Acoustic tube amplifiers
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- Published: Wednesday, 10 March 2021 18:54
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Acoustic tube amplifiers
Radioamator, Rok XI, Luty 1961, Nr 2 (Radioamateur, Year XI, February 1961, No. 2)
Simple 2-tube amplifier
The output power of this amplifier is 3W with a harmonic distortion coefficient of 2.5%; the sensitivity of the amplifier is 150mV. In order to minimize mains hum, the cathode of the first tube is grounded (Fig. 1), and the negative voltage is obtained due to the voltage drop caused by the grid current in such a system is very small, so the input resistance of the tube is approximately half the leakage resistance.
The output stage is conventional with negative feedback for adjusting the frequency response. In the left position of the potentiometer in the negative feedback circuit, the frequency response is raised for the lowest and highest frequencies of the acoustic band. In the right position of the potentiometer slider there is a significant weakening of higher frequencies, starting from 1000Hz. Any rectifier with a voltage of about 240V and a current of up to 40mA can be used to power the amplifier. The rectifier should have a ripple smoothing filter. The output transformer of the amplifier can be made on a core with a cross-section of 16x16mm, the primary winding should have 3500 wire turns 0.15 in diameter, and the secondary winding - 165 wire turns 0.65 in diameter (for a loudspeaker with a resistance of 4 ohms).
Amplifier with an output power of 3W
This amplifier has better quality paremeters than those described previously, and besides, a separate adjustment of the frequency response in the range of low and high frequencies of the acoustic band. The output power of the amplifier is 3W with a harmonic distortion not exceeding 1.5%. The frequency response is adjustable within ± 16dB at 100Hz and within ± 14dB at 10kHz. Amplifier sensitivity - 100mV.
The schematic diagram of the amplifier is shown in Fig. 2. The negative feedback loop contains RC elements selected in such a way that the strongest negative feedback falls on the middle part of the amplifier's passband. As a result, the gain in the 400-2000Hz range is lower by about 16dB than for low and high frequencies of the acoustic band. To adjust the frequency characteristics of the amplifier, two potentiometers at its input are used. With the help of a potentiometer with a resistance of 1Mom, the characteristic can be adjusted in the high frequency range. Similarly, the 4.7Mom potentiometer controls the characteristics in the low frequency range.
The adjustment range is shown in Fig. 3.
As can be seen from the diagram, the first stage of the amplifier is not covered by the negative feedback loop. However, it works at very low voltages, so that the harmonic content factor does not exceed 0.1%.
The previously described output transformer or another designed for the 6P14P (EL84) output tube can be used for this amplifier..
10W push-pull amplifier
This amplifier has an output power of 10W with a harmonic distortion factor of 3%. The sensitivity of the amplifier is 0.7V.
After initial amplification, the signal is fed to the phase inverting stage (Fig. 4). The circuit of this stage differs slightly from the known phase inversion circuit with distributed load. Namely, positive feedback was applied by connecting the 200-ohm resistor to the cathode of the previous stage, and not directly to the ground. Such a system increases the amplification by about 2.5 times. All stages of the amplifier are covered by deep negative feedback (26dB), which ensures stable operation despite the use of some positive feedback.
The output stage of the amplifier is ultralinear and works in the AB class. As it is known, such a system requires good filtering of the supply voltage, otherwise significant distortions may occur due to the modulation of higher acoustic frequencies by the hum of the power network. Therefore, a good choke should be used in the smoothing filter.
In order to simplify the design of the output transformer and save the choke in the filter, the layout of the output stage can be changed to a conventional one. Then, instead of the Dł choke, a 1.2-ohm resistor should be switched on. The power of the amplifier will be the same, but the distortion will increase to at least 0.7%.
The output transformer is made on a sheath core with a cross-section of 25x30mm. The diagram and arrangement of the windings for the second variant of the output stage is shown in Fig. 5. The halves of the primary winding (Ia and Ib) have 1500 turns each wound with a wire with a diameter of 0.1. The secondary winding consists of 4 sections of 40 turns each wound with 0.65 wire - for a 16-ohm loudspeaker. The most advantageous working resistance of the final stage (from anode to anode) is 6 kohms. The arrangement of the windings shown in Fig. 5 is to ensure a low leakage inductance of the transformer, which is necessary to obtain a good frequency response and low distortions. The transformer ratio is determined by the ratio of the number of turns of the primary winding (3000) to the number of turns of the series connected secondary windings (80).
In practice, such a transformer is wound as follows: section 1, 7, 2, 8, 3 is initially wound, then the body is removed from the winder, rotated by 180 degrees and sections 4, 9, 5, 10, 6 are wound. section starts and ends according to the scheme.
