I have the Ilkin-Norton preamp built, using 2SC5551 transistors, 9dB version. There are some minor deviations from the original, the biggest one being bigger BN73-202 cores used (this is what I have on hand).
Got the transformer windings right the first time, no oscillations and a nice 9dB gain, rolling off to 7.5dB at 30MHz (maybe because of the oversized transformers and the 73 material). -3dB point is around 35MHz. No measurable rolloff on the low end to at least 100kHz (NanoVNA limit).
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My IMD setup is finicky after laying on the shelf for more than a year, need to spend some more time to get reliable numbers out of it. I can already see that IP3 is good, but IP2 is not what I expected from a symmetrical design (worse than a push-pull resistive feedback preamp I compared it with). Surprisingly, the optimum current (lowest IMD) is not the highest I tried, ~40ma per transistor, but somewhere around 15mA per transistor. Is this a known 2SC5551 property? Ferrite bead saturation maybe?
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More numbers to follow after I completely revive and recalibrate my IMD bench.
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73, Mike AF7KR
