Skeeter
Member
Both MM and MC have specific challenges related to noise because one is a low source impedance sensor (MC) and the other is a high source impedance sensor (MM). There are two input noise mechanisms in an amplifier: input noise voltage and input noise current. For high-impedance sources like an MM cartridge, you want to use a part that has LOW noise current because the noise current flows through the source impedance creating an additional noise voltage that is added to the device's native noise voltage. The OPA1612 features a noise current of 1.7pA per root hertz (pA/rt Hz) which is high. It does however feature very low input noise voltage. So, if you feed it with a low source impedance device like an MC cartridge, you get very little noise current x source impedance contribution, and because you have low noise voltage (1nV/rt Hz) it is a perfect match for MC. However, if you feed it with an MM cart, the source impedance is about 20k in the upper half of the audio band. The 1.7pA/rt Hz noise current flowing through this gives rise to a significant additional noise voltage. A much better part for MM is the NE5534 with an input noise current of 0.4 pA/rt Hz (so about one-quarter of the OPA1612) or a JFET input opamp like an OPA1642. The OPA1612 is a great part for source impedances up to about 2.5k Ohms which is what you get with a 10k pot when set to the electrical mid position if fed from a low source impedance - this is typically what you would see in a line preamp taking in CD, DAC or a phono preamp.OK - school time for me. Why is the 1612 worse for MM cf MC?
I've always understood that the higher gain needed for MC makes it more challenging wrt to noise, not less.
Also - what is a volume lot?
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To get low input noise voltage, opamp designers run the input device emitter currents at high levels and the transistors are large. Unfortunately, this has the side effect of increasing the noise current - i.e. there is a trade-off. For this reason, if a phono designer is looking for good noise performance, they have to match the type of device being used in the amplifier with the type of source.
Here is a ~27 minute video discussing some of these aspects in amongst other things (see from 3.45 to 13.00)
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