Cartridge impedance loading question

Running a typical MC cart. with a load of 47K is kind of like driving your car without shock absorbers. The inductance of the cart. in combination with the load capacitance forms a resonance circuit. With a 47K load and typical values for inductance and capacitance the peak is usually well past the audio band. The audio band may still be flat, but he response above the audio band will be rising. This will make the system more sensitive to surface noise and pops and other ultrasonic disturbances. If rise intrudes into the upper audio band any excitation of the resonance (ultrasonic surface noise and pops) will be made more audible.

Load the cart. down with a too small value resistor and the frequency response starts to drop in the audio band. Get it right and the response will be maximally flat. Load it with a high value resistor and get an ultrasonic peak in the response.

"These analogies between cartridge loading and automobile technology have their limitations."

In the old days before digital computers automotive companies use to model their suspension designs with analog circuits using inductors, capacitors and resistors. In the spring/mass system energy sloshes back and forth between the kinetic energy of the moving mass and the potential energy of the spring. In the electrical analog energy sloshes back and forth between the electric field in the capacitor and the magnetic field of the inductor. In the mechanical system the resonance is damped with a shock absorber, in the electrical circuit the resonance is damped with a resistor. Both systems are governed by the same differential equations. So the analogy is very apt.

The fact of the matter is a cartridge has inductance and forms a resonance tank in combination with the input and cable capacitance. The resonance peak will occur at a very high frequency, well beyond the audio band. Depending on the Q of the resonance you will see a rising frequency response well down in frequency from the peak. The Q of a resonance is determined by the source resistance of the cartridge and any load resistance. Now I will readily admit that if your phono pre is susceptible to RFI and your cart. is undamped, RFI will excite this resonance and raise all hell. That said, if your phono pre is not susceptible to RFI the resonance will still be there and the rising high end response it causes.

MC cartridges also have mechanical resonances that with some cart. need to be damped with a load resistor. Think speaker resonances and a low source impedance (high damping factor) amplifier.

But look, if you like the sound of your cart. running it wide open then go for it. Just don't make some kind of religion out of it.

I suppose if you were to use a transformer placed close to your TT to keep leads very short you could push the resonance up past 1mHz. In a more normal situation I would guess it would be hard to push cable and input capacitance much below 100pF. Without an input transformer the input stage would have to be probably a cascode for the high gain needed without getting killed by the Miller effect.

A cartridge with 5mH inductance in combination with 100pF has a resonance of 225kHz. A typical system may have more like 200pF. This results in a resonance of 160kHz. A sharp impulse like from a scratch will generate a lot of high frequency harmonics and could conceivably excite the resonance and cause it to ring generating bursts of high frequency noise. Now as long as your phono pre is hunky dory dealing with bursts of high frequency signal (no IM with audio signal, or maybe it shunts these signals to ground with a ceramic disc cap at the input?) you're good. The 75uS time constant will roll this stuff off, usually after the input stage. In the above examples the resonance is 6 to 7 octaves above 2.122kHz so the ringing will be attenuated by 36dB - 42dB (unless your phono stage uses the 4th pole and shelves the response at 50kH). Not gone by any means. Could still pollute gear downstream. That said, there is enough going on that in the end one's own ear should be the final arbiter.

Some might find the thread below interesting.

thread