Tube Preamp Paired with Tube Phono Stage?

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

Chervokas, to be clear: if you load the cartridge with a lower resistance than your 10:1 or 11:1 ratio, all that happens at audio frequencies is you may reduce the output voltage of the cartridge. Its bandwidth will be unchanged.

So in theory you would want to load it as lightly as possible such that the RF issues are managed. The RF issues *can and do* act like changes in tonality. I admit that its confusing, but if you study this stuff on the test bench the fact of it comes out in spades.

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.

Chervokas, As far as an undamped LOMC goes, there is no effect whatsoever in the audio band, other than the voltage output- with no load its output will be a little higher, not much.

The issue is that the coils in the cartridge have so little inductance that the ringing issues that often affect inductive devices do not come into play until you are at very high frequencies, well into the MHz region. It is this ringing that supplies energy for the tuned circuit that results from the cartridge and the interconnect cable. Note that the capacitance of the cable always plays a role, the lower it is the higher the frequency of the resulting tuned circuit. So its rare that this can be solved by reducing the capacitance of the cable!

What is paramount, IOW, is that the phono section be stable with RF at its input.

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.