Cartridge Matching Part 1

Cartridge matching part 1

The Moving Magnet cartridge

Cartridge output

The most common phono cartridge is the moving magnet type. Prices range from about £10 to £400. Some people consider moving magnet cartridges to be cheap!

The output of a moving magnet cartridge can be between 2.5mV and 10mV depending on the model. The output is given in the cartridge specification. Because the output of a magnetic cartridge (whether moving magnet, moving iron or moving coil) is dependant on its velocity and the bass and treble on your records have a different velocity to the mids, its output is quoted at a standard spot-frequency which is 1 kHz (1,000Hz). The velocity at this frequency is 5cm/sec. Therefore you will often see the output described like this

"Output voltage at 1000 Hz, 5cm/sec. 5 mV"

Now as the velocity increases to the treble end, the cartridge output at 20 kHz will be approx. 50mV, and as it decreases to the bass end, the cartridge output at 20Hz will be approx. 0.5mV. It can be seen that bass boost and treble cut is required to restore the tonal balance.

The 5mV in the example above is 0.005 volts. That's roughly 155 times less than a line-level device and therefore some pre-amplification as well as tonal balance correction is required for the cartridge output to match a line-level device. This is done with a phono preamp or phono stage. In the case of a moving magnet cartridge, a phono preamp or phono stage of moving magnet sensitivity is required.

Cartridge load

The moving magnet cartridge output is specified when driving a load and that load is universally 47k (47,000 Ohms) these days. Why? Because it is an adopted standard and since moving magnet sensitivity phono preamps or phono stages use 47k the cartridge manufacturer will do well if he makes his cartridges to work best into 47k. Likewise with cartridge manufacturers specifying 47k, the phono preamp or phono stage manufacturer will do well if he uses a load of 47k.

There is also a capacitive load. The one usually stated is 200pf (200 pico-farads) or a range of acceptable capacitive loads such as 150 - 300pf. This is a bit tricky because phono preamps or phono stages often state an input load capacitance of only 100pf - clearly there is a mismatch? Not exactly! The turntable connection lead (interconnect) contributes some of the required capacitive load – something that the cartridge and phono preamp stage manufacturers are aware of, but you may not be. The average turntable interconnect is 100pf, and when connected to a phono preamp having a 100pf input capacitance the required 200pf capacitive loading is "seen" by the cartridge.

These values are given so that the cartridge will give a "flat" output right up to its high frequency turnover point – the frequency after which its output falls off because it can do no more. This is because a magnetic phono cartridge has a thing called inductance. It can't escape having inductance because the current it generates is from a coil of wire which is influenced by a changing magnetic field - it is changing because the stylus is being moved by the record's groove. All coils of wire have inductance - the more turns they have makes more inductance. A moving magnet cartridge has the highest output of all magnetic cartridges and hence has the highest inductance of all, the median being 500mH (500 milli-Henries or half a Henry).

Cartridge manufacturers manipulate inductance, magnet strength, and mechanical resonances to give a "flat" response with the cartridge loaded by the specified 47k resistive and 200pf capacitive load, and as such you may see inductances of 350 to 650 mH quoted in specifications (that is for those manufacturers who go to the trouble).

Taking the median 500mH and a 47k load, and using inductance equations it can be seen that the moving magnet cartridge can only go up to 15kHz before rolling off at a natural 6dB per octave (In fact 15kHz used to be considered high fidelity in the valve days). Capacitance however, is unavoidable, and as we've heard, the turntable interconnect contributes around 100pf which resonates with the cartridge inductance, taking it to 21 kHz, but then rolling-off at 12dB per octave. To avoid radio breakthrough which is what you'd get from a high impedance input, a moving magnet phono stage finds an input capacitor a "must-have" and 100pf is about the smallest that can be gotten away with. The result is 200pf in parallel with the cartridge and that results in a turnover frequency (-3dB) of 20 kHz with a slight 1dB peak at 10 kHz. This may be OK for a slightly sluggish sounding system, but otherwise, really sensitive ears can hear this and it has been suggested that the 47k input resistance is reduced to around 35k, which takes the -3dB roll-off back to 15 kHz, in effect giving the same turnover as having no capacitive load, but still with a 12dB per octave roll-off.

The opposite – increasing capacitance and resistance - boosts the output through resonance with the cartridge inductance, and is akin to turning up a treble control: a 47k plus 450pf load gives 3dB of boost at 10 kHz, and a 100k plus 450pf load gives 6dB of boost at 10 kHz. This can cause clipping and if this course of action is found necessary then something about the system is obviously wrong and needs attention - either that or the listener should consider tone controls.