Articles

Universal 15VA HI-FI amplifier

Universal 15VA HI-FI amplifier.
Radioamator, Rok X, kwiecień 1960, Nr 4 (Radioamateur, Year X, April 1960, No. 4)

Editorial office: The following description concerns a system, the model of which was built at our request and practically tested by the designer.

The amplifier described here is perfect as a final power amplifier for playing music from records or tapes in the apartment. It is also suitable for broadcasting dance music in medium-sized halls and clubs.

Construction is easy; it can be performed by any radioamateur with basic theoretical and practical knowledge.

Output stage.

Due to the need to obtain a power equal to at least 10VA with very low nonlinear distortions, I decided to use the output stage amplifier in a push-pull configuration with negative feedback. This is called "ultralinear" system. The feedback voltage in this system is obtained from taps, or from separate windings on the output transformer. By changing the ratio of the alternating voltage of the screen grid to the anode alternating voltage, we change the working conditions of the end tubes. With this ratio equal to one, the tubes work as triodes, because the shielding grids are connected with the anodes, while with the ratio equal to zero, they act as pentodes (Fig. 1).


Fig. 1. "Ultralinear" system.

For the ratios with intermediate values, the circuit has a number of advantages over push-pull circuits, both with triodes and pentodes, and above all, less nonlinear distortions at large and small signals, at the cost of low power loss. Moreover, such feedback significantly reduces the internal resistance of the system.

For EL34 tubes, the 20% tap is the most advantageous (Usz / Ua = 0.2, of course, from the power supply side).

The "ultralinear" circuit with EL84 tubes gives about twice less nonlinear distortions compared to the conventional push-pull circuit with 10% lower power. The "ultralinear" circuit requires a special design of the output transformer.

Output transformer.

Both halves of the anode windings of the transformer should be placed symmetrically by dividing the window into two sections, each for one half of the anode winding. The unbalance has a significant impact on the phase shifts and amplitude differences between the anode current and the screen voltage, so that even harmonics of higher frequencies may not reduce each other.

Fig. 2 shows a diagram of the windings of such a transformer, and Fig. 3 shows the arrangement of the windings.


Fig. 2. Diagram of the windings of the output transformer.
Primary windings: 0.18mm copper wire in enamel.
Secondary windings: 0.75mm copper wire in enamel.

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Portable universal amplifier

Portable universal amplifier.
Radioamator, Rok X, wrzesień 1960, Nr 9 (Radioamateur, Year X, September 1960, No.)

This description concerns the model awarded with the consolation prize in the Great Radio Amateur Model Contest.

From the editorial office:

The amplifier described below is an example of a well-thought-out design for a specific purpose. A simple but full-fledged system, proper use of power thanks to the use of efficient domestic speakers, good adaptability to portability, cheap and effective finish - these are the main features of the device. We can recommend the construction of this device to anyone who wants to have a good portable amplifier for playing recordings from discs, enhancing solo performances and increasing the acoustic power of the receiver when using dance music..

The AMPLIFIER was designed and made for a stage singer producing with an electric guitar. On this assumption, the device should meet the following conditions:

  • small dimensions and weight,
  • high quality playback,
  • high (10W) output power,
  • easy to carry and easy to use.

It would be very difficult to meet the above conditions to the full extent, therefore a compromise solution was necessarily applied in the model made. First of all, a system with a mains transformer was adopted (Fig. 1), although in the universal version the weight of the apparatus would be absolutely lower.


Fig. 1. Schematic diagram of the amplifier.

Nevertheless, it was found necessary to galvanically separate the amplifier, microphone and guitar circuits from the network. In contrast, the reduction in dimensions and weight was achieved by a different route. The output power of the amplifier was limited to 5W, filling its final stage with a single EL84 pentode. The reduced output power, in turn, was used as rationally as possible, feeding two lightweight speaker sets with relatively high efficiency.

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New types of tubes for push-pull and stereo amplifiers

New types of tubes for push-pull and stereo amplifiers.
Radioamator i Krótkofalowiec, Rok XI, Marzec 1961, Nr 3

Among the many new types of receiver tubes that have recently been introduced to the market by Western European manufacturers, the ELL80 and PLL80 dual speaker pentodes deserve special attention. The idea of placing two tube systems in one bulb has been known since the birth of the now obsolete ECL11 tube, which at the time was a kind of revelation; it was followed by further, more modern types, such as ECL82 ... 86 and their equivalents in the U and P series. Placing two end pentodes in one balloon is, however, something completely new, resulting from the current needs of the electronics industry, in particular from the needs of modern stereo technology.

The final stages of the stereo amplifiers were initially filled with the standard ECC83 + 2xEL84 set, consisting of a total of three tubes. The use of ECL82 tubes made it possible to reduce the number of  tubes to two, and - with the same number of stages - had an impact on the cost of the device. However, equipping the same circuit with the ECC83 - ELL80 set is more rational at the same cost, as it allows for a favorable and transparent assembly, allowing you to easily avoid undesirable microphonics, various types of couplings, etc. As you know, two stages with a very strong overall amplification (in the case of an ECL tube) is quite critical and requires careful elaboration of both electrical and mechanical systems in terms of the stability of the system. The popular ECL11 tube was particularly capricious in this respect.

Fig. 1. Schematic diagram of the 2 x 3W amplifier with ECC83 and ELL80 tubes and its technical parameters.


