I recommend that you use the moving magnet phono cartridge like it was intended to be used: 47K (more or less) and 100-500pf load capacitance. Not ALL MM's need both conditions, but most do.
Trying to load with Zero ohms just throws everything off, and how are you going to get back on a flat frequency response course without extensive measurement and added equalization. When they make many MM phono cartridges, they adjust the internal damping, etc to match the standard conditions, and unless you are just an experimenter, you should use it this way.
MC is different, BUT a Zero ohms input is but one solution and not always the best. Take it from me, I have tried to use it exclusively and failed to be 100% successful, sonically. Current through a cartridge? Are you crazy?
Trying to load with Zero ohms just throws everything off, and how are you going to get back on a flat frequency response course without extensive measurement and added equalization. When they make many MM phono cartridges, they adjust the internal damping, etc to match the standard conditions, and unless you are just an experimenter, you should use it this way.
MC is different, BUT a Zero ohms input is but one solution and not always the best. Take it from me, I have tried to use it exclusively and failed to be 100% successful, sonically. Current through a cartridge? Are you crazy?
Depends on the mechanical resonances being compensated by the electrical, and how close one gets to the manufacturer's recommendations---but I agree the options on most preamps are very limited. One can get lucky. See van de Gevel's Linear Audio* article showing the effect of loading on cartridge frequency response for examples of the magnitudes involved. The changes are not small.
It would help if more of the MM/MI makers provided inductance data. Resistance is easy to measure, but few have L meters.
Brad
*from whom I receive no promotional consideration, despite plugging a lot lately.
Moi?
I have nothing against the resistively (over)damped approach, just concerned about the need for knowing the inductance, which as mentioned could be determined automatically, but would be "out of the way" in normal operation. Bob's issues with noise could be addressed with a synthetic "cooled termination" for the damping resistance, once the desired value was fixed.
Ideally, I would have a comprehensive database of cartridges and allow the customer to secure a loading impedance at least in accordance with best frequency response. The cable and tonearm capacitance would be measured and factored in. Maybe the inductance would be measured directly as well. The loading resistance would be the synthetic "cooled termination" (using the preamp's own gain for best results---see van de Gevel again for a discussion and his particular implementation).
I'm breadboarding some of this as we speak, in sand state, with a few details to be kept quiet in case someone might like to support the development and would wish said details to remain proprietary for a while. The noise benefits are primarily with MM/MI, owing to the high inductances and sensitivity to termination noise and other parallel (i.e. "current") noise. But the heart of the system would be very good for MC, while the synth term advantages will be negligible.
I had a student intern look at this 30 years ago. The ratio on a moving coil transducer was so big that the damping argument could never work. it was something like 1000:1 as in 1 mW of energy in was 1 uW of energy out.
You could approximate it from the compliance number plus deflection to get nominal output.
Also, if you model a cartridge as a voltage source in series with the internal magnetics it should be clear that as you lower the impedance the magnetics will interact with the current. That will bring the magnetic nonlinearity (eddy current losses, hysteresis etc.) into play making the output intrinsically less linear. Claims of lower distortion really need examination. The distortion numbers I remember were always pretty high. Possibly the groove has significant distortion.
I think it would be possible to test some of this using a cartridge in series with a voltage source into different types of preamps and see what changes. This would assume perfect transduction from the stylus to the field changing in the cartridge. If damping were possible you would possibly see and hear the stylus move when the signal is applied (reciprocity) and you should see the shorted output change if the stylus is not free to move (sitting on a disk vs. on a tonearm rest).
Cartridge demagnetizers work by sending a current of some magnitude through the cartridge. Then reduce the field which is supposed to demagnetize the cartridge (actually the magnetic bits with the coil of wire around them, not the magnet). In fact that is absolutely how you would magnetize those parts, immerse them in a strong dc field and raise the internal ac field enough then remove it and the part will hold the DC field. The same way magnetic tape works. All this is to point out that people have been abusing cartridges with currents for years and claiming they sound better afterwards.
hear, hear !
it would be nice if reviewers published their hearing test results.
mlloyd1
it would be nice if reviewers published their hearing test results.
mlloyd1
With transimpedance MM loading, one finds that the ratio of L/R determines the required eq correction. Many MM carts have this ratio about the same mark, so it's typically possible to trim with a simple small series R if one designs for a single ratio of L/R.
Most MM carts emperically obey a law 2L>R where R is in ohms, L is in mH. One can design for 2L=R, and trim R with small external series resistance for total R until that condition is met.
If the law isn't obeyed, ie L/R is out of range, eg HOMC or some MI carts, some rework of the correction network is necessary. But then these don't suffer the vagueries of C loading and audioband LCR resonance in any event.
As you say Bill, perhaps not for the great unwashed. But as a one off or DIY, a topology can be devised to be readily configurable to accommodate most MM carts. And it's nothing if not different !
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As bill posted, you have to be a bit crazy to post on this thread.......
You didn't provide any reason why not, John. As replied to bcarso, I accept it is iconoclastic, and contrary to common lore, but that hardly seems a sound basis not to consider it on merit. It stands scrutiny - really. You'd have to explain, at least in outline, technically why not ?
Other than Nelson's observation about requirement for good connections to avoid noise, no-one has yet posted a tangible downside.............
Perhaps it's sort of surprising that dc seems indicatively well tolerated for MM IME. But this genuinely presents fresh preamp topology opportunities.
Crazy, moi ? Don't think so, I'm just not afraid to scrutinise established wisdom.
I've been measuring a bunch of players lately wit very different cartridge/tonearm combinations seeing very little variation in THD*. My strong impression (no proof yet) is that the actual record is the limiting factor for THD, not the cartridge etc
*Except the laser player which had clearly lower distortion on this particular test record, though still not much better than -50dB.
Jan
Well, it will hold a part of it, depending on the total applied field at the moment of external applied field collapse and the remnance of the material. But that will neither demagnetise nor magnetise the permanent magnet, beyond any tiny extent that happens for first time at dc switch off. ie it's not progressive, and IME there is no alteration to MM cart performance over multiple cycles and in use. In this case, the ac component is small and irrelevant.
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May I add that it doesn't need to a be gross problem, at those voltages and gain factors a relatively small off may generate a very audible differnce in the end.
I was taught this way back in 1975 or so, by an older and much more experienced audio buff. He pointed out that we (he and I) were smokers, and that alone will produce settled layers acorss the cartridge connection pins. He suggested that every 6 months or so I disconnecte the cartridge wiring and wash the pins with medicinal alcohol, I picked up the habit and kepp it to this day.
Non-smokers can get away with once every year or so.
Later on, I expnaded trhe cleanup to each ane every connector I have, RCA, 6.3 mm headphones jacks and even binding posts. I am always amazed at the black dirt and grime I take off.
In case this helps for reference, here's a plot of harmonic distortion versus frequency for an OM40 into one of my transimpedance preamp prototypes. The floor is around -45 to -50dB. Recorded level, location on the disc, and 33/45 rpm recording makes a huge test difference, of course. This was at 1.2cm/s at 33rpm sweep outer to inner hf at outer. This example is hard to beat, IME.
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Indeed hard to beat, better than anything I saw except for the laser player.
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