AEL

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====== Discrete Stereo Preamplifier ======

The circuit shown below is based on the discrete [[discrete_opamp|operational amplifier]] circuit described earlier in this Wiki.

I actually designed this circuit as I wanted a fully discrete stereo amplifier from preamplifier to power amplifier, kind of reminiscent of the early 1980's! None of that integrated circuit rubbish, here! And, I used the [[2026|AEL-2026 power amplifier]] as the outputs in the finished product.

{{:discrete_preamp:schematic.png}}

===== Circuit Description =====

This circuit is a fully discrete stereo audio preamplifier. The upper half of the schematic is the left channel and the lower half is the right channel. Both channels use the same design, so we'll just look at the left channel.

The audio signal enters through the input connector and passes through the coupling capacitor C101 to block DC voltage from the source equipment while allowing the AC audio signal to enter the amplifier.

The first amplification stage is built around Q101-103 and this is the input gain stage of the preamplifier. It provides the initial voltage amplification for the weak incoming signal while maintaining high input impedance and low noise. The stage is configured overall as a non-inverting amplifier, so the output signal remains in phase with the original input signal. The voltage gain of this stage is moderate, typically around 2.

The amplified signal then enters the Baxandall tone-control section, which is built around Q104-106. The bass and treble controls are formed by RV1A, RV2A and their associated capacitors and resistors. This section allows selective boosting or cutting of low and high frequencies. With the controls centered, the response is approximately flat. The Baxandall stage is configured as an inverting amplifier, so the signal undergoes a 180-degree phase inversion here. Depending on the control positions, this section can usually provide around +/-10 dB to +/-15 dB of bass and treble adjustment while also providing moderate voltage gain.

After the tone-control stage, the signal passes through the final voltage amplifier and output stage built around Q107-109, and this stage further amplifies the signal and lowers the output impedance so the preamplifier can properly drive a power amplifier or long audio cables. This stage is also configured overall as an inverting amplifier, introducing another 180-degree phase shift. Since the Baxandall stage already inverted the signal once, this final inverting stage restores the signal polarity back to the same phase as the original input signal. The voltage gain here is typically around 2.2.

The signal then passes through the balance control RV4, which adjusts the relative level between the left and right channels, and finally through the volume control RV3, which sets the overall output level. The output coupling capacitors C109 and C209 block any DC voltage from reaching the next piece of audio equipment.

At the bottom-left of the schematic is the power supply section. The Zener diodes ZD1 and ZD2, along with resistors and filter capacitors, generate stabilized positive and negative supply rails of approximately +/-15V. The capacitors smooth ripple and reduce noise. Using symmetrical supply rails allows the audio signal to swing equally above and below ground, improving headroom and reducing distortion.

===== Test Results =====

I built this circuit up on breadboard initially, then finally designed a PCB for it. However, I didn't simulate the circuit in TINA-TI, so I can't actually give you any definitive answer to its total harmonic distortion (THD). The original discrete op-amp this is based on had a THD of around 0.0021%; however, as there's multiple stages, this will likely increase. I would say THD to be under 0.02%; one day I may simulate the entire thing (at least one channel), and update this article accordingly.

===== Conclusion =====

The preamplifier works surprisingly well with low noise. A PCB has been designed for this circuit, but is currently not available for purchase at this time.

Please note, onsemi BC559C and BC549C transistors used in the design are now considered obsolete and may become increasingly difficult to source over time. At the time of writing this article, a few thousand devices were still available from Mouser Electronics manufactured by Diotec Semiconductor, although there is no guarantee of long-term availability. If these exact parts cannot be obtained, possible substitutes include the BC558 and BC548 transistor series. However, these alternatives generally have slightly higher noise characteristics, which may affect performance.