Fig. 2. Schematic diagram of the 2 x 2.6W amplifier with ECC83 and PLL80 tubes and its technical parameters.
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Fig. 3. Schematic diagram of the push-pull amplifier with ECC83 and ELL80 tubes and its technical parameters.

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Design of output transformers

DESIGN OF OUTPUT TRANSFORMERS

Radio amateur and amateur radio operator of Poland, Year 24, December 1974, No. 12.

Low-frequency tube amplifiers, especially with higher power, are still built by radio amateurs. The output transformer is the most difficult element to design and manufacture. This is evidenced by inquiries and requests for help in the calculations sent to the editorial office. The basic principles of designing transformers intended to be made in amateur conditions presented here briefly should satisfy the wishes of interested readers.

The principles of designing low-frequency transformers in amateur conditions are slightly different from those used in industry. First of all, it is determined approximately what core is needed for the designed amplifier. Then a more or less suitable core is searched for, and after it is acquired, further winding calculations are made. After establishing approximate data as to the necessary winding wires, wires with diameters similar to the selected ones are purchased and only then the number of turns of individual windings is finally determined.

The basic relationships linking the phenomena in a transformer result from the following formula:

Etr = 6,28⋅f⋅n⋅Q⋅B⋅10-4        (1)

where:

  • Etr - the amplitude of the reverse-electromotive force induced in the primary winding, approximately equal to the amplitude of the supplied voltage [V],
  • f - frequency [Hz],
  • Q - core cross-section [cm2],
  • n - number of turns of the winding,
  • B - the highest induction value in the core [T].

The value of the reverse electromotive force is related to the alternating voltage of the final amplifier stage and results from the power and operating resistance. The highest and the lowest frequency of the passband results from the assumptions. The highest allowable induction value in the core should not exceed 0.6T. For Hi-Fi amplifier transformers, it is recommended to take 0.4T. Two unknowns remained in the formula given: the core cross-section (Q) and the number of turns (n). We determine the core cross-section approximately from the formula:

where:

  • Pwy - the output power of the amplifier.

As far as possible, we aim to build a transformer with a large core cross-section, which will allow to reduce the number of turns in the windings. This is important both due to the undesirable leakage inductance of the transformer and the degree of difficulty of its manufacture. In transformers composed of sheets with holes for fastening bolts, it is necessary to check that the core cross-section near the bolts is not smaller than that of the main column.

The simplified transformer substitute circuits are shown in Fig. 1. At the lowest frequency, the influence of the inductance of the transformer primary winding, which is connected in parallel to the appropriate amplifier load, should be taken into account. In most cases, it is the necessity to obtain a sufficiently large value of this inductance that determines the number of turns of the primary winding. At medium frequencies (1000Hz is assumed), only the winding resistances play an important role. At high frequencies, the influence of the leakage inductance is noticeable, the value of which depends on the number of turns, the transformer winding scheme and its quality. This inductance, in combination with the inter-winding capacitances, creates a low-pass filter limiting the transformer bandwidth..


Fig. 1. Simplified equivalent transformer diagrams.
a - equivalent circuit for the lowest frequencies,
b - equivalent circuit for medium frequencies,
c - equivalent circuit for great frequencies (treble and ultrasound).

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Class D transistor amplifiers

Class D transistor amplifiers

Radio amateur and short wave amateur 1971/03
Wojciech Czerwiński

Jerzy Kwaśniewski

Pulse circuits with transistors switched from cut-off to saturation have found wide application in digital and automatic control technology, among others due to their high efficiency. The search for a way to increase the efficiency of transistor acoustic amplifiers has led to an application for amplifying low frequency signals switching amplifiers working in class D. Such amplifiers show efficiency of 90%, unattainable in conventional systems of class A, B or C.

Principle of operation

Let us briefly consider the principle of amplifying harmonic signals in pulse amplifiers. Class D work is a type of amplifying element operation in which the element in the working cycle is only in two states: complete blocking when no current flows through it, or complete unblocking when the voltage drop on it is close to zero. If a current flows through the resistor in the form of rectangular pulses shown in Figure 1, then the average value of the current Io marked with a dashed line may be equal to zero (Figure 1a), greater than zero (Figure 1b) or less than zero (Figure 1c) . The ratio of the pulse duration to the repetition period is called the duty cycle γ.

For the waveforms in Figure 1, the γ factor is respectively: γ = 0.5, γ> 0.5 and γ <0.5. If the change of γ from the maximum to the minimum value is carried out according to the function e.g. sin (ωt), then in such a series of pulses the average value of the pulses will be the low frequency harmonic component sin (ωt).


Fig. 1. Rectangular waveforms with different fill factors.

Otherwise, the stage operating in the impulse system can amplify the low frequency vibrations, if we apply a pulse train in which the positive half-period of the low frequency signal is the pulses with the value of γ> 0.5 correspond to the negative half-period, and the pulses with the value of γ <0.5 correspond to the negative half-period (Fig. 2). We are dealing here with pulse width modulation. It goes without saying that the pulse repetition frequency should be much greater than the highest frequency of the amplified low frequency waveform.

 
Fig. 2. Pulse width modulation with a sine wave>
a - modulating waveform, b - impulse modulated waveform.

Pulse width modulation can be implemented by various methods. The simplest way is shown in Fig. 3. The triangular voltage is compared with the sinusoidal voltage representing the low frequency signal. Square-shaped pulses appear at the modulator output, the length of which depends on the amplitude of the amplified signal. The triangle waveform can be obtained from square pulses using a Miller integrator.

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