Preamplifi ers and Input Signals 171Typical signal output levels from these cartridges will be in the range 0.02–0.2 mV/cm/s
into a 50- to 75-ohm load impedance. Normally, a very low-noise head amplifi er circuit
will be required to increase this signal voltage to a level acceptable at the input of the
RIAA equalization circuitry, although some of the high output types will be capable
of operating directly into the high-level RIAA input. Such cartridges will generally be
designed to operate with a 47-K load impedance.
7.4 Input Circuitry .......................................................................................................
Most of the inputs to the preamplifi er will merely require appropriate amplifi cation and
impedance transformation to match the signal and impedance levels of the source to those
required at the input of the power amplifi er. However, the necessary equalization of the
input frequency response from a moving magnet, moving coil, or variable reluctance PU
cartridge, when replaying an RIAA preemphasized vinyl disc, requires special frequency
shaping networks.
Various circuit layouts have been employed in the preamplifi er to generate the required
RIAA replay curve for velocity sensitive PU transducers, and these are shown in Figure
7.4. Of these circuits, the two simplest are the “ passive ” equalization networks shown in
Figures 7.4(a) and 7.4(b) , although for accuracy in frequency response they require that
the source impedance is very low and that the load impedance is very high in relation
toR 1.
The required component values for these networks have been derived by Livy^2 in terms
ofRC time constants and set out in a more easily applicable form by Baxandall^3 in his
analysis of the various possible equalization circuit arrangements.
From the equations quoted, the component values required for use in the circuits of
Figures 7.4(a) and 7.4(c) would be
RR 12 /sands6 818. CR 11 2187 μμC R2 2 109For the circuit layouts shown in Figures 7.4(b) and 7.4(d) , the component values can be
derived from the relationships:
RR 12 /sands12 38..CR 11 2937 μμC R 22 81 1