Loudspeaking radio receiver with a bridge amplifier powered by "free energy"

Radio 2001, 12

Receivers without power supply are interested for radio amateurs. This article describes an improved radio receiver powered by radio waves.

While experimenting with different receivers and amplifiers powered by "free energy", it was found that it is more convenient to connect the audio amplifier to the receiver by using only two wires for audio signals and supply voltage. This would allow to use the radio receiver with no switches, just connecting headphones to the output of the receiver.

In general, this receiver reminds the previously described version of "crystal" radio receiver, but it has some interesting features.

loudspeaking radio receiver circuit schematic

Fig. 1
VT1, VT3 - MP37 (Germainum, hFE = 15...30, ft = 1MHz); VT2, VT4 - MP41 (Germainum, hFE = 30...60, ft = 1MHz);
VD1-VD4 - D18 (Germainum); T1 - transformer with ratio 30:1;
L1 - LW loopstick ferrite antenna.;
* - tweak the value (see text).

The schematic diagram of the receiver is shown in Figure 1. From the detector bridge the circuit is completely symmetric, the detector is connected to the amplifier by two wires (the terminals A and B) and the output of the amplifier is connected to the loudspeaker (the terminals C and D) by two wires.

The resonant circuit of the receiver comprised the antenna capacitance and inductance of the coil L1. This solution provides a maximum power of the signal in the resonant tank circuit. The switch SA1 and the neon lamp HL1 are used to protect the receiver during thunderstorms. The static charge doesn't build up in the antenna because the antenna connected to the ground through the coil L1.

A bridge detector circuit (VD1 - VD4) is used in this receiver, it works very well for the inductive load. The detector connected to the antenna through the capacitor C1, this capacitor is matching impedances between them. Once adjusted for maximum voltage across the amplifier, the capacitor C1 may be replaced with a constant capacitor with proper value. The optimal capacitance of the capacitor C1 is about 47 pF for LW band.

The output voltage of the detector is symmetric with respect to ground. Through the wires A and B the voltage passes from the detector to the input of the audio amplifier. At the input of the amplifier the voltage decomposes into AC and DC parts. The AC part feeds through the coupling capacitors C3 and C4 to the transistor bases of the bridge amplifier. The DC part charges through the low-frequency chokes the capacitor C6. The DC part is used for power supply. The receiver doesn't have a common wire. The arms of the amplifier balances automatically, because the bases of the complementary transistors are connected together.

But transistors in this type of amplifiers don't have a bias, they does not work in the class "B" but rather in the class "C". This leads to crossover distortion of the signal waveform, as shown in Figure 2(A).

 the dependence of the output current versus the input voltage of the audio amplifier

Fig. 2

The graph shows the dependence of the output current in one arm of the amplifier (for example, VT1, VT2) on the input voltage. We see a distorted output current for a sinusoidal input voltage. These distortions are especially noticeable with silicon transistors that have higher junction drop voltage of about 0.5 V. Germanium transistors has lower junction drop voltage of about 0.15 V, so they are used in the audio amplifier.

Crossover distortion is related to the moments when voltage crosses zero point, that is very unpleasant to the ear. Crossover distortions can be reduced by using a slight forward bias Ubias, as shown in Fig. 2(B). The distortions disappear but some initial current i0 appears, it makes the amplifier less efficient.

The same result can be obtained by other means. If mixing the audio signal with a high frequency signal, as shown in Fig. 2(C). This method is used in tape recorders with AC bias, because the magnetization curve of the type is very similar to the amplifier transfer characteristic of a push-pull stage without bias. By adjusting the amplitude of the "high frequency bias" the desired initial current (quiescent current) can be set, this current should not be too high, but sufficient to eliminate the distortion.

But we already have high frequency bias, we got the detected RF voltage ripple. In the bridge detector circuit the ripple has twice the frequency of the carrier signal. We just need to tweak the value of the smoothing capacitor C2 (Fig. 1) to obtain the desired quiescent current. It's better tweak the capacitor C2 when there is no audio transmission (but there is a carrier frequency of the radio station) because if there is an audio signal then the current of the audio amplifier increases. At the output of the audio amplifier the ripple don't need anymore, so there is the smoothed capacitor C5.

The coil L1 is wound with litzwire 7 x 0.07mm (7 wires x AWG 41) on a cardboard pipe with a ferrite slug of 8 mm in diameter and 160 mm long (the permeability of the slug is about 1000). The coil has about 200 turns. Actually, any other litzendraht may be used or any copper wire with silk insulation of 0.15...0,25 mm in diameter (AWG 35...AWG 30). A standard loopstick antenna with the LW band can be used as the coil L1. C1 is a ceramic or air trimmer capacitor.

In the detector circuit the best result was obtained by using diodes D18, diodes GD507 works not too bad, and the worst result was obtained by using diodes D311 (D18, GD507, D311 are germanium diodes). In the detector circuit may be used any germanium diodes.

The transformer T1 has ratio 30:1. The primary winding has 2700 turns of the wire with diameter 0.12 mm (AWG 37) and the secondary winding has 90 turns of the wire with diameter 0.5 mm (AWG 24) wound on a former which is mounted on a core made of permalloy E-shape plates of 15 mm2. Any suitable output audio transformer can be used here. A primary winding of the same transformers can be used as chokes L2 and L3. The inductance of this chokes should be not less than 6..7 H. Any low frequency germanium transistors may be used in this circuit. If possible, match the transistors with similar hFE.

The receiver can be adjusted in a few minutes. Disconnect the audio amplifier from the detector and connect a high-impedance headphones to the terminals A and B, check the detector part of the receiver, try to tune to a powerful radio station, if necessary change the number of turns of the coil L1. The tuning is performed by moving the ferrite rod in and out the coil L1. Next, connect the amplifier to the receiver and connect a high-impedance DC voltmeter across the capacitor C6 to monitor the voltage, tune the receiver to the frequency of a powerful radio station and adjust the capacitor C1 for the maximum reading of the voltmeter. Keep in mind that the voltage across C6 increases slowly because of the large capacitance of the capacitor C6. Connect across the capacitor C2 another capacitor with a value of a few thousand picofarads and wait for some seconds, read the voltmeter. Then tweak the capacitor C2 to get the voltage 20...30 % below the nominal value. In the author's version of this receiver the voltage was 5.5 V and 4 V. There is nothing more to adjust in this circuit.

The receiver was tested in the city apartment located in the eastern outskirts of Moscow. An external antenna was used. The antenna has length 30 meters of copper enameled wire with a diameter of 0.7 mm (AWG 21). The maximum height of the antenna above the roof does not exceed 7 meters. Metal pipes of a central heating system was used for grounding.

Even with this antenna it was possible to receive signals of a five radio stations with loud speaker volume. The loud speaker volume means that the volume is sufficient for normal listening in a small room when there is no ambient noise. The values of the detected voltages, currents and power, extracted from the air by the receiver of the above mentioned radio stations are shown in Table 1. The voltage was measured across the capacitor C6, and the current was measured in series with any of the wires A or B, while the receiver is working.

Frequency, kHzVoltage, VCurrent, mAPower, W

Table. 1

It should be noted that the audio amplifier loads sufficiently the detector, because the value of the capacitor C2 that was chosen provides the best quality of the sound, so with this value the quiescent current of the amplifier is sufficient.

The widespread opinion that the quality reception of long and middle wave signals is impossible especially in the night time are wrong, and this receiver disproved this mistaken opinion. This receiver does not suffer from interference because of its low sensitivity. The quality of the sound cannot even be compared to the sound quality of conventional portable receivers.

V. Polyakov