This particular diamond buffer board was populated with 2N5087/5088 for the input and current mirror transistors, and MJE243/253 for the outputs. 2N5486 JFETs are used for the adjustable current source. These are all "standard configuration" parts as specified in the diamond buffer schematic diagram.
With the discrete diamond buffers, the Millett Hybrid exhibits less upper order harmonic distortion and intermodulation distortion than the BUF634P. The THD and IM percentage numbers don't tell the whole story. You must compare the graphs to see this. In fact, these measurements produced virtually identical results to those with the OPA551PAs (which has the benefit of being closed loop and thus very low distortion). The tubes remain the dominant source of distortion, so the differences in the output buffers is masked. What the results do tell us is that both the OPA551PA and the discrete diamond buffers outperform the BUF634P in this application.
I tested with the diamond buffers set to 15mA, 20mA, 25mA and 30mA bias (only the first three are shown below, the fourth is no different than the rest). The differences between these bias points were insignificant, thus I recommend setting to between 15-20mA for the lowest heat dissipation on the output transistors.
As expected, the frequency response, noise, and stereo crosstalk measurements were not affected by the choice of output buffers.
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