Universal 15VA Hi-Fi amplifier

Andrzej Depczyk "Uniwersalny wzmacniacz Hi-Fi o mocy 15VA"
Radioamator 1960/04

The amplifier described here is ideal as a final power amplifier for playing music from discs or tapes in an apartment. It is also suitable for broadcasting dance music in halls and common-sized rooms. Construction is easy; it can be performed by any radioamator with basic theoretical and practical knowledge.


Output stage

Due to the need to obtain a power equal to at least 10VA with very small non-linear distortions, I decided to use in the amplifier the final stage in a push-pull system with negative feedback in the secondary grid. This is called the "ultralinear" system. The feedback voltage in this system is obtained from the taps, if necessary. from separate windings on the output transformer. By changing the ratio of the variable voltage of the secondary grid to the anode voltage, we change the working conditions of the output tubes. With the relation equal to one - the tubess work as triodes, because the screening nets are connected with the anodes, while with the ratio equal to zero - as pentodes (Fig.1).

Fig.1. The "ultralinear" system

For relations with intermediate values, the system has a number of advantages over push-pull circuits, both with triodes and pentodes, and, above all, less distortion in the non-linear, with large and small signals - at the expense of a small power loss. In addition, such feedback reduces the internal resistance of the system significantly.

For EL84 tubess, a 20% tap will most advantageously be used:

(of course from the power supply side). The "ultralinear" system with EL84 tubes gives about double the non-linear distortion at 10% less power than the normal push button circuit. The "ultralinear" system requires a special output transformer.

Output transformer

Both halves of 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 asymmetry has a very significant effect on phase shifts and amplitude differences between the anode current and the screen voltage, so that the even harmonics of the higher frequencies may not abate.

Fig. 2 shows the scheme of windings of such a transformer, and in Fig.3 - the arrangement of windings.

Fig.2. Diagram of output transformer windings
(primary winding: 0.18mm wire in enamel, secondary winding: 0.75 wire in enamel)

Figure 3. The location of the windings on the output transformer's body
(arrows indicate directions of winding)

I used to make a core with the dimensions shown in Fig.4. Of course, another core can also be used if its cross-section is large enough and the windings fit into its window.

Figure 4. The dimensions of the output transformer core
(iron cross-section - effective - 7.9 cm2, composite core with no gap)

Phase inverter

In modern amplifiers with push-pull output, the phase reversal takes place without the help of a transformer or choke, because the stage of the phase inverter is subject to negative feedback, and systems containing inductances prevent the use of sufficiently strong feedback.

There are a number of phase inverting circuits, from which I have chosen the arrangement shown in Fig.5.

Fig.5. Phase inverter diagram

The quality of the phase inverter system can be evaluated from its symmetry, which can be expressed by the ratio of E1 / E2 voltages. For the system shown in Fig. 5, this relation expresses:

where: k2 - amplification of the V2 tube.

If R3 = R4 = R5 (as is usually used in practice), then:

As it appears, the symmetry of the system is the better, the higher the amplification of the V2 tube. For example, let's calculate the layout for the 6J5 tube (half tube 6SN7). Resistance R2 = 0.1M and resistance R3 = R4 = R5 = 0.25; then k2 = 14, and the voltage ratio E1 / E2 will be 1.21, which is inadequate for the Hi-Fi amplifier. In our case, we dispose one half of the ECC83 tube. The amplification factor of this tube is 100. Since R2 = 0.2M and the internal resistance of the tube is 80K, its voltage gain will be:

Therefore symmetry:

is quite good.

Amplifier circuit

The amplifier system is shown in Figure 6.

Figure 6. Schematic diagram of the amplifier

The number of elements used in the system is as small as possible to reduce the construction cost of the amplifier as much as possible without lowering its quality. The amplifier is adapted to work on the load of 3,5 ÷ 4ohm speakers, which does not mean that when the speaker is loaded with, for example, 7 ohms, it will work badly. In this case he may not give full power.

As can be seen from Fig. 6, in addition to the feedback in the output stage, feedback was applied to the entire amplifier. The voltage divider creates resistors: 800 and 180 ohms. A resistor of 800 ohms is blocked by a capacitor with a capacity of 1000pF in order to correct the already existing phase shift at the largest frequencies of the band. The feedback has more than 20dB, the amplifier sensitivity for 10VA at the output is about 1.2V (at 5 ohms).

Due to the low sensitivity, a pre-amplifier is needed that would allow the final amplifier to be actuated from an adapter or tape recorder.

