There ought to be a law!

January 27, 2017
 by Paul McGowan

…and there is. Ohm's.

Ever wondered why phono cartridge loading is so important? We want to make sure a moving magnet is properly loaded at 47KΩ, the moving coil perhaps 100Ω.

Get those input impedances wrong and the system won't sound right.

A phono cartridge is little more than a coil of wire. This coil of wire is where the electrical signal is generated. A magnet attached to the end of the cantilever is surrounded by this coil in a moving magnet. This coil is attached to the end of the cantilever and surrounded by a magnet in a moving coil.

In either case, (the magnet moving closer to the coil or further away) a voltage is generated in response to the movement of the needle in the record groove. Apply this tiny signal voltage to the input of a preamplifier equipped with the proper equalization, and voila! we get music.

Seems simple enough. But there's a catch. When we use magnetics (coils of wire) to either transfer or generate energy, we run into a hitch in the get along. These devices are not flat in the frequency domain. At their frequency extremes, they work differently than in the middle of their range. At the lowest frequencies, they don't work at all. At the higher frequencies, they lose steam and begin rolling off—but not before they get louder—sort of a last gasp before their swan song.

This louder signal at higher frequencies is called a resonance, a peaking. It is to this peaking we apply our buddy, Mr. Ohm.

In a moving magnet cartridge, we want to terminate with a 47KΩ resistor and a small capacitor. This damps out the peak and makes for a smooth transition as the cartridge rolls off on the top end. Too high a terminating resistor and we get an unnatural boost at the top end from this peak. Too low a resistance and we lose the top end. The same applies to the moving coil, just on a different scale.

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6 comments on “There ought to be a law!”

  1. This is a very important subject for all of us who like to play records. I have discovered that 47k ohms seems to work well for both MM and MC cartridges. Since I use both and switch cartridges a lot, this discovery has simplified my life.

  2. Paul, admittedly 47K is the most common loading for MM and MI cartridges, but it may not be "proper" for all of them. There is a very long MM post on Audiogon where several respondents reported loading up to 100K provided more favorable sonics. Others reported lower values such as 15K-30K seemed more optimal for some models of Grados. More specifically, some manufacturers recommended loadings of 100K, such as certain models of Empire and Pickering. This was common enough that some Japanese preamps offered 100K in addition to 47K loading on their MM cartridge inputs.

  3. Please note that there are also phono preamps out there that amplify on the basis of current, rather than voltage. I have an Aqvox phono preamp that requires no loading for moving coil cartridges.

  4. There is a lot of variation among MC cartridges depending on the cartridge design and output. I had a high output MC (2.5 mV) and used the MM inputs on my phono stage. I now use a Lyra Atlas (.56 mV) and found 750 ohms to produce the best sound in my system (Lyra recommends 104 to 887). I am familiar with MC cartridges that sound best in a range from 10 to 1000 ohms.

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