doctorcilantro Posted July 15, 2009 Share Posted July 15, 2009 Hi all, I'm trying to calculate, based on the following information, the load on my cart in my setup. I have wired my tonearm in a balanced configuration as described on PureVinyl site, AudioKarma, etc. and I utilize the following. Linear Gain AEA TRP ribbon pre: 36K input impedance Grado Reference Statement (.5mv): Inductance: 2mH Resistance: 2 Input Load: 47K Here is an archived AA post in which the poster was using the higher output cart. I'm not sure about the capactance of my cable; it is very short maybe 1ft and, as mentioned, terminates with XLR (3rd pin no ground or connection to shield - although I have tried that). I get about -70db noise floor with about 70-75db of linear gain applied - this is prior of course to RIAA EQ which is done via a VST plugin in Samplitude. The AEA TRP ribbon pre is listed at 18K input impedance but I have verified it is actually 36K from the manufacturer. I have had the system offline for some time due to a move, but it sound really good before; right now I am just curious as to the resulting electrical specs.. I'm also planning to look into how the Grado low-output moving iron specs differ from those of a true MC because lately I have been reading about (here) an argument about using MC carts at 47K which would open up the possibility of trying some other carts with this mic preamp. "GRADO Inductance: 45mH Resistance: 475 Input Load: 47K I cannot reproduce here the VISUAL results of the program used to render the following calculation, but here are the results. They will, as have other real world tests, throw into question the rationale that drives manufacturers to provide a standard 47k load for MM cartridges, as well as the recommendation that users so load ALL MM cartridges (The Shure V15VxMR, for example, would not behave anything like we see here; their engineers seem to have designed it to work into a 47k load). Here are the results and something I think you might find worthwhile and relatively inexpensive to try out: GRADO (45mH / 475d) With 200pF cable & stray capacitance and 47K load resistor, output starts to rise at ~10KHz to +1dB at 20KHz and a +9dB peak at 50KHz; With 300pF cable & stray capacitance and 47K load resistor it gets worse. Output now starts to rise at ~10KHz to +2dB at 20KHz and a +10dB peak at 42KHz; With 200pF cable & stray capacitance and 10K load resistor things look much better. Output is pretty flat to 20KHz and rolls off to –3dB at ~45KHz; With 300pF cable & stray capacitance and 10K load resistor things look better yet. Output is ruler flat to 20KHz and rolls off to –3dB at ~46KHz. In this scenario, the added capacitance actually helps. Whether or not your ears will hear the peak (it is ultrasonic & outside the audible range), it is probably not good anyway. You WILL hear a difference, and I think one which you will find favorable, at least based on what information we presently have about your system and what you are now hearing as "wrong." If you place a 13K resistor across the phono input plug, it will be in parallel with the existing 47K which is then 10K. It will let you experiment without modifying the preamp." from http://db.audioasylum.com/cgi/m.mpl?forum=vinyl&n=196869&highlight=grado+impedance+47k&r=&session= Quote Link to comment Share on other sites More sharing options...
LarryC Posted July 16, 2009 Share Posted July 16, 2009 DC: I'm trying to figure out if I have a worthwhile comment. Mdeneen might have something to add. How do you get your RIAA equalization with a microphone preamp? I'm under the impression that low-impedance carts that specify 47k (MM's, some MC's) are designed to sound best with that. Impedance loading with the preamps I've had is done with a second pair of phono jacks wired in parallel with the ones labeled "phono." (Actually, with that arrangement, you could plug the phono into the "loading" jacks and it wouldn't make any diff.) Anyway, since they are in parallel, the total resistance is lower than the smallest resistance. I use a pair of 100 ohm plugs in my loading jacks, so the total resistance is minutely less than 100 ohms. I, like most LOMC owners, do trial and error to see what SOUNDS best. I myself would never choose based on supposed measurements. No doubt the kind of tweaking you're doing could sound better, but I suggest making your ears the final judge. I have no idea how a low-impedance MM fares in all that. Based on not much experience with a variety of cartridges, I'm an LOMC guy all the way. It would be interesting to know how the low-output Grado compares. Is capacitance more important with that kind of cart than with LOMC's? Sorry I can't be of more help. Quote Link to comment Share on other sites More sharing options...
doctorcilantro Posted July 16, 2009 Author Share Posted July 16, 2009 Thanks Larry. I do want to try som LOMC if it's even theoretically possibly they will sound good. Luckily, I do have a group of audiophiles in Tampa (Tampa BayListening Society) I could ping for some carts to try. The way RIAA is implmented is in real-time with n EQ implemeneted in a VST plugin named IIEQ: IIEQ is a 10 band parametric track EQ especially designed for an optimal balance of CPU usage and sonic control. Main features: Minimum phase IIR EQ. Low shelf with gain and and cutoff frequency, high shelf with gain andcutoff frequency plus 8 peaking filters with gain, center frequenciesand Q. The gain can be ± 20 dB. Overall output level control with VU meter to avoid overshoots. The person who implemented it did the math and compared it to the results Robert Orban is getting with PureVinyl. I will post the specs but I have an improved version which I run at 96kHz. The 44kHz version pretty good: .05db from 20hz-18kHz, however it has -.2db to -1.3db from 18-20kHz. DC Quote Link to comment Share on other sites More sharing options...
