High-quality 2x10W stereo reproduction set
Michał Gołębiowski, Radioamator i Krótkofalowiec 1970/05
(The description concerns a model made in cooperation with the editorial office of the magazine and practically tested by the designer)

  Despite the rapid progress of transistor technology, electroacoustic devices equipped with electron tubes are still quite often made by radio amateurs. This is justified due to the still high cost of semiconductor elements and the fact that starting transistor devices is generally more difficult and troublesome than their tube counterparts and requires good knowledge of the issue and extensive practical experience.

  This description is addressed to radio amateurs who already have some achievements in the construction of electroacoustic devices and would like to obtain a higher quality reproduction set at a relatively cheap cost.

Technical data of the stereo amplifier

  • Maximum output power with non-linear distortions in the band 40Hz ÷ 16000Hz and Rload = 7.5Ω less than 1% (sinusoidal signal): 2x10W.
  • Frequency characteristics (1.5dB band): 30Hz ÷ 20000Hz.
  • Sound tone adjustment in relation to the frequency of 100Hz:
    at 60Hz: + 6dB ÷ -12dB
    at 12kHz: + 6dB ÷ -15dB
  • Crosstalk attenuation between the channels in the 30 ÷ 16000Hz band: ≥ 40dB.
  • Stereo balance control: ± 6dB.
  • Input resistance:
      "magnetic adapter" input - 100kΩ
      "crystal adapter" input - 100kΩ
      "microphone" input - 80kΩ
      "radio" input - 1MΩ
      "additional" input - 0,5MΩ.
  • Input voltage for maximum output power at f = 1000Hz and Rload = 7.5Ω:
      "magnetic adapter" input - 5mV
      "crystal adapter" input - 70mV
      "microphone" input - 4mV
      "radio" input - 330mV
      "additional" input - 150mV.
  • Signal-to-noise ratio at maximum output power: ≥50dB.
  • Change in output voltage when the load is disconnected: ≤1dB.
  • Mains power: 220V, 50Hz, maximum power consumption 55W.
  • The remaining parameters are presented in the form of appropriate characteristics in Figures 5, 6, 7, 8 and 9.

The principle of operation of a stereo amplifier

  The amplifier circuit can be divided into two parts:

  1. input stages shaping the frequency response and enabling obtaining the appropriate input resistance and voltage amplification,
  2. output amplifier (power stage).

  The schematic diagram of the preamplifier is shown in Fig. 1. Only one channel is shown between the dashed lines: to obtain the complete diagram, this part of the drawing should be repeated.

Fig. 1. Schematic diagram of the preamplifier (channel "A")

  The amplifier has 5 inputs: for radio, microphone, magnetic pickup, crystal pickup and auxiliary input. Switching of inputs as well as feedback circuits is done by a 4-section 5-position Pk1 switch - A and B.

  Each preamplifier channel has two voltage stages with EF86 pentodes with low microphoning effect. The frequency response correction elements are located in the first stage and are connected between the anode and the grid of the tube. Due to the strong feedback, low resistance of the grid circuit of the tube control was obtained, which significantly reduces the interference voltages. The R1 ÷ R5 resistors connected in series with the appropriate inputs adjust to the given type of transducer. The values of these elements were selected in terms of the amplifier's adaptation to work with a wide range of electro-acoustic transducers.

  The second stage is built for maximum amplification. The output voltage after the tone control system is equal to about 200mV. The tone control system uses coupled linear potentiometers P2 A and B as well as P2 A and B. Due to their characteristics, linear changes of frequency characteristics are obtained as a function of potentiometer rotation.

  The balance control (stereo balance) is realized by the oppositely coupled linear potentiometers P1 A and B, which makes it possible to equalize the gain of both channels without changing the overall power given by both channels.

  Gain control is done with a double logarithmic potentiometer P4 A and B.

  The amplifier is also equipped with two filters shaping the frequency response: "noise" and "ripple" switched on by the Pk2 A and B switch. By changing the value of the RLC elements of the shaping elements included in these filters, you can influence the shape of the characteristics (Fig. 5).

  The Pk3 A and B switch allows the operation of both channels in a stereo system, as well as changing channels in places or connecting them in parallel for monophonic reception.

