A zero impedance preamp and CMRR

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koko5

Member
2012-02-27 10:40 am
Sofia
Hi all,

I found this circuit - Model 4712 Phonocube . As you can see this preamp is recommended for MC phono cartridges because of its very low imput impedance.
I need your assistance about: is this unbalanced inverted zero impedance input preamp an alternative of balanced line systems-at a first look it seems that interference signals appears to both inputs (as common mode signals) and I expect that these must be suppressed by opamp CMRR-is this right?Or the same question with other words: unbalanced zero impedance input preamplifier, working as current to voltage converter, suppresses common mode interference signals as good as balanced circuits,is this right?
Since it is a current amplification device with 0 ohm input impedance, output level is determined by the output amperage of the cartridge (output voltage divided by internal impedance)
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The voltage gain is Vout / Vs = Rf / Rs, where Vs is the open circuit output voltage of the cartridge. A "zero impedance" input stage allows an inverting topology to be used without adding extra noise generated by the input resistor. The impedance of the cartridge must be relatively constant over the audio band, however, or else the frequency response will deviate significantly from the manufacturers specification.
If we try such topology (expecting lower gain of the stage) for MM,mic. and any universal purposes, how to determine / how to test the signal source's impedance consistence over audio band?

Thanks in advance for your input :)
 
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