English (United Kingdom) 
Polski (PL) 
Grzegorz "gsmok" Makarewicz (prepared in 2007)
The EgoVox tube amplifier is one of the most "mysterious" products on our market in terms of the amount of information available about it. My searches in my home library resources ended with one article in the professional press, namely a description of a listening system test entitled "Close Specialization" in the April issue of the Audio-Video monthly from 2006. The evaluated listening system was based on the EgoVox amplifier with EL34 tubes in the output stage.
The AI-generated malicious drawing is not random
A bit more information about the first EgoVox design, this time with EL84 tubes in the output stage, can be found in the June 2004 issue of Audio-Video, but considering the fact that the test presented there concerns a different version of the amplifier (analogous circuit but different output tubes), the information presented there can be treated only as a supplement and a source of some interesting curiosities about the joint Polish-Chinese business venture. I will refer to some information later in the description.
While the lack of information in the "paper" literature does not surprise me, the "dryness" in the results of searches in online resources is quite a surprise. Why? Well, because this amplifier is offered in several stores and often appears on online auctions. It seems that the owners of this amplifier are deprived of even basic documentation. Honestly, I am surprised that the distributors and the designer of EgoVox, in their own, well-understood interest, do not provide not only documentation, but even any user manual.
In appearance, the EgoVox amplifier is a classic-shaped design. A distinctive, eye-catching feature is the exceptionally solid cover of the electron tubes.
The tube cover is so rigid that it surpasses the basic chassis of some tube amplifiers imported from China. A special hole is cut out in the upper part of the cover through which the "badge" with the amplifier's name is visible. It can be seen in the photo below showing the amplifier after removing the cover (to do this, unscrew four powerful Allen screws). The cover can be dismantled very easily - it takes literally a few minutes. Reinstalling it is a completely different matter. Hitting invisible holes with long screws can take an hour or more.
The cast of electron tubes is truly impressive - there are ELEVEN of them. The tubes are 1x6N9S (6H9C), 4x6N8S (6H9C), 2x 5C4S and 4xEL34. In the described amplifier, all the tubes were made in China.
The following elements are located on the front of the amplifier, looking from the left:
- an input selector that also acts as a switch for the tube that is monitored for the value of the quiescent current,
- a beautifully illuminated yellow indicator for checking the correct operation of the output tubes (the voltage for the illumination is "stolen" from the transformer filament winding), and
- a volume control potentiometer.
The power switch is located on the wall of the housing on the left side of the amplifier. This solution is often used, although not very practical, because it prevents another element of the electroacoustic path from being moved closer to the amplifier, e.g. a CD player, etc. On the other hand, such a location of the switch allows for shortening the wiring from the primary side of the mains transformer and reducing the interference associated with it. Something for something.
The rear panel features RCA sockets for four signal inputs, speaker sockets with the option of connecting speakers with an impedance of 4Ω and 8Ω, and a socket for connecting a power cable. All sockets seem very solid, although they do not come from audiophile manufacturers.
The right side of the case in the photo below is completely empty - and rightly so, because there's nothing left that makes sense to put there. At least on this side, there's no problem "hugging" some audio device to the amplifier.
Before I move on to the gradual dismantling of the covers and an insight into the amplifier's interior, a few words about the lack of documentation mentioned at the beginning of the description. Namely, a certain "opinion-forming" discussion forum can "brag" about a thread in which participants analyze the issue of regulating the quiescent current of the output tubes in the EgoVox amplifier. Does the amplifier require regulation or not? - that is the question. Deprived of any support, they finally come to the conclusion that this is an amplifier that requires such regulation. But where are the regulating elements? Among the various ideas, I liked the one most, according to which the "bias" is adjusted using special screws with unusual heads placed near the output tubes, so a special "audiophile" Allen screwdriver is needed to turn them.
And here is a view of the aforementioned "bias adjustment elements" (in yellow outlines). There are even two for each tube. And now seriously - I would not want to mislead anyone under any circumstances. These are of course the screws with which the tube bases are screwed to the chassis, and the amplifier does not have any elements for regulating the quiescent currents of the output tubes.
After removing all the tubes, we see that each socket is screwed on with solid screws, the only function of which - let me emphasize once again - is to mechanically attach the tube bases to the chassis.
Enough of the jokes. Since the tubes have been removed, the amplifier can be turned over without fear of damaging them. Here is a view from the bottom. A solid cover with ventilation holes ensures proper air circulation. The amplifier, even after several hours of continuous operation, does not heat up too much.