  The non-linear distortions caused by the preamplifier do not exceed 0.15% under normal operating conditions. The corresponding frequency characteristics are shown in Fig. 6.

  The power stage (Fig. 2) is a class AB push-pull amplifier, in which the pentode systems of ECL86 tubes operate in an ultralinear system.

Fig. 2. Schematic diagram of the output amplifier (channel "A")

Supplying the shielding grids of the power tubes from the taps of the output transformer introduces negative feedback. This reduces non-linear distortion and lowers the internal resistance of the amplifier.

  The ECC83 tube, common to both channels, works as a voltage amplifier, and the triode parts of the ECL86 tubes form a phase inverter. The coupling between the input stage and the phase inverter is direct to reduce the phase shift errors at the lowest frequencies and thus increase the stability of the amplifier.

  The entire amplifier is covered by a strong negative feedback loop from the transformer output to the cathode of the ECC83 tube. This coupling reduces non-linear distortion to about 1% and sets the output resistance of the amplifier to about 0.2 Ω.

  When building the amplifier, special attention must be paid to the correct construction of the output transformers. Due to the negative feedback effect, they should transfer frequencies up to about 100kHz, and therefore have a low leakage inductance. The method of winding the transformer is shown in Fig. 3.

Fig. 3. Output transformer. The windings were wound with a DNE type wire.
a - winding diagram, b - method of winding

    The individual arrangement of sections, their beginnings and ends, should be symmetrical, and the number of turns of analogous sections, especially the secondary winding, should be strictly the same. To meet this condition, the transformer should be finished very carefully, and the winding should be done with turning the body over - as shown in Fig. 3. It is best that the sections have an even number of layers, then the outputs are located at the outer wall of the body. If by no means full layers can be obtained, then the last incomplete layer extends over the entire length of the winding, or it is better to decide to proportionally change the number of turns, choose a different wire diameter. or the use of a different core.

  The output transformer is best wound on a core with a cross-section of 9-12 cm2 (eg from the mains transformer of the "Tatry" receiver). Individual layers should be insulated with condenser tissue paper, and sections with a double layer of oilcloth.

  The sensitivity of the output amplifier without feedback is about 8mV, and after the feedback loop is closed - 200mV.

  The power supply (Fig. 4) works in a conventional half-wave rectifier circuit and provides an average current of 150mA at a rectified voltage of 265V. The filament winding should withstand a load of up to 4A. It is shorted by potentiometer P6 (removing mains hum).

Fig. 4. Schematic diagram of the power supply of the amplifier.

  The mains transformer was wound on the core from the "Symphony" receiver. Another core with a center column cross section of at least 12 cm2 may also be used. The choke can be made using a loudspeaker transformer from the "Stolica" receiver or a similar one with a cross-section of about 4 cm2. The winding should be wound massively with DNE Ø0.45mm wire until the body is full. To increase the safety factor, instead of individual DK62 diodes, you can use two DK61 diodes connected in series, shunted with resistors of about 1MΩ / 0.5W.

Fig. 5. Characteristics of shaping filters
a - ripple filter, b - noise filter

Fig. 6. The dependence of the input voltage as a function of frequency
1 - "radio" input, 2 - "magnetic adapter" input

Fig. 7. Characteristics of the non-linear distortion of the amplifier as a function of the output power
(percentage of harmonics)

Fig. 8. Characteristics of the non-linear distortion of the amplifier as a function of frequency (Note: Rload = 7.5Ω)

Fig. 9. Frequency characteristics of tone control

 Details of the design of the amplifier

  The amplifier with the power supply is placed in a wooden housing with dimensions of 640x210x350mm (box from the "Arkona" receiver). The following control elements are mounted on the front panel: Pk1 A and B switch (a switch from the "Opal" TV set), Pk2 A and B, Pk3 A and B switches (a switch from the "Lazuryt" TV set), gain and tone controls , balance, as well as the speaker outputs of both channels and an activation indicator light. On the rear wall there are input sockets, the grounding terminal and the power supply fuses.

  The assembly of the amplifier was made in the form of three components, namely:

  • preamplifier,
  • output amplifier (power stage),
  • power supply.

