LOAD RESISTOR FOR PHONO CARTRIDGE

Impedance matching. 47K was standard for dynamic, 100-250K for ceramic. Why?

The load resistor for the MC is low some 100 ohms or less. use manufacturers recommendations. I have a MC that was designed to MM input. I don't know how its wired, but has enough output, or barely. I would have to crank the volume knob to max and it would not clip. That preamp had no specified MC input and thus no load selector.

greg

"RealInfo"

MC cartridges have very low inductance and low resistance so any electrical resonances are well above the audio band - resistive loading is not near so important. Suggested load values range from 10 to 100 ohms - often this is provided by a matching transformer.

..... Phil

There are a couple of reasons.

The less important is that if you don't have a load resistor, and thus have an input whose impedance is extremely high, then the input will tend to be noisy and prone to RF pickup if you don't have a phono cartridge plugged into it. Not a really big issue.

The more important is that having a suitable input resistance, and a suitable amount of shunt capacitance, allows the cartridge to deliver as flat a frequency response as possible. A phono cartridge has a complex (not-purely-resistive) output impedance - it's actually quite inductive. This inductance interacts with the load (the preamp's resistance, in shunt with its input capacitance and the capacitance of the cable) to create a resonant circuit that influences the cartridge's frequency response.

A good cartridge is designed to deliver the flattest (or, sometimes, the subjectively most pleasing to the manufacturer) frequency response when working into a specific load. With an inappropriate load, the high-frequency response will usually suffer - it may roll off prematurely, or have a pronounced peak prior to rolloff.

That depends on the cartridge.

Most moving-magnet and moving-iron cartridges are designed to work into a load of 47k ohms, in shunt with a specific amount of capacitance (typically somewhere in the 200 - 300 pF range, if I recall correctly). Remember that the total capacitive load consists of both the capacitance of the cables, and that of the preamp input.

For MC cartridges - there seems to be a lot more variation in the recommended load. High-output MC cartridges may be designed to work well into a 47k load, in order to allow them to be used with MM phono preamps without a separate gain stage being required.

Low-output MC cartridges typically have a lower impedance (fewer turns of wire on the coil), and the ones I've seen have been designed to work into loads ranging from as low as 30 ohms up to a couple of hundred ohms. Because of their low output impedance these MC cartridges tend to be less sensitive to variations in the shunt capacitance.

If you're building a preamp for use with a specific cartridge, I'd advise starting with the cartridge manufacturer's recommendation, and perhaps having a way to tweak the shunt capacitance (MC) or resistance (MM) to suit your own personal tastes.