Circuit : Andy Collinson
The audio amplifier is built around a single LM380. This IC will deliver up to 2 watts into an 8 ohm speaker. C1 provides extra supply decoupling from mains hum, whilst the zobel network formed by R2 and C5 prevent a wideband oscillation at 10MHz. (Authors note: I have once experienced the 10MHz wideband oscillation. This was on an LM380 amplifier without the zobel network; when oscillation occurs, the audio is distorted and current consumption is excessive.)Input Circuit
The electret mic is amplified by a two stage, direct coupled preamp comprising two BC549C transistors. Electret mics have two terminals, older types may have three terminals,see this page in the practical section for more detail.
Gain is controlled by the 10k preset, once set for a particular room environment listening volume is controlled at the remote location using the 10k potentiometer. The preamp power supply is decoupled by the 1k and 200u capacitor, the first stage is run at a low collector current to ensure a high signal to noise ratio. The second stage is an emitter follower which ensures a low output impedance for driving long cables. As the output impedance is low, hum and noise pickup are minimum and therefore screened cable is not required.
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