atrocity wrote: > > I have found a claim out there that the mic input impedance of my device > (a Roland R-05) is 7,000 ohms. Phono cartridges generally want 47,000 > ohms. It's nice to know that, but what I don't know is what the > real-world ramifications of that mismatch are. Too many highs? Not > enough highs? Something wildly non-linear that would be extremely > difficult to fix via digital EQ? >
I think you can expect a roll off that is shifted, possibly significantly, towards the lows, because of the additional damping of 7,000 ohms versus 47,000 ohms. That is a guess based on the calculations below. I looked up your cartridge. It is, apparently, of the moving magnet type. https://www.audio-technica.com/cms/cartridges/2cafc1c694070c54/index.html Amongst other things is quoted: > > Coil impedance : 3,200 ohms at 1 kHz > Coil inductance (mH, 1 kHz) : 490 > This implies a coil resistance of 872 ohms, by my calculations. If we make the thoroughly unreasonable assumption that the mechanical parts plus the magnet and coil act as a simple voltage generator (they won't), and that the cable plus mic input present no significant capacitative load, we can model the arrangement as a voltage generator (Vo) driving the coil (complex impedance Zc, with resistance Rc and inductance Lc) in series with the resistive input load (Ri). The load will therefore see a voltage of Vo x Ri / (|Ri + Zc|). At DC, that will be Vo X Ri / (Ri + Rc). The 3dB (half power) point of the roll off happens when Frequency = (Ri + Rc) / (2 x pi x Lc). Numbers: > > Ri = 47,000 ohms, Rc = 872 ohms, Lc = 0.49H. > Frequency when 3dB down = (47,000 + 872) / (2 x 3.14 x 0.49) = 15,557 > Hz. > DC voltage = Vo x 47,000 / (47,000 + 872) = Vo x 0.98 > > Ri = 7,000 ohms, Rc = 872 ohms, Lc = 0.49H. > Frequency when 3dB down = (7,000 + 872) / (2 x 3.14 x 0.49) = 2,558 Hz. > DC voltage = Vo x 7,000 / (7,000 + 872) = Vo x 0.89 > That's 0.8dB below the 47k case. > I hope I got all of that right... In practice, I would expect that mechanicals will also play a significant part, although I have no idea what the impact would be, as I have no experience of the matter. It is possible to create a model in which mass, springiness, and friction are combined with the electrical components to give additional elements to the "electrical" network, with inductance, capacitance, and resistance replacing the mass, springs, and friction of the mechanicals. The above numbers would change, perhaps substantially. An experiment: (A) Record something directly into your mic. (B) Record it again, through a 40k resistor in series with the input (thereby creating a 47k load). Get the levels sorted, and see if you can form any conclusion about the respective frequency responses. I wonder if, in practice, they would bear any resemblance to my unreasonable calculations. atrocity wrote: > > Making it even more interesting, I've read (but have no idea if it's > true) that impedance/capacitance can actually have a -mechanical- effect > on the cartridge/stylus. > Yes. It is a composite system. The cartridge/stylus is generating electricity which is then passed through the load. If you think of the load as imposing friction (through its resistance) and springiness (through its capacitance), and even having mass (through inductance), you might expect this to impact the cartridge/stylus just as if there were mechanical friction, springs, and masses in play. It gets quite complicated to model these things, though. I believe that motor car manufacturers used to use analogue computers operating under these principles to model car suspensions and the like. ------------------------------------------------------------------------ mrw's Profile: http://forums.slimdevices.com/member.php?userid=38299 View this thread: http://forums.slimdevices.com/showthread.php?t=110295 _______________________________________________ ripping mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/ripping