  The general view of the amplifier and the subassemblies is shown in Figs. 10, 11 and 12. The electron tuning indicator ("magic eye") visible in the photo is designed to signal the operation of the FM decoder, which can be mounted as an add-on to the described amplifier, or it can also work. as an indicator of a stereo signal in the circuit shown in Fig. 13.

Fig. 10. General view of the amplifier.

Fig. 11. The inside of the amplifier - bottom view.

Fig. 12. Inside the amplifier - rear view.

Fig. 13. Stereo signal indicator.

  The indicator works in such a way that if the same signal comes from both channels A and B, the illuminated part of the indicator screen does not change its surface, otherwise the illuminated surface changes.

  In order to avoid the influence of interfering signals, the preamplifier was placed in a separate box made of aluminum sheet, where the first amplification stage (ECC83 electron tube) was also placed. The casing of the preamplifier is attached to the front plate of the amplifier.

  Low noise and hum is achieved by using careful and thoughtful assembly, i.e. by connecting the individual elements to a common grounding bar, connected to the chassis near the power supply, as well as using the shortest possible connections. Due to the wide frequency response, the use of shielded cables should be limited to the necessary minimum.

  The output amplifier (power amplifier) and the power supply are mounted directly on the chassis. The mains transformer is magnetically shielded and placed so that its stray field has as little effect as possible on the first stage of the preamplifier.

Amplifier startup

  When preparing the amplifier for operation, perform the following steps:

  1. Check the correct assembly of the entire device.
  2. Start the power supply, i.e. check the correct voltage at nominal load (+ 265V, 150mA and 6.3V AC, 4A).
  3. Start the power amplifier. The operation is as follows: the outputs of the loudspeaker transformer should be closed with a 7.5Ω load resistor and the preamplifier control disconnected. If the amplifier will oscillate at the over-acoustic frequency, the elements C22 and C29 should be selected accordingly. If the oscillations occur at the subacoustic frequency, the capacitances of C21, C26 in the decoupling filters should be increased. Disconnecting the terminating resistor should not cause overvoltage in the output transformer. However, if this phenomenon occurs, the value of the damping resistor R48 should be reduced. Then the symmetry of the anode currents of the pentode parts of the ECL86 electron tubes is determined by adjusting the potentiometer P5
    The R37 and R38 resistors should be highly stable with a tolerance of 5%. The values of these resistors should be measured before assembly and the larger one should be put in place of R38. The best working conditions are achieved when the difference between the resultant anode resistances of the ECL86 triodes is 3%. It follows that also the resistors R39 and R40 should be properly selected.
    A properly operating power amplifier should deliver a power of not less than 10W to a load of 7.5Ω, with a 200mV sinusoidal signal controlled in the frequency range 40Hz ÷ 16000Hz, without noticeable nonlinear distortions and parasitic oscillations.
  4. Starting the preamplifier. It consists in checking the correctness of the voltages at characteristic points, checking the operation of the tone, balance and gain regulators. Noise and mains hum is checked after connecting the power amplifier to a 7.5Ω load resistor at full amplification.

Loudspeaker housings

  The amplifier works with two sets of speakers. There are two GD20 / 5F or GD31-21 / 5 speakers and two GDW6.5 / 1.5 speakers in each housing. The connection of the loudspeakers to the separating filter with a slope of 12dB / oct is made as in Fig. 14. The cut-off frequency of the filter is about 4000Hz. Coil inductance L = 0.4mH, capacitance C = 4μF. On a carcass without a core, shown in Fig. 14, 125 turns of DNE wire Ø1.4mm were wound. The use of a separating filter enables high acoustic power to be processed without excessive distortion and with a better average efficiency.

Fig. 14. Loudspeaker connection layout and dimensions of L coil body.

  The speakers and filters are placed in a two-chamber housing with dimensions: 750x350x230mm. The use of this type of housing ensures reproduction of a wide frequency band using mid-range loudspeakers. The housing is made of 10mm thick chipboard. From the outside it is covered with a layer of "skay", and inside it is lined with a layer of mineral wool 2 cm thick.

