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Phono Cartridge Loading For Moving Magnet?

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    Posted: 10 Oct 2010 at 4:13am
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... Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 2010 at 6:00am

I am glad you have cleared that up for me   Confused  Wacko

I knew there was a good reason to use MCs - I can work them out (mostly) Wink

Seriously though, more good info - thank you.

Perhaps a little harder for the average bodger to readily utilise.

BTW, WRT the VMS20, does this go any way to explaining the difference you have noted elsewhere between the performance on the Rega vis a vis the Technics ? - thinking here of the possibility of differing tonearm cabling capacitance contributing.




Edited by tg - 10 Oct 2010 at 6:03am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2010 at 12:56am
Originally posted by tg tg wrote:

...BTW, WRT the VMS20, does this go any way to explaining the difference you have noted elsewhere between the performance on the Rega vis a vis the Technics ? - thinking here of the possibility of differing tonearm cabling capacitance contributing.


Possibly but...

However, the VMS20EII was bought to replace a high output moving coil of which I cannot remember the name (that was 1979 when the Planar 3 was bought). The VMS20EII wrought an improvement, taking away the bass lightness and forceful treble to a degree. In 1979 aged 24 and back living with parents I was unable to provide the Rega with the hefty shelf the Hi-Fi press were suggesting. It was not until around 12 years later I was able to furnish it with such a shelf. On that "long journey" the RP3/VMS20EII saw numerous phono inputs most of them self-builds (but not all) of one design or another. Eventually I bought the Roksan Tabriz arm, and the removal of the Acos Lustre meant inflicting fatal damage to it because it was never intended to be removable (this gave me the impression that the RP3 was a toy...), so I was never able to test it further. The Roksan arm wrought a further improvement. So by that time I had the RP3/Roksan Tabriz/VMS20EII/Hefty Shelf. I splashed out on some tiny JPW speakers after tiring of my Goodmans DIY bass reflex "monotones" in a bid to get greater three dimensionality. I'd purposely bought the most bassy sounding "book-shelfs" then available, but still tonally, the results were only just balanced. What I did achieve was getting Hendrix walking around the listening room as my pal (and now co-director) John C will vouch for.

A bit of a lengthy reply, but I'm getting to the point. It was then I stumbled upon the Audio Technica AT95E (moving magnet again) which proved to be the icing on the cake, and much better tonally balanced than the VMS20EII. Having since measured the Roksan arm lead capacitance, and also since, the Technics arm lead, I found them both in the 100pf region, so I assume the original Rega arm was in the same ball park. Most of my phono inputs were 100pf or 220pf, so the total capacitive load would have been on the light side for the VMS20EII anyway. And that tends to suggest I was not giving the VMS20EII the optimum loading conditions, and I'm still not. At roughly 300pf total cap load the VMS20EII fairs better than 200pf. On the Technics there is no hint of 8-10kHz peaking that the simulation suggests, which tends to bear out the argument that stylus tube tuning mirrors the electrical output. I shall have to find a couple of wide bodied phono plugs and make a 100pf load plug set and for the first time hear the results of 400pf loading.

I shall report back on this.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2010 at 5:39am

These plugs would have the capacitor across the plug as for a load resistor ?

Perhaps a nice Panasonic 1206 film cap would fit nicely ?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2010 at 11:12am
Originally posted by tg tg wrote:

These plugs would have the capacitor across the plug as for a load resistor ?

Perhaps a nice Panasonic 1206 film cap would fit nicely ?




A web search suggests these are surface mount?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 2010 at 12:46pm

Yes, they are - 3mm x 1.6 or thereabouts - should fit in almost any plug.

Don't sneeze while they are loose on the bench though.  Ouch

Chosen by a consensus of DIYers on SQ grounds as bypass caps for a TDA1545 DAC chip.

Bit more pricey than ROTM caps but considered worth it. (in that application at least)

I suppose any kind of film could be OK - it was the size issue I was considering.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Oct 2010 at 12:05am
Well tg, good idea but I have to wear two sets of specs (spekky six eyes...Ouch) as it is...Wink

My choice for the load plug set (something I've been trying to put together now for the last 12 months) includes the Switchcraft phono plugs (picture on its way soon), and I'd thought my favourite PFR's wouldn't fit, but lo and behold, tried it today, and they fit a treat, and I only had to wear one set of "bottle-bottom" spectacles!

I fitted a 100pf and put it in parallel with the input to a Reflex M which now has 220pf loading, using a Tee adapter (per channel) and it made a difference. The total loading is now near the recommended 400pf for the old VMS20E II with arm cable capacitance factored in.

I'd previously been increasing the tracking weight to try and rid it of the slight bit of spit it had operating into roughly 300pf, but currently the additional 100pf has enabled me to set it to 1 gram (0.8-1.2g specified) - quite amazing, especially considering this is a bog standard Technics arm! Big%20smile

So here we have a case of increasing capacitance to rid a treble peak (instead of what you'd expect). The hunch paid off with this cartridge.

The clarity is staggering and the tonal balance quite fantastic. It's smooth too. Not bad at all for a cartridge of its age - and an MM at that!



Edited by Graham Slee - 13 Oct 2010 at 12:07am
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