RIAA Amplifier

RIAA is actual an abbreviation of Record Industry Association of America, an organisation, as the name indicates, has its roots back in the childhood of the gramophone industry.

riaae

However, the designation is often used about amplifiers with the characteristic equaliser characteristic curve (very distorted!) shown at left.

This is used in playback amplifiers of the old gramophone records, where the last generation of pick-ups is either magnetic or moving coil type.

The necessary RIAA playback equaliser characteristic curve has three frequency references: 50, 500 and 2120 Hz. These frequencies correspond to the time constants 3180, 318 and 75 µs respectively. In the figure below is the asymptotic (blue) and real (green) characteristic curve drawn.

riaakar

The reference frequency is 1 kHz with the value 0 dB. This implies that an eventual additional gain will increase this value with an equal amount for all frequencies. For completeness the equaliser characterization is shown in the table below.

Hz

dB

Hz

dB

20

+19.3

800

+0.7

30

+18.6

1k

0.0

40

+17.8

1.5k

-1.4

50

+17.0

2k

-2.6

60

+16.1

3k

-4.8

80

+14.5

4k

-6.6

100

+13.1

5k

-8.2

150

+10.3

6k

-9.6

200

+8.2

8k

-11.9

300

+5.5

10k

-13.7

400

+3.8

15k

-17.2

500

+2.6

20k

-19.6

Magnetic pick-ups have a relative low output voltage. It is common practice to use about 2 - 10 mV output voltage at 5 cm/s as a rule of thumb. This implies that RIAA amplifiers have to be designed as low noise amplifiers to avoid a degradation of the system's noise properties.

Moving coil pick-ups have a very low output voltage, 20-40 dB below the magnetic types is a typical value. This calls for the need of an additional amplifier – or transformer – ahead of the RIAA amplifier. Here the frequency characteristic should be linear. The reason why transformers often are used here is the difficulties in designing adequate noise weak amplifiers for this type of pick-ups.

The output voltage is specified at a given modulation speed. For magnetic pick ups the output voltage is proportional to the speed, e.g. a pick up delivering 5 mV at 5 cm/s will have 1 mV output voltage at 1 cm/s. This pick up is specified to have a sensitivity of 1 mV/cm/s.

To be able to transform the sensitivity to useful information in order to design RIAA amplifiers one needs to know typical and maximum modulation speeds for the stereo records.

The RIAA playback characterization allows a maximum speed of 25 cm/s in the range 800 to 2500 Hz. For a pick up with the sensitivity of 0.5 mV/cm/s the maximum output voltage is accordingly 12.5 mV. Good stereo recordings are recorded at a typical speed of 3 to 5cm/s.

The RIAA amplifier’s nominal output voltage and its sensitivity determine the amplifier’s gain. For a pick up with the sensitivity 0.5 mV/cm/s, the output voltage is 2.5 mV  (for a speed of 5 cm/s). If the gain is 40 times (as 0 dB reference at 1 kHz), the nominal output voltage is 100 mV. At the pick up’s maximum output voltage of 12.5 mV, the amplifier’s maximum output voltage is 500 mV. Observe that an increase of the nominal gain to 200 times (in order to convey the output signal directly to a power amplifier with high sensitivity), the amplifier in worst case have to handle an output signal of 2.5 V. Even this value makes it possible to design amplifiers running on relatively low supply voltages.


 

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Copyright©1998
Knut Harald Nygaard