Grzegorz Makarewicz ("gsmok"), This email address is being protected from spambots. You need JavaScript enabled to view it.

   At first glance, another stereo tube amplifier in a push-pull circuit. Classic look with three boxes containing a mains transformer and two loudspeaker transformers and a beautifully displayed "battery" of electron tubes. The manufacturer also presented us with a set of electrolytic capacitors - maybe a slightly less common, but also not very innovative design approach. To sum up - the amplifier is nice, but it is boring. The last glance at the vacuum tubes used and suddenly a surprise - the set of tubes a bit strange, not to say crazy. And it is here that the secret of this construction is hidden. But let's start at the beginning.

   The manufacturer of the amplifier, the company "Audio Aero" was founded in 1997 and its formal association concerned the aviation industry more than audio devices. Well, it happened, and an important French player appeared on the market. "Audio Aero" does not specialize in the production of tube amplifiers at present. The presented amplifier called "Audio Aero Capitole PA" is an example of an ephemeris born at the moment of a flash of a creative genius of a designer and forgotten during the struggle with competition on the difficult audiophile market. A small number of these amplifiers remained on the ruins of the lost war, among them the one that ended up in my hands. Here are its basic technical parameters:

  • Output power: 50W (Class A, 8 ohms load),
  • Tubes used: 6SN7 x 4, E34L x 4, KT88 x 4,
  • Frequency response: 7Hz - 35kHz (-1dB),
  • Input impedance: 22K,
  • Input sensitivity: 800mV,
  • Signal / noise ratio: 70dB,
  • Harmonic distortion: 0.2%,
  • Power consumption: 400W,
  • Dimensions: 520mm (width) x 395mm (depth) x 285mm (height),
  • Weight: 38 kg.

   You can say that I was lucky that the amplifier found its way into my hands, because even on the Internet there is very little specific information about this amplifier. The photo below shows the amplifier in all its glory. It is not a copy that I had the opportunity to repair - the photo comes from company materials (unfortunately I do not know its exact source and I cannot provide the author's data). Two photographs taken by me, showing the appearance of the amplifier, are presented in the further part of the description. Why? Well, because they're not very good and you don't see all the details on them.

   Let us return to the issue of the unusual design of the electronic circuit of the amplifier. This unusual feature consists in the use of a parallel connection of triodes and pentodes in the output stage. In fact, only pentodes work here, but in each of the four sets of tubes - one of them (in this case E34L / KT77) is connected to a triode circuit, while the other (KT88 / 6550) works as a pentode in "ultralinear" mode . The previous photo and the two below show the full set of tubes used. The photos taken by me are a bit too dark and not very detailed. Unfortunately, once again, driven by curiosity, after receiving the amplifier, I immediately started to disassemble it and document the interior, and after the repair I forgot to take pictures and took them just before handing the device over. I cannot learn to be systematic and in many reports in the Gallery section I have a problem with showing nice photos. Well, no more self-pity. Coming back to the matter, the photo shows this extremely interesting combination of output tubes in each channel of the amplifier.

Photo 1.

   In the shot from the side of the amplifier, the set of tubes looks even more impressive.

Photo 2.

    The aforementioned work system of the output electron tubes used in the amplifier is called by the manufacturer the "TRAC" system (short for Tube Relay Amplification Concept) and its roots are apparently derived from the "triode-pentode" concept developed in the 1950s. Unfortunately, I have not found any information on this subject in the literature I have. So it is difficult for me to refer to the technical advantages and disadvantages of such a system. My plan is to perform simulation calculations - maybe something will result from them. Meanwhile, I must admit that my subjective listening experience is amazing. So, regardless of whether it is just a marketing gimmick or a solution based on technical foundations, the effect has been achieved.

    Time for a "strip session". I start with removing the tubes. It should be emphasized here that the manufacturer took care to help the user keep the original position of the tubes in the sockets and equipped the amplifier with two tasteful sponge packages (one for each channel) with holes for the tubes. After taking out, it is enough to place each electron tube in a corresponding "pocket" to easily put them back into the appropriate sockets of the amplifier. Bravo, bravo. My admiration is somewhat muffled by the thought that the development of sponge houses for electron tubes is due not so much to the care for the user, but to the fact that the amplifier, due to its unusual circuit, is very sensitive to the selection of electron tube parameters - which I found out when starting an amplifier with electron tubes of different parameters.

