Capacitance loading for AT 150 mlx cart

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

Al:

Thanks for your comments. To address your points:

The phono stage of the GCPH is bipolar. PS Audio uses and Analog Devices microphone pre-amp SSM2019 which has a bipolar input.

If it were an FET stage, it would be a non-issue, as FET input stages have extemely high inherent input impedances, so a source impedance as high as 100K would not impact them much. But bipolar stages, on the other hand, are quite different. Depending on bias conditions, and the gain of the input stage, input impedance may only be a megohm or less, and 50-100K source impedances can make a big difference. The input stage corner frequency is a function of source impedance, and may affect stability in closed loop mode, depending on the design.

Besides the sound signatures of the two types, generally designers use an FET input for MM carts and biploar for MC, because the lower noise current of an FET gives better noise performance with MM, and the noise voltage created by higher noise current of a bipolar is reduced by the lower source impedances of MCs.

Now to the attenuator. You are right about the DC resistance and that the MM impedance will be low at low frequencies. But the response pole created by the RL network in the MM will have the impedance climbing starting at about 200 Hz. It will climb at 6 dB/octave until it reaches the resonant point, and by that time it will be close to 50K. This is exactly where you don't want a high source impedance, at the highest frequencies in the audio band. If I use a 5:1 attenuator, with a total load impedance of 47 K facing the MM cart, the preamp will see a more constant 8-10 K source impedance, all the way up to the cartidge resonance and beyond. Plus, any capacitance at the input of the preamp will be isolated from the cart by the 40 K divider resistor, leaving only the native capacitance of the tonearm cable and interconnects loading the MM cart. IMHO, it is better for a bipolar input stage to see a constant impedance, independent of frequency, throughout the audio band, even if its higher than one at low frequencies.

Re the noise performance, yes there will be a bit higher noise from a 10 K source than from a 500 ohm - 1K source. And, I will need to use a higher gain as well, which could also increase the noise. But my noise measurements of the GCPH (see my post in the amplifiers section) indicate that even at 60 dB of gain in the GCPH, the noise levels are not bad. So today (as opposed to when I did this in the 70's) we are blessed with high gain low noise designs from both the FET and bipolar camps which makes using this attenuator principle more attractive.

Re input capacitance, I would build the attenuator in a RCA loading plug, with a male RCA on one end and a female RCA on the other. The loading plug would be connected directly to the pre-amp input RCAs. The two resistors would be wired in the plug itself. I doubt if the stray capacitance would exceed about 5-10 pF. Even if you use a "Y" connector (which I do not prefer) with a loading plug on one leg of the Y, the added capacitance would still be under 30 pF IF you use a good quality Y adapter. In either configuration, shielding is not a factor.

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.