Input amplifier stage

The diagram of the pre-amplifier is shown in Fig.7. This amplifier consists of two amplification stages, between which there is a resistance - capacitive characteristic equalizer in a known and widely used system.

Figure 7. Scheme

aticof the pre-amplifier and equalizer frequency response

In the upper position of the potentiometer "W" we have raised high volumes, because resistances 50K and 100K only for high frequencies are shunted by a 200pF capacitor. In the lower position of the slider of this potentiometer, the high tones are cut off, a resistor-capacitive voltage divider consisting of 100K resistive and 3000pF capacitor, which cuts off the high frequency of the band.

The operation of the bass regulator is similar. In the upper position of the potentiometer "N" resistance 50K, the capacity of 20000pF in series with the resistance 10K form a divider, which lets only low tones, and strongly weakens the high tones. In the lower position of the potentiometer of the "N" potentiometer, a divider is created: 50K, 3000pF and 10K, on which the small frequencies are weakened, so we have low bass clipping. For medium frequencies around 1000 Hz, the system has more or less constant damping, regardless of the position of the individual regulators. The gain of the first stage of the amplifier for 1000Hz frequency is approximately 2.

The amplification of the second stage of the pre-amplifier due to the current negative feedback (resistor in a cathode not blocked by a capacitor) is only about 30.

The sensitivity of the entire device (for 10VA and 5 ohms at the output) is approximately 20mV. Because the pre-amplifier has a fairly high sensitivity, all the elements of the equalizer must be carefully shielded.

Power supply

The schematic of the power supply is shown in Fig.8. As you can see, it is a completely conventional power supply. The filter consists (Fig. 6) with a 5 ÷ 10H choke and two electrolytic capacitors of 32μF.

Fig.8. Diagram of the power supply for the amplifier

In order to remove the hum of the network getting from the cathode side, the winding of the tubes is symmetrized with a 50 ohms potentiometer, the center of which has a voltage of about + 12V (end-cathode cathode). This is to reduce the fiber - cathode emission.

Mounting the amplifier

All the above described elements of the amplifier were assembled on a common chassis with dimensions of 200x300x60mm. The layout of the elements on the chassis is shown in Fig.9.

Fig.9. Arrangement of elements on the chassis

The assembly starts with the mechanical fixing of the elements to the chassis. Make sure that the mains transformer is not too close to the output transformer and that the potentiometers are located away from the power supply wires (especially heating). Then we proceed to perform the so-called grounding rails made of thick copper wire. The sketch shown in Fig. 10 will help us find out how to perform the earthing bar.

Fig.10. The method of making the earthing bar

We start this work from the input socket. It should be remembered that the level of hum of the network in the amplifier depends on the connection to the grounding rail. The splint can connect to the chassis only at one point, preferably somewhere near the power supply. The electrolytic capacitors of the filter as well as the decoupling circuits should be mounted on insulating washers (minuses) from the chassis. The housings of these capacitors are grounded by connecting them to the rail.

After completing this work, we connect the circuits of incandescence of the amplifying tubes, the rectifier tube and then the power supply to the anode circuits. Now we can proceed to the proper electrical installation. We start from the end, that is from the power amplifier. We connect the anodes and end lamp screens with the shortest possible wires with the appropriate connectors on the output transformer. Next, we gradually move to the phase inverter and pre-amplifier circuits, fixing resistors and capacitors with connectors wherever short-circuits could occur due to the displacement of the elements (eg when moving the amplifier).

Grounding of individual resistors (cathode, leakage) is done by soldering the terminals directly to the grounding bar, if possible at one point for one-stage circuits.

Earthing the screen of the cable connecting the input terminals of the amplifier with the potentiometer controlling the sound power should be made at the very entrance.

After performing (careful) assembly, it is also necessary to carefully check the system, and then insert the rectifier lamp into the stand and after switching on the amplifier to the network, check that the amplifier tubes are not high voltage. It should be recalled here that ECC83 lamps have a glow plug attached to other contacts than other types of "80" series lamps.

After this initial test, discharge the electrolytic capacitors, shorting them through a 100K resistor to ground (not a screwdriver, because a large current will flow, which can damage the internal electrolytic capacitor connections), then insert all the tubes, turn the loudspeaker on and try to make the amplifier incorrect connection of feedback. Take care not to damage the loudspeaker, as the excited amplifier generates approximately 20W of power.

While everything is fine, we measure anode voltages and cathodes. The cathodes of the output tubes should have a voltage of 11 ÷ 12V. The voltage on the positive side of the power supply (after the filter, on its second capacitor) should be about 325V. When controlling the amplifier, this voltage should not drop by more than 5 ÷ 10V.

