Capacitance loading for AT 150 mlx cart

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

Al:

Not to worry.

This should not be a problem because the physical layout of the system w/ attenuator is not much different than the stock system without it. The attenuator plugs are short (about twice as long as the standard locking RCA) and connected to the pre-amp RCAs. Everything then connected to attenuators is identical to the stock layout. So the only difference from a noise pick up point of view is the short length of the attenuator barrel, which is of coarse metallic and grounded to the shield.

The impedance up to the attenuator input would be the low MM cart impedance. Its only the impedance after the series divider resistor of 40K (which would be 8-10K) that would be susceptable, and that would be about 1/4" inside a shielded RCA plug, Not a significant area for noise induction IMHO.

But your concern is exactly why I would not use a "Y" adapter. Those are much larger, and even though still shielded, are more susceptible to induced noise.

The other issue I think you are implicitly raising is that the interconnect wiring from the RCA to the actual input of the IC within the preamp would also be susceptable. PS Audio has done a good job here, judging from my noise measurements with a 47K input impedance and the input RCAs open. Very slightly different than the 1K loading choice, which can be easily explained by the increased thermal noise of a 47K resistor vs a 1K resistor.

An update.

I built the 5:1 attenuator and connected it with custom cables I built using DH Labs BL-1 interconnect wiring. Got the capacitance of the cables and the arm wiring to 155 pF. With these cables connected to the ADC LMF arm and the AT 150 mlx cartridge mounted, everything connected to the attenuators and the PS Audio GCPH phono preamp, I can use the 66 dB gain setting with very acceptable noise levels. I probably only need 55-60 dB of gain to get a 300 mV into my line pre-amp in any case. The 100 pF input capacitance in the GCPH is no longer an issue, as its now isolated from the cartridge by the 37K series resistor in the attenuator.

The attenuator was built inside a male/female pair of locking RCA connectors similar to the DH Labs versions. So these are plugged into the GCPH and the IC BL-1 interconnects are pugged into the female RCA.