Audio Engineering

Preamplifi ers and Input Signals 213

system, causes each signal to be modulated by the other. This is conspicuous in the
drawing in respect to the HF component, but is also true for the LF one.

This can be shown mathematically to be due to the generation of sum and difference
products, in addition to the original signal components, and provides a simple method,
shown schematically in Figure 7.43 , for the detection of this type of defect. A more
formal IMD measurement system is shown in Figure 7.44.

With present circuit technology and device types, it is customary to design for total
harmonic and IM distortions to be below 0.01% over the range 30 Hz–20 kHz, and at all
signal levels below the onset of clipping. Linear IC op-amps, such as the TL071 and the
LF351, will also meet this specifi cation over the frequency range 30 Hz–10 kHz.

Output

Input

Figure 7.42 : Intermodulation distortions produced by the effect of a nonlinear input/output

transfer characteristic on a complex tone.

19 kHz

oscillator

LPF

1.5 kHz

Amplifier under test

Mixer

1 kHz

mV

20 kHz

oscillator

Figure 7.43 : Simple HF two-tone intermodulation distortion test.