This is a complex and complete power audio station (at the bottom in above image) with one stereo output and 6 stereo channels input (three of them can be mixed) that includes several stages of amplifiers. It took me a couple of years to build this unit and in 1988 was ready for use.

Technical specification includes:

-power supply: mains 230V/50 Hz
-max current load from mains power supply: 1A
-output impedance for loudspeakers: 2-50 ohm
-recommended impedance for loudspeakers: 2-8 ohm
-maximum measured audio output power (before limitation): 64 W SIN / 4 ohm (16 V ef / 4 ohm at 1000Hz)
-input adapters for: microphones, electromagnetic audio transducers, quartz piezoelectric transducers, line-in standard inputs etc
-mixer amplifiers: 6 inputs on each channel switchable in different combinations. On each channel there are potentiometers that allow the mixing of three signals simultaneously.
-audio filters: 3 active filters on each channel: RUMBLE, LOUDNESS, HISS
-tone correction, Baxendall type with BASS and TREBLE
-one level indicator on each channel, with the possibility of measuring output level on loudspeakers or on mixer output.
-monitor connector and switch with the possibility to insert other audio stages in the signal chain ( EQUALIZER, DNL or guitar effects for example).
-several electronic protections for loudspeakers, including DC signal protection, timer on power on, instant disconnect on dangerous events
-metal aluminium case

Figure 1 shows the schematic diagram of the power stage. This is a circuit built with discrete parts, 1970s technology. It was designed by me according to the schematic of a power integrated circuit. If it is powered with ±50V, you can obtain 100W on audio output.

In figure 2 is the schematic of an audio active filter. Audio "rumble" is a low-frequency noise, typically below 75Hz, often caused by external sources like traffic, HVAC systems or analog sources like vinyl records, while "hiss" is high-frequency noise from electronic components or electromagnetic radiations. "Loudness" filter is centered at 1000Hz and it makes the system sound better because it compensates the human hearing response.

Figure 3 shows the classic Baxandall EQ tone control circuit made with transistors for "bass" and "treble", which allows us to control their respective EQ bands with potentiometers.

In figure 4 is the schematic diagram for microphone amplifier module, made with two transistors. The gain is linear in the entire audio band.

Figure 5 shows the amplifier for pick-up magnetic head. It uses the RIAA type equalization to improve the frequency response of vinyl records.

In figure 6 is the schematic diagram of the mixer with three inputs. There are two mixers in this audio station which allows you to mix six different input sources.

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Links:
[1] - BC415 datasheet
[2] - BD182 datasheet