Secrets Q & A
- Written by John E. Johnson, Jr.
- Published on 12 August 2008
Part 2: The Technology
The MT10 comes with what's called an MC cartridge. This means "Moving Coil", and that brings us to the stage of defining the terms (and there are lots of them to plow through).
Phono cartridges (the thumb-sized thing with the needle on it) come in basically two flavors. One is the Moving Magnet (MM) and the other is the Moving Coil (MC). Both work principally in the same way. The tip of the needle has a small diamond, called the "Stylus", that is shaped like the groove of the LP. It is attached to the cantilever which is a long thin - and very light - rod. It is really not much wider than the diamond stylus. It is attached at the rear of the cartridge, and it goes up and down in the LP groove as the LP is spinning.
When the stylus is moving, the interaction between a magnet and nearby coils of wire creates a very small signal (millivolts) in the coil, and that is sent to the phono stage.
So, here is the difference. With a moving magnet cartridge, a magnet is attached to the cantilever, while the nearby coils are stationary. With the moving coil cartridge, coils of wire are attached to the cantilever, and a magnet nearby is stationary. There are advantages and disadvantages to both types, and that is why they both still exist. For MM, the cantilever with its magnet has more mass, and thus, is a bit more sluggish to respond in the groove. Therefore, it tends to have slightly less detail in the sound. However, because the coils can be large, the resulting voltage is high, e.g., 4 millivolts (mV). This is important, because the lower the output voltage, the more likely the signal is to be swamped by electrical noise and hum. Remember, the output of a CD player's analog jacks is on the order of 2 volts. That is 500 times higher than the output of even the MM cartridge.
The MC cartridge has tiny coils of wire attached to the cantilever, often just a few turns of very fine wire. The magnets are stationary, nearby. Because the cantilever has a very low mass, it responds to even the smallest detail in the LP groove. The result: better fine detail in the music. However, its output is usually less than 1 millivolt. The MT10 cartridge, for example, made by Clearaudio for McIntosh, outputs only 0.75 mV. Talk about having to be very careful about where you put the interconnects - Wow!
MM cartidges have high voltage, low current output, while MC cartridges have low voltage, high current output.
Phono preamplifiers need to have an extra gain stage for use with MC cartridges, and for example, the BP-1.5 uses a transformer to boost the voltage from an MC cartridge to the same level of an MM cartridge output, and then it is fed to the same gain stage circuits for both types of cartridges.
One rare variation is the Moving Iron cartridge, where the cantilever has a small piece of non-magnetized steel on it, and when it moves, this disturbs the magnetic field set up between stationary magnets and coils, generating the signal voltage. This type is generally not available now. Lastly, there are ceramic cartridges that were used in children's record players, and are still available as replacement parts. They can put out several volts, so a preamplifier is not really even needed.
Unlike CDs where you just pop the disc in the player and press the Play button, there are a bunch of other things you have to be concerned about with a modern turntable, and the LP itself, in order to get the best out of it (them).
One of these is that once you have decided on using either an MM or MC cartridge, you can't just use any old tonearm on the turntable. It has to balance what is called the "Compliance" of the cartridge. Ready for another definition? Here we go.
Think of the stylus dragging the cantilever as it wiggles through all the grooves on the LP. And, think of the cantilever dragging the cartridge. OK, now imagine you are holding the end of a 10 foot rope that is attached at the other end to a wall about waist height. You begin moving your end up and down, faster and faster. At some frequency, the up and down force you are applying to your end of the rope will match the mass of the rope, and it will move in a wavelike form, with a peak in the exact middle of the rope. This is the resonant frequency of the rope and the force you are applying. It's the same with the stylus, cantilever, and cartridge. At some frequency - the resonant frequency - the force of the stylus to cantilever to cartridge will be "just right" so that the entire mechanism is swinging back and forth with the movement of the stylus in the groove. That resonant frequency depends on the "compliance" value of the cartridge and is specified in centimeters per dyne, which refers to how far it will move with a specific value of force applied.
The range of compliance values is from about 5 x 10-6 cm/dyne up to 25 x 10-6 cm/dyne. If the value is less than 12, it is considered a low-compliance cartridge, 13-18 is mid-level, and above 18 is high compliance.
Now, the reason this is worth paying attention to, is that when selecting a cartridge, you want to know the "mass" of the tonearm - in grams - to which it will be attached. And, the idea is that you match a low compliance cartridge to a mid or high mass tonearm, and a high compliance cartridge to a mid or low mass tonearm. A mass value of 10 grams or less for the tonearm is considered low, 11-15 grams mid, and above 15 grams, a high mass. These numbers and categories are not engraved in stone, but just a general guide.
The combination of cartridge compliance and tonearm mass will result in a system resonant frequency, and you want that frequency to be between 10 Hz and about 18 Hz. If it is above 20 Hz, you will get too much audible deep bass. If it is lower than 10 Hz, this will make the rumble (the motor and platter make very low frequency noises of their own) audible.