Dedicated phono-pre for MM only?

Kirkus

I'm not sure I understand your point about dynamic capacitance of JFET vs V/T inputs, and if I do I'm not sure I agree with it.

Assuming that you are talking about junction thickness dependent capacitance varying with drain - source voltage, I thought this effect was pretty well analogous to Miller capacitance in V/Ts. In both cases cascoding reduces the variation in drain / source (read anode / grid for V/T) voltage.

Is there a secondary effect in JFETs I need to know about?

Mark Kelly

Hi Mark -- yes we're indeed talking about the same thing, its just that the Miller capacitance is usually much lower. The reason I made the (admittedly rather imprecise) statement about vacuum-tube vs. JFET junction capacitance is from some generalizations about exactly how much this capacitance is actually reflected at the input when these parts are used in a circuit.

That is, 2-3 pF from the interelectrode capacitances of i.e. a 12AX7 are significantly less worrisome than the 30-40 pF or so from something like a 2SK389 set up as a diff-amp. I also think that cascoding JFETs (either with bipolars, or more JFETs) is a simpler, more elegant implementation than the vacuum-tube counterparts, which usually involves bootstrapping and multiple/floating heater supplies.

So I would personally always cascode JFETs in this application, but probably wouldn't with most vacuum-tubes. Just my preferances.

So it seems that a quality SUT, or, a high quality dedicated MC phono stage, is the key for MC cartridges. Since MC cartridge designs include a fairly wide range of output voltages and internal impedances, and thus gain and loading requirements, it may necessitate a range of dedicated MC phono stages if someone wants to listen to a variety of LOMC cartridges.

Too bad, because that sounds expensive. Is the added expense (of having multiple dedicated MC phono stages) worth the perceived improvements in sound?

Tom

Tom, I did not get that message (that the range of output voltages and inernal impedances of MC cartridges presents a problem per se) from Kirkus' long post. I will have to re-read it, but I think he is saying that the much higher output impedance and reactance of an MM cartridge poses a problem for MM phono pre design.
Kirkus, Given your depth of knowledge of this subject, are there any commercially available phono preamps that seem up to snuff for use with MM cartridges exclusively?

Lewm, my mistake for not including the intended recipient in my post. (I don't post here at Audiogon very often so am not used to the continuous thread format.) I was trying to get a sense from Axel (and others) if it was worth the added expense of having dedicated phono stages for several LOMC cartridges, each with a unique amplification and loading requirement. That's making the assumption (and the analogy to a dedicated MM stage) that to get the best sound from a phono stage, the design must be specific to the cartridge.

If so, then the question might also be asked: Dedicated phono-pre for (specific) LOMC only?

Tom