Clearaudio in Germany has two expensive phono stages with current amp. input. Erno Borbely Proteus phono stage is also of the same type.Are they wrong, according to some designers?Is it the future trend of high-end phono stages? I am going to build Phonoclone as soon as possible, just to see the type of sound. It is very relaxing not to think if cartridge loading is right or no.
"current" inputs
I would be cautious in supposing that a near-zero input impedance for MC cartridges is always beneficial. To the extent that it can be realized electronically, the cartridge's self-resistance (and the tonearm and interconnect wires' resistance) becomes the loading R. If the cartridge manufacturer developed the cartridge with that as the operating condition, then fine. If not you are going to have frequency response anomalies which may well be audible and which you may even prefer. But they are not necessarily more accurate. I'd recommend a careful comparison with accurate level matching and if possible a good test record, to see what is happening to frequency response.
Some time ago in here I proposed simulating a negative resistance at the preamp to back out some of the internal resistance, thereby enabling what Cordell has described in Linear Audio for introducing an overdamped response in (partial) realization of the 75us time constant of RIAA. Cordell does this with MM cartridges, where the requisite R is quite a bit less than the standard 47k. The approach requires that one knows the actual cartridge's inductance and resistance, so may be impractical for many unless they can make those measurements.
For the technique to be used with low-inductance MC cartridges, the internal R alone is already too large to get the 75us tau, hence the need for a negative resistance. Unfortunately, although the negative R can be synthesized, and can have very low noise, the distortion attendant on the positive feedback topology tends to be high. And with any negative impedance synthesis, the specter of oscillation looms.
I lost interest in this primarily for those reasons. But your mileage may vary.
Brad
I would be cautious in supposing that a near-zero input impedance for MC cartridges is always beneficial. To the extent that it can be realized electronically, the cartridge's self-resistance (and the tonearm and interconnect wires' resistance) becomes the loading R. If the cartridge manufacturer developed the cartridge with that as the operating condition, then fine. If not you are going to have frequency response anomalies which may well be audible and which you may even prefer. But they are not necessarily more accurate. I'd recommend a careful comparison with accurate level matching and if possible a good test record, to see what is happening to frequency response.
Some time ago in here I proposed simulating a negative resistance at the preamp to back out some of the internal resistance, thereby enabling what Cordell has described in Linear Audio for introducing an overdamped response in (partial) realization of the 75us time constant of RIAA. Cordell does this with MM cartridges, where the requisite R is quite a bit less than the standard 47k. The approach requires that one knows the actual cartridge's inductance and resistance, so may be impractical for many unless they can make those measurements.
For the technique to be used with low-inductance MC cartridges, the internal R alone is already too large to get the 75us tau, hence the need for a negative resistance. Unfortunately, although the negative R can be synthesized, and can have very low noise, the distortion attendant on the positive feedback topology tends to be high. And with any negative impedance synthesis, the specter of oscillation looms.
I lost interest in this primarily for those reasons. But your mileage may vary.
Brad
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