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)
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:
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).
Class D transistor amplifiers
Radio amateur and short wave amateur 1971/03
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.
Radioelectronics, December 1990, volume XLI (139)
FROM THE COUNTRY AND FROM THE WORLD (cover page 2)
ELECTROACOUSTICS - "Duetto-Stereolith" SPEAKER SYSTEM (2)
The company REVOX, known for its high quality products, offers a stereo loudspeaker system operating in accordance with a new concept. As it is easy to reproduce and gives interesting sound effects, it is worth getting to know it better.
The classic way of stereo reproduction requires the use of two loudspeakers separated by a distance, the so-called base. The optimal stereo listening position is in front of the loudspeakers, symmetrically to them. It has long been noticed that when using loudspeakers with a wide angle of sound or omnidirectional radiating units, the position of the listener in relation to the loudspeakers is less critical, but the stereo image of the sound becomes less accurate, especially in terms of the location of the apparent sound sources.
The Swiss acoustician - Walter Schupbach - proposed a solution consisting in placing the speakers of both channels in one housing in such a way that they radiate in opposite directions, i.e. to the left and to the right of the listener. It turned out that the best results are achieved by a housing with a triangular cross-section (see the drawing), divided by a wall into two parts. This solution can be compared to the radiation of sounds by one source, in principle omnidirectional, but the sounds radiated to the left and right and their reflections differ due to the fact that they correspond to different stereo channels.
DISTORTIONS CAUSED BY ELECTROLYTIC CAPACITORS (2)
When designing electronic circuits, capacitors are considered linear elements. This does not correspond to reality, especially in the case of electrolytic capacitors, which under certain conditions may cause nonlinear distortions. This is the subject of this article.
We know that the capacitance of electrolytic capacitors varies significantly with temperature changes (it decreases rapidly at subzero temperatures) depending on the frequency of the alternating current. The capacity of a good, fully functional and formed electrolytic capacitor changes slightly depending on the value of the DC bias voltage, as shown in the figure.
It follows that in the case of electrolytic aluminum capacitors there is an increase in capacitance with an increase in the polarizing voltage, and in the case of tantalum capacitors - on the contrary, there is a decrease in capacitance.
Radio engineering time travel (1930)
I invite you to look at the "pictures" related to radio technology in 1930. They come from various sources. The gallery has its sister version in Polish and Russian. Versions in other languages will be added successively. More detailed descriptions can be found in other language versions (articles are related and can be displayed by clicking on the icons with flags on the left, top of the TRIODA website). Of course, I will complement translations of signatures quite slowly. All descriptions are displayed using the present tense and not the past - this is not a mistake only deliberate action - a reference to the original descriptions from those years. In order not to delay the possibility of viewing the gallery, this site is made available as a "site under construction" - for which I apologize to all guests. At the beginning and end of each set of photographs there are links allowing to change the period - calendar year.
Radio engineering time travel (to 1921)
I invite you to look at the "pictures" related to radio technology. They come from various sources. The gallery has its sister version in Polish and Russian. Versions in other languages will be added successively. More detailed descriptions can be found in other language versions (articles are related and can be displayed by clicking on the icons with flags on the left, top of the TRIODA website). Of course, I will complement translations of signatures quite slowly. All descriptions are displayed using the present tense and not the past - this is not a mistake only deliberate action - a reference to the original descriptions from those years. In order not to delay the possibility of viewing the gallery, this site is made available as a "site under construction" - for which I apologize to all guests. At the beginning and end of each set of photographs there are links allowing to change the period - calendar year.
Grzegorz "gsmok" Makarewicz
Very interesting and exclusive design. I must admit that until the amplifier hit in my arms I do not even realize that any copy of it is located in Poland. And suddenly such a pleasant surprise. I invite you to analyze the photographs showing construction details.
Grzegorz "gsmok" Makarewicz,
Yaqin MC-100B is a Chinese stereo tube amplifier. It's original name, which is a registered trademark, has some strange symbols and you can not write it using "traditional" alphabet. Willy-nilly, in this relation I am forced to use the Yaquin name. This approach is justified because even in the manual, strange symbol appears on the cover, but in the inside you can find the name Yaqin MC-100B. The amplifier does not has sophisticated look. The distinctive frame constituting the tubes cover reminds me of a container for storing bread and willingly I would use this amp with a frame permanently dismantled and hidden in some inaccessible place.
Description and photos of details by Andrzej Żółtowski, photos by Piotr Stanisz Łódź.
[page was updated on April 3, 2005.]
At the end of the first half of the sixties a small audio amplifier appeared in small electronics stores. It had an avant-garde for those times appearance and shocking pink colored casing.
This was, produced by Warsaw manufacturer called "ELZA", the "WS17 Luna" amplifier designed for ad-hoc sound reproduction in medium-sized rooms: school auditoriums, smaller churches, small clubs etc.
Photographs of WR75-62 "Ampli75" amplifier ( PZT Warsaw 1964) made and gave to us Mr Piotr Kowalski
The happy owner of an amplifier is Maciej Myga.
I would like to present some photos of a very interesting STA25 Renaissance tube amplifier manufactured by Woodside Electronics. It's basically a modernized version of the amplifier produced by Radford. The main difference relates to the regulation of quiescent current of output tubes. The amplifier is characterized by interesting and controversial design line. Like previous Radford products, it raises big emotions - from admiration to total criticism.
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