Amateur acoustic amplifier "Melodia"

Amateur acoustic amplifier "Melodia"

Lech Krzymowski
Radioamator i Krótkofalowiec, Rok 20, Wrzesień 1970r., Nr 9
(Radio amateur, Year 20, September 1970, No. 9)

   The amplifier, the diagram of which is shown in Figure 1, is not a revelation or a novelty, but due to the results obtained, it may be of interest to amateurs of good music and enthusiasts of small music bands.

   The use of such units as a 3-channel mixer system and a key switch for changing the amplifier's frequency characteristics with simultaneous smooth adjustment in the range of extreme frequencies of the acoustic band gives good results, which was found using the ZK-120 tape recorder and a turntable. The results were incomparable with the capabilities of average systems, even factory production. Also the attempt to use the amplifier by the small music band was positive.

Fig. 1. Schematic diagram of the "Melodia" acoustic amplifier
(The drawing is large so you can copy it, for example to a graphics program and view the details)

   The block diagram of the amplifier in Figure 2 explains the purpose of each stage of the circuit.

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We are building a stereo amplifier

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:

  1. tone correction system (preamplifier),
  2. two-stage power output amplifier,
  3. 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.


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.

Read more: We are building a stereo amplifier

Assembling the simplest low-frequency tube amplifier (II)

Assembling the simplest low-frequency tube amplifier (II)

Radioamator i Krótkofalowiec 1961/06. Author: K.W.
(A corner for beginner radio amateurs)

  The previous issue of the magazine gave novice radio amateurs a description of the operation and construction of a tube amplifier with a very simple design with an input transformer. At the same time, a diagram of a similar circuit equipped with a potentiometer for volume control was presented. Now, as announced, we will discuss the amplifier layout in this modified version, with assembly instructions and drawings as usual. Undoubtedly, they will facilitate the correct construction of this simple amplifier.

   The operation of the amplifier, the diagram of which is shown in Fig. 1, is of course analogous to the operation of the previously discussed circuit, and we refer to it all Readers interested in its construction.

Fig. 1. Schematic diagram of the amplifier

The input circuit of the system requires a separate discussion, not only because it is still unknown to us, but above all because of its great popularity. As we know, each radio receiver or amplifier is equipped with a knob with which you can adjust the volume of the received broadcast or playback. This knob is nothing more than a component part of an element called a potentiometer.

Read more: Assembling the simplest low-frequency tube amplifier (II)

Two-stage mains powered amplifier

Two-stage mains powered amplifier

Radioamator i Krótkofalowiec 1961/11. Author: K.W.
(A corner for beginner radio amateurs)

   The simple one- and two-stage battery amplifiers described in the previous issues of the magazine helped us to get acquainted with the basic circuits of this type of amplifiers. We must say, however, that battery power, apart from its specific advantages, has a major disadvantage: it is uneconomical. Therefore, wherever possible, radio equipment is powered from AC power circuits.

   AC powered amplifiers, popularly known as "mains" amplifiers, differ from battery amplifiers in that, apart from the actual amplifying circuit, they are equipped with a power module, usually composed of a mains transformer, a rectifier tube and a rectified voltage smoothing filter. Some details about the layout and operation of the power supply were given in the previous issue when discussing the design of the power supply, intended for cooperation with a two-stage low-frequency amplifier. The power supply is usually constructed as one unit with the amplifier or receiver system (e.g. radio receivers), and only in special cases it constitutes a separate element. The latter solution is used, for example, in the popular tourist receiver "Szarotka".

   There is also a second, fundamental difference between a mains and a battery amplifier: the use of other types of tubes. This issue requires further discussion due to its crucial importance.

   As we remember from the short explanation of the principle of operation of the electron tube ("Radioamator" No. 5/61), the cathode is the source of electron emission inside it. In the case of battery-operated tubes, it is simply a thin filament, heated to an appropriate temperature. The design of the cathode of the vacuum tube adapted to AC power is more complex.

   Figure 1 shows us in cross-section the cathode of such a modern vacuum tube. It is an "indirectly heated" cathode. As we can see, it consists of two basic elements: an electric heater made in the form of a spiral of resistance wire and the actual cathode. The latter, usually made in the form of a ceramic tube, is covered on the outside with a suitable substance which, when heated to an appropriate temperature, emits electrons. As you can see, the filament circuit does not directly participate in the work of the amplification circuit.

Fig. 1. Cathode of an indirectly heated electron tube (cross-section)

Indeed, the fragment of the amplifier diagram with the tube in question shown in Fig. 2 has a filament circuit completely independent of the rest of the circuit.

Fig. 2. Part of the schematic diagram of an amplifier with an indirectly heated electron tube

   Now we can present our readers a schematic diagram of one of the mains amplifiers. As shown in Figure 3, it is a simple and economical system, as it uses only one modern ECL82 electron tube.

Read more: Two-stage mains powered amplifier

Assembling the simplest low-frequency tube amplifier (I)

Assembling the simplest low-frequency tube amplifier (I)

Radioamator i Krótkofalowiec 1961/05. Author: K.W.
(A corner for beginner radio amateurs)

   Despite the constant progress in the production and application of semiconductor elements such as diodes and transistors, the electron tube is still an essential component of most radio engineering devices. As we know, the electron tube, invented about fifty years ago, created great development prospects for radio engineering and became the basis of its extraordinary career. The knowledge of the construction and principles of operation of the vacuum tube is the first step of "initiation" of each radio technician and therefore it is also valid for beginner radio amateurs. We will establish our knowledge of electron tubes in the simplest way, i.e. by hand-assembling and testing a single-tube low-frequency amplifier. This amplifier can be used as a detector receiver and be - despite its simplicity - very useful, for example, if you need to listen to a broadcast using a larger number of headphones (2 - 6 pairs).

   The schematic diagram of the amplifier is presented in Fig. 1 in two variants, which differ in the way of feeding the signal from the detector to the amplifier circuit. In the first case (Fig. 1a), a low-frequency coupling transformer with an appropriately selected ratio is used. This system should be used when the signal obtained from the detector receiver is very weak, and we want to obtain the highest possible gain. No volume (gain) control is provided here. The use of the amplifier in the circuit shown in Fig. 1b is, however, advisable when the receiver plays programs at a relatively high volume; this amplifier is slightly simpler in design, and at the same time allows you to adjust the volume. However, we must remember that the gain provided by this system is lower than the maximum one provided by the same electron tube coupled to the detector by means of a transformer. Of course, in both cases the acoustic signal from the output of the detector receiver is connected to the same electrode, the so-called the "control grid" of the vacuum tube.

Read more: Assembling the simplest low-frequency tube amplifier (I)