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

Al:

Thanks for your input. Since posting I have done more digging into this and it's a bit more complex than I first thought.

The site you reference is a bit misleading because his plots don't take into account the effect of the series resistor on the shape of the peaking (he assumes a Q factor of 1).

I found this reference by Rod Elliot to be a bit more realisitic:

http://sound.westhost.com/articles/cartridge-loading.html

The problem is that some manufacturers use the electronic tuning response to compensate for mechanical resonances of the stylus tip/cantalever/motor devices in the cartridge, so that the overall actual response is not just predicted by the RLC network of the elctrical components. See, for example:

http://www.tnt-audio.com/sorgenti/load_the_magnets_e.html

So the electrical modeling would suggest that the lower the capacitance, the better. But for some MM carts this might result in a reduced high end response due to mechanical effects.

Not sure how any of this is related to the performance of the AT 150 mlx, but I am begiining to remember why I don't like the concept of MM carts, and may re-consider using them. MCs don't have this issue due to the much lower inductances and impedances. They are tolerant of higher cap loading because of this.

Swap to a 100 pF arm cable if you can. I've been using an AT150MLX for 4 years and love it. I currently have a JoLiDa phono stage with highly adjustable capacitance. I have a Technics SL1210 M5G TT whose arm cable capacitance is 100 pF. I have played with the input capacitance in my preamp and found that 0 through 100 pF works OK, but my personal preference is an input of 47 for a total of 147 pF (or 150). However, 200 total is good enough that it could come down to system matching. I definitely wouldn't want to run it into 300 pF or above.

I hope you can get it to work for you. The AT150MLX has a very musical balance of robustness and body on the one hand and spatial cues and detail on the other. It's also an excellent tracker.

Johnyb:

Where can I get a 100 pF RCA/DIN phono cable? Most manufacturers of aftermarket versions don't spec the capacitance. ADC made a 100 pF special version back in the day, but this is NLA and I have never seen a used one.

FYI the ADC arm's internal wiring is 30 pF (headshell to DIN in the arm), so the DIN/RCA cable has to be about 70 pF to have a total interconnect of 100 pF. The input capacitance on the GCPH is fixed at 100 pF. They use all surface mount capacitors so exchanging or modding input caps in this phono preamp is near impossible.

I think your setup with a total of 147 pF is right on the money, as the references indicate the lower the better.

To get to 70 pF in the interconnect would mean I have to build my own cable, or build an interface box with a short cable to the arm/DIN and RCA bulkhead connectors. I have low cap RCA/RCA interconnects I can use from the box to the pre-amp. Or I could shorten the cables I have to under 12" and mount new RCAs. But that limits where I can place the preamp; it would need to be immediately adjacent to the turntable.

A lot of work just to use this cartridge though. I am leaning back to a HO MC or maybe MI