Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Posted by Jon Risch (B) on December 5, 2002 at 20:05:07

In Reply to: Re: My experience with current sensing MC phono pre's posted by RJM on December 5, 2002 at 00:59:21:

How the virtual ground works.

With the positive input of the op-amp grounded, the feedback from the output to ther input attempts to keep the negative input terminal at ground potential too.

Whatever signal current is input into the negative terminal, the output provides an equal amount of current of the opposite polarity to end up equaling zero.

Whatever the feedback resistor is, with a given amount of current required, there is a proportional voltage occuring at the output.

For the purposes of computing effective gain, you can use the MC cart coil resistance as an input resistance, and this will be used as the inverting input resistance to be plugged into the classic voltage gain formula. As an example, if we have an MC cart with a coil resitance of 35 ohms, and use a feedback R of 350 ohms, the effective voltage gain will be 20 dB. If the cartridge puts out 0.3 mV from a 0 dB recorded level, then the output will be approx. 3 mV.

Looked at another way, an MC cart with medium to low coil resistance will put out current into a virtual ground as if it were a short.

So for our 35 ohm MC cart that outputs 0.3mV at 0 dB recorded levels,
it will put out 8.57 uA's into the virtual ground input. This amount of current into the op-amp negative terminal (with a 350 ohm feedbackl R) is Iin X Feedback R = 3 mV output.

Essentially, the MC cart coil resistance limits the amount of output current that can flow.

Used as a signal source into a virtual ground, an MC cartridge is primarily a current producing device.

It helps to keep in mind that ALL dynamic (magnetic) transducers are current driven and sourced. That means that the signal that comes out of the MC cartridge that truly represents the information in the groove, is current. The voltage is a derived quantity, and as such, represents one step away from the original signal.

With current sensing, you find that not only is the noise greatly reduced from that of normal voltage operation WITH THE PROPER OP-AMP or input circuitry, but that the noise that is present from the record surface seems to be disconnected from the music, and as such, is a lot easier to ignore or dismiss mentally.

RE your PS, I believe that you meant 0.4 mV? (Even very low output MC carts put out at least 0.05 mV or so. uV is too low). Into a virtual ground, that would equate to 40 uA's. Using a 22 k ohm feedback resistor, this would end up with approx. 46.8 dB of effective voltage gain (NOT RECOMMENDED), and would require the op-amp to put out 0.88 volts.

However, the higher the MC coil resistance, the less that the cart acts like a current source, and the closer to voltage mode operation the op-amp will be. I have not had good results with MC carts with coil resistances above 100 ohms or so, operating into a virtual ground input.

Jon Risch

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