Preamplifier A.R. Electronics PL-02
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Under the name of A. R. Electronics hides Audio Electronics Company - a small workshop producing tube amplifiers in Gdańsk. For the first time I saw a product of this company at the end of the 90-ies of the last century, when I became the happy owner of the amplifier with the symbol SE-41. Unfortunately, I do not know if the company still exists, because for some time I have no information about new A.R. Electronics amplifiers. Anyway, in the secondary market one can meet the A.R. Electronics amplifiers characterized by good sonic parameters and interesting design line. Despite the lack of information in the official offer, A.R. Electronics produce or produced not only amplifiers but also other audio equipment such as tube preamplifiers. They were made to individual order or in a small number of units. To this type of products belongs a tube preamplifier with the symbol PL-02.
PL-02 preamplifier is shown in Figure 1. It is connected to the mentioned SE-41 tube amplifier. As an example of the reliability of output tubes used in the amplifier, I proudly declare that these are the same tubes which were used in the amplifier when I bought it almost twenty years ago. It is no suprise considering the fact that these tubes are known for their reliability EL34 made by RFT company.
PL-02 preamplifier is a small, sleek device (Figure 2) with a separate plug-in power supply. The housing is made of wood. The interior is covered with two metal powder painted metal plates. A gold-plated badge with the logo of the company is glued to the top cover. This item is very characteristic for A.R. Electronics products. So is the wooden knob with gold-plated pin indicating the position of input source selector.
Exposed tubes are unprotected and during operation of the preamplifier everyone can enjoy their natural illumination. I emphasize the word "natural" as more and more so-called "tube amplifiers" utilizes some kind of boosters in the form of light-emitting diodes (terrible embarrassing!).
According to the manufacturer, preamp has the following parameters:
- Input impedance: 470KΩ,
- Output impedance: 20Ω,
- Frequency range (-0.5dB): 0.5Hz - 1MHz,
- Dynamic range: 110dB(A),
- Inter-channel separation: 90dB,
- Distortions THD: <0.005% in the range 10Hz-100kHz,
- Amplification 1.1,
- Power requirements: 5W.
The preamplifier has three inputs selectable via rotary switch. Each of them is described as different signal source, but from the electrical viewport (sensitivity, input impedance) all inputs are equivalent. The device does not have its own power switch - it's confusing economy. His role is played by the user inserting and removing the power supply from the wall socket. Praemplifier input sockets and switch are shown in Figure 3.
The preamplifier is based on four small rubber feets, which are also used to fix the bottom cover. There is a sticker glued on it (Figure 4) indicating the functions of the applied input sockets. A very practical approach - not often encountered in audio devices produced in single pieces - they are usually stripped of all descriptions, and users are led to believe that the lack of descriptions is a sort of audiophile fashion.
Opening the preamp is very simple. This avoids any problems with servicing. Unscrew the four decorative caps on the front panel (Figure 3) and after removing it the inside of the preamp appears in all its glory - as shown in Figure 5 As you can see all elements are assembled on a single-sided PCB. Tube sockets and most of the parts are soldered on the same side of the PCB. Nevertheless, thanks to the "low" elements used, it wasc possible to expose the tubes in the same manner as is the case of devices in which the elements are mounted on the opposite side of the plate than the tubes.
As far as the signal capacitors, the axial elements are usedl (and that's OK), the electrolytic capacitors used in the power supply are not axial and had to be "located" on the side (which is already not so OK). Viisual feelings are spoiled by ceramic resistor (the white cube in the bottom left corner), which looks like it has been temporarily retrofitted rather than intentionally installed in the device. It is even more aberrant that the place on the board was provided for him - indeed nothing surprising because of the fact, that it is used to lowering the filament voltage, the value which is frequently too high.
Equally simple is the procedure for accessing the interior of the bottom of the preamplifier. Unscrew the four feet (Figure 4), take off the cover, and here in front of us there is a view from the side of the PCB tracks (Figure 6). There are four capacitors soldered on the underside of the PCB - two green MKS capacitors filtering anode supply voltage (C7 and C8 in the schematic diagram) and two blue electrolytic capacitors filtering tubes filament voltage.
I did not have access to the manufacturer's documentation, so I created the schematic diagram from real device, just looking at the mounted PCB. One channel of the preamplifier is shown in Figure 7...
... and power supply in the Figure 8.
I repeat and emphasize that above diagrams are written by nature, with all its consequences. I tried to recreate it as accurately as possible, but it is possible that I made some mistakes. If someone find them, please let me know.
The following photos presents parts of the printed circuit board containing various functional preamplifier blocks. In Figure 9 one channel is shown as seen from the components side of the PCB. In the foreground you can see the 9-pin bakelite tube socket (noval) and two axial polypropylene capacitors: 0.22μF/630V (C1) and 2.2μF/250V (C5). The third capacitor in the signal path is 0.22μF/630V (this little red "cone" near the tube socket - wish the manufacturer used the same type as the input capacitor).
In Figure 10 part of the PCB with elements providing positive voltage supply is shown. Negative power supply voltage is the same except for the type of transistors used (see diagram on Figure 8).
In Figure 11. a view of the section of the PCB with preamplifier signal path is shown. Please pay attention to the place of joining filament voltage filter capacitosr. The first one (the one in the lower right corner) is connected directly to the heater pins, while the second has been soldered directly into the ground fixed to the input jacks. Mounting suggests that the connection of the capacitors were chosen experimentally. The experiment was a success and the preamp is virtually free of hum - hip hip hurray!.
Figure 12 shows a section of the PCB containing the preamplifier power supply. The yellow wires are designed for serial connections of preamp tubes heaters.
I invite you all to share opinions and information about the PL-02 preamplifier.
Written by Grzegorz "gsmok" Makarewicz, www.trioda.com