Handbook for Sound Engineers

Analog Disc Playback 1033

as it is reproducing the frequency of known amplitude
at which the measurement is being made.

Vertical Resonance. The second variable in the equa-
tion is the tonearm/cartridge vertical resonance. Tone-
arms and cartridges resonate between 5 Hz and 15 Hz;
the most desirable range is between 8 Hz and 12 Hz.
Resonance below 8 Hz will produce instability of the
tonearm and will result in poor tracking of moderately
warped records.
Stereo cartridges have fairly uniform compliance in
all planes of stylus motion. Cartridges with higher
compliance work best with light tonearms, and heavy
tonearms should be set up with cartridges having low
compliance. If the stylus compliance is low, the tracking
force applied to the stylus should be higher than for a
high-compliance stylus.

27.6.2 Cartridge Voltage Output

The output voltage of the cartridge depends on its
design and the type of generator system used. Ceramic
or crystal cartridges produce the highest voltage. Next
are the moving-magnet cartridges and then the
induced-magnet pickups; the last group is the mov-
ing-coil cartridges. The moving coil cartridge produces
higher power output than other types so they can work
with step-up transformers to increase the output voltage
10–20 times or 20–26 dB. On the other hand, some high
output voltage ceramic cartridges are connected to the
loss pads and response-shaping networks to reduce the
voltage down to the average output level of the moving-
magnet cartridges. Today most of the preamplifiers are
designed to accept moving-magnet cartridges.

27.6.3 Electrical Loading

With various output levels and different source imped-
ances, cartridges respond differently to electrical loads.
For instance, crystal or ceramic cartridges are the most
susceptible to capacitive loading. The entire frequency
response is dependent on the loading of the cartridge. In
the moving-magnet cartridge, only the highest portion
of the frequency range is affected by the capacitive
loading. The moving-coil cartridge is almost completely
immune to the loading effects. Once it is connected to
the step-up transformers, the secondary of the trans-
former becomes very sensitive to loading, and excess
capacity can play havoc with transformer resonance and
the impedance of the secondary transformer winding.
Therefore, cartridge manufacturers specify the recom-
mended resistive and capacitive loads.

The most common resistive load is 47 k: (50 k: for

Europe), paralleled by 200–400 pF of capacitance for

the moving-magnet cartridges, depending on the manu-

facturer and on the cartridge model. The capacitive

loading for the cartridge includes capacitance of all

interconnecting cables and tonearm wiring to ground (or

between the conductors), capacity added by the connec-

tors and switches. Finally the internal wiring of the

preamplifier circuit and preamplifier input circuit

capacitance, which varies widely depending on the

circuit design, adds capacitive loading to the cartridge,

Fig. 27-27. In many cases the total capacitance that

appeared as a capacitive load for the cartridge exceeded

1000 pF, which resulted in an electrical resonance peak

around 7–8 kHz followed by premature response rolloff

at frequencies above this point.

27.7 Phono Preamplifiers

Phonograph cartridges require a special type of amplifi-

cation to reproduce the recorded sound the way it

existed during the recording session. The electrical sig-

nals from the cartridge, measuring only a few millivolts

rms have to be amplified into signals of many volts.

This has to be accomplished with:

• Minimum distortion.


• Flat frequency response.


• Excellent SNR.

The phono preamplifier has to amplify a cartridge

signal without:

• Changing its phase.


• Adding more than a small percentage of harmonic
  and intermodulation distortion.


• Adding to the noise content of the original signal
  from the cartridge.


• Needing enough reserve power to handle any unusu-
  ally high transient signals.

The average required voltage amplification is

40–50 dB and is dependent on the output of the

cartridge. The dynamic cartridge produces 4–5 mV of

output for the average recording signal. A preamplifier

gain of 45 dB will boost the signal output to nearly 1 V,

the level required to drive most power amplifiers to full

output. The noise contribution of the cartridge and of

the recording medium requires the preamplifier noise

level to be at least 70 dB below the average input signal

of 10 mV.

The frequency response of the circuit should follow

the RIAA characteristics, with the low frequencies