Matching the cartridge to the phono stage

Hi @lewm 

"That’s often tolerable but also there is a gradual high frequency roll off."

The statement quoted above will be true if the cartridge is an MM or MI with hundreds of mH inductance.

In contrast, MC cartridges generally have far less inductance (by a few decimal places), and therefore will not behave as claimed.

For example, if the frequency response of a Denon DL-103R (14Ω coils) is measured when the load is 47kΩ, then 47Ω (a thousand times less), the two curves will track each other to within a fraction of a decibel (at the most 0.5dB at 20kHz).

Keep in mind that a low-inductance MC cartridge is a terribly inefficient passive generator that is mechanically driven by the LP groove. As a result, its behavior changes hardly at all when the electrical load is altered.

As a handful of engineers (including Ralph and myself) have been saying for many years, the sonic differences perceived by many audiophiles is caused not by differences in the behavior of the MC cartridge as its electrical load is altered, but rather distortion in the phono stage that is triggered by the electrical reactance / ultrasonic resonance occurring between the inductance of the cartridge coils vs. the capacitance of the tonearm internal wiring, tonearm output cable, and input circuitry of the phono stage.

If a phono stage designed to be insensitive to ultrasonic or low radio-frequency energy, the tonal balance will change very little when the input load is altered, with the side benefit of reduced LP surface noise (much of which occurs within a frequency range that overlaps with the ultrasonic resonances occurring between the inductance of the cartridge and the various capacitances present between the cartridge and the amplification circuitry of the phono stage).

I was meaning to speak in general terms of matching impedances between any two components but thanks for correcting me as regards an LOMC cartridge. I am very aware of the vast differences in inductance among the 3 major types of cartridge but didn’t figure that in. However, many do report that the sound got dull or closed in when they reduced the load drastically below the 10X rule of thumb, say down to 2 or 3X, and I’ve always attributed that observation to an effect of the impedance match. Not meaning to argue the point as your expertise far exceeds mine.

@jcarr +1

The basis for my reasoning is as follows:

First, we can group LOMC cartridges as having similar very low inductance in the 1 to 50 microHenry range. Typically more like about 10 uH for a very low output MC. So I would imagine that differences in inductance factor out, and no one is talking about using a less than 47K ohm load with an MM or MI type (and parenthetically, I agree with you and Ralph; I use 47K with LOMCs.)

Second, the effect to which I refer would have to do with parallel capacitance not inductance. Your quoted data re the DL103R (off by 0.5db at 20kHz) shows that the effect I cited is small, but do we know what were the capacitance values in that test? The idea is that impedance due to parallel capacitance will change from a high value to a lower value as frequency increases from 20Hz to 20kHz but even at 20kHz paralllel capacitance is not the dominant source of impedance (typically in the pF range). As impedance due to capacitance goes down with increasing f, then proportionate to impedance at 20Hz, net impedance goes down at 20kHz, shunting slightly more of the signal voltage to ground. Thus, compared to signal voltage at 20Hz, signal voltage at 20kHz would be reduced. Hence the potential for HF roll-off if cartridge internal R and phono stage input Z are mismatched to begin with.

Here ya go!!

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