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Audio Amplifiers

# Equalization Networks

It is often desirable to shape the frequency response of the amplifier to a prescribed characteristic. This may be done by a method similar to that used in the design of the gain (volume) control circuits discussed previously, except that the gain control resistance must be replaced by a combination of frequency-dependent elements. An example will illustrate the design procedure.

**Example.**

It is required to compensate the two-stage amplifier for the RIAA playback
characteristic for microgroove records. The preamplifier is to operate from
a variable-reluctance cartridge having the following specification:
peak output at 1 kHz = 50 mV; terminating resistance for a
high-frequency roll-off at 2120 Hz = 6200 ohms.
The figure below shows the RIAA playback characteristic.

Since the high-frequency roll-off is obtained by terminating the cartridge in a 6200 ohms load, it is only necessary to design a compensating network for the response characteristic of the figure below. We choose to place the compensation networks in the first stage in order to obtain a minimum operating dynamic range for the transistors.

The circuit will take the form of the figure above. The equivalent circuit is
shown in the figure below (view A), with *R*_{2}, *R*_{3},
and the 6200 ohms resistance combined in parallel in a common term *R*_{g}.

This may be further redrawn in the form of the figure above (view B) where *R*'
is the parallel combination of *R*_{g} and *h*_{11e}.

Then

Since

the current entering the transistor (*i*_{in}) has the same
frequency characteristic as *i*'.

At high frequencies, the output is down to 0.1 of its initial value. Therefore

The frequency *f*_{2} occurs when *R* is equal to the reactance
of the capacitor; whence *C* = 1.7 μF.

A study of the RIAA playback characteristic shows that the
location of *f*_{2} determines the amount of bass boost over
the entire low-frequency range, whereas the location of *f*_{1}
determines only the gain at very low frequencies. Thus, while *f*_{1}
may be varied considerably without much apparent effect, it is important that the
location of *f*_{2} be maintained critically. This will be
accomplished if the product of *R* and *C* is maintained constant.
With this restriction in mind, minor changes in *C* are permissible.
In general, it is preferable to increase *C* since this will lower
*f*_{1} slightly. In the final design, this may result in a reduction
in the size of the emitter bypass capacitance. The final version is shown
in the figure below.