SHA1 Stereo Headphone Amplifier Kit

What is it and how does it work? Let’s take a look at the schematic diagram, and we will follow through from input to output to get a general idea how this kit works, and why. We will look only at the right channel circuitry starting at J1 since the left channel is identical to the right. A line level audio signal is placed in J1. Line level means an audio signal of around 1V peak to peak and will give a reading of 0dB on a VU meter. The audio passes through R1 the input audio dropping resistor. If your audio source has a hot output level this resistor can be increased in value to give better control over output volume. It can be decreased if your source has a low drive level. R3 is the front panel right channel volume control. The volume control allows the user to variy the incoming signal level that is passed on to R5. C1 is a coupling capacitor. This capacitor prevents DC from entering the circuit from the audio source. It also prevents the setting of R3 from changing the bias on the amplifier U1. Without C1 both input DC and varying R3 would cause unwanted changes in audio quality. U1, the LM386, is a fully integrated audio amplifier, capable of driving low impedance loads. It requires very few external components, runs very efficiently and has great fidelity. With the sibilance switch open, the audio passes directly into U1 and the signal is amplified to drive a low impedance speaker like those in your headphones. With the sibilance switch closed, the capacitor C4 is switched into circuit. This capacitor shunts some of the higher frequencies to ground but leaves the lower frequencies alone. C4 creates a low pass filter when combined with R5 and reduces the level of high frequencies that are amplified in U1. This is really handy if you have a noisy weak station coming in or the signal source is really Ramsey Customer Use Only Not For Publication

SHA1  4

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