LarryC Posted July 16, 2009 Share Posted July 16, 2009 OK, I can only suggest giving an MC a try. Some sound better than others, though -- I, Gary, Travis, Nu2, MarkD and others I'm probably neglecting might be happy to offer comments on the cart(s) you can try. I dunno about trying to get an RIAA curve out of a 10-band equalizer. I'd expect it to sound a little ragged to me. Maybe your local audio society can scare up a phono stage with enough gain to loan you at the same time, as it might sound smoother. I have never heard a LOMM. I go by intuition a lot (as you can tell), and have always imagined having the coils moved directly by the stylus replicates a more direct signal than picking it up indirectly from a moving magnetic flux. But that may not be true, of course. However, I DO know that high-output, or even "medium output" (like 1.0 or 1.2 mv) MCs can start sounding opaque to my ear. That's why I stick with LOMC's. I did like a MM Ortofon I heard, an OM-40, I think. Wasn't that what you had, Marty? Quote Link to comment Share on other sites More sharing options...
doctorcilantro Posted July 16, 2009 Author Share Posted July 16, 2009 I think the MAX implementation may be better but I have used the VST plugin. I need to get in touch with the designer and ask him directly about the VST vs. MAX. Here's some info on the MAX version: "As seen in picture, I have multiplied the gain coefficients by 10. This gains the filter output signal over +30dB. Harmonic distortion stays still under -100dB but, if you feel it's gained too much then, just change those three first values on each RIAA filter. By dividing values by 10 you'll get the original level (~0dB @ 1kHz (as seen in the background image of tis filter software)). Do not touch into those two last filter coefficients !! Quote: RIAA Filter software specs: 44.1kHz Maximum error from 0.00 Hz to 20000.00 Hz = ~0.224dB Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 30 degrees where the "phase error" is computed after a constant delay is added or subtracted to make the phase error equiripple 48kHz Max error from 0.00 Hz to 20000.00 Hz = ~0.14dB Max phase error from 0.00 Hz to 20000.00 Hz = ~+/- 24 degrees 88.2kHz Maximum error from 0.00 Hz to 20000.00 Hz = ~0.0082dB Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 3 degrees 96kHz Maximum error from 0.00 Hz to 20000.00 Hz = ~0.0057dB Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 2.4 degrees" http://img135.imageshack.us/img135/9095/riaafilterscemazi2.jpg DC Quote Link to comment Share on other sites More sharing options...
thebes Posted July 17, 2009 Share Posted July 17, 2009 You are right Larry, that was an Ortofon OM 40. You've also heard a variation on it with the OM 30 sytlus installed. The OM 30 is probably the better all -around cart, but the OM 40 (it's really just a needle change) is more refined, better overall soundstage and more detail. From my own recent and ongoing experience, dialing in a MC is hard work and hopefully when I'm done, my ZU/Denon103 will beat out the Ortofon. So far the Zu is losing. Quote Link to comment Share on other sites More sharing options...
doctorcilantro Posted July 23, 2009 Author Share Posted July 23, 2009 Good news. There are two (one unpublished) VST plug-ins for digital RIAA and the math is excellent. http://www.kvraudio.com/get/3550.html RIAA Filter software specs (of unpublished - but he says the math of the one above is comparable ):44.1kHzMaximum error from 0.00 Hz to 20000.00 Hz = ~0.224dBMaximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 30 degreeswhere the "phase error" is computed after a constant delay is added or subtracted to make the phase error equiripple48kHzMax error from 0.00 Hz to 20000.00 Hz = ~0.14dBMax phase error from 0.00 Hz to 20000.00 Hz = ~+/- 24 degrees88.2kHzMaximum error from 0.00 Hz to 20000.00 Hz = ~0.0082dBMaximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 3 degrees96kHzMaximum error from 0.00 Hz to 20000.00 Hz = ~0.0057dBMaximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 2.4 degrees Quote Link to comment Share on other sites More sharing options...
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