Differential Input Phono Preamp

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If capacitance sucks:

Eliminate the 2KHz pole in the RIAA preamp

Load-down the cart with a smaller resistor, 5K-10K.

That's how the early Shures were supposed to be worked.

Doing that loses the chance to ring-up the 15KHz zone with a perfect combo of L R and C. Grados don't do that; many Shures do. Given some other reference (calibration disk) you could EQ this after the preamp.

I do MUCH prefer putting most of the preamp IN the record-player. This only made sense when tubes were king. I mean, how often do you find tape-decks without internal preamps? (I may have retired some of the last of that bad idea.) Back in 1976 my preamp was 1"x3" (plud 4 inches of optional bypass caps). As JR says, today SMD could be smaller.
 
Honestly, I have mentioned the driven shield as anecdotical, since it is quite complex in the implementation. I agree 100% that the drive must be LPF'ed and care must be taken of the propensity of opamps to oscillate when driving capacitive loads.
Just to add some confusion, don't forget about the option of a VE input, which would almost completely eliminate the capacitance problem.
 
Kingston said:
And what is "negative capacitance" here?
Just consider John's answer; driving the shield with 50% of the active signal reduces capacitance by half, 75% reduces by a factor 4, 90% factor 10 and 100% gives zero capacitance. Now, if you have gain, the capacitive current becomes negative, acting like a negative capacitance. Think of bootstrapping capacitance. Since it is actually a form of positive feedback, it is prone to oscillation/instability.

I used active shield at the Opera Bastille, where each party line of the RTS intercom system runs on 6.4km of shielded cable. The nominal impedance is 200 ohms. The capacitance of the line would create a roll-off at 1.3kHz (RTS specify a max lengthof about 1200 meters). The RTS system is unbalanced, so I used 2-conductor shielded cable (standard multipair) and I drove one of the conductors with an amplified version of the signal, collected at a central position.
This allowed extending the frequency response to about 6kHz, which is more than enough for intercom. The amplifier had an output impedance of about 20 ohms, which created a natural roll-off of the shield-driving signal at about 10kHz. I had tested the system on reels of cable and it worked as sson as it was installed.
 
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