Dispelling a myth (?) about phono-cartridge loading

Pink noise spectrum of Ortofon Windfeld Ti phono cartridge loaded with 47k ohms (blue) and 30 ohms (red).

In the midst of his December 2023 Gramophone Dreams column, Herb Reichert presented the results of an experiment. He was listening to the most recent version of Zu Audio’s Denon DL-103, installed on his new-old Lenco. He hooked it up to the moving coil input of his SunValley SV-EQ1616D phono preamp, which apparently is intended for use with low-output MC cartridges since it loads them down with a 50 ohm shunt resistor—a heavy load for all but the lowest-impedance MCs. The rough rule of thumb for loading an MC cartridge, as many readers are aware, is that the load resistance should exceed the cartridge’s internal impedance by about a factor of 10.

As MC cartridges go, the DL-103 has a high source impedance, around 40 ohms—almost equal to the load impedance imposed by the SunValley phono pre. The result—loading at nearly 1:1—is very unconventional!

So, Editor Jim, doing his editorial due diligence, started asking questions. He found himself involved in a three-way conversation, with Herb plus Dave Slagle, proprietor of Intact Audio and partner in EMIA. Herb brought to my attention a 1980 article by J. Peter Moncrieff, published in International Audio Review #5. Moncrieff started by noting the “common speculation in the audio community that a moderately low-value resistor … damps the high-frequency peaks which most moving coil cartridges exhibit in their frequency response.” Yep, that’s what I thought. While admitting that it’s plausible, Moncrieff concludes, “in this case it turns out that common speculation is substantially untrue.” He presents measurements of cartridge response as a function of loading from 100 ohms down to 5 ohms, with no change in frequency response.

Slagle became involved in our conversation because, as Herb already knew, he has independently corroborated Moncrieff’s result. Both Moncrieff’s and Slagle’s work—and Herb’s subjective observations—support the idea that what I have long taken to be conventional wisdom about MC cartridge loading—described by Moncrieff in the paragraph above—is wrong.

It is true that, when viewed as purely electrical circuits, MC cartridges have a resonant peak. Physics provides a simple formula for predicting the resonance of an electrical system. By plugging in typical values of inductance and capacitance for an MC cartridge, we find a resonant frequency not in the treble but in the low MHz range. (A mechanical resonance in the near-ultrasonic range has been hypothesized for a stylus moving in a record groove—the so-called stylus groove resonance—but assuming it exists, loading down the cartridge with resistance will not dampen it.)

Such values are so far above the audible range that even if they were real, damping them is unlikely to have any direct effect on the audible-range frequency response. But it doesn’t matter: A phono cartridge is an electromechanical system, not just electrical. It isn’t possible to vibrate a phono cartridge at MHz frequencies. Any electrical resonance will be mechanically damped.

The behavior in the audible range is easily tested. I had an Ortofon Windfeld Ti MC cartridge set up on my turntable. The Windfeld is a low-output MC cartridge with a low source resistance: 7 ohms. In my system, the Windfeld was feeding a Pass Laboratories XP-27 phono preamplifier, which has a range of passive loading from 30 ohms to 47k ohms. I connected the phono preamp’s output to my Focusrite Scarlett 2i2 audio interface and connected that by USB to my Macintosh laptop. I played the pink noise track from the Analogue Productions Ultimate Analogue Test LP and recorded it at a sampling rate of 96kHz using Adobe Audition. I did this twice, first with the load resistance set to 47k ohms, then with it set to 30 ohms. I repeated the measurement with a few different values of capacitive load to confirm that this wouldn’t affect the result. (It didn’t.)

You can see the results in the figure at the top of this page. In the audible range, the only effect of heavier loading (lower R; red trace) is a slight reduction in cartridge output. Over the range measured, the frequency response is unchanged.

Yet anyone with ears can hear that loading down an MC cartridge does something. If it doesn’t affect the frequency response, what does it do? Moncrieff suggested that proper loading reduces distortion—especially intermodulation distortion—perhaps by reducing the tendency of the stylus to flit about inside the record groove. Makes sense to me.

Where did the conventional wisdom come from? Perhaps it was helped along by the fact that it’s qualitatively correct: There is indeed a high-frequency electrical resonance, and a resistive load can dampen it. It’s just that the peak is too high in frequency to matter. A second source of confusion may be the use of step-up transformers (SUTs). Just as a transformer reduces the effective load resistance by an amount equal to the turns ratio squared, a SUT multiplies cartridge capacitance by the same amount. That’s enough to bring the resonant frequency much closer to the audible range.

I find results like this exciting—it’s fun to be freed up from convention. If one accepts this result, then it’s time to rethink the whole subject of MC cartridge loading. Does an optimal value of the load resistance exist at all? Moncrieff says no—that there is no lower limit: The lower the load resistance, the better.

I was unaware of any of this until a few weeks ago—yet Moncrieff did his work 43 years ago. Has anyone since then solved the mystery of MC cartridge loading? If you know the answer, drop me a line at [email protected].

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