In the case of using an ultralinear system, the most advantageous working resistance of the output stage (from anode to anode) is equal to 8kohms. Accordingly, the secondary winding should have 140 turns wound with 0.65 wire diameter. As before, the primary winding should have 3,000 turns, wound with 0.1mm diameter wire. Shielding nets should be connected after winding 650 turns, counting from the power point. The method of winding the transformer for the ultralinear system is similar to that described above, but the complete symmetry of the windings of the individual push-pull tubes must be maintained. The primary winding can be divided into 6 sections, two of which will have 650 turns each, or into 8 sections.
The amplifier, the diagram of which is shown in Fig. 6, has a power of 12W with a harmonic distortion factor not exceeding 0.4% at 1000Hz, and 1.3% at low frequencies. The characteristics of nonlinear distortions are presented in Fig. 8. The sensitivity of the amplifier: 100mV. Two potentiometers enable independent adjustment of low and high frequencies of the acoustic band according to the characteristics presented in Fig. 7. In the middle position of the regulators, the frequency characteristic runs in the range from 20Hz to 30Hz with the non-uniformity not greater than ± 1dB. The arrangement of the preamplifier stage and the phase inverting stage are conventional.
The output stage works in AB class. High qualitative indicators are obtained thanks to a sufficiently deep negative feedback, covering the 3rd stage of the amplifier. A necessary condition for the good operation of the amplifier is the careful execution of the output transformer. The previously described 10W amplifier transformer may be used in the amplifier.
It is worth emphasizing that the quality of almost every acoustic amplifier depends to a large extent on the design of the output transformer, and therefore it is worth spending more time on its proper design and execution. The reason why it is impossible to obtain the required quality indicators is often too high leakage inductance. It depends on the number of turns in the windings and the mutual coupling of the windings with each other.
In order to reduce the number of turns in the windings, cores with a sufficiently large cross-section and small holes should be selected, because the amount of copper then decreases, and large holes extend the average magnetic flux path in the core.
For the best possible coupling of the windings, the previously described sectioning is used, carefully winding the turn by the turn, and the geometric and electrical symmetry of the windings is maintained..
The diagram in Fig. 9 shows the circuit of a three-stage push-pull amplifier with a power of 20W. The total harmonic distortion does not exceed 1.2%. Amplifier sensitivity: 0.5V. The characteristics of nonlinear distortions are presented in Fig. 10. The frequency characteristics are uniform from 20Hz to 25kHz. The phase inverting circuit is designed to ensure good symmetry of the output voltage over a wide frequency range. In the output stage, there are two 6P 14P tubes connected in parallel, in the AB class. The negative voltage for these tubes is obtained automatically on a common resistor. The resistors in the control grids prevent harmful oscillations. The negative feedback loop covers the entire amplifier.
The amplifier is powered by a rectifier with a 5Ц3C tube. The transformer of this rectifier is made on a 40x50 core and has the following windings: the primary winding has 220 + 34 + 186 turns wound with 0.8 wire diameter; secondary winding - 2 x 700 turns wound with wire 0.3; winding "xx" - 11 turns wound with wire 1.5 in diameter; windings "yy" and "zz" - 13.5 turns each with a wire of 1.2 diameter.
The output transformer is made on a 30x35 core, the primary winding having 2400 windings wound with 0.25 wire, and the secondary winding - 85 windings wound with 1.0 wire. The transformer should be wound as shown in Fig. 5.
The diagram of the two-channel amplifier is shown in Fig. 11. This amplifier has a power of 4.5W with a harmonic distortion factor of 3% - low-frequency channel, 3W at 2% - high-frequency channel. Amplifier sensitivity: 250mV.
The channels are separated after a common volume control. Weakening of low frequencies in the tweeter channel (in the upper diagram) is due to the small capacitance of the capacitor blocking the cathode resistor and the small capacitances of the coupling capacitors.
In the low-frequency channel (in the lower diagram), the weakening of high frequencies is obtained thanks to the appropriate negative feedback characteristics. The applied negative feedback is stronger for high frequencies. The circuit of this coupling connects the anode of the output tube with the grid of the output tube through a capacitor and a potentiometer. Additionally, the output transformer is shunted with a capacitor.
In order to obtain sufficiently good quality indicators in each of the channels, negative feedback was used, obtained by connecting the anode of the output stage tube with the anode of the preceding stage tube (resistor R8 and resistor R16). The value of these resistors should be selected experimentally, bearing in mind that a strong feedback (low resistance of the resistors mentioned) reduces the gain, improving the parameters of the amplifier.
The output transformer of the tweeter channel Tr1 is made on a 12x16 core. Its primary winding has 1500 turns wound with 0.1 wire, and the secondary has 56 turns wound with 0.51 wire. Gap in the transformer core: 0.1mm.
The Tr2 low-frequency channel transformer is made on a 19x28 core. The primary winding of the transformer contains 3,000 turns wound with 0.12 wire, and the secondary - 52 turns wound with 0.64 wire. The gap in the core is 0.12 mm.
Prepared by A.W. on the basis of the "Radio" magazine No. 7/60