Amateur electroacoustic amplifiers - Universal amplifier with a power of 2x4W

Aleksander Witort, Amateur Electroacoustic Amplifiers
Third Edition, Warsaw 1972

I recommend Appendix 2 of the book on electroacoustic amplifiers. You can read the full table of contents here (for the curious).

Appendix 2

Universal amplifier with a power of 2x4W

   For good playback of dance music records in a room larger than 50 m², a 3 ÷ 4 W amplifier is insufficient. Building a push-pull amplifier with higher power is quite difficult. What solution can be found? A good solution is to build a 2×4 W amplifier, the schematic of which is shown in Fig. D2-1. It consists of two completely identical two-stage power amplifiers.

   The parts intended for this amplifier should be carefully inspected and matched in pairs. This applies especially to resistors, coupling capacitors, and output transformers. It's also a good idea to choose the output tubes.

Fig. D2-1. Schematic diagram of a 2×4 W amplifier

   Of course, the Za1 and Za2 power supplies should have the same voltage. Two separate power supplies were used because it is easier to use components from typical radio receivers. The power supplies can be the same as those used in the amplifier described in Appendix 1. A single, higher-power power supply (voltage 250 ÷ 280 V, permissible load 120 mA) can also be used.

   Variable resistor R₁ is used to equalize the gain of both amplifiers. This is accomplished as follows: an AC voltage source (an acoustic generator or, as a last resort, a 50 Hz AC voltage drawn from the filament winding via a 5 ÷ 10 kΩ potentiometer) is connected to the parallel inputs of both amplifiers. The output voltage of each amplifier is then measured with a universal volt-ammeter, at rated load and with a drive equal to 50% of the maximum value. The slider of variable resistor R₁ is set so that the output voltages of the amplifiers are the same.

   The depth of negative feedback depends on the value of the R_sz resistor. As a guide, the following values ​​can be assumed: 700 Ω for a 4 Ω output, 100 Ω for an 8 Ω output, and 1500 Ω for a 16 Ω output. The correct value should be determined during the final amplifier tests. The amplifier's sensitivity, depending on the depth of negative feedback, is 0.5 ÷ 1 V.

   Both amplifiers and their power supplies should form a single structural unit. The enclosure can be a metal cabinet, wooden box, or suitcase, depending on the amplifier's intended use. The amplifier's inputs and outputs, as well as the feedback circuit terminals (Sp1 and Sp2), should be connected to terminals on a Bakelite board, positioned so that the appropriate switching is possible. Several possible uses for the amplifier are shown in Fig. D2-2.

   The first amplifier can be used to amplify only low frequencies and drive the subwoofer, while the second amplifier can amplify midrange and high frequencies. In this case, a frequency separation filter (F_roz in Fig. D2-2a) should be connected between the signal source and the amplifier input. Such a filter can be assembled using RC elements on a separate Bakelite board. The optimal cutoff frequency depends on the speakers used.

Fig. D2-2. Connection methods for a 2x4 W amplifier
a – two-channel amplifier, b – amplifiers connected in parallel, c – separate use of amplifiers
F_roz – filter separating lower and higher frequencies, M – microphone, Git – guitar pickup

   Both amplifiers can, by connecting their outputs in series, power a single loudspeaker or loudspeaker array (Fig. D2-2b). The impedance of the connected loudspeaker should be twice as high. Otherwise, the amplifiers' feedback circuits must be connected. Terminal Sp1 should be disconnected from the output of the first amplifier (Wy1) and connected directly to terminal Sp2. The output power of the amplifiers connected in this way is approximately 8 W. The output of the first amplifier (Wy1) is disconnected from ground and connected to the output of the second amplifier (Wy2).

   When amplifying a soloist's voice and the sounds of musical instruments, the amplifiers can be used independently, powering the loudspeakers separately (Fig. D2-2c). Other combinations are also possible, for example, two guitars can be connected via a mixer to the first amplifier, and a bass guitar to the second amplifier.

   When using a single broadcast source, both inputs should be connected in parallel and the outputs can power two loudspeakers.

   The amplifier can also be used to play stereo records. In this case, the first amplifier is used for the right channel, and the second for the left.

   As a supplement to the amplifier described, various transistor preamplifiers with an output voltage of at least 1 V can be built.