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In Reply to: A current input MC phono preamplifier project posted by rjm on December 04, 2002 at 06:37:01:
Based on a 90 dB gain version, with a 60 dB gain input stage (75 dB version with 45 dB input stage in brackets)1. Low DC offset. Input offset voltage below 10 uV (100 uV)
2. High GBWP. Above 200 MHz (20 Mhz)
3. High PSRR.
4. Low distortion into 600 ohm loads.
5. Low voltage noise. Below 10 nV/rootHz, the lower the better.
That's sets the bar pretty high. I'm sitting here looking at the AD797 datasheet and it's the only one I have left that passes all the above. I know its a pain to use and hard to get to sound good, so I'm open to suggestions, but dang it the AD797 seems to have been born for this circuit.
rjm
Follow Ups:
One more time.It is not the input offset voltage that is critical for a current sensing virtual ground input, it is the current offset.
For the noise, it is not the voltage noise that is critical, but rather the current noise.
The AD797 has a HUGE amount of input bias current compared to any FET input op-amp.
Further more, if you want to get all the gain of an MC head amp (typ. 36-40 dB), AND a typical phono preamp (40 dB at 1 kHz), and a preamp (typ. gain 20 dB), all rolled into one box, then I strongly recommend that you not try to get it from just two op-amp stages. I certainly would not try to get 60 dB, or even 45 dB of gain from just one op-amp.
This kind of MC level to preamp out level really should have the gain spread out over three op-amps if the typical 106 dB of total gain is desired.
Even 90 dB is too much for just two op-amps.
Just to make sure:
THE AD 797 IS NOT OPTIMAL FOR CURRENT SENSING VIRTUAL GROUND INPUT CIRCUITS DUE TO VERY HIGN BIAS CURRENTS AND RELATIVELY HIGH INPUT OFFSET CURRENTS.
Jon Risch
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