Dedicated phono-pre for MM only?

Hi, Axel and Lewm. My comment on the need for separate MC phono stages is based on Axel's comment that a SUT degrades the signal, thus my assumption that a dedicated phono preamp (for a specific range of LOMC parameters) would be a better way to handle the amplification and loading requirements. One gain/loading setting probably isn't the best way to handle a wide range of LOMC cartridges, not if optimal performance is desired. And if switches must be used to optimize the LOMC signal, it is not acceptable for a high end preamp, which was the premise of this thread. So, it appears that if the logic is used that a dedicated MM stage is needed (to avoid having additional switching for MC cartridge requirements) then dedicated LOMC stages are needed to avoid having additional switching.

I bring this up because it seems that what is implied, Axel, is that MM is inherently a better cartridge because it doesn't require additional switching or transformers. But a dedicated LOMC phono stage would eliminate the need for switching or a transformers as well. Wouldn't that put a MC cartridge on par with a MM cartridge?

Tom

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.