- Category: Radioamator i Krótkofalowiec
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We are building a stereo amplifier
Eng. Zbigniew Faust
Radioamator i Krótkofalowiec, Rok 22, Maj 1972r., Nr 5
(Radio amateur and amateur radio operator, Year 22, May 1972, No. 5)
(Prepared on the basis of "Funktechnik" no. 23, 24/1965 and no. 2/1966)
Here is a description of the construction of a stereo amplifier intended for cooperation with a stereo turntable. The amplifier has two channels: left and right. Each channel consists of an input stage, volume and balance controls, and an output stage. In the input stage, the weak signals from the turntable are pre-amplified, as well as the correction of the frequency characteristics of the reproduced recordings from gramophone records by appropriate raising or lowering of bass and treble. The volume control system allows you to continuously change the sound strength of recordings, while the balance control allows you to equalize the playback volume of both channels. The output stage is no different from a similar mono amplifier stage.
Taking into account the design, the amplifier has been divided into 3 parts:
- tone correction system (preamplifier),
- two-stage power output amplifier,
- power supply system.
The individual elements of the system are mounted on separate bakelite plates, thanks to which experimenting with the system is very easy.
Basic technical parameters:
- Output power: max 2×4W
- Sensitivity: about 100mV
- Frequency response: 5Hz÷20kHz (-3dB)
- Nonlinear distortions: less than 5%
- Power supply: 220V/50Hz
- Power consumption: about 80W.
SOUND TONE CORRECTION
The circuit includes two levels of voltage amplification in each channel and a tone control system, separately for low and high tones.
The schematic diagram of the system is presented in Fig. 1. Both amplifier channels are the same and therefore it is enough to describe only one of them. The input from the stereo turntable passes through the standardized input jack [We] and the coupling capacitor C1 to the sound level controller R1, and then through the capacitor C2 to the tube control grid L1a. The grid leakage resistor is 1MΩ. There is a resistor R6 in the cathode circuit of the electron tube to generate the grid bias voltage, blocked by the capacitor C4. The signal amplified in the anode circuit is fed through the resistor R4 and the capacitor C6 to the tone control system. So that the treble is not weakened too much, the R4 resistor is bypassed by the C5 capacitor. In the first amplification stage, there is furthermore a feedback between the anode and the grid of the tube L1a (resistor R5). Due to this coupling, a more linear transmission characteristic and a lower coefficient of non-linear distortion are obtained.
Fig. 1. Schematic diagram of the tone correction system
The tone control system consists of two RC circuits. The first circuit includes elements R7R8R9C7C8 for bass adjustment, and the second circuit (C9R11C10) allows the treble response to be changed. The R10 resistor decouples the bass control circuit from the treble control circuit.