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It’s quite possible that some MC phono pre-amplifiers may not have variable settings for impedance and capacitance, especially if already built into the main pre amp. The good news is that the likely fixed load impedance is between 50 and 100 ohms, and this is a useful figure for many of the moving coil cartridges available. The bad news is that you may need a different cartridge or more flexible pre-amp or transformer.
In addition to the subject of cartridge loading, the input loading impedance of the main pre-amp to the step-up device should be taken into account. Again, the correct loading will control excessive “ringing” and prevent frequency response anomalies. An excessively low load impedance for the transformer, whilst controlling ringing and electrical resonance effects, can cause a significant drop in output from the step-up transformer. 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 loading. This again is a significant issue where cartridges are concerned as it can affects the frequency response in combination with impedance. Take a look at the graph below which shows the effect on a moving-coil cartridge frequency response with different capacitance and impedance settings. A significant rise in the high frequency response begins from 3 Khz onwards, and this will be apparent on any system.
The phono inputs on your pre-amplifier may have variable capacitance settings which could help produce a “flat”response directly from a moving magnet cartridge or from a step-up device.
On a general note, use high quality, low capacitance cables for all phono connections to the pre amp/step-up device, and keep these cables as short as possible for minimum capacitance, which increases with cable length. Cable capacitance should not be used as a means to increase capacitance loading.
With thanks to extremephono.com for the graphs
© Red Diamond Audio 2013