Higher Impedance MC Carts on Transimpedance Stages?


Can anyone explain what happens if one pairs a transimpedance / current injection phono stage with a moving coil cartridge whose impedance may be higher than optimal? What would the result be?

This question arose from someone who wanted my thoughts on the BMC MCCI Signature ULN phono stage that I use as my reference, but that individual is using a Kiseki Blue which is spec’d to have an internal impedance of 40 Ohm, which I’ve found is higher than typical MC cartridges. 

@lewm and @rauliruegas, you guys likely can answer this easily, but of course open to anyone else that can explain.

Thanks!

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Showing 1 response by atmasphere

@hagtech +1

Transimpedance works nicely if you have a nice low output. Not so well as the impedance of the cartridge goes up.

In a transimpedance circuit, the cartridge replaces the input resistor of an opamp. The gain of an opamp circuit is the ratio between the input resistor and the feedback resistor. Since the feedback resistor is fixed, this means that the lower the impedance of the cartridge, the more gain the circuit will have (and also greater distortion since there is a trade-off between the two).

The loading is less than ideal. The cartridge has a certain compliance which interacts with the mass of the cartridge and tonearm, resulting in a mechanical resonance. This resonance should fall between 7 and 12 Hz for best tracking. When you load the cartridge with a low impedance, the result is the cantilever becomes harder to move (so has less compliance). This results in a very measurable loss of ability to trace higher frequencies. So at best the cartridge choice has to match the transimpedance circuit so this doesn't happen in the audio range.

As Deep Thought once said, 'Tricky'.