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El156 Audio Power EL156 AUDIO POWER Gerhard Haas Thanks to its robustness, the legendary EL156 audio power pentode has found its way into many professional amplifier units. Its attraction derives not just from its appealing shape, but also from its impressive audio characteristics. We therefore bring you this classical circuit, updated using high- quality modern components. 28 elektor electronics - 3/2005 AMPLIFIER Return of a legend The EL156 was manufactured in the enough to give adequate sensitivity, electrolytic capacitor: this voltage is legendary Telefunken valve factory in even before allowing any margin for further filtered on the amplifier board. Ulm, near the river Danube in Ger- negative feedback. The ECC81 many. The EL156 made amplifiers with (12AT7), however, which has an open- It is not possible to build an ultra-lin- an output power of up to 130 W possi- loop gain of 60 and which can be oper- ear amplifier using the EL156 with a ble, using just two valves in the output ated with anode currents of up to high anode voltage. The same goes for stage and one driver valve. Genuine 10 mA, can be used to build a suitably the EL34. The output transformer is EL156s are no longer available new at low-impedance circuit. therefore connected in such a way that realistic prices, and hardly any are Two EL156s can be used to produce an the impedance of the grid connection available second-hand. The original output power of 130 W with only 6 % to the output valve is much lower than devices used a metal valve base which distortion. To improve reliability and in conventional valve circuits, and con- is still available, but a new design increase the life of the valves, however, siderably lower than the maximum using original valves and metal valve we have limited the maximum power. permissible value of 100 kΩ. This bases would anyway be impractical, A genuine hi-fi output at 100 W with relaxes the requirements on the toler- given the lack of availability at reason- low distortion is better than 130 W at ance of the valves, and select-on-test able prices. 6 %, especially when there is a large of the valves is not required. component at the unpleasant-sound- Coupling capacitors C9 to C11 have ing third harmonic. relatively large values. This is needed Made in China The whole circuit is built on four to ensure that sufficiently low fre- For tunately this valve is still being pro- printed circuit boards, forming a quencies can be handled in the low- duced in China, using the original Tele- monoblock. Figure 1 shows the power impedance circuit. The input and the funken machines. A normal octal base supply and the amplifier together. The phase inversion stages (V1 and V2 is now used, with pinout the same as power supply capacitors are cascaded respectively) have relatively low that of the EL84, 6L6, KT88 and similar to filter the high anode voltage in order anode and cathode resistors. The sup- valves. The devices are still not exactly to obtain the required voltage stability. ply voltage for the input and phase- cheap, but the price is not too unrea- To supply the relatively high currents inversion stages is regulated by Zener sonable and the valves are generally required by the screen grids of the diodes D1 to D4. The operating point supplied with bases included. Compar- EL156s two separate high voltage sup- of V1 is therefore independent of sup- ison with original Telefunken valves plies are produced using bridge recti- ply voltage fluctuations caused by the shows that the devices are a successful fiers from two isolated transformer output driver stage. R1 and C2 block mechanical and electrical copy and are windings (‘hi’ and ‘lo’). Immediately high frequencies. Capacitor C4, con- suitable for use in a hi-fi amplifier. after the rectifiers these supplies are nected in parallel with negative feed- Before we proceed to describe the connected in series and individually fil- back resistor R11, suppresses high design, we should first look at a few tered. Choke Dr1, with a value of 2.3 H, frequency oscillations. C3, between special features of these valves. In the is rated for a current of 0.3 A and filters anode and grid of V1a, performs the text box we compare the basic specifi- the anode supply, while Dr2, with a same task. R4 and R6 are effectively in cations of the EL156 with those of the value of 4 H and rated for 0.18 A, filters parallel to AC signals, and, in