Eico's HF-85 circuitry is unique and very versatile. The phono stage can measure and sound phenomenal, whether you use the Tape Output or Main Out jacks. Directly after the phono stage are the tapped, dual volume controls. Using them this "dual mono" way alleviates the stereo tracking imbalances. The almost useless Loudness switch can easily be made into a Low Pass Scratch Filter. Being close, electronically, to the phono stage output, this is the perfect location for a phase accurate "Scratch Filter." The next gain stage is a special, plate follower with feedback, providing some gain and lowering impedance for Tape Out duty. Note that this is one of only a few preamps offering a Tape Output after the Volume controls. (BTW, The HF-81, with very similar phono cktry, actually sends its' Tape Out after the Volume plus the Tone controls). After the "special" plate follower with feedback stage, The next HF-85 gain stage is for Line, then the Tone stages and Main Out. Very simple paths, high gain Line stage, favored Baxandall tone controls, low impedance output without using cathode followers, plus reference grade RIAA phono EQ make the HF-85 a super sounder.
Working originals might deserve installing some better coupling caps and rebuilding/enhancing the power supply. The unused Tape Head and Microphone inputs, also switched in as high gain, Low Level inputs like the Phono, can be redesigned for a one or two more phono stage retrofit in addition to the original's single Phono input. I did that with my HF-85 and also utilized the unused Tape Head EQ switch, labelled 3 3/4 and 7 1/2, for switchable NAB/Columbia "LP" phono EQ or "AES" (the Audio Engineering Society 1951 pre-RIAA "averaged" playback curve) phono EQ. Even with AC filaments, my HF-85 adds no noise or hum in my systems.
The HF-85 is a great platform for an excellent preamp...
