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In Reply to: thanks posted by Leo loves music on April 24, 2025 at 01:18:15:
"Busy" - and you haven't even seen the inside! In addition to the buffered MM phono preamp and switchable unity-gain tone controls, under the skin there's also an OTL/OCL line stage. This uses large 90H chokes to load the cathode follower output stages and an auto-bias circuit that holds the cathodes at 0V DC. The cathodes are connected directly to the output through mercury-wetted relays. The relays are activated by a protection circuit that provides a startup delay and also quick disconnect if voltage rises above a few millivolts. B+ has a separate regulator for each stage to minimize crosstalk and other unwanted coupling, as well as a pre-regulator to accommodate wider line voltage variations. The filaments are powered with DC that's regulated by a buck converter. Despite this overall complexity, the line stage signal path is short and extremely transparent. It uses an EF86 in triode mode feeding a high-current pentode-as-triode follower. Nested feedback reduces maximum gain to about 6dB, and there is only one small polystyrene coupling cap in the entire chain. Frequency response is beyond audibility at both ends and distortion at 1V RMS output is triple digit. This project consumed several years of my spare time, and although I probably wouldn't build it as one unit again, the effort has proven to be worthwhile. This preamp took the sound quality of my system to a whole new level.
Edits: 04/24/25
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