Raul, I did not realize your new phono pre was a "current" device. I think that is wonderful. I did see another one recently, and if I can find it again ;~)) I will send you the link.
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Nil, the 25X rule (that I have been endlessly promoting and which you'll find discussed in depth on other threads) is, I keep reminding people, a theoretical optimum, based on the more or less universal objective of good impedance matching.
However, based on the other elements of a given system, I always urge people to explore loading by ear using a range +/_ 50%. In your case this would be from roughly 70 - 200 ohms.
Everything else about your cartridge setup should be right beforehand (VTF, SRA, AS, overhang, azimuth, etc). Start at or near the optimum (125 -150) If you listen and it's bass-shy, reduce the load. If the bass is muddy or bloated, increase the load until the bass tightens up. If the load gets too high, you'll start to lose the bass, so back it off a little. |
Hi Jeff, re: impedance matching of cartridges:
A device which is generating a voltage into a load (could be a source like a tuner or CDP into a preamp, or a preamp into an amp, or a cartridge into a phono preamp) has to "see", or work into, enough of a load (resistance or "impedance") so there will be voltage registered at the other end. Or to flip it over, if the other end were just a short circuit (no load) no voltage would register and there would be nothing to amplify.
Depending on the difference between the ouput device's inherent impedance and the input (load) impedance of the driven device, there is an optimum combination that will deliver the best signal transfer, i.e. the most voltage and the most faithful (to the source) frequency response.
You hear more about "impedance matching" between preamps and amps, when someone experiences problems like not enough gain out of their amp, or poor frequency response (usually loss of highs) when their impedances are not optimally matched. Another instance where you often run into this issue is amp-to-speaker impedance matching, which is the reason for the use of auto-formers like the ones Paul Speltz markets.
The cartridge-to-phono preamp interface is no exception. Although to be absolutely fair, I should point out that cartridges are really current producing devices, but there are very few current amplifying phono preamps, so we always speak of a cartridge's output in terms of (micro) volts.
If one takes the time (and it is a bit tedious I admit) to experiment with a variety of load values, it's actually quite easy after awhile to hear when you're getting the flattest response and strongest output from a given cartridge.
The "25-times cartridge impedance (or coil resistance)" convention is a bit like setting an initial 1.5 degree Stylus Rake Angle. It's just a quick way of getting yourself firmly "in the ballpark" so that you don't have to go too far one way or the other in order to find just the right spot. |
Jeff: Well, yes, you are endeavoring to impedance-match the cartridge to the phono preamp. The 25x value may turn out to be right on the nose (after experimenting a little up or down and making listening comparisons, taking notes -- always take notes!)
But if it isn't right on the nose, the 25x value will still be pretty close, thus saving you a lot of time.
If you're using a MC cartridge with a typical output and coil resistance of +/_ 10 ohms (optimum loading around +/_ 250 ohms) you could also follow another approach which would take about the same amount of time. And that would be to start at the low end of value range (i.e. optimum less 50%) and work your way up in increments of say 25 ohms. In this example, 125, 150, 175, 200, etc.
I've done it this way too, and it may actually work better (in terms of training one's hearing) for someone who has never done it. You are guaranteed to start with sloppy bass which slowly tightens up and (if you keep going) eventually gets very thin, or disappears. That's when you back down 25 or 50 ohms and can be confident you've found the best match. |
Nil, the 25x rule emerged from experience and experiment and also compiling data from those (few) cartridge manufacturers who actually publish optimum load values for their products (like van den Hul).
Anyway after compiling a lot of data (on normal output MC cartridges that is) and comparing it with various internal resistance figures, the multiple always seemed to come out at or near 25. To save time, since most cartridge maker's load specs are so vague, it seemed like starting with a value around 25X the internal resistance would put you at or very near the goal; instead of stabbing all over the place from 5 to 47K ohms! |
Raul is correct, though I think I already covered all those same points.
It is important to remember however, that installing load resistors of the maufacturer's specified value will most likely not result in your cartridge actually "seeing" that exact load impedance, because there are other factors involved. It will be close, but could be off by as much as +/_ 30 ohms, which is enough to alter the desired flat frequency response. I'm sure Raul knows this, he just forgot to mention it.
My point is, once you get it close, then use your ears to make the final adjustment. (Now I am sounding like Raul!!) |
Jeff, there are three kinds of mechanical damping I can think of, are you including all of them in your wonderment?
1.) Cart. suspension
2.) Tonearm effective mass (static inertia)
3.) Possible silicone trough damping or magnetic damping. |
Jeff, re: mechanical damping affecting total load impedance, I suppose it does contribute by a small factor (you are talking about a reactance-type current in the coils damping the cantilever movement, right?)
Anyway, the effect on the freq. response curve would be small I should think, compared to the deviation from flat that would be caused by a load change of even 20 or 30 ohms.
It's one of those factors like VTA, where a less than optimum configuration will only yield 4th (or was it 8th) order distortions which are essentially inaudible. Whereas even a slight mis-adjustment of SRA is quite audible. It's sort of like that I think. |
Larryi, negative SRA will compensate for the HF tilt at higher loading values, but it won't really restore the bass balance, only roll off the highs a bit. Also you pay a price in record wear because the sharp edges of the backward-tilting (micro-ridge type) stylus are scraping across the tops of the forward-slanting groove undulations, instead of locking into them, as they will at a nominal SRA of 1.5 degrees.
Check out this thread for a very interesting discussion on the subject (complete with photos and illustrations ;-)
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1140840022&openmine&Nsgarch&4&5&st0 |
