Higher Impedance MC Carts on Transimpedance Stages?

Hagtech, You wrote, "There is no real contribution from phonostage, since it appears as a dead short, we are talking transimpedance, right?"

I learned my audio electronics from a few good books and from reading what others say on the internet, always a risky way to learn anything. So I do not mean to be snotty; I am only trying to learn, and I realize your level of knowledge is far superior to mine, assuming you are THE Hagerman. I am not ashamed of being ignorant, but in fact don’t all "transimpedance" phono stages have some finite, albeit very low, input impedance? Else the signal from the cartridge would be lost to ground, as in a mute switch. I do realize that with an op amp, you can wire it to present a "virtual ground", but even in that case the signal has to get past the input. There are also a few current driven phono stages that do not use an op amp input, the BMC MCCI being one of those. I suppose you can derive a virtual ground using discrete transistors, as the MCCI must do. I think of current driven phono stages as devices that have an I/V converter at the input which drives a conventional voltage amplifier that includes the RIAA correction circuit downstream. All of that said, how can it be the case that the phono stage characteristics are not important? By the way, I hate the term "transimpedance". It leaves the false impression that such a device is unaffected by the internal resistance of the cartridge, which is not true.

In your equation, G = 5R/V, G would be in terms of ohms per volt, not ohms alone. You can’t just throw away units like that. An ohm/volt is the inverse of current, in amperes. Likewise, in your equation, f = R/6.3L, which I still don’t understand, f would be in ohms per Henry (or millihenries, or whatever henries). I cannot make sense of that. How do you get f in Hz from ohms per Henry?

The inductance of an LOMC is trivial, in the sense that it’s very low, on the order of 1000 times lower than MI which is already much lower than MM. Your acquaintance is probably happy because it works and he has yet to try an LOMC with truly low internal resistance. I feel that I must be misinterpreting Hagtech’d equation .

Hagtech, In your equation, I assume R is the internal resistance of the cartridge and L is the cartridge inductance. Since you speak of "bandwidth", "f" must be bandwidth. Further I assume R is in ohms and L is in henries. I must have something wrong, because if those assumptions are correct, then LOMCs with high internal R (>>10 ohms), like the Kiseki, would have much greater bandwidth than a typical LOMC with an internal R of ~10 or less. This happens because LOMCs have very low inductance, usually less than 50 microhenries, and therefore the denominator is always going to <<1.0. Further, I would think an equation to determine bandwidth resulting from a specific match between cartridge and phono would have to include parameters of both the cartridge and the phono stage.

For example, the Kiseki with an internal resistance of 40 ohms and an inductance of say 50 microhenries or .000050 Henries (cannot find the actual value on line) would have a bandwidth in excess of 100KHz. Whereas a more typical LOMC with an internal R of 4 ohms and similar inductance would have a ten-fold narrower bandwidth.  Even if such a cartridge has also a lower inductance, that would not help much; the predicted value for f would still be well below 100kHz.

Nothing blows up. The mating would be inefficient as the internal impedance of the cartridge exceeds the input impedance of the BMC, which we think is around 2-3 ohms. Thus some of the signal voltage derived in the Kiseki would be lost to ground.  But also, the Kiseki probably produces very little signal current; you can estimate that by dividing its output voltage by it internal impedance.  I don't know the output of a Kiseki but for example if it is 0.4mV then its signal current is around 0.4mV/40 ohms = ~10 micro-amperes.  For comparison, even my Ortofon MC2000 which produces only .05mV of signal voltage makes 25 micro-amperes of signal current because its internal impedance is only 2 ohms. Nevertheless, one might take advantage of the BMC's adjustable gain (0, +7, +11, or +14db) and set the gain high using the internal jumpers. My prediction is it won't be satisfying but worth trying.