Published: Wednesday, 01 July 2020 08:23
Written by Super User
Renat Terlecki,DC power amplifier, Antena 1934/11
Radio engineering has recently gained a completely new field of activity in the form of high power loudspeaker installations. Apart from the film of high-power loudspeaker installations, we also often meet in a wide range of public utilities such as train stations, guest houses, hotels, sports grounds, concert halls, even churches and parliaments.
Several years of practice crystallized the concept of a modern power amplifier and today we require from such a device:
- Complete electrification.
- Uniform gain and reproduction of a wide frequency range.
- Versatile use, i.e. for both the adapter and the microphone, photocells, radio and so on.
- High efficiency possible.
- Transparent design.
- Simple operation and minimal maintenance, and finally
- Carefully fitted equipment.
However, all this is successful when we have an AC source available, usually in the form of a lighting network. Alternating current, as we know, can be transformed into any high or low voltage; so we can build economical filament circuits without losses on reducing resistors, and having any high anode voltage we use more efficient systems, eg "class C", i.e. directly coupled.
In many cities, however, we have power plants, supplying lighting or industrial DC, and in this case building a high power amplifier, we face special difficulties. In fact, for the reasons given above, high power amplifiers are usually calculated on the power network. variable, so wanting to power them from the power grid fixed, we must use converters whose cost for small installations may exceed the value of the amplifier. So sometimes it pays to give up the economics of the system and settle for a relatively high power amplifier with a very low anode voltage of 150 ÷ 200 Volt.
The amplifier described below is designed for a 250 ÷ 150 Volt DC power supply. Understandably, by reducing the value of the main reduction resistance "R" we can connect the amplifier to a network with a lower voltage, but then the output stage power would drop too much.
Before we get to our amplifier, let's start by studying the most important system - the filament circuit. This circuit, as we can see from the simplified diagram in Fig. 1, consists of adjustable resistance R5, at whose ends, according to Ohm's law, the flowing current (1.4 Amp.) Creates the potential difference "Eg", used here as the negative pre-grid of output tubes. Then the filament current branches off after 0.65 Amp. on two cathodes of output tubess in parallel. The rest of the current, according to Kirchhoff's law, flows through the regulated shunt resistance R4, which regulates the filament voltage of both tubess. Then again, the sum of the filament current flows through the heater of the indirectly-heated input tube (ca 1 Amp.), whose heater is controlled by the R3 shunt resistance. Next we have an iron - hydrogen tube "1331" automatically regulating the filament current to 1.3A - and finally the main reduction resistance R.
It should be noted that the sum of resistance of this circuit between points A and B must be a constant value for a given network voltage. Not very large fluctuations in the network voltage and changes in R5 and R resistances will automatically compensate the resistance of 1331 tube, however, e.g. larger changes in R5 resistance must be compensated with R resistor. As for the essential part of the amplifier, as we can see from the complete diagram in Fig. 2, it is very simple.
At the input we have a switch that allows you to quickly transfer the amplifier from one pair of input sockets to another. From the switch, the current of audible frequency runs through the Pot potentiometer, regulating the force, from where it goes to the primary winding of the transformer Tr1, whose secondary winding is the grid circuit of the amplifier's input tube. The potentiometer can be successfully lowered if the audible current source (adapter-microphone) used has its own power regulation. The resistance R2 inserted into the cathode circuit of the first tube gives it the necessary negative pre-netting. The input tube anode through the primary winding Tr2 is connected to the choke Dl, which together with the C2 capacitor is used to smooth the pulsation of the network current. The Tr2 secondary winding has three terminals.
Read more: DC power amplifier RA360ST (1934 r.)