combi- well-known and widely-used EL34. the screen grid voltage. The driver nation with negative feedback resis- This information will to a large extent valve is also powered from the screen tor R11, set the overall gain. The determine the design of the amplifier. grid supply. The screen grid voltage amplifier is designed with only a mod- In order to obtain sufficient output must be well filtered since any hum erate amount of negative feedback: power, the anode voltage must be at present on it will be amplified through this improves the resulting sound. least twice as high as the screen grid to the output: the screen grid has some An E-1220 transformer (Tr1) with a 1:2 voltage. The driver circuit must be control effect. The values suggested turns ratio is fitted at the input to the designed to cope comfortably with the give good filtering and hence low hum. amplifier. This gives adequate input comparatively low-impedance load Radial 100 µF/500 V electrolytic capac- sensitivity as well as providing isola- presented by the grid leak resistors. itors are recommended to make the tion. Differential or quasi-differential The popular ECC83 (12AX7) is ruled power supply compact; a working volt- audio connections are in theory less out, since it operates at only around age of 500 V ensures adequate margin susceptible to interference and prevent 1 mA. The ECC82 (12AU7) audio dou- to give reliable operation even in the earth loops. Also, the 1:2 ratio gives an ble triode can be operated with an event of mains overvoltage. Note the extra 6 dB of sensitivity without added anode current of 10 mA, and so would discharge resistors in parallel with the noise, leaving a little more margin in appear to be suitable; however, its electrolytics. The negative grid bias hand for negative feedback. The open-loop gain is only 17, which is not voltage is provided by a diode and printed circuit board also allows for a 3/2005 - elektor electronics 29 R7 R23 +UB 2k2 10k FIL1 HT supply board 2 R5 R10 A1 7 V2 4 D1 18k 56V 10k 9 3 V2 V1 5 Dr1 D2 1 7 R24 SG1 100V 4 HI 2 V3 C10 R18 100 Ω D-2360 R8 5 C5 C6 1k DRLO DRLO1 D3 UBHI FIL2 470n 8 100V 310V 100k 400V 10µ µ B1 22 EL156 450mA 450V 500V D4 R16 R15 R21 amplifier board 100V R2 R1 Ω C8 C9 C5 C7 F1 HI 68k 10k 10 V1.B 150k 150k B500C1500 ECC 81 EL156 1 Tr2 0A63 T P1 100µ 100µ 100µ 100µ f g2 g1 500V 500V 500V 500V f a C9 8Ω 2 5k k g a 470n Ω R22 –UG 4 400V 3 g ff k ff 2k7 C3 V1 = ECC81 P2 C8 a fM g3 k R12 R13 Dr2 V1.A 5k 33p 6 Ω 100µ D-4060 LO 680k 220 63V R1 C1 R17 R14 R20 DRLO DRLO1 7 B2156 UBLO 1k Ω B Tr1 C7 270V 68k 10k B2 B2' 2µ2 220µ R9 10 450mA 50V 8 R26 10k R4 R3 A2' EL156 25V C13 C12 C10 C11 F2 LO 8 R2 R3 R4 R6 C11 R19 B1' 100k C2 5 150k 150k B500C1500 Ω Ω 0A63 T 1k R25 SG2 100µ 100µ 100µ 100µ XLR 4 100k 100k 2 C12 220 180 470n 100 Ω 500V 500V 500V 500V 10p 1 A1(M) 400V A 3 2n2 C4 V3 3 A2 D1 1 UG– UG 1n5 R11 GK 1N4007 1k3 R5 35V C6 100mA 47k S1 +12V 100µ ground 63V STAND BY R27 10k 030334 - 11 Figure 1. The heart of the valve power amplifier with its output transformer and high-voltage power supply. 1:1 connection, in which case twice the stage valves, normally by switching off books this is if anything preferable to input signal level will be required for the anode supply, while the heater and switching off the anode supply as pro- full drive. The desired ratio can be other voltages remain. On leaving longed operation with the heater on selected using wire links. The combi- stand-by the amplifier is immediately without applying an anode voltage nation of C12 and R26 compensates for ready for action. gradually reduces the emissivity of the the response of the transformer at In view of the high anode voltage, an cathode. An LED connected to the sec- higher frequencies. ordinary switch or relay is not suitable. ond pole of the double-pole switch We take a different approach, shorting indicates when stand-by mode is out R22 using the stand-by switch, so active. The LED can be powered from Stand by me that the negative grid bias voltage on the heater supply. Valve power output stages are often the output stage valves is raised. Only designed with a stand-by function. a very small quiescent current now This prolongs the life of the output flows. According to the valve data DC heater supply To minimise hum a regulated low drop- out DC heater supply is provided, using the familiar 723 voltage regula- T1 BUZ12 KBU8B R4 +12V6 tor and a MOSFET (Figure 2).
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