1970s Design Indulgence

Whilst on the subject of bootstrapping, referring to the article at https://www.americanradiohistory.com/Archive-Wireless-World/60s/Wireless-World-1965-01.pdf, page 7 (page 10 of the PDF), quote:

"(ii) The bootstrap capacitor C6, while linearizing the l.f. response may also induce distortion by pulling Vt3 into the non-linear (bottoming) region of its characteristic on large negative-going signals. It has therefore proved worthwhile to connect a 1k resistor in series to minimize this effect."

This might be an option for the power amplifier of this discussion. Contrary to the above I do not think it will improve distortion - it will most probably make it worse, but taking a look at the first image on page 22 of this topic, it can be seen that the typical open-loop slope is curtailed rather abruptly by the a.c. coupling, and although its low frequency fall here is a controlled 20dB/decade, it is rather much steeper since catering for the low frequency anomalies discussed since.


Reducing open-loop gain by inserting some resistance in the bootstrap, and in this case 1k (the equivalent of the 1k in the article would have been 1.6k here by the way) shows in simulation flatness from around 100Hz to 2kHz. This reduces 1kHz negative feedback from around 48dB to 42dB, which is half, and might result in 1kHz THD increasing to 0.07% from roughly 0.035% at present. If however it gives a subjectively better bass response it will in my opinion have made an improvement.

I shall report my findings.

Sounding quite nice. Plenty of o's in smooth - but still extremely detailed and "nicely clean". It has "big boys" bass which is quite impressive coming out of the Harbeth M20's (the pro version of the P3ESR).

That's probably classified, knowing might cost you your life!!