Need good EF86 based RIAA preamp.

[MarcelvdG]

Yes, it's certainly quirky and has a noise penalty. However, in practice the noise penalty is fine IME, especially in semiconductor variants. I played around with the concept out of curiosity, got the whole gain job done in a single pentode, saved a gain stage and simplified RIAA - though obviously still needs a CF buffer.

As to motivation, some of the alleged advantages are set out in this (lapsed) patent from 1984 which seems to be a variant of similar principles:

Patent US4470020 - Virtual ground preamplifier for magnetic phono cartridge - Google Patents

Interesting patent! As long as the feedback resistor is large enough, a transimpedance amplifier will hardly give you any noise penalty, unlike a voltage amplifier with a low-valued resistor shunting its input.

The earliest moving-magnet transient response improver I know of is Hans van Maanen's 1979 circuit:

H.R.E. van Maanen, "Compensatie van mechanische resonantie van pick-up elementen", Radio Electronica, 1979, no.15/16 pages 25...29 and no. 17 pages 35...40, available on his website Temporal Coherence - Home

This circuit does not mechanically damp anything, but just damps the electrical resonance and compensates for the mechanical resonance. Steven van Raalte published an improved version a few years ago in Jan Didden's magazine:

Steven van Raalte, "Correcting transducer response with an inverse resonance filter", Linear Audio, vol. 3, 1 April 2012, pages 69...90

 

[luckythedog]

Yes, it is curiously interesting I find. And different. Feedback impedance in combination with cartridge coil impedance determines gain too, of course. In practice gain-bandwidth performance of op-amps I found to be an issue , and that feedback stage also re-forms the 75us RIAA time constant as set out in the text. One of the reasons I looked to pentodes intially.

The Barney Oliver preamp is also a virtual earth amplifier where virtual input impedance is designed to naturally form the 75us time constant. This is easier to realise, but has a noise penalty - albeit fine in practice IME.