8.2.1.2 Detailed Design Procedure

Vinyl records are recorded using an equalization curve specified by the Recording Institute Association of

America (RIAA). The purpose of this equalization curve is to decrease the amount of space occupied by a

groove on the record and therefore maximize the amount of information able to be stored. Proper playback

of music stored on the record requires a preamplifier circuit that applies the inverse transfer function of the

recording equalization curve. The combination of the recording equalization and the playback equalization

results in a flat frequency response over the audio range, as Figure 8-3 shows.

-20

-15

-10

-5

0

5

10

15

20

10

100

1000

10000

Frequency (Hz)

C009

Recording Curve

Playback Curve

Combined Response

Figure 8-3. RIAA Recording and Playback Curves Normalized at 1 kHz

The basic RIAA playback curve implements three time constants: 75 μs, 380 μs, and 3180 μs. An IEC

amendment was later added to the playback curve and implements a pole in the curve at 20 Hz with the

intent of protecting loudspeakers from excessive low frequency content. Rather than strictly adhering to the

IEC amendment, this design moves this pole to a lower frequency to improve low frequency response and still

provide protection for loudspeakers.

Resistor R1 and capacitor C1 are selected to provide the proper input impedance for the moving magnet

cartridge. Cartridge loading is specified by the manufacturer in the cartridge datasheet and is absolutely crucial

for proper response at high frequency. 47 kΩ is a common value for the input resistor, and the capacitive

loading is usually specified from 200 pF to 300 pF per channel. This capacitive loading specification includes

the capacitance of the cable connecting the turntable to the preamplifier, as well as any additional parasitic

capacitances at the preamplifier input. Therefore, the value of C1 must be less than the loading specification to

account for these additional capacitances.

The output network consisting of R5, R6, and C5 serves to ac couple the preamplifier circuit to any subsequent

electronics in the signal path. 100-Ω resistor R5 limits in-rush current into coupling capacitor C5 and prevents

parasitic capacitance from cabling from causing instability. R6 prevents charge accumulation on C5. Capacitor

C5 is chosen to be the same value as C4; for simplicity however, the value of C5 must be large enough to avoid

attenuating low-frequency information.

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SBOS901C – MARCH 2019 – REVISED SEPTEMBER 2022

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