  The blue tube sockets with gold-plated contacts are not original. They were replaced by someone, which can be clearly seen in further photos.

Photo 3.

    After turning the amplifier over, a solid base appears. Three legs ensure a stable position even on uneven ground. The cover seems a bit rough but it does its job well - and here it is, after all.

Photo 4.

    After removing the cover, you can see the inside of the amplifier. This is probably one of the few photos of the interior - at least I have not found such a photo anywhere, despite the fact that I searched the Internet resources for quite a long time. As you can see, all electronics are placed on three printed circuit boards.

Photo 5.

   Here is the PCB for the power supply and the left channel of the amplifier.

Photo 6.

  A closer inspection of the entrance step shown in the photo below ...

Photo 7.

... enable the reconstruction of the schematic diagram of this fragment of the electronic circuit ...

... and unfortunately evoke mixed feelings about the input impedance value given by the manufacturer. By the way, this amplifier can be a big problem for the signal source.

   And this is the PCB for the right channel (and of course the power supply).

Photo 8.

    Now the circuit boards under a slight magnification. Left amplifier channel.

Photo 9.

    Anode power supply. High power "glazed" resistors were used here. I love them .

Photo 10.

   Enlarged photo of the left amplifier channel. Oh, I would have forgotten. The elements not mounted on the PCB, despite being included in the description layer of the PCB (e.g. resistor R33), were intended to be used in the amplifier with a symmetrical input - as you can see, the manufacturer gave up this option. There are clear traces of soldering of new sockets for electron tubes on the printed circuit board - too powerful soldering iron, a lot of rosin and, unfortunately, little patience .

Photo 11.

  One more view of the PCB in the vicinity of the voltage gain stage. Here we see a sloppy soldered assembly potentiometer marked with a red circle.

Photo 12.

  And yet the necessity to use a potentiometer in such a system seems obvious...

... and it was possible to predict the right place already at the stage of designing the printed circuit board and not at the stage of assembling the final version of the amplifier. Could the constructor run it for the first time in the "factory" version .

  Enlarged photo of the power supply. You can see here - under the power cord - a small printed circuit board with a delayed anode voltage switch-on system placed vertically.

Photo 13.

  Right amplifier channel PCB (top view).

Photo 14.

  For several photos, many people have probably wondered what these chokes are doing here. Well, it's actually hard to say. They were placed in the anode output triodes, which, unlike the pentodes, are not connected directly to the output transformer, but through such chokes, as can be seen from the schematic diagram.

  Perhaps this is some form of frequency correction - although at first glance it is only a way to deteriorate the parameters of the output transformer seen from the side of the triodes anodes.

  Left channel amplifier PCB - horizontal shot.

Photo 15.

  Anode power supply printed circuit board - horizontal shot.

Photo 16.

  Now it's time for detailed photos. This is the first of many shots showing large coupling capacitors "flying" in the air (Kimbercap 1uF / 600V). Below is a photo of a part of the printed circuit board of the left channel of the amplifier.

Photo 17.

  Another photo with "flying" capacitors (PCB for the right channel of the amplifier) ...

Photo 18.

... and one more photo (left channel PCB).

Photo 19.

  Now an anode power supply with visible elements of a printed circuit board with a delayed anode voltage switching system.

Photo 20.

    And now a dozen or so photos of printed circuit boards - piece by piece. I'm silent - admire or criticize yourself.

    A fragment of the PCB for the right amplifier channel.

Photo 21.

   A fragment of the PCB for the right amplifier channel.

Photo 22.

   A fragment of the printed circuit board of the left channel of the amplifier. I am amazed by the belief in solid mounting of the capacitors with double-sided adhesive tape. I have not yet come across an amplifier based on electron tubes in which this would work. The sad truth is that if something is just stuck, it will come off sooner or later - the sooner the higher the temperature of the surface to which the "patch" is stuck.