  The whole structure was made very carefully, fitting the structural elements exactly, screwing them in many places with wood screws and pouring the joints with Epidian 5 epoxy resin. The edges and corners were additionally reinforced with pins and strips. The holes (tunnels) are made of round bakelite pipes with a diameter of 70mm and a length of 180mm. The location of the speakers and the most important dimensions of the housing are shown in Fig. 15.

Fig. 15. Loudspeaker housing.

  The woofers are bolted from the inside of the housing with screws to metal, and the tweeters are placed "shallow" so that a channel does not form in the thick wall of the housing and also attached to metal with screws.

  A thorough examination of the loudspeaker system is difficult and possible only in an electroacoustic laboratory. Therefore, you have to limit yourself to the following attempts:

  • measurement of the loudspeaker input impedance module in the entire frequency band,
  • reproducing different frequencies in the entire frequency band (so-called "whistling"), thus catching unwanted resonances or "holes",
  • playing music from good playing records or other source, especially works for large symphony orchestra and large jazz groups.

  The two-chamber housing was described in the book by A. Witort "Elektroakustyka dla wszystkich".

List of electronic components for one channel of a stereo amplifier


All resistors, unless otherwise stated, are of the MŁT type 0.5W 5%.

R1, 21 - 0,5MΩ
R2, 5, 15, 18÷20, 35, 50 - 1MΩ
R3 - 56kΩ
R4, 26, 30 - 68kΩ
R6, 12, 22, 25 - 0,1MΩ
R7 - 0,15MΩ
R8, 9 - 0,56MΩ
R10 - 5,6MΩ
R11, 13 - 0,22MΩ
R14, 51 - 2,2kΩ
R16 - 33kΩ
R17 - 27kΩ
R23 - 1,2kΩ
R24 - 0,39MΩ
R27 - 6,8kΩ
R28 - 10kΩ
R29, 34, 44, 47 - 120Ω
R31 - 0,2MΩ 2%
R32, 42, 43, 48 - 1kΩ
R33 - 3,9kΩ 2%
R36 - 150kΩ 1W 2%
R37, 38 - 68kΩ 1W 2%
R39, 40 - 470kΩ 2%
R41 - 12kΩ 1W
R45, 46 - 220Ω 1W
R49 - 5Ω 2W (wirewound resistor)
R52 - 470kΩ 1W


C1 - 150pF/100V
C2 - 560pF/100V
C3 - 220pF/100V
C4 - 2200pF/100V
C5, 7, 14 - 0,1μF/250W KSF
C6 - 25μF/12V
C8 - 8μF/350V
C9 - 4700pF/25V
C10 - 3300pF/25V
C11 - 6200pF/25V
C12 - 820pF/100V
C13 - 25μF/12V
C15 - 0,1μF/250V
C16 - 560pF/100V
C17 - 8200pF/25V
C18 - 2200pF/25V
C19 - 0,02μF/25V
C20, 21 - 5μF/350V
C23 - 0,1μF/250V
C24 - 0,022μF/250V KSF
C25 - 0,022μF/250V
C26 - 5μF/350V
C27, 28 - 50μF/25V
C29 - 430pF/500V
C30, 34 - 0,022μF/630V
C32, 33 - 100μF/450V
C34 - 0,1μF/100V


P1, 2, 3 - PA-102 - 500kΩ A 1W
P4 - PA-102 - 500kΩ C 1W
P5 - DG 101T - 27Ω 1W
P6 - DG 101T - 470Ω 1W

Electron tubes

L1, L2 - EF86
L3 - ECC83
L4, L5 - ECL86
L10 - EM84

Semiconductor diodes

D1, D2 - DK62

Other items

B1 - fuse 1A
B2 - fuse 0,3A


  • A. Witort: "Elektroakustyka dla wszystkich (Electroacoustics for everyone)" WKŁ Warszawa 1963
  • A. Witort: "Amatorskie wzmacniacze elektroakustyczne (Amateur electroacoustic amplifiers)" WKŁ Warszawa 1968
  • M. Słaby, P. Kozłowski: "Przetworniki elektroakustyczne (Electroacoustic transducers)" WKŁ Warszawa 1969

Material made available to the joy of electron tube enthusiasts: Grzegorz 'gsmok' Makarewicz