Audio Signal Conditioner Circuit

This discussed audio signal conditioner circuit is great for sweeping weak and noisy audio or Morse-code signals. The circuit can also be utilized to clean up digital audio signals which are of the same frequency in a remote-control extender circuit.

How it Works

Referring to the circuit diagram below, the audio signal is attached through C1 and R9 to the input of U1 (at pin 3), which is a 567 phase-locked loop (PLL). The values of R1, R7 and C2 settle the detector’s operating frequency.

The moment a tone is recognized, U1’s output at pin 8 is attached to ground for the interval set by the input signal. If the audio wave is pulsed on and off, U1’s output follows in step with the input signal.

The output of U1 is supplied to the base of Q1, which is a 2N3906 general-purpose PNP transistor.

This BJT is applied to switch power to a second 567 PLL (U2) on and off. LED1 starts to flicker on and off in the same step with the coded input signal. Integrated circuit U2 functions as a set oscillator, producing a new steady-amplitude output signal.

There is no need for the enhanced tone to have the same frequency as the input.

However, it can be set to a different frequency using resistor R8. The circuit’s output frequency is dictated by R4, F8 and C4. Transistor Q2 is applied to separate the output of U2 at pin 5 via external loading.

The real values of frequency-determining components (as shown in the parts list) for this audio conditioner circuit permit the two PLL’s to tune from a low of a few hundred hertz to a high of several thousand hertz.

The simplest way to elevate or decrease the running tuning range is by increasing or lowering the values of C2 and C4.