This preamp can be used to attack our power amplifier tube EV8010 (or another). The lamps give a blast of heat incomparable with that of bipolar transistors or MOS. Original design and aesthetics (see by yourself), this Hi-Fi stereo preamplifier has four selectable input buttons and relays and two outputs, monitor and amplifier.
Technical
Input Sensitivity: 200 mVrms
Input Impedance: 50 kilohms 
Output Impedance: 10 kilohms Output Impedance
monitor: 1 ohm Output Impedance: 10 kilohms Output Impedance Harmonic Distortion: 0.04% (@ 1 kHz)
Separation between channels: 85 dB Signal to Noise Ratio: 100 dB Frequency Response
: 7 to 150 000 Hz (-3 dB) Voltage gain : 2 Voltage gain (+6 dB inserted): 4 Voltage: 230 VAC.
This is indeed just a fad: the preamp / tube amplifiers (electronic or thermionic, English "valves" short tube) make it a more supple and warmer than fleece transistors. And, fortunately, today we no longer need as before to cut round holes in the folded sheet to secure the brackets lamps: large printed circuit boards are glass-epoxy board perfectly frame (see Figure 4 ).
For us innovative electronics, preamplifier is the Hi-Fi audio device that is before the power amplifier for (pre) increase the level of signals from the FM tuner, CD player or audio cassette and also to perform some corrections tone by tunable filters (control treble, mids and bass) and fixed ("loudness" bass enhancer). We usually ensure that this circuit is possible if done with a chip to do anything (in this area, of course) and represents the latest technology.
But here we have preferred to entrust this preamp to a pair of lamps (one per channel D and G) giving his famous fedora before the war! The preamp
described in this article is nonetheless a genuine link Hi-Fi (see specifications) for challenging any amp (tube, as our EV8010 or not ... but it might be injury) followed by its pair of speakers (in fact, we are in stereo). It is even more remarkable that many preamps transistors or integrated circuits in which the structure is not able to provide the sound that only tubes can provide electronic. This device, we wanted to "essentialist" or able to satisfy the most discerning audiophiles, those who in any case for an amplifier to amplify the stereo component without inflicting the slightest alteration. So here no "loudness" or "bass boost" or other Baxandall: do not worry, the signal is routed directly to the selector input stage preamplifier.
The wiring diagram in Figure 1
you find the wiring diagram in Figure 2 General and that of the secondary plate comprising the left channel input (identical to the right channel, mark) : The main scheme also includes the power and logic control relay selecting entries (which are also mounted on the large main deck (see Figure 5). Why two turntables? Well this allows small to superimpose great and save space in depth but still have the RCA "phono" inputs of both channels superimposed on two lines (which is aesthetic and rational).
We can connect to the preamp up to four stereo audio sources with output levels or means capable of providing between 150 and 600 mVrms. By no cons RIAA preamp input for playheads MC or MM vinyl records: if a module must be installed RIAA or buy ready (there are in all price ranges depending on the services offered, see our advertisers).
The choice of audio source equipment connected is by selector relay, equivalent to a four-position switch, two tracks: the eight reed relays are activated both, so both channels D and G are selected together, using the buttons associated with status LED (lit or not selected = = off).
The excitement of the relay is made by Microchip microcontroller which, at least for the selection of signals, acts as a "latch" (toggle) with four inputs triggered by pressure switches: each time one acts on one of them (SW1 , SW2, SW3, SW4), the PIC controls the corresponding pair of relays, the activation occurs in bistable in that the relay log entries in sections pre-amp until another button is pressed . These buttons can be used to manage three functions: the one just described is obtained by pressing and releasing them, if you press the button a second time the selected entry (the one you are listening), it empowers the monitor output, that is to say that the source sends one listens to the input of the any recorder connected. To do so puts the microphone in his line logic low RB3 (which turns LD7), which saturates T5 and will stick RY5 sections whose input contacts connect the MONITOR selector lines. If you press the button and maintain the channel listened to the ignition of LD5 (+6 dB) activates the exaltation of 6 dB (corresponding to a voltage gain of two times compared to the normal situation) designed to compensate for level differences between the various input signals (this exemption must touch the volume when switching from one source to another). The excitement is obtained by the microphone commander, through the line RA2, relays and RY7 RY8 whose contacts short-circuit R26 and R17, which reduces the value of cathode resistance and thus increases the gain of the tube stage .
