Design Ideas: July 7, 1994
Testing the frequency response of an audio device typically requires a sine-wave oscillator, an ac voltmeter or an oscilloscope, and an operator to record the response of the device at several frequencies across the audio band. Complicating this test is the inability of many audio oscillators to maintain a constant output level as the frequency varies, particularly when switching between frequency ranges.
To speed testing, the circuit in Fig 1 generates a composite audio signal comprising 10 sine waves of 10 different equal-amplitude frequencies across the audio band. Values stored in EPROM determine the frequencies. Driving an audio device-under-test with this circuit allows you to test that device's responses quickly by connecting its output to an audio-spectrum analyzer. A quick check of frequency response requires only a few seconds. The output amplitude at the LINE output is 1V rms, and harmonics are all below -20 dB.
IC1 generates a 128-kHz clock, which R1 adjusts, to drive counter IC2. The count from IC2 addresses successive locations in EPROM IC3. The EPROM contains the data values to generate the 10 octaves of equal-amplitude sine waves (see listing in EDN BBS /DI_SIG #1559). In this case, the frequencies are 1 octave apart from 31.3 Hz to 16 kHz. IC4 and IC5 latch the 8-bit data from IC3. The latched values drive IC6, an 8-bit DAC.
The first amplifier of IC7, a quad op amp, converts the current output of IC6 to a voltage. The next two amplifiers filter the resulting signal, removing all frequencies above 20 kHz. The signal then goes through a level control and pad resistors to permit connection to the line or microphone inputs of the audio device. The signal also goes through the last section of IC7 to an inverse RIAA filter. This filter compensates for the magnetic phono input of an audio amplifier, resulting in an overall flat frequency response of the test setup.
Two 9V batteries power the circuit. Voltage regulator IC8 provides 5V power for IC1 through IC5. The resistors between -9V and each of the LM324 outputs prevent crossover distortion. All of the ICs are low-power to increase battery life. (DI #1559)