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 R₂, R₃, 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 f₂ 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 f₂ determines the amount of bass boost over the entire low-frequency range, whereas the location of f₁ determines only the gain at very low frequencies. Thus, while f₁ may be varied considerably without much apparent effect, it is important that the location of f₂ 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 f₁ 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.