Capacitance: a question of tolerance?

There's a lot been talked about on moving magnet cartridge capacitive loading.

Why bother with it?

Well, there's one reason why we have to bother with it - the capacitance of the tonearm cable.

Another reason is in the blocking of radio frequencies at the preamp input.

Therefore the magnetic cartridge manufacturer has to cater for capacitive loading.

But has it become a question of tolerance or a question of necessity?

Often we'll be told that we have to use a specific capacitive range to get the best out of a cartridge - now isn't that the wrong way up?

Most good tonearm manufacturers go to the trouble of giving us a nice consistent 100 pico farads, plus or minus say 10 pico farads.

By rights, you need up to 100pf of capacitance to deck at the input pin of the amp of a phono stage to block RF.

Taking board and connector capacitance into consideration that gives us around 220pf of capacitive loading. So that should do it?

So what effect does that have on the highs from our MM cartridge?

First let's consider a capacitance free world. The cartridge would just see the prescribed 47k Ohm load. The cartridge has inductance, usually around half a Henry (the unit of inductance), that's 500mH.

By a simple formula we find to our dismay we have a 1940's high frequency roll-off at around 15kHz!

So how come the cartridge manufacturers claim greater than 20kHz for their MMs?

If you add capacitance into the equation you find you have a damped tuned circuit (a tuned circuit damped by the 47k resistor). With 220pf it will take the highs that little bit further, but with a slight peak just before they fall away, but then they fall away twice as rapidly as would be the case if it were not for capacitance.

It's still not sufficient to get as high as the specs say. The rest of it is done by cantilever tuning - a bit like a tuning fork. Hit it with the frequency of interest and it resonates - it holds onto the highs.

The problem is that sometimes this "ringing on the note" can be obnoxious! It sometimes has the effect of making the harmonics of what our hearing range lets us hear change the shape of what the sound should be. It can be painfully obvious on studio monitoring headphones.

However, when it comes to loudspeakers in the listening environment it can be beneficial in sound staging, and then again it can do the opposite. When it does the opposite it tricks the ear into believing the sound stage just went mega-large, and then it's back to normal. Sometimes it does the opposite. That's what happens when a MM cart is too dependent on capacitive loading.

Reducing capacitive loading can sometimes destroy the illusion. Perhaps this is why so many up sticks and convert to the less fussy MC? The problem then can still exist in phono stage overload due to even higher frequency glitches.

If only capacitors could be made like potentiometers? Then we could tweak to suit ourselves. Well, they can, and they used to be used to tune radios, but probably not to the range of capacitance we need and probably not of the quality we need either. Some stages allow you to tweak by flicking "piano key" switches. Mostly the exercise ends up in confusion and exasperation. That's why we just use a fixed capacitance.

Careful adjustment of other factors such as tracking force, VTA and the likes usually results in more palatable highs.

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