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grindrod · 3rd Sep 2008, 8:35 pm

I did a lot of experimentation on a 'hifi' valve phono stage some years ago and the optimum configuration was an EF86 with a gain of X20 fixed using some undecoupled cathode resistance and with the 75us pole (use a 75k resistor and 1nF 1% polystyrene) by adding a capacitor from anode to earth (the HF role off in the RIAA) this was followed by a double triode cascade amplifier with feedback from final anode to first cathode that incorporated the 3.18ms pole/0.318ms zero giving a midband gain of X10 and an LF gain of X100 (as per RIAA). It worked extremely well using an OA2 to regulate the HT at 150V and was low noise enough to use with some moderate output moving coil cartridges (0.5mV). The signal/noise was terrific using a typical 5mV O/P moving magnet. The output impedance is low enough (due to the negative feedback) not to need a cathode follower.

3rd Sep 2008, 8:35 pm	#24
grindrod Retired Dormant Member Join Date: Feb 2005 Location: Stroud, Gloucestershire, UK. Posts: 57	Re: Phono preamp valve kit? I did a lot of experimentation on a 'hifi' valve phono stage some years ago and the optimum configuration was an EF86 with a gain of X20 fixed using some undecoupled cathode resistance and with the 75us pole (use a 75k resistor and 1nF 1% polystyrene) by adding a capacitor from anode to earth (the HF role off in the RIAA) this was followed by a double triode cascade amplifier with feedback from final anode to first cathode that incorporated the 3.18ms pole/0.318ms zero giving a midband gain of X10 and an LF gain of X100 (as per RIAA). It worked extremely well using an OA2 to regulate the HT at 150V and was low noise enough to use with some moderate output moving coil cartridges (0.5mV). The signal/noise was terrific using a typical 5mV O/P moving magnet. The output impedance is low enough (due to the negative feedback) not to need a cathode follower.