On this you could end static tests of the amplifier.

In the event of oscillation, the feedback must be disconnected and the excitation has to be checked. If so, change the direction of the feedback connection by changing the secondary winding terminals.

At home, further attempts are limited to checking the hearing amplifier. So we will check whether the amplifier is not distorting, whether the characteristic control is working properly and whether the hum of the network is not too big.

In case of careful execution according to general assembly rules and with proper voltages on lamp electrodes, the amplifier should work well right away.


The described amplifier has been studied in detail. I give the results obtained below:

1. Power amplifier test (without pre-amplifier)

Amplifier loaded with real resistance 5ohms. The frequency response (Fig.11) is linear from 20Hz to 20000Hz (-1dB) with a 10VA output power of 1000Hz.

Figure 11. Frequency characteristics of the amplifier

Harmonic content ratios at 1000 Hz, depending on the output power: 1VA - 0.15%, 5VA - 0.15%, 10VA - 0.3%, 12VA - 0.4%, 14VA - 0.9%, 15VA - 2% (Fig.12).

Fig.12. Distortion characteristics of amplifier

The sensitivity of the amplifier at the output power of 10VA - 1.2V.

The PPAE generator with an average harmonic content of about 0.12% was used to test the amplifier.

2. Testing the pre-amplifier

The frequency response can be adjusted within the following limits: 30Hz +/- 16dB; 20kHz +/- 18dB. Nonlinear distortions do not significantly increase the harmonic factor of the final amplifier.

Supplementing the article, which appeared in the Editor's answers section in the journal Radioamator i Krótkofalowiec 3/1961.

eng. Adam Zeyland

The discrepancy between Fig. 2 and Fig. 3 in the description of the 15VA amplifier (Radio Amateur No. 4/1960) does exist, however, Fig.3 is incorrect. It should look like shown in the picture below. The directions of the windings, of course, as in Fig.2.

Distribution of windings on the skeleton of the output transformer Hi-Fi 15VA

As for the way of isolation of the output transformer's windings, the individual winding layers should be interlaced with possibly thin waxed (condenser) paper, but can be wound in bulk using a good enamel wire. A thin sheet should be used between the windings.

In the device made by the author of the description, the bass-reflex cabinet is equipped with an 8-watt speaker from the "Beethoven" receiver and two tweeters, also from "Beethoven". The tweeters are embedded in the housing front panel, connected in parallel and through a 4μF capacitor.

Frequency characteristics of the entire set were not removed by the author. The characteristics of the speakers themselves are with deviations of + 5dB from 70 to 17,000 Hz.

The ECC42 tube does not appear in the catalogs we have, so it is difficult to say whether it can replace the ECC83 tube. It would not make sense to use such an amplifier exclusively for the ordinary superheterodyne receiver with respect to the frequency band. A normal receiver of this type transfers to 4.5 ÷ 5KHz. But there are other sources of control that give a much wider bandwidth, such as full-length discs and FM receivers (where it transmits FM stations with frequency modulation). In addition, the use of a detector attachment for the reception of a local station allows the use of a full band radiated by the transmitter. From the point of view of non-linear distortions, the use of a Hi-Fi amplifier always gives good results.

T. Pawluk

The most important values ​​of supply voltages are given in the diagram (fig.6). The remaining tensions were not measured at all, and because the system was dismantled by the author, we can not give them to you. In the system of the described amplifier we have two feedback circuits: one in the shielding screens of the end tubes, reducing the output resistance, and the other from the secondary winding of the output transformer to the phase inverter cathode, which reduces nonlinear and linear distortions of the whole amplifier and reduces its output resistance. The most favorable feedback for the amplifier in shielding the end tubes, which is a compromise between the reduction of nonlinear distortions, decrease of output resistance and power drop, is usually given in the factory catalog of tubes; this size depends on the construction of the tube and varies from 10 to 45%. Calculating the feedback around the whole amplifier (second feedback) is complicated and tedious, as it requires determining the amplification and total shifting of the amplifier phase from the input to the output (the output transformer is also subject to feedback). This issue is too broad to be explained as part of the response. You will find a comprehensive explanation in the book by Witort and Trechcinski entitled "Broadcasting principles", PWT, 1959, (chapter on MW amplifiers).

"Stali czytelnicy"

All resistors in the phase inverter can have a load capacity of 0.25W, likewise leakage resistors and protection against parasitic vibrations in the final stage. The 130-gauge cathode resistor should have an acceptable 3W load capacity, and 1W decoupling filter resistors.

Andrzej Depczyk