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By: ARNOLD SCHWARTZ [President, Micro-Acoustics Corp., Elmsford, N.Y.]
With the exception of the cartridge designer, the effects of the cartridge's electrical network are most often totally overlooked.
The characteristics of the phono cartridge electrical output net work have been almost totally neglected and have become something of a skeleton in the audio technology closet. Hi-fi equipment is often described as "the audio chain." This is a very appropriate description because it reminds us that the performance of each component depends on the components pre ceding and following it, and my purpose here is to describe the electrical network coupling the cartridge to the preamplifier, an important, if somewhat neglected, link in the chain. The function of the cartridge can best be understood if we examine the phonograph record under, let's say, 1000 times magnification. If we did this, we would see a continuous undulating groove. These undulations are musical waveforms, a mechanical replica of the musical performance. The job of the cartridge is to retrieve the musical information from the record, where the musical data exists in mechanical form, and deliver an electrical signal which is a replica of the mechanical in formation to the phono preamplifier stage. In doing this, the cartridge per forms three distinct but related functions.
First, the stylus tip scans the mechanical waveforms. Second, the transducer mechanism converts the mechanical vibrations of the stylus to an electrical signal. Third, the cartridge is part of an electrical network which transmits this electrical signal to the preamplifier input stage. This network consists of the electrical element internal to the cartridge, in series with the signal; the sum of tonearm cable capacitance, interconnecting cable capacitance and preamplifier input capacitance, in shunt with the signal; and the input resistance of the preamplifier, also in shunt with the signal.
With the exception of the cartridge designer--who carefully attempts to use the characteristics of this electrical network in his overall design--the effects of this circuit on cartridge performance are most often overlooked. In fact, the performance of this electrical net work can be as important as, and some times even overshadow, the high frequency and transient performance of the scanning and transduction systems of the phono cartridge.
Recognizing the importance of this network, we have used a unique approach in our System II cartridge which has a passive network incorporated in the body of the cartridge in the form of a thick film hybrid micro circuit. It serves to match the electrical characteristics of the electret transducing element to the external circuit. The electret, the electrostatic counterpart of the magnet, functions as a transducer by directly converting mechanical vibrations to an analogous electrical signal. This internal matching net work presents a purely resistive 4,000-ohm impedance to the capacitive-resistive external circuit. The circuit itself and the method of measuring the frequency response of the System II cartridge electrical output network is shown in Fig. 1.
The transducer signal is simulated by an oscillator with an output impedance of less than 10 ohms; the sum of the tonearm cable, connecting cable, and preamplifier shunt capacitances Is shown as a single capacitor, and the typical preamplifier input resistance of 47,000 ohms is included. The cartridge itself is used as the series impedance in this test circuit so the conditions are al most precisely those in actual operation.
Although the range of capacitance in user playback systems may vary from as much as 250 pF to 1000 pF, the circuit response is measured for capacities of 300 pF and 500 pF, the more usual values for better quality playback systems.
The frequency response of the System II electrical network (see Fig. 2) is essentially flat from 20 Hz to 20 kHz (down less than 0.25 dB at 20 kHz with a 500 pF load). The circuit time constant with 500 pF is approximately 2 micro seconds so that we know the response is -3 dB at 80 kHz. The flat frequency response in the audible range means that the signal will be delivered to the preamplifier exactly as it appears at the System II cartridge output terminals, and, just as important, it will not be significantly affected by the wide variations in capacitive load found in user playback systems.
The internal electrical element of the magnetic cartridge is an inductance.
Magnetic cartridges convert mechanical vibrations to an electrical signal by inducing a signal in a coil of wire located in a magnetic field. The electrical net work of the moving magnet, moving iron, and induced magnet (all three types have essentially the same general circuit configuration) is shown in Fig. 3. This circuit is a low-pass filter with a cut-off frequency somewhere below 20 kHz, and, depending on the value of the circuit elements external to the cartridge, will exhibit something approximating a 12 dB/octave roll-off beyond its cut-off frequency. Variations in cable capacity affect the cut-off frequency and will have a pronounced effect on the frequency response.
