.
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