A lots of radio amateurs have an interest to power a simplest radio receivers with the "free energy", i.e. the energy, taken by the receiver antenna directly from the air. The circuits described here can provide a radio reception using a loudspeaker.

The question of how much power can get out of a signal from an antenna, and how to build a loudspeaking crystal set, was already discussed in the author's articles [1,2]. However the questions "how much power we need for loudspeaking reception?" and "how to better use the power from the antenna?", still remain.

According to the old reference books, to listening a voice of a broadcaster from the distance of 1 meter it takes the sound level of the loudspeaker about 60 dB. In this case, the radiated acoustic power is 12.6 μW. The necessary electrical power can be calculated by dividing the radiated acoustic power by the energy conversion efficiency of the loudspeaker. For the common loudspeakers the energy conversion efficiency is about 1%. Thus we get the electrical power about 1mW. It is interesting to calculate the required power for loudspeakers to get the sound level of 60 dB. The calculation results for the different loudspeakers are presented in table 1.

 Model Power, mW 0,025GD-2 3,6 0,05GD-1 1,8 1GD-5, 1GD-28, 2GD-7 1 5GD-1, 6GD-1PP3, 6GD-30 0,25 8GD-1PP3 0,2

Table 1.

From the table 1 we see that we need to use the high-efficiency loudspeakers. The acoustic design of the loudspeaker systems is very important, the bigger speaker cabinets is better. In the experiments the author used two loudspeakers type 4GT-2 in wooden cabinets with the enclosure volume of 50 liters.

Horn loudspeakers has three times better efficiency because of the improved coupling efficiency between the speaker driver and the air and because of the directional characteristics of the produced sound waves. The simple and effective loudspeakers was built by radioamateurs, they used paper, cardboard and plywood [3]. Horn loudspeakers with a bass reflex system with U-shaped design provides with the loudspeaker 6GD-1 efficiency of about 2.3%, and at the low frequencies about 3.4%. So, we found that the audio signal of 0.2 mW is sufficient for the sensitive acoustic system.

The second part of this "research" is related to electrical circuits of the loudspeaking detector radio.

Analysis of the detector circuit leads to the conclusion that the current should be amplified, but not the voltage, because the voltage amplification could limit the peaks of the signal. Because of this it is wise to use in this circuit the push-pull emitter follower, based on the complementary pair of transistors working in class AB. This amplifier has good efficiency and low current consumption while the quiet sounds and pauses of the signal, this allows to store the energy of the carrier and use the energy at the peaks of the audio signals.

The circuit diagram of the receiver with the amplifier based on the push-pull emitter follower is shown in Fig. 1. The AC component of the detected signal passes through the coupling capacitors C3, C4 to the bases of transistor amplifier, and the DC component passes through the choke L2 to the storage capacitor C5. This capacitor cannot be directly connected to the detector because in this case the audio signal will be smoothed and suppressed. The parameters of the choke are not critical, so you can use any choke or any transformer with a winding, containing not less than 2000 turns wound on the magnetic core with cross section not less than 1 cm2.

Fig. 1.
VD1 - D311 (Germanium diode); VT1 - MP37 (Ge, hFE = 15..30, Ft = 1 MHz); VT2 - MP39 (Ge, hFE ≥ 12, Ft = 0.5 MHz);
R1, R2 - 560K; C1 - 17..500 pF; C2 - 680 pF; C3, C4 - 0.68 μF; C5 - 68μF x 6.3 V; C6 - 22μF.
L1 - magnetic loop antenna for MW with a moveable ferrite core; L2 - an audio choke; T1 - transformer with ratio 30:1;
Loudspeaker BA1 - 4 ohms.

The optimal transformation ratio of the transformer T1 is about 30 for the load of 4 ohms. It is convenient to use a small power supply transformers from transistor radios with the voltage ratio 220 V to 6.5..9 V. A suitable output audio transformer can be used too.

The large size of the device (due to the heavy transformer and choke) is not an issue, because it uses the large antenna and a floor standing speaker system, so this is not a portable radio!

The use of a voltage doubling rectifier allows to increase the supply voltage of the circuit. Distortion on peaks of signal will be decreased. A bridge amplifier loads the voltage doubling rectifier symmetrically and furthermore decreases the distortion. This allows to get rid of the capacitive coupling at the output.

The circuit schematic of the receiver with the voltage doubling detector and the bridge power amplifier is shown in Fig. 2. The positive half-wave of the signal detected by the diode VD1, smoothed by capacitor C2 and filtered by low-frequency choke L2 and the capacitor C8, so it creates a positive supply voltage. Similarly, the components VD2, L3, C3 and C9 produce a negative supply voltage. The emitter followers based on the VT1, VT2 and VT3, VT4 are working in opposite phases, the signals to this emitter followers feeds from the different detectors. The emitter followers are loaded with the transformer T1. Just like in the previous circuit, the transformer ratio is about 30, but due to the bridge circuit the output power of the amplifier is higher than in the previous circuit. The purpose for the other components of the circuit shown in fig.2 is the same as the circuit shown in fig. 1, and the recommendation about the chokes is the same.

Fig. 2.

Adjustment of the receivers powered by the "free" energy has some features. This receivers will not work until they are tuned to a powerful radio station, because there is no power supply. But after tuning it will take some time to charge the capacitors (C5 - in Fig. 1 and C8, C9 - in Fig. 2). Charge time is directly proportional to the capacity of this capacitors, so in the first experiments the capacity should not be too large. But in this case while receiving a long loud sounds (especially in the musical passages), the power supply voltage and the detected voltage drops significantly due to increasing current of the audio amplifier, the result of it is the limitation of dynamic range. This does not lead to issues, but even improves speech perception.

When the receiver will be completely adjusted, the capacity of the smoothing capacitors can be increased even up to several thousand microfarads, it will improve the dynamics of the receiver and the audio amplifier would work out the peaks of the signals. In any case, all the capacitors should have a small leakage current (check it with an ohmmeter), to avoid the necessary load of the power supply.

Tweaking of the bias resistors in the receivers are based on the next reasons: the greater the resistance, the less the current consumption (the current when there is no signal in the receivers - see fig. 1 and 2), and the less the gain of the transistors but higher the supply voltage! A compromise can only be found empirically for a particular antenna, by getting the maximum sound volume and quality of the radio reception. The bias resistors for the circuits shown in Fig. 1 and Fig. 2 may have different values, it depends on the parameters of the transistors. The voltage at the emitters of the transistors is half of the power supply voltage (Fig. 1) and zero (Fig. 2).

It's better to start the experiments without bias resistors, and then try to use this resistors with value from 2.7 MΩ to 1 MΩ; if there is a "powerful" antenna than use the bias resistors with value of hundreds of kilohms, because the power supply voltage can be dropped. If a pair of complementary transistors have a sufficient initial current, it can be reduced by inserting a resistor between the bases of the transistors, or the bases could be connected together, and one of the base capacitors could be removed from the circuit. It has not mush sense to use in this circuits a bias thermal stabilization networks due to the low power of the amplifiers (some milliwatts).

This radio receivers has been tested in a country house (33 km to south-east from Moscow). The audio volume level has been enough for a small quiet room. The second circuit has been tested with especially good results. The antenna type the end fed half wave with the length of 12 m was used. The antenna was stretched from the window to the tree outside. The receivers was grounded to the water pipes of the well. The receivers was tuned to "Radio Rossii" 873 kHz, the radio stations "Radio 1" and "Radio Mayak" was received with loud volume too. The quality of the sound was excellent.

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