The circuit of the moving-magnet cartridge is shown in Fig. 3. The cartridge itself is used as the series inductance to reproduce the actual playing conditions as closely as possible. Figure 4 shows the frequency response of a high-quality moving-magnet cartridge connected into the circuit as the cartridge impedance; with a 300-pF load the response is -3 dB at 15 kHz and falls rapidly to-6 dB at 20 kHz. At 500-pF load the response changes -- ± 1 dB at 7 kHz,-3 dB at 13 kHz, and -8.5 dB at 20 kHz. The signal from this moving-magnet cartridge is altered considerably by the time it arrives at the preamplifier, and it will change as the capacitive load varies from one playback system to another playback system.
The electrical output network of a moving-coil cartridge, a form of magnetic cartridge, is shown in Fig. 5 (upper circuit). Because the moving-coil cartridge has a low output, a step-up transformer is often used to boost the voltage fed to the preamplifier.
Transformers have leakage reactance which appears as a series inductance in the equivalent circuit, as well as a shunt conductance which appears as a shunt inductance in the equivalent circuit (lower circuit of Fig. 5). We have taken one channel of a high-quality moving-coil cartridge feeding a transformer, which was designed by the manufacturer to work with the cartridge, and connected it as shown in Fig. 5. The frequency response of the circuit is shown in Fig. 6 for capacities of 300 pF and 500 pF. At 300 pF the electrical response falls to -3 dB at 12 kHz and -5 dB at 20 kHz. With a 500-pF load, the response falls to -3 dB at 11 kHz and -6.25 dB at 20 kHz.
At low frequencies, due to the shunt conductance of the transformer, the out put falls about -0.5 dB at 100 Hz and -3 dB at 30 Hz. Here again, we see that the quality of the sound is altered when it arrives at the preamplifier, and it will change as the capacitive load varies from playback system to playback system.
If we compare the System II electrical system to the electrical systems of magnetic cartridges, we note two important differences:
A) While the System II frequency response is flat within the audible range, the moving-magnet and moving-coil cartridge circuits are not, and B) System II circuit response remains flat with changes in playback systems, while changes in the playback system capacitance do affect the response of magnetic cartridges.
A major significance of the 80-kHz pass band of the System II cartridge electrical network lies in the speed of its response--how fast it reacts to instantaneous changes of the music. Response speed is generally referred to as transient time, and transient time is the figure of merit for comparing the ability of the cartridge to reproduce the transient characteristics of music. Musical sounds are, actually, a series of transients. Transient response or rise time is determined by using a square-wave test signal (see Fig. 7) in our test circuits; in this case the simulated transducer feeds a square wave to the cartridge circuit.
The transient rise time is the time elapsed from the point of the leading edge of the signal when the waveform is at 10 percent of maximum height to the point when the waveform leading edge is at 90 percent of the maximum height.
This widely accepted engineering measurement method uses the most uniform segment of the leading edge to measure the transient rise time. If a transient is slow to pass through an electrical net work, only the slow-moving low-frequency components pass through unaffected, while the high frequencies, which contribute to the attack and brilliance of the music, are eliminated or delayed. To actually measure the rise times, the out put of the cartridge networks (see Figs. 1, 3 and 5) are displayed on an oscilloscope with the simulated transducer generating a 1,000-Hz square wave.
The time it takes from a point on the wave front that is 10 percent up from the bottom of the wave to a point 90 percent of the maximum height of the waveform is the transient rise time.
Figure 8 shows the oscillogram of the output of the System II electrical net work. The waveform at the left is the conventional representation of square-wave response. However, if we expand the time scale by 20 times and then look at the leading edge, we improve our resolution. The transient rise time is 2 microseconds. The time constant of the System II circuit is 2 microseconds, reflecting the 80-kHz bandwidth. The very fast rise time contributes to the ability of the System II cartridge to reproduce the attack of sharp musical sounds that are so important to the dramatic quality of the original musical performance. The comparable rise time for the moving-magnet cartridge (Fig. 9) is 25 micro seconds, while the moving-coil cartridge (Fig. 10) has a rise time of 30 microseconds. The significantly slower rise time of magnetic cartridge circuits, less than one-tenth the speed of the System II, reflects the limited bandwidth of their low-pass filter circuits. The slow rise time reduces the ability of the playback system to faithfully reproduce the attack times of the musical performance.
These measurements of cartridge circuit are of great significance to the user and illustrate why the cartridge's electrical output network has such a profound effect on the overall performance of the playback and why the cartridge designer must take the characteristics of this net work into account in the most thorough way possible in his design. We think we've done that in our System II cartridges.
(Source: Audio magazine, March 1981)