    In my opinion, the use of adhesive tape fastening in systems with high voltages and temperatures is a sign of a lack of common sense and professionalism.

Photo 23.

    A fragment of the printed circuit board of the left channel of the amplifier. Two mysterious white elements were already visible in the previous photos. I will reveal the naked truth to you. These are electrolytic capacitors shunting the cathode resistors of the output tubes. The capacitors are placed in something that resembles thick heat shrink sleeves. This procedure was performed only on the PCB of the left channel of the amplifier. Why? Well, we have two options.

    The first possibility - the manufacturer placed all electrolytic capacitors in such sleeves to protect the capacitors from high temperatures from practically contacting cathode resistors - someone could remove the sleeves from the capacitors on the PCB of the right channel of the amplifier.

    Second possibility - these jackets are not original and were put on by someone as protection against the temperature coming from cathode resistors. Apparently there weren't enough sleeves for all capacitors.

    Either way, the electrolytic capacitors are placed too close to the hot resistors. The circuit board was not very well designed. There is a lot of space on it and the capacitors could be placed further from the cathode resistors - even by a few or a dozen millimeters. The disastrous effect of temperature is evidenced by the fact that the capacitors could not withstand the increased temperature and I had to replace all four. I was surprised by the low value of the rated voltage of the capacitors used - 35V with the voltage reaching almost 33V - so the voltage reserve is rather symbolic.

Photo 24.

    A fragment of the printed circuit board of the left channel of the amplifier. The transistor "supporting" the Zener diode D1 visible on the left side of the heat sink is screwed to the heat sink.

Photo 25.

    Relay and delayed anode voltage circuit of the amplifier.

Photo 26.

   A fragment of the printed circuit board of the anode power supply.

Photo 27.

   A fragment of the printed circuit board of the power supply and the right amplifier channel.

Photo 28.

  A fragment of the left amplifier channel board. Here you can see the connection details between the PCBs. Some of the wires are soldered directly to the PCBs, some are fixed in really solid sockets with spring clips.

Photo 29.

  A fragment of the printed circuit board of the left channel of the amplifier - the input voltage amplification stage.

Photo 30.

   A fragment of the PCB for the right channel of the amplifier. C4R coupling capacitor mounting details. Traces on the printed circuit board indicate that it was glued "pointwise" directly to the leads of the vacuum tube socket .

Photo 31.

  A fragment of a printed circuit board of an anode power supply.

Photo 32.

  A fragment of a printed circuit board of an anode power supply.

Photo 33.

  A fragment of the printed circuit board of the anode power supply and the circuit board of the delayed anode voltage switching system. As you can see, the relay (RL1) was originally supposed to be mounted on a dedicated vertical PCB and not on the PCB of the anode power supply.

Photo 34.

  A fragment of a printed circuit board of an anode power supply.

Photo 35.

  A fragment of a printed circuit board of an anode power supply.

Photo 36.

  A fragment of the printed circuit board of the anode power supply and a cutout in the chassis, through which you can see the power transformer.

Photo 37.

  A fragment of a printed circuit board of an anode power supply.

Photo 38.

  A fragment of a printed circuit board of an anode power supply.

Photo 39.

  A fragment of a printed circuit board of an anode power supply.

Photo 40.

  Right amplifier channel input and speaker terminals.

Photo 41.

  Left channel amplifier input and speaker terminals.

Photo 42.

  Input socket, right channel loudspeaker sockets of the amplifier and mains power socket.

Photo 43.

  AC power socket and speaker terminals.

Photo 44.

  Power plug of the electronic system of delayed anode voltage activation. Unfortunately, this plug does not pair with the socket used. You can insert it as shown in the photo or turn it 180 degrees. Unfortunately, there is no original socket that forces the correct direction of insertion of the plug. Fixing the wires in the plug is uncertain - it is done on the basis of a mechanical pressure of the wires to the sharp edges of the contacts. The lack of an original socket causes the wires to move away from the contacts and the contact is lost. The strangest thing is that there are no signs of unsoldering the original socket. This connection appears to have been made "at the factory".