Outside of inputs, the microcontroller manages to push fifth of stifling amplification: This function is useful when you must, for example, answering the phone (usually based MUTE named). This mute is obtained through RY6 the two channels which, when you press SW5, are activated by T6 (saturated by the logic high level at which the PIC pin RB4 is his), the closing of the contacts makes the mass release of preamplifier (LINE OUT LINE OUT L and R).
This method was chosen to avoid damaging the lamps: indeed, it is in series with each line before the corresponding relay, a resistor. Each activation involves a signaling LD6.
The mute is automatically activated whenever the PIC is an entry pass to the other: it stifles amp for about three seconds. SW5 has a dual function: pressed and released, he operates a "muting" circuit, it is maintained throughout the preamp in "standby" (waiting), by interrupting the anode.
To remove the "standby", just press one of the input selector buttons: Standby LED lights LD6 and that of "standby" is off. Note that the ignition using the buttons for selecting inputs to the microcontroller configures a range of self-extinction (this helps to automatically "stand-by" the preamp instead of leaving it on for more than 24 hours).
addition to reading the pushers, the PIC handles the application of anodic current to the lamps after a period of 30 seconds from when the entire circuit is powered up: that because electronic tubes can amplify signals correctly before their cathodes have reached their operating temperature (hence you must first feed the filament and allow the passage of time for the cathode is at the correct temperature, otherwise the sound will be distorted). Let again a look at the wiring diagram and note that power is in three sections: one providing 5 V for operation of the logic and relays, and that of providing heating to 12.5 VDC filaments are energized immediately while the third (high voltage anode) passes through contacts RY9 whose winding n ' is supplied only after the period mentioned above has elapsed and therefore the filaments have heated enough cathodes. Section feeding logic consists of a diode bridge followed by smoothing capacitors C25 and C15 filter and a 7805, that of heating is similar except that the controller is a 7812 with, in series with its paw M Central, a diode to silicon (voltage output is stabilized thereby to 12.6 V).
But why feed the continuous filaments? Well, that's for not doing near the AC at 50 Hz with tracks carrying the precious audio signal and avoid the painful snoring characteristics of the sector (although the cathodes are shielded to prevent this phenomenon but too good not harmful and the signal-noise amplifier is never big enough). Time to
amplifier stages, exactly. Each channel D and G its dual triode ECC82: we chose to develop the signals of both channels with a lamp to achieve the best possible crosstalk. Both amplifiers are strictly identical: the explanations we give for the left channel (L) are valid for the right (R).
dual triode V1 is mounted in a common cathode, which receives on its gate (pin 2) the left component of the selected source, this component modulates the current flow and determines the anode plate (pin 1) a component audio frequency amplitude and larger phase opposition. BM thus obtained is applied through C18 to the gate of the second triode, the latter is mounted plate (limited follower, that is to say "Buffer") with network boot (bootstrap).
It only amplifies the signal current, leaving the stage as they are at the entrance. The voltage delivered to the grid (pin 7) module and determines the flow anode to the cathode, downstream of C21, a signal of the same magnitude as the output of the first triode. The volume is controlled by a section double potentiometer RV1 (RV1A) upstream of the lamps, so as to avoid congestion and to give them a level still maintaining a faithful reproduction audio signal source. The LF component reaches the amplified output (LINE OUT L) from which it may be amplified by the power amplifier. Our
also has a preamp output for MONITOR to a recording device: it is in parallel with the line selector input sources and is normally connected to the input of the cassette deck or CD recorder, who use it for recording. It is activated by the buttons, as explained above.
Figure 1: Diagram of the preamplifier (see Figure 2 entries left channel).
A peculiarity of this preamplifier lies in the differentiation of masses: the diagram distinguishes between four symbols to indicate the masses in order to avoid noise and interference, the signal ground (the output of the input and output and food anodic lamps) is separated from the mass of power (power supply, filament lamps, coils relays, LED). The signal ground is, in turn, connected to metal housing and land Cord 230 V with a resistance ensuring galvanic connection and a pulse capacitor eliminating parasites.
Figure 2: Diagram of left channel input.
Figure 3: The lamps.
They were the first active components of the history of electronics.
