Phono Cartridge Loading For Moving Magnet?
Thousands ask the question "What is the correct loading for a moving magnet phono cartridge" and never get a straightforward answer.
IEC (CEI) 6-1938 gives the answer as being 47k Ohms and 420pf. IEC (CEI) 6-1938 is usually promptly "trashed" by all and sundry!
I mean, how do they have the nerve to suggest anything contrary to the forum masses?
It's obvious that a collective of any number will concur the correct answer is that put forth by the most prominent/strongest member... isn't it? Hang about! That's not very scientific!
Plugging in the IEC values to my phono cartridge model (see
http://www.gspaudio-community.activeboards.net/forum_posts.asp?TID=964&PID=7158#7158 for some background) the -3dB high frequency response is only just 15kHz! But most moving magnet cartridges specify at least 20kHz - that can't be right?
And this is exactly where the "little knowledge is a dangerous thing" goes on the rampage thinking the wrong thoughts.
Taking a look at the first page of the aforementioned topic a chart can be seen showing response curves for different capacitive loadings of an average moving magnet cartridge - the resistive load maintained at a constant 47k Ohms. It can be seen that with increasing capacitive load the response kicks up (there's a peak in the response) just before it starts cutting (falling off). To the uninitiated any peak in the response is wrong, but delve into filter design and it will be noted that the steeper the roll off, the greater the peaking prior to roll off. If we were talking about a simple RC network giving a single pole filter with slope -6dB per octave we wouldn't expect a kick, but capacitive loading of an inductive source results in a double pole -12dB per octave (LCR) filter - a kick in the response is inevitable.
So what else can we deduct? My old mentor Dr Jones always said "from what we know...". So what do we know? Well, if we knew something about history, we would know that old father hi-fi - real high fidelity of yesteryear, had a frequency response of 30Hz to 15kHz.
15kHz? Yes, 15kHz, the -3dB frequency you'd get for a 420pf (with 47k) capacitive load. So maybe the IEC standard 6-1938 has a point?
But in that case how can a moving magnet cartridge manufacturer claim an upper frequency response of 20kHz or more? Either he breaks ranks and dictates a different load - and a few "newbies" to the cartridge scene seem to get away with this - or there's something more we need to understand?
And the answer is yes, we need to understand stylus resonance (more correctly put as stylus cantilever or stylus tube resonance). Stick a stylus on the end of a tube (which you have to do anyway, to give some distance between the cartridge body and the record surface) and every time the stylus moves there will be some reluctance for the stylus tube to bend so that the power transfer is optimised in the cartridge coils. The stylus tube is considerably stiff, but its reluctance will give at some point. To get a frequency response higher than the yesteryear's 15kHz the manufacturer's only serious approach is to make the tube's resonance happen in such a way it carries the response above 15kHz.
Reluctance is a word also used to describe the behaviour of an inductor, and therefore the resonance could very well be arranged to mirror the LCR behaviour of the electrical load - the 500mH inductance of the average MM cartridge (L), the 47k load resistor (R) and the 420pf capacitive load (C). It would make sense if it did. And if that were to be the case, the up-kick in the electrical response would be countered by a down kick in the electro mechanical response, making the actual response flat!
If this did not make sense then loading a 600mH cartridge such as the old Ortofon VMS20E II moving magnet cartridge with Ortofon's suggested 400pf would be crazy. But if you don't load it like that you'll find this quite brilliant cartridge will hiss a spit like a rattle snake!
What is happening by unloading the capacitance is that the highs are increasing beyond 15kHz due to stylus tube resonance. By unloading the capacitance the electrical response peak is "cured" (more likely negated much more) but the response doesn't fall at the expected rate until say an octave above so the surface noise headroom of the preamp is compromised by 6dB ensuring that the noise will cause good wide-band preamps to clip on really loud passages. Furthermore, just like any generating system, without any load (expected in this frequency range) the cartridge output rises compounding the problem.
By rejecting common sense such as the IEC 6-1938 matching system standard (to which many European manufacturers comply), we again, through use of uneducated ears, run the risk of mob-enforcing tone controls
where we say "there are no tone controls".
Yes, it's amazing where a little treble boost leads... Without proper understanding we in hi-fi often end up trying to milk the bull!
And often it results in the same...