Audio Tube Amplifiers

A.W., "Audio Tube Amplifiers"
Radioamator 1961/02

Simple 2-Tube Amplifier

The output power of the amplifier is 3W.  Harmonic distortion coefficient is 2.5% and the sensitivity of the amplifier is equal to 150mV. In order to minimize hum, the cathode of the first tube is grounded (Fig. 1). A negative voltage is obtained due to voltage drop caused by the grid current. This current is very small, therefore the input impedance of the tube is equal to approximately half of the leakage resistance.

The output stage has a conventional system with negative feedback utilized to adjust the frequency response. For the left position of the potentiometer in the negative feedback loop, frequency response is raised for the minimum and maximum frequency range. For the right position of the potentiometer slide, higher frequencies from 1000 Hz upwards are strongly reduced. To power the amplifier every rectifier with a voltage of about 240V and currents up to 40mA can be used. Rectifier should be equipped by ripple smoothing filter. Amplifier output transformer can be made on the core with cross-section 16x16mm. Primary winding should have 3500 turns of wire diameter of 0.15, a secondary winding - 165 turns of wire diameter of 0.65 (for the speaker of resistance 4ohms).


Fig.1.

3W Amplifier

This amplifier has better quality indicators than previously described, and besides, a separate frequency response regulation 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.

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 central part of the amplifier's bandwidth. As a result, the gain in the 400-2000Hz range is smaller by approx. 16dB than for small and high frequency bands. Two potentiometers at its input are used to adjust the frequency response of the amplifier. With a 1Mom potentiometer, you can adjust the characteristic in the high frequency range. Similarly, the 4.7Mom potentiometer regulates the characteristics in the low frequency range.


Fig.2.

The adjustment range is shown in Fig. 3.


Fig.3.

As shown in the diagram, the first stage of the amplifier is not included in the negative feedback loop. However, it works at very low voltages, so the harmonic content factor does not exceed 0.1%.

A previously described output transformer or other designed for the 6P14P output tube (EL84) can be used for this amplifier.

10W Push-Pull Amplifier

The amplifier has a 10W output power with a harmonic distortion coefficient of 3%. The amplifier's sensitivity is 0.7V.

After initial amplification, the signal is fed to the phase inverting stage (Figure 4). The system of this stage differs slightly from the known phase inverter system with the distributed load. Positive feedback was used by combining the 200-ohm resistor with the previous degree cathode and not directly with the ground. Such a system increases the amplification about 2.5 times. All stages of the amplifier are covered by a deep negative feedback (26dB), which ensures stable operation despite the use of some positive feedback.


Fig.4.

The output stage of the amplifier is made in an ultralinear system and works in the AB class. As it is known, such a system requires good filtration of the power supply, otherwise there may be significant distortions caused by the modulation of higher acoustic frequencies by the hum of the network. For this reason, a good choke should be used in the smoothing filter.

In order to simplify the construction of the output transformer and save the choke in the filter, the output stage system can be changed to a conventional one. Then, instead of the Dł choke, you need to turn on the resistor 1.2. 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 core with a cross-section of 25x30mm. The diagram and arrangement of the windings for the second embodiment of the output stage are shown in Fig. 5. The primary winding halves (Ia and Ib) each have 1,500 windings wound with a wire diameter of 0.1mm. The secondary winding consists of 4 sections of 40 coils wound in wire, 0.65mm in diameter - for a 16-ohm speaker. The best resistance of the final stage (from anode to anode) is 6 sticks. The winding system shown in Fig. 5 is to ensure low transformer leakage inductance, which is necessary to achieve good frequency response and low distortion. The transformer ratio is determined by the ratio of the number of windings of the primary winding (3000) to the number of turns of the serially connected secondary windings (80).

In practice, the transformer is called as follows: first sections 1, 7, 2, 8, 3 are wound up and then the body is removed from the winder, rotated by 180 degrees and sections 4, 9, 5, 10, 6 are rolled. beginnings and ends of the section according to the scheme.


Fig.5.

In the case of using the ultralinear system, the most favorable resistance of the output stage (from anode to anode) is equal to 8K. Therefore, the secondary winding should have 140 coils wound with 0.65mm wire diameter. The primary winding should have 3000 coils, wound around 0.1mm wire, as before. Screening 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 the one described above, except that the complete symmetry of the windings of individual push-button tubes should be maintained. The primary winding can be divided into 6 sections, two of which will have 650 coils or 8 sections.

