Handbook for Sound Engineers

1034 Chapter 27

boosted about 20 dB, and the high frequencies attenu-
ated by the same amount, with respect to 1 kHz, which
implies that the preamplifier with 40 dB of gain at
1 kHz will have as much as 60 dB of gain at 20 Hz and
only 20 dB of gain at 20 kHz.

It is not unusual for a cartridge producing an output

of several millivolts for the average modulation to

produce 100 mV voltage peaks. Cartridges are designed

to produce an output voltage of around 1 mV for each

centimeter per second of recorded velocity or for the

average recorded level of 5 cm/s, the cartridge output is

5 mV. Some preamplifier circuits when overloaded by

fast spikes can recover in a matter of microseconds and

resume their normal operation while others are inca-

pable of recovering fast and once overloaded stay in this

unbalanced state long enough to produce audible distor-

tion of lower-level signals that may follow. Direct

coupled stages, which don’t employ large capacitors

and inductors, have much higher slew rates and conse-

quently react much faster and with less distortion to

audio signals.

The average moving-coil cartridge produces from

0.1–0.6 mV output with the source impedance of a few

ohms and an inductance of a few millihenries, so

20–30 dB of additional voltage gain is required from the

pre-preamplifier. Because the output level of the

cartridge is so low, an extra demand for low-noise

performance is placed on the circuit. To maintain the

same SNR as in high-output moving-magnet cartridges,

the pre-preamplifier (or head-amplifier) circuit should

have 20 dB lower noise than the preamplifier for the

moving-magnet cartridges. One of the ways to achieve

this lower noise is by using a step-up transformer. The

power supply for the low-level amplifiers requires

excellent regulation and extremely low ripple voltage.

The preamplifier input for the moving-magnet

cartridges requires a 47 k: input resistance and a low,

preferably adjustable, capacitive load. The proper termi-

nation of the moving-magnet cartridge is very important

for the correct performance of the transducer.

Moving-magnet cartridges have a resistive and induc-

tive nature, so designers specify the capacitive load. If

the specified capacitive load is higher than the total

capacity of the circuit, the preamplifier should have a

provision to add capacitance to the cartridge termination

as required. If the total capacitance is larger than

needed, cables can be made shorter or replaced with

ones having lower capacitance.

27.8 Laser Turntable System

The Laser Turntable (LT) is manufactured by the ELP

Corporation, Japan, Fig. 27-28. It features a contact-free

optical pickup system allows records to be played thou-

sands of times without damage to the record. The LT

operates with five laser beams, three of which stabilize

the groove and two that read the analog audio informa-

Figure 27-27. Effect of cartridge loading on frequency
response. Courtesy Stanton Magnetics, Inc.

A. Effect of varying capacitive load on resistively

unterminated cartridge.

D. Various load combinations.

C. Effect of varying capacitive load on cartridge

terminated with 47 k7

B. Effect of varying resistive load on capacitively

unterminated cartridge.

CL

70 pF

170 pF

270 pF

370 pF

470 pF

RL = 10 M 7

+10

1 2 5 10 20 kHz

Frequency—Hz

RL

10 M 7

150 k 7

100 k 7

47 k 7

23 k 7

CL = 70 pF

0

5

Frequency—Hz

CL

170 pF

270 pF

370 pF

470 pF

570 pF

RL = 47 k 7

0

5

10

CL RL

170 pF 100 k 7

pF 100 k 7

pF 47 K 7

pF 47 K 7

Frequency—Hz

1 2 5 10 20 kHz

1 2 5 10 20 kHz

Frequency—Hz

1 2 5 10 20 kHz

0

5

10

5

+10

+5

5



+5

dB

dB

dB

dB