Earphone Intercom

This article is about an earphone intercom that works without the delay line.

Figure 1: Device.

You can see the circuit working in this video:

Step 1: Design the Circuit

I designed the circuit via PSpice simulation software.

Figure 2: Circuit Design Preamplifiers.

I designed a class A power amplifier:

Figure 3: Circuit Design Power Amplifiers.

Step 2: Simulations

I simulated the two circuits:

Figure 4: Preamplifiers Transient Simulations.

Figure 5: Preamplifiers Frequency Simulations.

Figure 6: Power Amplifiers Transient Simulations.

Figure 7: Power Amplifiers Frequency Simulations.

Step 3: Make the Preamplifiers

I made two circuits. I took a photo of only one circuit.

Figure 8: Preamplifier.

Step 4: Make the Power Amplifiers

I used a big heat sink with heat transfer past for the power amplifier.

Figure 9: Power Amplifier.

I got away with using two 22 uF bipolar capacitors for input signal DC signal elimination when I should have used two 470 nF capacitors instead. Yet the circuit was not oscillating. I wanted a wide frequency bandwidth.

Step 5: Encasement

You can see that I have an LED connected in series with two 1 kohm resistors (total 2 kohms) being supplied 20 mA.

Figure 10: Encasement.

Step 6: Testing

Testing showed that there is a lot of noise in the low-cost 10 metre cable that I used.

Conclusion

An audio amplifier with a microphone input usually needs a delay line to avoid feedback oscillations, caused by sounds from the speaker entering the microphone, propagating through the amplifier stage (from microphone to speaker) to complete the feedback loop.