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Line Stage Output Z

Posted by Triode_Kingdom on June 6, 2017 at 12:42:12:
This is a problem with any tube-type line stage, not only followers. It isn't as apparent with common cathode designs, because their output Z is usually fairly high, even at 1kHz.

There are only two solutions I know of for flattening the output Z of a cathode follower. One is to make the cap larger. That's what most designers do. The other is to apply a negative voltage under the cathode resistor (or choke), then direct-couple the cathode to the output. This requires a servo circuit to apply negative bias to the grid, in order to keep the cathode at exactly zero volts DC. It also requires adding a small blocking cap at the grid in exchange for the larger output cap. Whether it's beneficial depends on the impedance of the loads being driven. A 100K resistive load can normally utilize a relatively small output coupling cap, so eliminating it probably has little or no advantage.

I thought of this technique and designed a circuit in SPICE while recently working on choke-loaded followers. The next day, I discovered that Sy at SYclotron Audio had done the same thing several years earlier. Oh well...