Made from a glass tube which was evacuated, they contain a number of electrodes which determine its operation. The simplest type is the diode, consisting of two electrodes anode and cathode named. All electron tubes are operated by the thermionic effect that metal plates (some metals more than others) show strongly when heated a few hundred degrees. The phenomenon involves the production of a flow of electrons torn from the plates under the effect of the heat energy received, with the materials used for the cathodes, this flow is of such an electric field intensity with respect low electrons can be easily swept away and attracted to an electrode biased positively with respect to the cathode (the electrode is the anode). The application of voltage between the anode and cathode in the tube determines the flow of current called "anode" is precisely the characteristic to block or allow the current English name that explains the thermionic tubes "valves". The triode has a third electrode (when the diode has only two): This third electrode is the gate and it is between the anode and cathode. Once the anode current flow established, we can modulate the intensity of the latter more or less negatively biasing the gate relative to the cathode, exactly, plus gate bias becomes negative, the lower the current as the negative potential tends to push the electrons and prevent them from reaching the anode. The reason why, after decades of hegemony of transistors and integrated circuits, Hi-Fi enthusiasts (audiophiles) are turning back to the lights, keeps warm, fleece only they can get in pregnant: this is due to the absence in the output characteristics of a saturation point net (typical BJT and, though somewhat less, FET). The triode, also, is the most linear, because its voltage / anode current is almost a straight line. 
Figure 4: Diagram of location of components the main board of the tube preamplifier. Figure 4b: Drawing scale 1, the printed circuit board of the main tube preamplifier.
Iist R1 ..... 10 k R2 ..... 10 k R3 ..... 10 k R4
..... 1 k R5 ..... 10 k
R6 ..... 10 k R7 ..... R8 1 k ..... 10 k R9 ..... R10 680 .... 1 M
R11 .... 27 k 0.6 W R12 .... R13 10 k .... 3.6 k 1%
R14 .... R15 100 k
.... 5.6 k R16
....
R17 680 .... 3.6 k 1%
R18 .... 33 k 1%
R19 .... R20 10 k
.... 1 M
R21 .... 27 k 0.6 W
R22 .... R23 10 k
.... 3.6 k 1%
R24 .... R25 100 k
.... R26 10 k
.... 3.6 k 1%
R27 .... 33 k 1%
R28 ....
R29 680 .... 5.6 k R30
.... R31 10 k
.... R32 10 k
.... 27 k 0.6 W
R33 .... 27 k 0.6 W
R34 .... 27 k 0.6 W
R35 .... 1
R36 .... R37 10 k
.... 4.7 k R38
.... R39 10 k
.... 4.7 k R40
.... R41 10 k
.... 1 k
R42 .... 1 k
R43 .... 1 k
R44 .... 1 k
R45 .... 1 k
R46 .... 1 k
R47 .... R48 10 k
.... 1 k
.... RV1 50 k potentiometer 90 ° above
C1 ..... 220 pF 100 V polyester 5 mm pitch
[...]
C7 ..... 220 pF 100 V polyester au pas de 5 mm
C8 ..... 100 nF multicouche
[…]
C15 .... 100 nF multicouche
C16 .... 2,2 μF 63 V polyester au pas de 15 mm
C17 .... 2,2 μF 63 V polyester au pas de 15 mm
C18 .... 68 nF 630 V polyester au pas de 15 mm
C19 .... 68 nF 630 V polyester au pas de 15 mm
C20 .... 22 nF 630 V polyester au pas de 15 mm
C21 .... 4,7 μF 160/250 V polyester au pas de 27 mm
C22 .... 4,7 μF 160/250 V polyester au pas de 27 mm
C23 .... 100 μF 16 V électrolytique
C24 .... 470 μF 16 V électrolytique
C25 .... 1000 μF 35 V électrolytique
C26 .... 1000 μF 35 V électrolytique
C27 .... 1000 uF 35 V C28 Electrolytic
.... 47 uF 350 V electrolytic
C29 .... 47 uF 350 V electrolytic
C30 .... 47 uF 350 V electrolytic
C31 .... 47 uF 350 V electrolytic
C32 .... 47 uF 350 V electrolytic
C33 .... 47 uF 350 V electrolytic
C34 .... 47 uF 350 V electrolytic
D1 ..... 1N4148
[...]
D9 ..... D10 1N4148
.... 1N4007
[...]
D20 .... 1N4007
LD1 .... LED 3 mm yellow
[...]
LD8 .... LED 3 mm yellow
RY1 .... reed relays
5 V normally open [...]
RY8 .... reed relays 5 V normally open
RY9 .... dual relay contact 12 V
T1 ..... BC557
[...]
T9 ..... BC557
T10 .... BC547
VR1 .... 7812
VR2 .... 7805
IC1 ..... PIC16C54-EV8020
V1 ...... 12AU7/ECC82
V2 ...... 12AU7/ECC82
SW1 .... push switch 90 °
it [...]
