Circuit : Andy Collinson
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:
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
>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.