Hi-Fi Preamp
HiFi Preamplifier

Circuit :  Andy Collinson
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Below is one channel of a hifi pre-amplifier. It uses discrete components, and has very low 0.05% THD and flat frequency response. The circuit is shown below.

This circuit is similar to some of my other preamps, with a few modifications. To get the best performance from discrete transistors, they must be biased into the linear region of the output transfer characteristic curve. For low noise performance, they need to be run at very low collector currents. The first stage of this circuit Q1, is run at a very low collector current of 65uA. This ensures low noise performance of the first stage of the first stage as shown on the output noise curve below:

Output Noise

A second benefit of low collector current in the first stage is to keep harmonic distortions to a minimum. In this preamp THD is around 0.05%

Total Harmonic Distortion

The collector load of Q1 is bootstrapped. Bootstrapping is a technique used to apply positive feedback (from output to input, but without causing oscillation). The collector load of Q1 is R1 and R3. The bootstrap is applied between R1 and R3 junction and coupled to the Q2's output at the emitter by C2. This raises the effectively increases the value of R3 as it appears to A.C. signals, and helps flatten overall frequency response.

Signal to Noise Ratio

A final look at the preamplifiers signal-to-noise ratio is shown above. Remember one channel only is shown and current consumption is around 670uA.

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