Optimizing MC cartridge step-up and loading in MM phono stages
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Abstract
A phono cartridge is nothing more than an "energy converter" - your speakers are another such converter, only doing the opposite – transforming the kinetic energy emanating from the diamond "reading" the groove, into minute voltages. These voltages, being as low as 0.1 mV for certain MC cartridges, must then "meet" their first – and most crucial – amplifying device, usually called the "phono stage". The latter has an extremely serious job to accomplish, as it should not only amplify the fragile signal (10.000 times for an output of 1V, if we consider the previous example...), but also preserve its integrity: That is, that a good phono stage should output a signal as clean as possible i.e. devoid of gross distortions, phase shifts and noise. To reach this goal, the design as well as the topology and build quality of the amplifying stage, are of paramount importance. With one caveat, though: The relationship between MC cartridge and phono stage input, can rarely be seen as "plug and play". The scope of this article is to examine this relationship, concentrating onto its most difficult iteration i.e. when MC cartridges attempt to "drive" tubed phono amplifiers.
1. About cartridges and amplifying devices
There are two common types of cartridges:
Moving Magnets (MM), which have typical outputs between 3-5 mV RMS. These require a standard loading of 47k Ohm, which is also the typical input resistance of MM phono stages. There is also some capacitance (pF's) parallel to this input, although this will not exactly cater to all MM's. In vinyl's heyday, most Hi-Fi buffs used to play with different capacitance values in order to optimize things to a maximum (there where even specially designed RCA plugs, for that). Today, nobody seems to care with optimum MM loading except, maybe, some GRADO owners...
Moving Coils (MC), which have typical outputs between 0.1-1 mV RMS. MC's are the current (and past...) fave of most Hi-enders. Only problem, here, is that their loading is not at all standard. And, to complicate things a bit more, it can very well be system-dependent! Explanation: MC's are usually not very good trackers and have a tendency to "ring", especially when their loading is totally open (i.e. 47k). Still, some MC's in some systems... like to see 47k-go figure! Theory says that a MC cartridge should not be loaded by a value which is less than ten times its internal impedance. Manufacturers even give loading figures which sometimes are precise (say, 100 Ohm) and sometimes of a more general nature (say, more than 45 Ohm). Then, you have MC's which are quite tolerant of loading and others which require a very precise "dial in" to sound their best. And all this, as I already said, could be system dependent, as the special relationship of your cartridge with your tone arm, also enters the equation. Not for the faint hearted, you will concede...
Let's now come to the relevant amplifying devices, a.k.a. phono stages:
Again, there are two main types of these-notably solid state and tube. Always thinking about MC loading, let me also "invent" two subcategories: The "easy" and the "difficult" ones. There could also be a third category called "impossibles" (like the LINN Linto...), but let's leave this out for the time being.
For a phono stage to be "easy" or "difficult" it all depends on the designer's ideas and priorities, concerning the final result. As I said earlier, we are talking here about minute voltages and these voltages do not really like going through multiple switches and the like. Noise and other nasties could be created, although these problems should be (and are...) less apparent in ss than in tubed designs. Still there are "easy" designs of both persuasions like, for example, the DACT or the MANLEY Steelhead. Both designs will permit dialing-in the correct values without too much fuss (the DACT), or with no fuss at all (the Steelhead). Needless to say that the latest AUDIO RESEARCH PH-5 is already in the "supereasy" category, as it permits to change the loading through... its remote control. (It is much less versatile than the other two, though...)
The purist approach, on the contrary, which is strongly recommended in case of an all-tube system driving ultra sensitive loudspeakers, calls for the "difficult" kind of gear (like, for example, the well known AUDIO RESEARCH PH-3) which requires to find the correct resistor value and – then – solder the item in place. The much more recent CONRAD-JOHNSON 15, on the other hand, has an internal dip-switch array for loading but – nevertheless – the manufacturer recommends that a proper resistor should be substituted, once the correct setting has been found. Ouch! And, please keep in mind that the PH-3, good as it was, had not enough gain for the so called "low output" MC's. And neither is the more modern CJ up to the task... Subsequently, to accommodate cartridges like the DENON 103 or the LINN Archiv etc the AR badly needed step-up services, either active (pre-pre of some kind), or passive (transformers). The PH-3 might be a ten year old classic but, believe me, the situation with tubed phono stages, has not changed much to this day!
2. Tubed phono stages and the Holy Grail!
Now, both the PH-3 and 5 are hybrid designs sporting jfet input circuitry combined with four 6922 tubes, for a total gain of 54 and 57 dB respectively. Exhibiting a different philosophy, an all tube design like the Conrad-Johnson 15 uses a total of 9 tubes for a gain of 53 dB, whereas simpler 3 tube designs never surpass the 43-44 dB barrier.
I have had on hands experience with both AR and CJ preamps and I can attest that even using cartridges around the 0.5 mV output mark, could result in rather noisy operation and this even with moderately sensitive speakers. But, things being what they are in the MC cartridge arena, higher output is again a trade off against the subtlety and delicacy these devices are normally capable of. Like you and me, cartridge manufacturers know it all too well and that's the reason why they tend to offer both low and high output versions of the same cartridge.
What to do, then?
Well, two things really:
1) Accept noise as a fact of life and pretend that – if you like tubes – this goes with the territory. And please don't laugh, as I read too many times this kind of apology in the specialized press...
2) Do what you have to do to remedy this unhealthy situation, i.e. use a correctly set-up step-up device.
3. Tubes & Transformers: It's only NATURAL!
Natural? Of, course... Because, contrary to an active pre pre, a step-up transformer (SUT) is a passive component doing its job with a minimum of alteration of the very weak and fragile signal, attacking it. And, as a bonus, the cost of two excellent quality SUT's is way lower than the cost of a – hard to find these days – active stage of respective quality. The reason for this is that, for technical reasons, active pre pre's can only be ss (There has been, to my knowledge at least, only one exception to this state of affairs: The short lived CJ Premier 6 nuvistor pre pre of the eighties). Given the fact that ss phono stages can easily be made complete (MM/MC), the market for ss pre pre's automatically "shrinks" to meet the demand from tubeophiles which, in turn, is very small, as the latter prefer transformers. So, transformers it will be!
Still, many people who have tried transformers – especially in their commercial "transformer closed in box" outboard iteration – had rather mixed feelings about them. Understandably so, I might add...
Let me explain: Like everything else, to make a transformer work as promised, one should spend some time and effort to properly understand the intricacies, as well as the working principles of the device. That said, simply inserting an outboard commercial product with one input and one output between your cartridge and MM phono stage, has all the prerequisites in the world for a sonic disaster. But I digress, so let's take things from the beginning:
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