Capacitance loading for AT 150 mlx cart

I can't say for sure, and I have no experience with the particular cartridge, but I'm highly skeptical of what the salesperson told you.

Although the Audio-Technica site does not indicate a spec for the inductance of the cartridge coils, this review, which is linked to at their site, indicates 450 mH, while the specs here for the VM version (not sure what that is) indicate 350 mH.

Plugging those values into the calculator provided here shows that in conjunction with the 150 pf mid-point of the recommended load capacitance range a high frequency resonant peak will exist at 19.4 kHz in the first case, and 22 kHz in the second case. Those seem like sensible values.

Increasing the total capacitance to 350 pf (250 pf cable + 100 pf phono stage) would reduce those numbers to 12.7 and 14.4 kHz, respectively, and would also increase the magnitude of the resonant peak. While it is possible that the design is intended to use that resonant peak to compensate for what otherwise would be a dip or rolloff in the upper treble, and my understanding is that it is not uncommon for that to be done, in this case I am doubtful.

And, of course, what sense would it make for them to provide a load capacitance spec that excludes cable capacitance, without providing an indication of how much cable capacitance is being assumed?

Personally, I wouldn't worry about going a little over the 200 pf number, perhaps to a total of 250 pf or so, but I would not be comfortable with 350 pf.

Regards,
-- Al

DHL, thanks for providing the references and the additional comments. Yes, the Hagerman paper is somewhat simplistic, most notably in not addressing the fact that for MM's too little capacitance can result in too little upper treble.

Good inputs from Johnny. His finding that optimal results occurred with 147 pf, right in the middle of the recommended range, would seem to further confirm that the salesperson you spoke with was mistaken.

Re MC's, the response of the cartridge at audible frequencies will of course be pretty much insensitive to load capacitance. A point to keep in mind, though, is that the amount of load capacitance may still have significant audible consequences (with lower capacitance generally being better), due to effects that the ultrasonic resonant peak may have on the phono stage, and due to constraints that it may impose on the choice of resistive loading. See the post by preamp and cartridge designer Jonathan Carr (JCarr) dated 8/14/10 in this thread.

Regards,
-- Al

Interesting thought about the 5:1 attenuator. It seems to me that it might very well do the trick, but I would have some concerns.

One concern is the possibility that the higher source impedance seen by the phono stage at low and mid frequencies, compared to direct connection of the cartridge, would result in some degradation of the phono stage's signal-to-noise performance, as well as increased susceptibility to hum pickup in the phono stage input circuit. Note in one of the links I provided earlier that the cartridge's DC resistance is spec'd at 530 ohms.

Also, obviously, the attenuator would have to be constructed so as to provide good shielding.

Finally, stray capacitance in the attenuator and its input connector would reduce the 100 pf saving at least a little bit, perhaps by 10 pf or so as a rough ballpark guess.

Best regards,
-- Al

Very nice analysis, DHL.

My only further comment is that when I raised the signal-to-noise concern I was not so much thinking of noise generated BY the input stage or by the source impedance, but rather that the increased source impedance at low and mid frequencies would increase susceptibility to pickup of EMI or hum that would be introduced TO the input stage. It could very well be that that won't be an issue, but that was the possibility I had in mind.

Best of luck, however you decide to proceed!

-- Al