Red Diamond Audio

Discover those classic audio gems.

I



Moving coil cartridges usually have a much lower output than their moving magnet counterparts by virtue of  having a coil moving, rather than the magnet to produce a voltage. Whether you are using a step-up transformer or a moving-coil  phono pre amplifier to achieve 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 a moving-coil cartridge and the device adding the extra gain can be the cause of  disappointing sound quality-high frequency audible “ringing”, a clearly uneven frequency response and generally high noise. The graph below shows the effect that cartridge loading resistance can have on frequency response.





















It's easy to attribute some of the potential performance problems of a turntable combo to a poor mechanical match between the arm and cartridge, or even to the tracking ability or “sound quality” of the cartridge itself. 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 loading, and what your pre amp or step-up transformer can actually provide. As a general rule of thumb, the source impedance (that of your cartridge) should be between 5x and 10x the load impedance, (that of your pre amp or transformer). So looking back to our Koetsu cartridge with a 5 ohm source impedance, a load impedance of 30-100 ohms should be about right.


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.


Moving-coil cartridge loading.

It's a fact that the issue of  moving-coil cartridge loading impedance is often misunderstood, but at worst ignored.
It’s an easy mistake - matching cartridge impedance  exactly with load impedance. For instance, a Koetsu moving coil cartridge typically has an internal impedance of 5 ohms. The correct load impedance is not 5 ohms but between 50 -100 ohms. If this is news to you , read on…
Yamamura cartridge step-up transformer

With thanks to extremephono.com for the graphs

more articles

© Red Diamond Audio 2013

Home    Products for sale    Selling on the site     Contact