Tube Preamp Paired with Tube Phono Stage?

Odd that Boulder would suggest 100 ohms generically for all moving coils.

The load impedance of a device should be at least 10X the source impedance of the device driving it to minimize insertion loss. Even at 10:1 you have a 1/10th loss of voltage IIRC. So a 0.3 mV, 10 ohm output impedance cartridge would lose 0.03 mV at the front of the phono stage with 100 ohm input impedance.

Now if you plugged a mid output Benz with a 24 ohm output impedance and 0.8 mV output into the 100 ohm phono stage I think you'd have around 0.5 mV insertion loss (if I'm remembering the math correctly), which is starting to get considerable, now it's like you have a .3 mV cart -- so you crank the gain to make up for the signal loss and you wind up with a higher noise to signal ratio than before. That's why Benz recommends greater than 200 ohms loading -- 250 ohms, 470 ohms, 1000 ohms, 47kohms even. 200 ohms is the minimum recommendation; not the preferred loading.

I know people like to load down their phono cartridges as a way of shaping the tone. I never quite got that. I'd rather just get flat frequency response and as little voltage loss as possible with as few resistors inline to add thermal noise. Greater than 10X source impedance is the starting point for me. There shouldn't be a magic number at which to load one kind of cartridge or another. It really depends on the cartridge's specs.

With MM and high output moving coils, there are other considerations. The higher inductance of those carts means you have to be careful about the capacitance loading or you'll change the frequency response of the system, but that has to do with the RLC circuit formed by the cart's inductance, the capacitance in line, and the phono stage's input impedance. That's a whole different calculation. But the impedance loading still should be at least 10X the source impedance. The source impedance is just typically much higher so the load impedance has to be much higher. You wouldn't want to load your 660 ohm output impedance Clearaudio Virtuoso at 1000 ohms.

Al, thanks for the clarifications. I just installed an AT33EV at home and noted that the specs supplied in the manual were pretty thorough on those scores -- 10 ohms impedance at 1 khz; 22 uH inductance at 1khz -- but I gather that's not always the case.

I'm trying to learn the tech of cartridge loading so it's less trial and error. I get impedance matching and insertion loss. I get how you have to manage the resonant frequency of the RLC circuit with higher inductance pickups by being careful about capacitance loading (that resonant frequency never seems to get close to the audio range with low inductance pickups and any amount of real world capacitance you run into). I get the need to watch out for the cutoff frequency of low pass filters formed by output impedances and cable capacitances, though again that seems to be a rare real world problem. I'm not sure what other tech stuff comes into play. I guess the need for an RC network at the front of the phono stage with a cutoff frequency sufficiently low enough to reject RF noise but not too low to affect audio signal. What I don't quite understand is what, if any, disadvantages there are to lightly loading the cartridge. Like what are the problems that arise when loading a low impedance MC with a high impedance? Why would you want to load it down?

Thanks, yeah, that's what I'm trying to get a grip on, if we're talking about a resonant peak way up above the audio band....maybe 600 khz a the low end or even pushing 2 Mhz at the high end; where presumably it's far enough above the audio band also to have limited if any impact on the phase performance of the audio signal, and even if it's basically undamped or at least certainly underdamped by the cartridge impedance loading (in many instances it seems like even 100 ohm loading would leave the ultrasonic peak underdamped), what precisely is the expected impact in the audio band?

I also realize that these aren't just electrical devices but also mechanical ones and the actual in-use performance isn't always perfectly predicted from trying to model the performance based on a circuit model of the device.

Right, I get that, it's an RLC circuit and the capacitance is tuning the resonant frequency, and the lower the capacitance the higher the frequency (and the R is damping it).

So what does it take in terms of good design for the phono stage to be stage be be stable at RFs or to RFs -- I assume there's some kind of RC filter at the input of the phono stage that's a low pass filter typically, to reject RF generally, no? Just trying to understand. Thanks for your help.