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Input Impedance & High Frequency Response of MC Cartridges ·
Due to mechanical
resonances of the stylus/cantilever assembly at very high frequencies, most MC cartridges
demonstrate a rising high frequency response starting somewhere between 10 Khz and 25 Khz.
Typically this rise will reach a peak of 6 to 12 dB at the resonant frequency which usually
occurs in the vicinity of 40 Khz. Above the resonant frequency, output response falls sharply
as the high frequency response limit of the cartridge is reached. This high frequency response
peak is audibly discernible in wide bandwidth systems and creates considerable phase error
of the signal at high frequencies. Coupled with the very wide bandwidth of many of these
cartridges, it can also needlessly stress high frequency performance of other components in
the music reproducing system.
Some cartridge manufacturers attempt to reduce this upper treble rise with additional
damping of the cantilever suspension system. While this approach may succeed in reducing
the extreme high frequency response, it frequently causes a dip of the response curve in the
“brightness” region, and a reduction of the natural “live” quality of music. Fortunately, recent
trends by cartridge manufacturers favors less mechanical damping and raising the resonant
frequency to higher values.
One popular approach employed by cartridge users to treat this problem involves loading the
low-output MC cartridge with very low value resistors (less than 100 Ohms). While this
approach has the effect of ameliorating the audible high frequency distortion (because of the
low reactive impedance of the cartridge generator the actual response curve is very little
changed), it has the undesirable side effect of reducing midrange dynamics, spatial ambience
cues, and the natural sense of “liveness” of the real performance. Very low loads can also
cause a signal loss at all frequencies, resulting in less than optimum signal-to-noise
performance of the electronics used.
A better approach to solving this problem involves using a relatively high load resistance
(1000 to 47K Ohms) and compensating for the high frequency response rise electronically
with a high frequency contouring network. A properly designed RC network as a load for the
cartridge can provide a high load impedance at low and middle frequencies, with a frequency
dependent and decreasing load as the frequency approaches the mechanical resonant
frequency of the cartridge generator assembly. This method provides both a low pass 6 dB
per octave filter, and a modicum of dynamic damping selectively applied to the region of the
resonant frequency.
With the
High Frequency Contour
capabilities of the Model 7PX3.5 Phono Preamplifier the
user can choose a quick, lucid sounding cartridge having low mechanical damping and
compensate electronically for any high frequency rise which might be present. The following
diagrams present a graphic confirmation of the results.
