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.

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

Chervokas, read my posts above and you will see why this is not odd at all.

Actusreus, the Atma-Sphere preamps are intended as stand-alone phono reproducers. People ask us all the time why we don't have a separate phono section; the reason has to do with interconnect cables. The idea behind our preamps is to reduce the signature of the interconnect cable between the amp and preamp. Without special circuity this cannot be done. It is included in our preamps. This allows the phono section to be more neutral.

05-20-12: Chervokas
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.

That's close. It's actually 1/11th, assuming that both impedances are essentially resistive at the frequencies being considered.

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)

Assuming both impedances are essentially resistive at the frequencies being considered, you would lose:

0.8 x 24/(24 + 100) = 0.15 mv

Resulting in (0.8 - 0.15) = 0.65 mv appearing at the input of the phono stage. Which can be calculated a little more directly as:

0.8 x 100/(24 + 100) = 0.65 mv

All of which brings to mind a pet peeve of mine. When the specs for a cartridge indicate "impedance," what are they talking about? Impedance at zero Hz (i.e., DC resistance), impedance at 1 kHz? 20 kHz? Other? For many and perhaps most LOMC's it won't make much difference, because of their low inductance. But it would certainly be nice if DC resistance and inductance were separately and explicitly specified (which they sometimes are, but all too infrequently), so we don't have to make assumptions and guesses.

Regards,
-- Al

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?

05-21-12: Chervokas
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.

Yes, those specs are unusually well defined.

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?

The one reason I am aware of (although I'm not completely certain that there are no other reasons) is the one Ralph (Atmasphere) described. Heavier loading will reduce the magnitude of the ultrasonic resonant peak you referred to, which as you realize is caused by the interaction of the inductance of the cartridge and the capacitance of the cabling plus the input capacitance of the phono stage.

The greater the magnitude of that resonant peak, the greater the amplitude boost that will occur, relative to other frequencies, of frequency components that may enter the phono stage that are near the frequency of that resonant peak. Depending on the design of the phono stage, that may increase the likelihood that intermodulation or other effects caused by those ultrasonic frequencies will affect audible frequencies. Extraneous ultrasonic or RF energy may be present as a result of tics and pops, ultrasonic musical information that is on the record, RFI pickup, etc.

Some references that may be of interest:

http://www.hagtech.com/loading.html

And the post dated 8-14-10 by JCarr (Lyra cartridge designer) in this thread

Best regards,
-- Al