In yesterday’s post about amplifier damping we covered what it meant and what happens when a power amplifier loses control of the loudspeaker – you get differences in amplitude that are anything but accurate.
There is another type of damping that I reminded of: cartridge damping. Phono cartridges have a problem similar (and for the same reason) to the loudspeaker damping issue but in reverse. A loudspeaker is a coil of wire driven by an amplifier while a phono cartridge is a coil of wire driving an amplifier.
So in a power amplifier situation we want the amplifier to have a low output impedance so that whatever changes happen in the loudspeaker coil don’t affect the output of the amp. In a phono cartridge setup we want the opposite – the amplifier to have a high enough input impedance so it doesn’t affect what the coil of the cartridge is doing.
A phono cartridge generates electricity in response to the movement of the needle in the record groove. This occurs because there is a magnet moving in concert with the record grooves in close proximity to a coil of wire. In the case of a moving magnet cartridge, the magnet is affixed to the needle and the coils are held steady in the cartridge while the magnet moves about. In the case of a moving coil cartridge the opposite is true – the coil of wire is affixed to the needle and the magnets are held steady by the head shell.
In either example it is then necessary to tame the impedance of the coil of wire in the cartridge either by adding a fixed resistor and capacitor across the coil or simply a fixed resistor. This process is called cartridge loading but more properly cartridge damping.
Phono cartridges act like any coil of wire producing a voltage differently per frequency. What we want is a constant output at any frequency so we damp the cartridge with resistors and capacitors. You’re probably familiar with the cartridge loading switches on the back of phono preamps like our GCPH.
Now you know.
Paul McGowan – PS Audio, Intl.