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"

** The high inductance of a MM cartridge ( circa 650 mH) resonates with the capacitance of the connecting cable at some high audio frequency - usually in the range of 12 to 20 kHz. A load resistor is needed to damp this resonance in a predictable way and make the overall response curve close to flat. ** Most MM cartridges work best with a 47 kohms load - see the maker's recommendation as there are a few exceptions.

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.

--
Dave Platt                                    AE6EO
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