MM Phono Cartridge Loading Capacitor
Inductance (L), resistance (R) and capacitance (C), when all placed in parallel with one another, form a resonant LCR ‘tank’ circuit. As frequency increases, so does the inductance – it becomes more ‘resistive’. As frequency increases, the capacitance becomes less ‘resistive’.
At some high frequency a point will be reached where the combination of inductance and capacitance resonates – it will produce a large peak or ‘spike’ in the frequency response. The resistor however, dampens the spike – reduces its amplitude (its height). When balanced up nicely you get an up slope to a particular frequency, followed by a down slope – both slopes being equal. The rate of rise and fall is 6dB per octave.
When this circuit is used as the input of a phono preamp, the inductance is the phono cartridge – it’s also generating the electrical signal which is the music. The capacitance is made up of the arm cable wiring capacitance, plus a real capacitor in the input of the phono preamp. The resistor (quite often 47k) is a real resistor (or combination of values) also in the input of the phono preamp. The response graph you would see from this particular LCR combination would not be rising, peaking and falling, but will be flat, followed by a peak, and then followed by a down slope twice as steep, or -12dB per octave. How come it’s different? That’s because of the equalisation in the phono preamp.
If we know the inductance and the capacitance, we can work out the frequency where it slopes down. This frequency is where the electrical output is designed to start falling – the upper end of the phono cartridge frequency response. There is a simple physics formula for this . . . ƒR = 1/ 2pi sqrt LC
Classic values are 500mH for a MM cartridge, and 200 pico farads for the total capacitive load. This gives a frequency of 16 kHz, which was considered excellent when hi-fi was defined as anything having a frequency response of 30 Hz – 15 kHz.
So, how come MM cartridges have a frequency response reaching higher than 16 kHz? Well, from what we know, and we know that physics tells us it can only go to 16 kHz, it must be something other than being electrical – it must be mechanical. Therefore, it has to be likened to a ‘tuning fork’, and the stylus assembly must therefore mechanically resonate to basically ‘cheat’ the response. Going back to the electrical physics, we can see that by varying the cartridge inductance and load capacitance, the high frequency turnover point can be changed . . . For example, if the inductance were to be 400mH, the high frequency turnover will rise to nearly 18 kHz using the same 200 pico farad capacitance load. If it were to be 600mH, it would become 14.5 kHz. By reducing the capacitance to 150 pf it would become nearly 17 kHz.
Because the vast majority of cartridges are 1] made to work with existing tone arm wiring capacitance plus phono preamp input capacitance – and that can add up to 200 – 300 pico farads, the sensible MM manufacturer will choose to make the cartridge inductance close to 500mH – plus or minus a little, and 2] because the output is close dependant on ampere-turns – how many turns of wire there is – it will follow that for a range of say 2mV to 10mV output, it’s going to end up as being in the region of 500 mH. As most tone arm cables plus internal wiring measures around 100 pf and most phono preamps use 100 – 200 pf, the total capacitive load is 200 – 300 pf. The manufacturer knows what the electrical high frequency limit it is going to calculate out at. He will therefore tune the mechanical resonance to reach as high as he can get it.
The question that usually gets asked is “what will happen if I use a cartridge specified for 400 – 500 pf into a phono preamp specified as having 100 pf input capacitance?”. Well, first you need to add the tone arm wiring and cable capacitance – that will be around 100 pf, making the total load 200 pf.
Then you have a difference of 200 pf. You can either try it like that and you may like the sound, and if you don’t you either need to add capacitance, or buy a phono preamp with different capacitive loading switches (but remember switches can go resistive through corrosion – they don’t last forever). You can also use an extension cable on the tone arm cable to increase capacitance.
But at what frequency do these differences take place and will you actually hear them? After all, the peak is damped by the resistance (the 47k). The frequencies are mostly going to be above 10 kHz with peaks or troughs of up to 2 dB. It is going to be very difficult for anybody in his/her 30’s to clearly hear much higher than say 16 kHz. It is going to be quite hard to tell the difference 2 dB is going to make.
The thing that could influence what you hear may be psychological – that being the specification said one thing and the phono preamp said slightly different.
