Cartridge Loading- Low output M/C

Thank you for joining the thread, Jonathan, and for providing the link. The text to the right of the last figure in the post Jonathan linked to is particularly relevant. Some excerpts [words in brackets are mine]:

The resonant peaks have too high of a frequency to hear directly, but the magnitude of the peaks and their high frequencies are likely to cause decreased stability and increased distortion and noise in many phono stages. Some phono stages will be fairly insensitive to these ultrasonic peaks, while other phono stages will show bigger effects....

... No additional capacitive loading was used [in the simulations] at the phono stage input.

Comparing the simulations of the 3 cables shows that higher capacitances of the tonearm-to-phono stage interconnect cable demand lower resistor values at the phono stage input to control the resonant high-frequency peaks. This, in turn, reduces the cartridge’s dynamics and resolution, and can also worsen tracking ability.

Also, regarding Wyn’s simulations, I would reiterate a point I made in an earlier post:

Almarg 5-26-2018
I believe what underlies the differing perspectives between your [Wyn’s] analysis and what I, Atmasphere, and JCarr have maintained is that while your analysis focuses on rejection of RFI per se, as reflected in your choice of 10 MHz in the analysis, I and the others have focused on energy that may be generated by the cartridge itself, at and near the resonant frequency.

Best regards,
-- Al

With the measured cartridge/minimum input cap (85pF) the response with a 47K R has a 29dB resonant peak at 4.3MHz and is -12dB at 10MHz.
With a 1k load it’s 4.2MHz and 9.5dB.
With 250 ohms it’s basically flat to 5MHz, with -14dB at 10MHz.
with 100 ohms it’s 1mdB down at 20kHz, with -17dB at 10MHz.
Let’s change the cap to 205pF, the original total input cap.
-1mdB at 20kHz, -20dB at 10MHz.

Now 1000 pF. 0mdB at 20kHz, -32dB at 10MHz, 0.5dB peak at 1MHz.

Now 10nF. 10mdB at 20kHz, -52dB at 10MHz, 3dB peak at 400kHz.
Now 22nF. 20mdB at 20kHz, -59dB at 10MHz, 2.2dB peak at 270kHz.
Now 47nF. 27mdB at 20kHz, -65dB at 10MHz, 0.8dB peak at 147kHz.
Now 0.1uF. -12mdB at 20kHz, -72dB at 10MHz. No peaking, -3dB at 150kHz.
And the actual load used by the designer, with an estimated cap based on the SUT ratio and a tube input stage of my knowledge.
+7mdB at 20kHz, -50dB at 10MHz, 1.7dB peak at 400KHz.
The same, with the recommended load cap of 0.68uF added.
-3dB at 20kHz, -87dB at 20MHz.
Incidentally, by my measurements the cartridge peaks by c. 5dB at 20kHz due to the cantilever resonance, so the extra cap makes the response more symmetric about 0dB, but at the cost of a dip close to 4kHz, which is not a great tradeoff in my opinion, so it’s no wonder the designer prefers to have no cap.

Personally, I’d go for the 100 ohms, 0.1uF load if I was using a non SUT input and 60 ohms no cap if using the SUT, which sounds about right.
Let’s now look at the driving into a low impedance current conveyor node case.

The lower inductance of course changes the R/L ratio by about a factor of 45, so driving it this way is more plausible.
However, a shunt cap between the input node and ground still would be beneficial as far as RFI is concerned.
For example, with the 10 ohms mentioned before the 20KHz loss is 14mdB and 10MHz is -29dB with 85pF, but with 1000pF it is still only 14mdB at 20kHz and -30dB at 10MHz.
With 0.1uF the loss is 17mdB at 20kHz, and at 10MHz it’s 65dB.
Personally I’d go with the 0.1uF cap in this case.

By the way, I still have no idea what you mean by energy of the cartridge itself etc.

By the way, I still have no idea what you mean by energy of the cartridge itself etc.

I didn’t say "energy of the cartridge itself." I said energy "generated by the cartridge itself," and I was referring to energy "generated by the cartridge itself" at RF frequencies. Which I was distinguishing from Radio Frequency Interference, which appeared to be the focus of your analyses.

Surely it must be more than obvious to you that a cartridge generates an electrical signal when it is playing a record, and electrical signals contain energy, and:

Energy = Power x Time

And for a resistive load:

Power = Voltage x Current = Voltage Squared / Resistance = Current Squared x Resistance

Not sure why what I meant by energy "generated by the cartridge itself" would not have been clear.

Regards,
-- Al

OK. Thanks for defining what you mean. So lets look at power.
I ran a simulation and calculated the power dissipated in the cartridge series R and the load R and plotted what happened to the total power as the load cap is varied.
The voltage and current must be in phase for a resistor so power remains V^2/R. We know that the voltage across the load R is reduced as the cap is increased- after all, that’s the objective- so the load power must fall- but what about the series R? I calculated this and added it to the load power to get the total power.
So, back to the "real" case with a 11.8uH winding inductance , 16 ohm Rcart, 85pF load and 100 ohms. I set the input to 1v rms and calculated the total power in the two resistors =10*log(((voltageacrossRcart^2/16) +(voltageacrossloadr^2/100))
The power plot starts at -20.6dB at LF then falls by 3dB at 1.7MHz and by 18dB at 10MHz. No peak is present.
I then changed the cap to 0.1uF.
The power at 1kHz was -20.6dB, it peaks at -13dB at 150kHz , is 3dB off the peak at 87kHz and 320kHz, then falls monotonically by 25dB at 10MHz.
So we’re measuring 1/7 the bandwidth and a bit less than 6x the power in that bandwidth- which is, again, hardly surprising, so the power is more or less constant, but the total power at 10MHz is reduced and the load power at RF is hugely reduced, so isn’t that better?
Is the increase in power dissipation in the cartridge at supersonic but not RF frequencies problematic?
Darn! I wish I had some way of showing plots.