I measured distortion at a variety of power levels with and without feedback. The equipment used was an HP 204D Audio Oscillator and HP 331A Distortion Analyzer. The lowest distortion that could be read on this system was about .1%.
Here are the distortion readings for the amplifier with an 8 Ohm load and feedback ON.
| Power | Distortion in Percent | ||||
| Watts | 100 Hz | 300 Hz | 1 KHz | 3KHz | 10 KHz |
| 0.1 | <0.2 | ||||
| 0.2 | 0.15 | ||||
| 0.5 | 0.12 | ||||
| 1 | 0.22 | 0.17 | 0.14 | 0.16 | 0.19 |
| 2 | 0.19 | ||||
| 4 | 0.26 | ||||
| 8 | 0.38 | ||||
| 15 | 6.7 | ||||
Below are the readings under the same conditions with feedback OFF
| Power | Distortion in Percent | ||||
| Watts | 100 Hz | 300 Hz | 1 KHz | 3KHz | 10 KHz |
| 0.1 | 0.29 | ||||
| 0.2 | 0.32 | ||||
| 0.5 | 0.28 | ||||
| 1 | 0.32 | ||||
| 2 | 0.52 | ||||
| 4 | 0.95 | ||||
| 8 | 1.9 | ||||
| 15 | 7.3 | ||||
You can readily see the improvement that feedback makes on the linearity of the amplifier. It would be interesting to get the comments of a skilled listener who could compare the two conditions.
Below is the frequency response of the amplifier. It is essentially flat from 10 Hz to 100 KHz and 3dB down at 6.5 Hz and 124 KHz
| Frequency response relative to 1 KHz | ||||
| Frequency | Response dB | PO = 1 Watt | ||
| 8 Ohm Load | ||||
| 6.5 Hz | -3 | Feedback ON | ||
| 10 Hz | -0.3 | |||
| 30 Hz | 0 | |||
| 100 Hz | 0 | |||
| 300 Hz | 0 | |||
| 1 KHz | 0 | |||
| 3 KHz | 0 | |||
| 10 KHz | 0 | |||
| 30 KHz | 1 | |||
| 100 KHz | 1 | |||
| 124 KHz | -3 | |||
I also tested it with up to 20 dB overdrive and it produces some wicked flat topping and crossover distortion as well but is stable and otherwise well behaved.
Two of the three amps I built were for gifts to musicians and high end audio enthusiasts. Both liked the sound of the amplifier and noticed a clear, even dramatic, difference between this amp and their high end solid state systems. I have used this amp for general listening and even an outdoor party and have been happy with the way it sounds. I would be interested in the comments of anyone wanting to duplicate and further test this amplifier design.