Dedicated phono-pre for MM only?

Tom,
I guess 'switching' means loading in your context, yes?

Loading an MC with 100, 500, 1k ohm for example makes little difference as to the phono-pre used as such.
The mechanical issues involved will.
Soldered Rs are best, than clamped in some binding posts, then the 'mouse-piano' i.e. the switching you seem to refer to.
The same would apply (to the phono-pre) using 47k, 50k, 75k or 100k phono-pre input impedance 'switching/changing' for MMs only.

The issue with MCs is rather to have a phono-pre that "can count electrons" (if no SUT is used).
Apparently only few designs are THAT good after all. Mostly noticeable in the difference of those parameters I mentioned that are improved, when an SUT is used with a Low MC.
Often a lack of dynamics is noted if an MCs output is on the low side - use an SUT, and THAT problem is GONE.

When using an MM there are different things at work.
Mainly due to the 'high' cart voltage output --- BUT also because they are a more 'reactive' load, usually 600 - 3.5 ohm DCR, 700mH as ~ 20uH with an MC and compared to only ~ 2 - 6 ohm with LOMCs).

That is why a TOP MM stage would ask for a different design if strictly dedicated to MMs.
No SUT can be used, single ended (SE) MIGHT be better. I'm as little sure about this as Lewm is and it might have to do with the 3-6dB lesser noise in SE design --- which then again might just be made worse by using common ground that can be noisy.
Also 'floating RIAA' as in a differential circuit between +/- is not an option either.

Greetings,
Axel

Tom and Axel, I heretofore have had only one LOMC cartridge, which seeems optimally loaded by a 100R resistor, which I soldered in place myself. But I recently purchased a (third) LOMC that will probably do best with 1000R. I have contemplated installing a switch, so I can change between 100R and 1000R, when I want to change cartridges. Obviously there is a risk of introducing noise at the switch. But in practice, have either of you, or anyone else, actually perceived degradation of the sound with such an arrangement? Many commercial products incorporate such switches in this critical part of the circuit, so most people would not know if they are losing anything in the way of S/N ratio as a result.

But this is off-topic. With an MM phono stage, we should all be using a 100K load, according to Raul. Based on very limited data, I have to agree with him; several years ago, I made a similar accidental observation with whatever MM I owned at that time. It sounded better with a 98K load compared to the standard 47K load, and the diff was not subtle.

Hi Lewm,
>>> With an MM phono stage, we should all be using a 100K load <<<
Another reason for this thread's subject.
I mean, WHICH phono-pre, of one-serves-all, has a 70k and/or 100k ohm input impedance option I ask?
I know Raul's Essential 3160 has, but that is a non-main-stream item.

As to 'switching degradation' I had a PS Audio GCPH using some pretty convenient switching, and now a pain-in-the-butt ML phono-module using internal binding posts or some jumpers.

But as you say, how could I have tested the GCPH's performance without the switches for gain and loading?

All I know is, that the ML modules sound better --- but because of the switches?

Raul and Co. might have some contribution in sharing their findings, wouldn't they?

Axel

Hi Mark - FWIW, my experimentation with discrete JFET MM stages included both the 2SK389 dual, and a pair of 2SK369 singles. I cascoded them with 2N5089 NPN bipolars, and loaded the collectors with 1K . . . there was also a single 2N5089 used as a simple current source for the tail. Rails were +/- 20V, and I put about 8mA through the tail (4mA through each JFET). VDS across the JFETS was about 5V.

I used this circuit in an unbalanced-input feedback amplifier, in the classic "hybrid-amplifier" (JFET-frontend feeds opamp) topology. Instead of a monolithic, I used a 990 discrete opamp, driving a low-impedance EQ network for the 318uS and 3180uS time-constants, with about 20dB of gain at 1KC. This fed a passive 75uS network and another 990 for 20dB more gain, same as my MC design.

I never really put in the time to fully optimize the circuit, but it was very low-distortion, and CRAZY quiet . . . like greater than -90dBV at the output when looking at a 1K source impedance. I think that the 2SK269s had a little better 1/F noise, and even though the offset on the 2SK389 was much better, it didn't really matter for my application. Of course, YMMV.

Hi Lewm - I will add my vote as well to the higher-loading approach, I think that 100K with minimal capacitance is much better (with virtually every cartridge that I've tried) than the common 47K/150pF.

But as far as commercially-available phono stages go . . . I don't really see enough of them anymore to have really strong opinions. I do think that in general, getting the subtle details right makes more difference than any generalisations about whether it uses JFETs, tubes, bipolars, opamps, etc. etc. There are certainly an unlimited number of ways to screw things up, or to avoid doing so.

I'm currently using my Beogram 4002/MMC20CL with a little phono preamp that's an opamp design, made on a Vectorboard . . . and the case and power-supply from a Lehmann Black Cube. Its a two-stage topology like my MC phono unit, with active 318/3180 in the first stage, passive 75uS, then more gain in the output stage. Opamps are AD745 (input) followed by AD797 (output), both of them use complementary discrete JFET buffers for high-current output, within each feedback loop.