High Fidelity Amplifier

The amplifier, the diagram of which is shown in Fig. 6, has a power equal to 12W with a harmonic distortion coefficient not exceeding 0.4% at 1000Hz, and 1.3% at low frequencies. The characteristic of nonlinear distortions is presented in Fig. 8. The amplifier's sensitivity: 100mV. Two potentiometers allow independent regulation of low and high frequencies of the acoustic band according to the characteristics shown in Fig. 7. In the middle position of the regulators, the frequency response ranges from 20Hz to 30KHz with unevenness no more than ± 1dB. The pre-amplification stage and phase inverter system are conventional.


Fig.6


Fig.7


Fig.8

The output stage works in the AB class. High qualitative indicators are obtained thanks to a sufficiently deep negative feedback, including the third stage of the amplifier. A necessary condition for the good work of the amplifier is the careful implementation of the output transformer. In the amplifier, the previously described 10W amplifier transformer can be used.

It is worth noting that the design of the output transformer depends to a large extent on the quality of almost every acoustic amplifier and therefore it is worth spending more time on its proper design and execution. The reason for the impossibility to obtain the required quality parameters is often the too large dissipation inductance. It depends on the number of turns in the windings and the mutual coupling of the windings between each other.

In order to reduce the number of windings in the windings, cores with a correspondingly large cross-section and small openings should be selected, because the amount of copper then decreases, and large holes extend the average path of the magnetic flux in the core.

For the best coupling of the windings, the previously described sectioning, careful winding of the coil and geometric and electrical symmetry of the windings are used.

20W Amplifier

The diagram in Fig. 9 shows the system of a three-stage anti-parallel amplifier with a power of 20W. The harmonic content factor does not exceed 1.2%. Amplifier sensitivity: 0.5V. The characteristics of non-linear distortions are presented in Fig. 10. The frequency response is uniform from 20Hz up to 25 kHz. The phase inverting system has been developed to ensure good symmetry of the output voltage over a wide frequency range. In the output stage, two 6P14P (EL83) tubes are operated in parallel in the AB class. The negative voltage for these tubes is automatically obtained on a common resistor. Resistors in the control grids prevent harmful oscillations. The negative feedback loop covers the entire amplifier.


Fig.9.

The rectifier with a 5Ц3C tube is used to supply the amplifier. The transformer of this rectifier is made on a 40x50mm core and has the following windings: the primary winding has 220 + 34 + 186 windings wound with 0.8mm wire diameter; secondary winding - 2 x 700 coils wound with wire, diameter 0.3mm; winding "xx" - 11 coils wound with a wire diameter of 1.5mm; windings "yy" and "zz" - 13.5 turns each with a wire diameter of 1.2mm.


Fig.10.

The output transformer is made on a 30x35mm core, the primary winding has 2400 windings wound with 0.2 mm wire, and the secondary winding - 85 windings wound with a wire diameter of 1.0mm. The transformer should be wound in the manner given in Fig. 5.

2-Channel Amplifier

The schematic diagram of the two-channel amplifier is shown in Fig. 11. The amplifier has a power of 4.5W with a harmonic content factor of 3% - low frequency channel, 3W at 2% - high frequency channel. Amplifier sensitivity: 250mV.

The separation of channels follows the common volume regulator. Weakening of low frequencies in the tweeter channel (in the upper diagram) occurs due to the small capacity of the capacitor blocking the cathode resistor and the small capacitances of the coupling capacitors.


Fig.11.

In the low frequency channel (in the lower diagram), the weakening of the high frequencies is obtained due to the appropriate negative feedback characteristics. The negative feedback applied is stronger for large frequencies. The circuit of this coupling connects the anode of the output tube to the grid of the tube through a capacitor and a potentiometer. In addition, the output transformer is coupled with a capacitor.

In order to obtain sufficiently good qualitative indicators, in each of the channels a negative feedback was obtained by connecting the anode of the 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 strong feedback (low resistivity of the resistors mentioned) reduces the amplification, improving the amplifier's parameters.

The output transformer (Tr1) of the high frequency channel is made on a 12x16mm EI core. Its primary winding has 1500 windings wound with 0.1mm wire and the secondary winding has 56 windings wound with 0.51mm wire. Gap in the transformer core: 0.1mm.

The Tr2 subwoofer transformer is made on a 19x28mm EI core. The primary winding of the transformer contains 3000 windings wound with 0.12mm wire, and the secondary - 52 windings wound with 0.64mm wire. The gap in the core is equal to 0.1 mm.

Prepared on the basis of the Soviet "Radio" No. 7/60
A.W.