MM Phono Input impedance change 47k to 100k ohms

Hi Erik,

I don't think it's possible to say without having a schematic and/or being familiar with the details of the design. There could very well be a 47K resistor between the input for each channel and ground, that could be changed to 100K. But even if that were the case, without knowing the details of the design I would not have confidence that the change would not result in unwanted side-effects.

Also, see my post here. Contrary to widespread belief, 47K could very possibly be a suitable load for the F9 cartridges.

Best regards,
-- Al

Raul & Pryso, I certainly don't disagree with your comments about the potential benefits that can result from experimentation, and from making the design as flexible as possible.

However, the original post appeared to reflect an understanding that 100K loading is a necessity for best results with this cartridge. For the OP to make an informed choice about how to invest his time, effort, and perhaps money (if he were to have a technician do the work), it would seem appropriate to advise him that that is not necessarily the case, and that a possibility exists that the performance of the cartridge or the phono stage or both will be worse with that loading. That is what I and others attempted to do.

Regards,
-- Al

Mofi, thanks for the excellent and valuable empirical information.

Lew, thanks for your inputs as well. I think that you are correct that in MOST cases changing the 47K resistor to 100K would not have any adverse effects on phono stage performance, especially if the input stage is tube or FET-based. But it goes against my technical instincts to incorporate a design modification, albeit a seemingly minor one, without having a full understanding of what the existing design is.

And in particular, I was envisioning the possibility that the active stage at the input may utilize bipolar transistors, and therefore most likely have a much lower input impedance than a tube or FET-based circuit. Conceivably, also, some of the passive components associated with RIAA equalization could be located at that point. I seem to recall having seen some Bryston schematics, for example, to which both of those points would apply.

Finally, given that the cartridge is a MM I would assume that its inductance is somewhere in the vicinity of 100 to 500 mH. That means its impedance at ultrasonic frequencies at which it may output significant energy may be upwards of several 10's of Kohms, perhaps even approaching 100K. That would clearly be significant in relation to the phono stage input impedances we are talking about, implying the possibility (if the input impedance of the active stage is not high enough to be insignificant) that the resistor change could result in adverse effects on noise levels, frequency response, intermodulation distortion, etc.

It all adds up to caution being called for, as I see it. Especially since, given Mofi's experiences and my theoretical speculation (in the other thread), 47K stands a good chance of working just as well or perhaps even better than 100K.

Best regards,
-- Al

An additional point that occurs to me. Tics and pops contain a considerable amount of energy at ultrasonic frequencies (i.e., greater than 20 kHz). The increase in bandwidth that would occur with 100K loading, presumably to the 45 kHz number that is specified for that loading, may result in tics and pops becoming significantly more objectionable. That would be particularly true if the phono stage implements RIAA equalization in a feedback loop, rather than passively. Atmasphere, for one, has emphasized that point a number of times in the past.

Regards,
-- Al

Thanks very much, Lew.

To clarify my comments, I too don't see any reason to be scared away from trying 100K or other loads. However, a lot of people do not have the time or inclination to experiment to the extent that you, Raul, and a lot of others here do. And there are other factors that will be concerns to some, such as the possibility of damaging their equipment while doing the work, effects on resale value, downtime and expense that will be incurred if the person prefers to have a technician do the work, etc.

The OP started the thread apparently under the impression that changing to 100K is a clear necessity, and asking how to go about doing that. I believe that my comments, and the experience-based comments from several of the others, provide perspectives from which he can make a better informed decision as to how to proceed, based on his own circumstances and preferences, not ours.

Best regards,
-- Al