The signal monitor does the work of the backend monitoring when you have a test setup going. In instances where a preamplifier is under test, the signal monitor acts as the convenient block because it has its own internal speaker.All youâ„¢re going to need from the signal monitor output is again some audio indication whether or not the circuit
under test is functioning.

Circuit Description
Figure shows the electrical schematic for the signal monitor. The trusty LM 386 (IC1) audio power amplifier is going to be used as our driving IC here. Itâ„¢s a circuit thatâ„¢s been cut down to include just the basic component count necessary to make it function. Only three components are used in this design: one IC and two capacitors.The incoming signal (from the device under
test) is fed into the signal input (pin #2) of IC1, entering via jack socket J1, and coupling into the audio input coupling capacitor C1 (0.1mF).The signal ground (pin #3) is returned to the circuitâ„¢s common ground rail. The audio signal output is taken from pin #5 via a coupling capacitor, C2
(100mF), to drive a very small internal speaker.Power for the circuit is supplied from a 9-volt battery, which is fed to the Vcc terminal (pin #6). The power supply return ground goes to pin #4. A small power switch, S1, completes the component. The power amplifier has a gain of ¥20, which is more than sufficient for our purpose. As in the previous project, the customary power supply components have been shorn off in this minimalist design.

Parts List

Semiconductors
IC1: LM 386 low-power audio amplifier

Resistors
All resistors are 5 percent 1/4W.
R1: 4.7kohm

Capacitors
All nonpolarized capacitors are disc ceramic. All electrolytic capacitors
have a 25 V rating.
C1: 0.1mF
C2: 100mF
C3: 0.1mF
C4: 100mF

Additional Materials
D1: LED
Subminiature speaker, preferably 1-inch diameter size
S1: single pole, single throw miniature switch

Power supply: 9-volt battery