Impedance Setting Of MC Phonograph Amplifier: Why Is It Important?

- Nov 21, 2020-

With L = 0.0075mh and R = 47K, the bandwidth is nearly 1000MHz. At this time, the ubiquitous radio waves will enter the singing and playing, causing abnormal behavior of the line overload. It sounds that there will be ringing at high frequency. This is why 47K impedance is indeed the best dynamic, but with the correct SRA ~ = 92 degrees, the high frequency will be sharp. The more accurate the SRA is, the closer the needle tip is to the groove, and a lot of UHF components will be sung. If the high frequency response of the singing and playing is not linear enough, it will certainly be harsh than the low SRA. Therefore, unless the location of a system is completely free of radio interference, appropriate impedance loading is necessary.

ClearAudio concept MC

However, if the impedance is too low, the work of the phonograph will be more difficult, affecting the dynamic.

At the same time, although the capacitance does not affect the bandwidth, it will work together with the inductance value of the phonograph coil, resulting in a peak value of extremely high frequency response.

The main source of capacitance value is the signal line connected between the phonograph and the phonograph. According to J. Carr's analysis on this page, the larger the capacitance value, the smaller the load impedance to suppress this peak value effectively. As can be seen from the figure, if the capacitance value of the signal line is very low 32pf, the 267 ohm impedance can completely eliminate the peak value. But if the capacitance of the signal line is the more common 103pf, 196 ohm impedance is required. If the capacitance of the signal line is as high as 197pf, the impedance is as low as 149 ohm.

However, the so-called "peak response" will have a substantial impact only when the radio waves of that frequency band enter. Therefore, if there is little radio interference in your home, you may be able to use a higher impedance value to get better dynamic performance, while still avoiding the overload of singing and playing. In my Taipei home, because the sound room is underground and the mobile phone and other signals can't arrive, I have no problem using 3K ohm. But in the United States, radio interference, you have to use 1K or less.In general, I would suggest that if the phonograph manufacturer has a fixed recommended value, then the value should prevail. However, you may as well test it both up and down to see if there is any difference in sound quality. After that, if you don't use 300 ohm for a while, listen to the low value. At the same time, if a low capacitance phonograph output signal line is used, the impedance value may be higher, and the dynamic performance should be improved