- Category: Radioamator i Krótkofalowiec
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Hi-Fi 20W amplifier with psophometric volume control
Author: Stanisław Głowacki
Radioamator i Krótkofalowiec, Year 15, December 1965r., No 12
The human ear is not equally sensitive over the entire range of audible frequencies. It shows maximum sensitivity for frequencies ranging from about 1kHz to 3kHz, and this property occurs the stronger, the weaker the intensity of the sound perceived by the ear. This non-linear frequency response of the ear reduces the perceived sound experience when listening to music at low volume levels. At high sound levels the differences in ear sensitivity decrease and the reception is more accurate. It follows from the above that the adjustment of the volume of the reproduced broadcasts should be related to the adjustment of the frequency characteristics of the loudspeaker amplifier.
At a low volume level, i.e. with lower amplification of the amplifier, the low tones and high tones should be emphasized in relation to the medium tones, or the medium tones from 1 ÷ 3kHz should be muffled in relation to the low and high tones, and so much more, the smaller it is. power output from the amplifier. As a result, it is possible to achieve such changes in the characteristics of the amplifier that the ear will perceive the emission with the full balance of sounds, regardless of the output power.
Fig. 1 shows a schematic of a high-quality amplifier that meets the above conditions. The 20W output power allows you to amplify large rooms or drive a large sound column.
Fig. 1. Schematic diagram of the 20W power amplifier
The first stage of the amplifier with the ECC85 electron tube is a voltage amplifier with a cathode follower that controls the low-resistance quadruple of the negative feedback circuit. The frequency response of the quadruple has a flat maximum in the range of 1 to 5kHz, so that the negative feedback at these frequencies is the strongest. The reverse voltage Uzw is fed to the opposite end of the volume control potentiometer in relation to the voltage Uo controlling the amplifier. The feedback voltage is deposited on the resistance of the potentiometer and the internal resistance Ro of the signal source Uo, as shown in Fig. 2.