Photo 45.

  Electronic circuit for delayed anode voltage switching.

Photo 46.

  Electronic circuit for delayed anode voltage switching.

Photo 47.

  Electronic circuit for delayed anode voltage switching.

Photo 48.

  I started disassembling printed circuit boards. Here is the interior without the left channel amplifier board...

Photo 49.

... and here is a view of the inside of the amplifier after removing the right channel amplifier board.

Photo 50.

   This is what the chassis looks like after removing the printed circuit board of the anode power supply.

Photo 51.

   More shots of the exposed sections of the chassis. Here are the holes for the tube sockets of the right channel.

Photo 52.

  Holes for electrolytic capacitors and a power plug for the delayed anode voltage switching system and LEDs signaling the operating status of the amplifier.

Photo 53.

   After removing the printed circuit boards, I made their detailed photographic documentation.

   Anode voltage power supply board. You can see here fast rectifier diodes installed by someone instead of the original rectifier bridge. The markings on the printed circuit board prove that there were rectifier bridges here.

Photo 54.

   Anode voltage power supply circuit board with visible tops of electrolytic capacitors. When I was looking at the inside of the amplifier, I wondered how the AC voltage was rectified using only THREE diodes.

Photo 55.

   After removing the printed circuit board of the power supply, the matter became clear - the "missing" diodes were soldered to the bottom of the printed circuit board.

Photo 56.

   The first time I saw these cut paths on a circuit board, I thought that the person swapping the bridges for diodes was a barbarian and the entire operation related to destroying the paths should be legally prohibited. A thorough analysis of the power supply system revealed the terrible truth - the paths were cut not because of the diodes, but ...

Photo 57.

... because there is an error on the PCB. The intersection of the tracks and the use of wire bypasses was probably done by the manufacturer. Were it not for these "corrections", the delayed anode voltage switching system would not work due to improper connection of the relay. By the way, the implementation of the "intersection" of paths is embarrassing .

Photo 58.

    Anode voltage power supply printed circuit board.

Photo 59.

   Anode voltage power supply printed circuit board.

Photo 60.

    Anode voltage power supply printed circuit board.

Photo 61.

    Anode voltage power supply printed circuit board.

Photo 62.

    Left channel amplifier PCB.

Photo 63.

   Left channel amplifier PCB. You can see that one of the resistors is soldered to the bottom of the PCB. Even a brief analysis shows that with a little effort this could have been avoided by slightly modifying the circuit board track design.

Photo 64.

    Left channel amplifier PCB.

Photo 65.

    Left channel amplifier PCB.

Photo 66.

    Left channel amplifier PCB.

Photo 67.

    Left channel amplifier PCB. Please look at the "floating" capacitors for the last time, because before mounting the PCBs, I removed the bands with slices and made dedicated fixings.

Photo 68.

    Left channel amplifier PCB after replacing electrolytic capacitors and installing clamps for coupling capacitors.

Photo 69.

    This is what a solidly mounted large-size capacitor should look like. For each capacitor, I made a stand in a form that matches the curvature of its housing, a support screwed (not glued !!!) to the PCB. The supports are made of a resilient, hard rubber-like material. They are equipped with through holes through which a clamp can be threaded. In this way, the fastening is on the one hand safe and durable, on the other hand it enables the capacitor to be replaced by simply removing the clamp.

Photo 70.

  And finally, I will proudly praise this phoography once again at the end of the presentation. What, in the end I got tired of making these supports and fixings.

Photo 71.

   Time for a quick summary.

   The described amplifier is an interesting and unconventional design. Despite a few shortcomings, there are no major objections to the quality of workmanship. Very good quality elements were used in it. Interesting appearance and good sonic properties make it an interesting purchase on the secondary market. It is not without significance that the risk that a neighbor has the same type of amplifier is minimal .

   I would be grateful for any information regarding the use of the triode-pentode configuration in amplification systems based on electron tubes. 

Prepared by: Grzegorz Makarewicz ("gsmok"), This email address is being protected from spambots. You need JavaScript enabled to view it.