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by HERMAN BURSTEIN (ab Seite 21)

Optimum performance from ceramic or crystal phono pickups may be obtained when they are matched to the amplifier input correctly. Here are the reasons, and some of the cures.

The piezoelectric phono cartridge, once disdained for high fidelity service, has become a full-fledged member of the high fidelity family, particularly in view of the facility with which it lends itself to stereo design.

However, its suitability very much depends upon its being properly loaded. Essentially, the loading problem is one of maintaining proper equalization of the RIAA recording characteristic; at the same time, output level and distortion are also at stake.

Voltage output of a piezoelectric cartridge (usually ceramic, though sometimes crystal) is proportional to recorded amplitude, rather than to velocity as in the case of a magnetic pickup.

Hence the equalization requirements differ radically from those for a magnetic cartridge. Whereas the magnetic requires bass boost and treble cut, the piezoelectric needs bass cut and treble boost in order to match the RIAA recording curve. See Fig. 1.

To obtain bass cut that reasonably compliments the RIAA recorded amplitude characteristic, the total circuit capacitance - cartridge, cable, and input tube - and the load resistance should have a time constant of about 1000 to 1300 microseconds.

For example, assume that a piezoelectric cartridge has 500 uuf capacitance, which is typical, and works into a typical load of 500 ohms. But in order for RC to be, say, 1300 microseconds, total circuit capacitance should be 2600 uuf instead of 500 uuf. Allowing 500 uuf for the cartridge and about 200 uuf more for the cable and input-tube capacitance, another 1900 uuf is required across the cartridge in order to achieve the correct amount of bass cut; that is to avoid too much bass cut.

The equivalent circuit is shown in Fig. 2. This of course is a high-pass filter, producing a decline in response below the frequency at which total capacitive reactance equals load resistance.

Instead of adding a shunt capacitor across the cartridge, the required time constant could be obtained by adding a 1.4-megohm resistor in series, as shown in Fig. 3. However, the disadvantage of this method is that the input capacitance of the input tube and the parallel resistance of the 1.4-megohm and 500-K resistors form a low-pass filter, causing a loss in response at high frequencies.

The equivalent circuit at high frequencies appears in Fig. 4. Assuming an input capacitance of 50 uuf for the input tube, a reasonable figure for a triode such as the 12AX7, response would be 3db down at about 8600 cps, declining thereafter at a rate approaching 6 db per octave.

Thus it is preferable to obtain the required time constant by means of a shunt capacitor. Both methods will result in signal attenuation because either a capacitive or resistive voltage divider is formed so far as signal output is concerned. Figure 3 already shows how a resistive voltage divider is formed, while Fig. 5 shows how a capacitive voltage divider results from the preferred method.

The loss in output voltage is in many cases a good thing, insuring that the signal is sufficiently low to prevent overloading the input tube. The writer has come across situations where cartridge signal was so great relative to the sensitivity of the control amplifier that the gain control could barely be turned up before volume became excessive. This inordinately large signal would often overload the input tube if it were located prior to the gain control. Even if the signal goes directly to the gain control, it is not a good idea to have the incoming signal so large that if someone, for example a child, were to turn the gain full on this would damage the speaker or one's eardrums or both.
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Most modern control amplifiers have fairly high sensitivity on high level input. Seldom is this less than 0.5 volt and often as low as 0.1 volt input for 1-volt output. Assume the control amplifier has sensitivity such that 0.25 volt input drives it and the power amplifier to rated output, anywhere from 12 to 60 watts or so.

A typical piezoelectric cartridge produces as much as 2 volts on peaks. To reduce this to 0.25 volt entails a reduction of about 18db. Referring to Figs. 2 and 5, shunt capacitance of 2100 uuf across a 500 uuf cartridge reduces its signal output 14db by voltage divider action. This goes a long way to prevent overdriving the control amplifier.

Although the modern piezoelectric cartridge can satisfactorily match the RIAA recording characteristic when properly loaded, many people still have old records which were recorded with LP, AES, and other recording curves, and they wish to reproduce them properly.

One way to meet this problem is to adjust for differences between the RIAA curve and other curves by means of the tone controls, using one's ears to decide when tonal balance is correct. Some persons prefer to convert the pickup into the equivalent of a velocity device inasmuch as most control amplifiers, stereo as well as monophonic, provide a variety of equalization characteristics for velocity (magnetic) pickups.

One method of converting the piezoelectric cartridge into a velocity device is by loading it with a relatively small resistor, on the order of 47K ohms or less. Such a resistor in series with the cartridge capacitance produces a high-pass characteristic, in the same manner as illustrated at (B) in Fig. 2 in connection with bass cut.

Now, however, the very small resistor causes high-pass action through all or most of the audio range, thus producing a rising response at the rate of 6 db per octave in the manner of a magnetic cartridge. Then the signal can be fed into an input designated for a magnetic pickup.

The above procedure contains a pitfall, and care must be exercised to avoid it, for otherwise one will have an undesirable treble peak.

High fidelity piezoelectric cartridges generally have a built-in treble hump, achieved by damped resonance, which produces a satisfactory approximation over most of the upper range of the treble boost required when reproducing an RIAA disc with an amplitude pickup (see Fig. 1).

If the pickup is used instead as a velocity device, this rising characteristic remains. However, by avoiding too small a load resistor, high-pass action can be limited in the treble range, offsetting the treble hump of the cartridge.

Manufacturers of piezoelectric pickups will generally supply users with information on the preferred load resistances for use of their product as a velocity device or as an amplitude device. Moreover, for smoothest response, they will frequently supply users with a schematic or ready-built adapter which contains a suitable loading network to convert the pickup into a velocity device and simultaneously reduce the treble hump to an insignificant quantity.

Bilder:
Fig. 1. RIAA amplitude playback equalization.
Fig. 2. (A), Loading for a piezoelectric cartridge to produce RIAA equalization and (B), its equivalent circuit.
Fig. 3. Alternative means of loading a  piezoelectric cartridge to produce RIAA
equalization.