After unscrewing the bottom cover, a view of the inside of the amplifier in all its glory is revealed.
The amplifier uses a mixed assembly. The power supply and the input switching system are assembled on two printed circuit boards. The amplification path is assembled using a spatial technique supported by soldering strips screwed directly to the chassis. The strips are selected so that all soldering points are used - where three points are needed, there are three-point strips, where four - four-point strips, etc. Very neat assembly.
Time for a few words on the circuit details. Based on a preliminary comparison of the interior of the amplifier described here with a photograph of the interior of its younger brother placed in AudioVideo 6/2004, I assumed that, omitting the output stage, the amplification path of the amplifier with EL84 output tubes should be similar to that used in the amplifier with EL34 tubes. Several pieces of information provided by the amplifier designer to the editors of AudioVideo should therefore be true for this design as well. What is this information? Here it is (please remember that this data concerns the version with EL84 tubes, not EL34):
- Each channel uses 5 triodes and 2 power pentodes.
- The first stage uses half of a 6H8C tube and is a classic voltage amplifier circuit, to which a negative feedback loop signal is fed.
- The signal is then fed to a phase inverter circuit operating in a differential configuration (two halves of a 6H8C tube).
- The signal from the inverter goes to cathode followers (another 6H8C) with a divided load, different for the constant and variable components of the signal.
- The circuit is powered in an unusual way and is the designer's own solution, which is an extension of a unique idea proposed in 1926 by P.N. Kuksjenko. The advantage of this configuration is good control of the output tubes, independent of their input capacitance.
And that's it for the information from AudioVideo. I tried to find something about Kuksjenko's circuit, but despite the fact that there are many of his archival studies available, I found nothing about the aforementioned circuit. I would be grateful if someone who has some information about it could supplement this description. In any case, I will probably do some simulation calculations sometime.
I started to try to recreate the schematic. Since I drew the amplifier schematic "from nature", it was not easy, because some of the connections to the mounting strips from the bottom are practically inaccessible. It turns out that my assumption about the amplifier configuration was correct. It is the same configuration that was described in AudioVideo. The only difference is that in the input stage in the specimen I tested, a 6H9C tube was used, not a 6H8C.
At the moment the diagram is not really suitable for publication. Just look at the sample below, which looks the best of all the ones I drew. The remaining fragments are a shame to show before calmly redrawing them cleanly. The presented fragment shows the voltage input stage (L9 tube) with negative feedback (from the 8Ω output of the speaker transformer) and the phase inverter (L8 tube).
I have made a kind of documentation connecting (using markings) the diagram with specific real elements. The photo below shows all electronic elements. I realize that with such dimensions of the drawing they are not clearly visible. To enable analysis of details, the drawing can be displayed in an enlarged form by clicking on it with the mouse, or...
...by familiarizing yourself with the component parts presented and described below.
The photo below shows the voltage stage (L9 tube), phase inverter (L8 tube) and the follower controlling the output tubes (L7 tube). Of course, only the elements of one channel have been marked. The input signal from the potentiometer slider (blue Alps with a resistance of 100kΩ) is fed directly to the control grid of the L9 tube. This tube is coupled directly to the phase inverter. The signal from the inverter outputs is fed in turn via capacitors C2 and C3 to the follower inputs.
The followers control the output tubes (L3 and L4) operating in a Push-Pull configuration as pentodes. The output transformer does not have a tap for ultralinear operation. The photo shows two test resistors (R16 and R17) designed to measure the quiescent current indicated by the indicator on the front panel during amplifier operation.
And here's a little assembly curiosity. The resistors through which the negative voltage biasing the grids of the output tubes is supplied are mounted "crosswise". For one channel, this is resistor R12 mounted near the output tubes of this channel, and resistor R13 mounted on a mounting strip placed near the opposite channel and vice versa. I don't really know why such assembly was used. It causes one of the connecting wires in each channel to be much longer than the other. This is probably not a coincidence, and there is something behind it - but what?
The tubes L10 and L11 visible in the photo are rectifier tubes.
Now the power supply. Here (on the right) we can see the delayed anode voltage switch-on circuit. Switching on is done gently. A semiconductor-tube rectifier is used (B1 bridge and L10 and L11 tubes visible in the previous photo). The rectifier output is fed to the first filter capacitor (C8) via resistor R25. As the rectifier tubes warm up, the voltage increases gently and is constantly reduced by the voltage drop on the series-connected resistor R25. After some time, the relay shorting resistor R25 is switched on and only then does full anode voltage appears on capacitor C8.
