Phono cartridge load impedance.
It's a fact that the issue of moving-coil cartridge load impedance is often misunderstood, but at worst ignored. It’s an easy mistake, to match the moving-coil cartridge impedance to that of the load impedance setting. Or what if there is no option to change the setting as with so many phono amplifiers and transformers. How will this be heard? Read on...
Moving coil cartridges, owing to their coil size, will generate a much lower output than their moving magnet counterparts, and so require additional gain to achieve a reasonable voltage for an amplifier phono input. There are high output moving coil cartridges available, but typically a MC cartridge may have an output of 0.3 mV compared to a figure of approximately 3.5 mV for a magnetic cartridge. Whether you are using a step-up transformer or a moving-coil phono pre-amplifier to attain the extra gain required for a moving coil cartridge (about 20dB more than a MM cartridge), the importance of cartridge impedance matching remains the same. A poor impedance match between the moving-coil cartridge and the device adding the extra gain can be the cause of disappointing sound quality. It's difficult to generalize exactly what you may hear (or not), but high frequency audible “ringing”, a clearly uneven frequency response and generally a high level of background noise are typical symptoms. Orchestral strings can sound "screechy", vocals muffled, and instruments not lively and interesting.
A moving coil cartridge may have an internal impedance of 10 ohms, so the correct load impedance is not 10 ohms but between 50 -100 ohms. Not only that, but the optimal load impedance will vary as to whether you are using an active moving-coil phono amplifier or a step-up transformer.
This graph below shows the effect that cartridge load impedance can have on frequency where the aim is obviously to achieve a flat response.
It's easy to attribute poor performance of a turntable combo to a poor mechanical match between the arm and cartridge, or to the tracking ability or even "the sound” of the cartridge itself. The audible effects of using an overly high load impedance setting can typically be a lack of low frequencies, a slightly hard or ”unmusical” sound with poor imaging. If the sound quality is somewhat veiled and lifeless, the impedance loading may be too low. But before taking any action, it's imperative to establish exactly what the cartridge manufacturers specify about the required load impedance, and of course what your pre-amp or step-up transformer provide. As a general rule of thumb, the impedance provided by the phono amp should be between 5x and 10x the output impedance, (that provided by your cartridge). In the case of transformers, this figure should be less, between 1x and 5x that of the cartridge. The reason for this is that induction within the transformer coils contribute to the loading.
Just a word about using a transformer as a step-up device. The inherent non-linearity of transformers and coils with respect to inductance causes complications in audio. Inherently, inductance changes with frequency with transformers, so the results obtained can be less than predictable. The plus side is that noise produced from a passive device is non-existent, as compared to an active pre-amp which will produce some degree of unwanted noise.
Some MC phono pre-amplifiers may not have variable settings for impedance and capacitance, especially if the MC phono stage is integrated as part of the main pre amp. The good news is that the likely fixed load impedance will be between 50 and 100 ohms - a useful ballpark figure suitable for many moving coil cartridges. The bad news is that you may need a different cartridge or more flexible pre-amp or transformer after all.
In addition to cartridge loading, the load impedance of the main pre-amp to the transformer should be taken into account. Again, correct loading will control excessive “ringing” and curtail audible frequency response anomalies. An excessively low load impedance to the transformer, whilst controlling ringing and electrical resonance effects, can cause a significant drop in output from the unit. Unless the manufacturers say otherwise, begin with the standard phono input impedance of 47k ohms standard used for moving-magnet cartridges. If the sound is too bright, forward and lean and the option is there, try reducing the input loading impedance. If the sound is dark, recessed and compressed, the impedance may be too low, but this isn’t likely to be the real cause as 47k-50k ohms is generally toward the upper limit expected from a phono input.
Finally, the subject of capacitance. This is a more significant issue where moving magnet cartridges are concerned as it will affect the frequency response in combination with load impedance. Capacitance follows primarily from the cable which connects the turntable to the pre-amp. A good cable should add no more than 150pF capacitance, maybe less. There may be an option on the pre-amplifier to add capacitance but I would suggest the minimum setting. There is a point of view that suggests an optimum figure of 400pF load capacitance for moving magnet cartridges, but in my view that is misguided.
Take a look at this graph which shows the effect on a moving-magnet cartridges's frequency response with different load capacitance settings. A significant rise in the high frequency response begins from 3 Khz onwards, and this will be apparent on any system. In the case of moving-coil cartridges, their coil inductance is so much lower than their moving-magnet counterparts that capacitance will have little effect on the frequency response unless grossly excessive capacitance is added - the circuit would be not so prone to resonance because of the low inductance of the cartridge.
MC cartridge load impedance is usually 5-10x the output impedance of the cartridge as specified by the manufacturer, 1x-5x for cartridges used with transformers. For MM cartridges load impedance is usually a standard 47k ohms.
The effect of total load capacitance is more critical for MM cartridges than moving-coil designs. My view is that this figure should be as low as possible, but never more than 200 pF.
If possible, use a phono pre-amplifier with a good selection of variable load impedances in place of a step-up transformer. The exception may be if the transformer was specifically designed for the cartridge you are using.
Low output moving-coil cartridges, by and large, sound more "refined" than their moving-magnet counterparts, but are often significantly more expensive and sometimes difficult to set-up optimally. Basically, if the sound you are achieving with a familiar disc sounds "right", i.e balanced and even across frequencies - dynamic and realistic, then leave well alone and enjoy the music!