SW5 .... Push switch to 90 ° below
TR1 .... Transformer 230 Vac / 18 + 18 Vac / 12 Vac 16 VA
Miscellaneous:
an integrated circuit support 2 x 9
2 lamp holders for this noval
6 RCA "RCA" for this 90 °
a female strip 20
a pole male chassis connector VDE for rear panel
2 terminal 2 pole 10 mm
a 3 pole terminal
an aluminum metal case specific
On the big circuit of the main board take up the components of both channels of the preamplifier, the power and logic control ; through the connector CN1 (SIL female at 2.54 mm) the small printed circuit board input left channel (see Figures 6 and 7) overlaps and connects to the main board. Before mounting the components, be sure to make many "jumpers" wired. As
suppor ts the two lights, take "noval" (nine pin) plastic or, better, ceramic.
Figure 5: Photograph of a prototype of the main board of the tube preamplifier.
Figure 6: Diagram of component layout on the deck of the left channel input.
Figure 6b: Drawing scale 1, the printed circuit board entries left channel. 
Figure 7: Photograph of a prototype of platinum entries left channel. 
Iist R1 ..... 10 k [...] R6 ..... 10 k R7
..... R8 1 k ..... 1 k
C1 ..... 220 pF 100 V polyester 5 mm pitch [...] C6 ..... 220 pF 100 V polyester 5 mm pitch D1 ..... 1N4148 [...]
D6 ..... 1N4148 T1 ..... BC557 [...]
T6 ..... BC557
RY1 .... reed relays
5 V normally open [...]
RY6 .... reed relays 5 V normally open
Miscellaneous:
6 RCA "RCA" for this 90 °
a strip 20-pole female
The practical
Build a tube circuit is now not much different than preparing a mounting transistors and integrated circuits, but still requires more attention: mainly because the lamps are glass, so fragile when hot and turned on it may get burnt and take electric shock unpleasant and dangerous (so do not touch the tracks and components).
First prepare the general circuit of the main board (you will find the scale drawing 1 figure 4b) or get it. Do the same with small printed circuit inputs left channel (you can find the scale drawing 1 figure 6b).
Then, carefully following the directions of 4a / 5, and 6 / 7 (and lists of components), install all the components starting with holders of ICP and two lamps (they are carriers "Noval" in ceramic preferred) and 20-pin SIMMs (female on the base plate and the small males).
The pin lamp holders are soldered to the PCB pads as you would any other pin. Check out these first welds (or short-circuit between tracks or pads or cold solder joints). Insert without further delay and solder the many "jumpers" wired on the mainboard.
Be careful guidance of many active components (electrolyte, diodes, transistors, LEDs, regulators).
The power transformer also sits on the big circuit board, top right, where it is secured by small bolts and pins are welded in place directly on the PCB.
Finish with components "bulky" and devices: the volume knob, five buttons, terminal blocks, the electromagnetic relay and RCA "RCA" (there are six on each plate). Attach the four standoffs on the base plate. Take early stage, component side up and in the right direction, press the 20-pin connector into the connector 20 pin female base plate and tighten the four screws holding the four standoffs.
At the very end, insert the PIC into the socket, keyed reference-U to R38 and push gently and thoroughly well in their lamps.
Regarding the transformer, nothing prevents you from mounting it outside, in a small annex housing, or even to provide more housing in the outdoors, provided they provide the required output voltage: 2 x 18 V 1.2 A, 220 V 60 mA, 12 V 150 mA.
Otherwise, the rear panel of the metal casing is aluminum with a base frame VDE three-pin male, think, once installed and set the turntables, to connect the terminal to the three poles HANDS base frame using son of three screw terminals and solder side plate side, do not forget to screw terminals in the other four poles HAND SELECTING a "strap" connecting the two poles of the medium. From
aluminum metal case pre-drilled out now from the front push-buttons and LEDs and the axis of the knob that you doterez its button. From the rear panel chassis connector with integrated fuse VDE and the two rows of RCA "phono" on the top two lamps protected by a ring capping spacers (see photo section). final point on the masses of the assembly (see Figure 1 and Figure 4a): the signal is different from others in order to prevent the metal from the RCA and the mass of food coming into contact with the housing metal. So connect to it the land of 230 V power cord (center of the base frame VDE) and the point of the integrated circuit noted (see Figure 4a) MASS HOUSING (Between CN1 and C16).
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