And here is the entire anode power supply with a full set of filters.
At the beginning, the CLC filter on the C8 capacitor, a choke placed outside the board (the photo is in the further part of the description) and the C9 capacitor. As a result, the supply voltage is obtained, which is then distributed to the amplifier from the point marked as Z1.
Subsequent filters - resistor R19, capacitor C10 and voltage Z2, and resistor R20, capacitor C1 and voltage Z3 from it.
The left side of the board is for the negative voltage power supply. Here we have a semiconductor bridge B3, filter C12-R22-C13 and after it two separated filter systems R23 and C14 and R24 and C15. Resistor R21 is not a part of the power supply. It is connected in series between the power supply ground and the amplifier chassis (the connection to the chassis is made using the black wire visible on the left).
And now it's time for a photo gallery with details. I've limited the commentary to a minimum. I'm starting, of course, with the part covering the signal amplification path.
After the amplification part, it was time for the power supply system. Before I show the details, as a reminder, I'm posting a view of the entire interior.
Here is the choke filtering the anode voltage. A solid beast on the so-called wound core in the shape of the letter "O".
In the photo below on the right side you can see the resistor through which the mass of the power supply is connected to the chassis. The way the connection point is processed on the chassis is embarrassing. In addition, this is only half of the sad truth - the other half is shown in the further part of the description.
Excellent quality of soldering components on the printed circuit board. There is something to look at. I couldn't resist posting a few photos. Bravo!!!
A solid ground "cake" and a series of black and white wires distributing ground throughout the amplifier circuit. A similar "cake" with red anode voltage wires. For "deception" one of the red wires turns out to be yellow.
And here is the aforementioned "other half of the sad truth". The "claw" method was used to remove the paint from the chassis. I suspected my friend's two cats of treating the amplifier chassis this way: Funia and Czaruś. Here they are. As you can see, they are pretending that nothing happened and, for the sake of convenience, are trying to be indistinguishable.
Intensive investigation revealed that the culprit may have been the claws of Funia, a cat commando.
And now, quite seriously. Couldn't it have been done professionally? After all, these two small imperfections spoil the whole effect of the amplifier's interior assembly. I have the impression that the connection of the system mass to the chassis was overlooked at the design stage and the visible solution is the result of desperate attempts to patch this hole in the amplifiers already produced - hence the "scratches" and the use of two, not a single, connection points for the system mass to the chassis, which is inconsistent with the accepted rules. By the way - the screws securing the soldering eyes also serve to fasten the transformer cover.
And now the elements related to handling the incoming and outgoing signals.
Let's start with the power switch. As you can see, it is protected by an additional flexible cover, thanks to which the places where the wires are connected are completely covered. The wires are placed in oil jackets - in fact, a real "pinnacle" of safe assembly. The switch is located in the immediate vicinity of the input selector. However, there is no fear of humming noise due to this.
This "red guy" (I mean the resistor) reminds me of something and doesn't quite fit in with the rest of the company. That doesn't change the fact that I obviously have nothing against these resistors - quite the opposite.
Input sockets and a set of relays for changing the active input channel.
A decent blue ("audiophile") Alps.
Switch for inputs (left-hand contacts) and measured electron tubes (right-hand contacts).
Relays again. An ingenious assembly has been used here to facilitate servicing of the amplifier. The wires can be soldered without having to dismantle the printed circuit board. For this purpose, solder loops have been placed on the board.
Here you can see the solder loops (connectors) in all their glory.
Since I am primarily interested in technical details, similarly to other descriptions I have placed in the Gallery, I will not refer in detail to the sound qualities of the equipment being analyzed. The EgoVox amplifier with EL34 tubes plays well. Due to a rather specific defect with which it came to me, I had the opportunity to listen to it for many hours, and I have no major reservations. My subjective perception of it is certainly influenced by my opinion regarding its design - it is simply well-made, so it should play well.
I invite those interested in listening tests conducted by experts to familiarize themselves with the results of the test published in Audio-Video no. 4/2006. However, I would like to point out in advance that the two audiophile opinions presented there traditionally lead to brain dilution and it is best to simply take advantage of some opportunity and listen to this amplifier in person.
Prepared by: Grzegorz "gsmok" Makarewicz,