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 ÐUG 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 UGÐ 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). The sup- 0Ω22 1 ply has been designed to minimise D3 R5 0Ω22 losses, and to this end the heater fila- 13V 18V R6 3A B1 Ω ments of the two EL156s are con-

R1 47 R16 C1 R7 C2 nected in series. The ECC81 can be 1k

2 4k7 2k2 10000µ 25V 47µ arranged so that it operates from a 40V R8 D2 R2 R3 C3 12.6 V heater supply. At double the Ω Ω 1k

470 100 voltage (using 12.6 V rather than 6.3 V) 220µ ZTK22 40V only half the current flows, which R9 D1 means that losses in the bridge recti- 1k 1N4007 12 11 fier are considerably reduced. With the V+ Vc 6 10 R17 REF OUT C7 given component values power losses R13 9

IC1 VZ 4k7 1 2 1n in T1 will be kept low.

2k7 CL µA723 3 R14 CS 5 (DIL) 4 R10 1k +IN ÐIN 24V 13

CMP 4k7 0,1A C6 R11 A further trick is used to reduce the VÐ C5

7 1k 1000µ 40V voltage drop due to the current limit P1 2 470p 1k circuit. The 723 includes a silicon tran- R15 R12 C4 sistor for current limiting, whose 4k7 2k7 2200µ 40V M base-emitter junction senses the volt- 030334 - 14 age across the current limit sense resistors R4 and R5. Normally the sili- Figure 2. This low drop-out DC regulator provides the heater supply. con transistor would switch off at

30 elektor electronics - 3/2005 IC2.C CTRDIV10/ 3 CTRDIV10/ 3 IC2 = 4011 8 0 0 IC3 = 4011 10 DEC 2 DEC 2 +12V 9 & 1 1 IC5 4 IC6 4 IC3.A 2 2 1 IC3.B 14 7 14 7 3 5 & 3 & 3 2 & 4 + 10 + 10 6 & 4 4 IC3.D T1 13 1 13 1 12 R3 IC2.A 5 5 11 IC2.D IC2.B 1 5 5 13 & 10k P2 12 5 3 4017 6 4017 6 11 4 2 & 6 6 13 & 6 & 7 7 15 9 15 9 IC3.C BD140 P1 CT=0 8 CT=0 8 8 R1 R2 11 11 10 C9 9 9 9 & D2 REL1 12 12 CT≥5 CT≥5 100k 680k 1µ

R5 1N4148 O1 100k S1 +12V +12V O2 1N4148 1N4148 S2 LED-A R10 R7 9 8 D10 D9 LED IC4 = 4011 IC4.C

10k 10k & LED-K IC4.B IC4.D IC4.A R11 R6 5 C11 12 1 4 11 3 10

10k 6 & 13 & 2 & 1k5 C10 10n R12 T2 R4 +12V 100n IC1 D4 10k 100k 1N4007* 7812 1N4007 BC549

B/D D3 D8 R9 * C1 C2 C3 C4 C5 14 14 14 16 16 C8

IC2 IC3 IC4 IC5 IC6 33k 13V GL1* 100n 100n 47µ 47µ 47µ 1µ 7 7 7 8 8 12V * C6 40V 40V 40V 63V 6V3 * B 22*µ 40V B80C1500 D B1 B1' C7 * D7 D6 D5 220µ * B2 B2' 3x see text 1N4007 * * 40V R8 *** 030334 - 13 2k2

Figure 3. This clever switch-on delay circuit prevents clicks, hum and rumble.

about 0.6 V. Here, however, the base the relay pull in, removing the short of this flip-flop at pin 4 thus goes low is provided with a stabilised bias volt- circuit. This approach avoids putting (if it was not already low). The output age from temperature-compensated relay contacts in the signal path. The of IC3.D is consequently high, T1 does Zener diode D2 via R7 and R8. This layout of the printed circuit board not conduct and the relay does not pull results in a smaller voltage drop allows a relay with two changeover in. The output of the amplifier is there- across R4 and R5 being needed to contacts to be fitted so that the circuit fore short-circuited. trigger current limiting. can be used for other applications, IC2.B and IC2.D form a 1 Hz clock gen- The reference voltage produced by the including with a stereo output stage. erator, the frequency being determined 723 is divided down by R13 and R15. In our case the circuit runs from the by C9 and R2. The flip-flop formed by C4 is in principle necessary to filter out 13 V heater supply, from which it will IC2.A and IC2.B makes this clock avail- any noise on the reference voltage, but draw a maximum of 200 mA. If the able to the cascaded counters IC5 and could be made considerably smaller. switch-on delay circuit is to be con- IC6. After 100 counts the flip-flop com- Because of the relatively high value of nected to an existing amplifier, the cir- prising IC3.B and IC3.C is set via the capacitor, the voltage at pin 5 rises cuit will work equally well from a 6.3 V inverter IC3.A. The output of IC3.D slowly, providing a ‘soft start’ to the heater supply winding, as long as then goes low, T1 conducts and the heater supply. A BUZ12 FET with an there is sufficient spare current capac- relay pulls in. The short-circuit is RDSON of just 28 mΩ is used for T1. ity. In this case a voltage doubler cir- removed and the audio signal is now It is important to ensure that the volt- cuit is used. Depending on the choice passed through to the loudspeaker. At age difference between the cathode of power supply voltage a number of the same time this high signal is used and the heater is not too high as other- components must be added to or to disable to counters via R5. wise arcing can occur. The negative removed from the circuit, as indicated side of the heater supply must there- on the component mounting plan and During the switch-on process the pin 8 fore be connected to the negative side in the parts list. input to IC4.C is high, and a 1 Hz sig- of the high voltage supply. nal is present on pin 9. The output on Switching on pin 10 will therefore also carry the 1 Hz When power is applied C8 immedi- signal, and so the LED flashes. Once Rattle and hum ately starts to charge via R8, taking the the switch-on delay is complete pin 8 When the output stage is switched on pin 13 input of IC3.D high. At the same goes low, forcing the output of the and while it is warming up the various time C10 charges via R10. As soon as NAND gate high. The LED now glows reservoir and coupling capacitors may the input threshold voltage of IC4.B is continuously. charge at different rates. This can give reached its output goes low and, via rise to hum and rumble. The circuit in the high-pass network formed by C11 Switching off Figure 3 can suppress these sounds and R7, generates a brief low pulse at When the amplifier is turned off, there effectively. The normally-closed relay the input to IC4.D. This signal is is no longer any voltage present on the contact shorts out the output trans- inverted and then used to reset the transformer. The transformer voltage is former for a set time (which the valve 4017 counter to zero, and then inverted monitored continuously by D5 and D6 output stage can comfortably cope again and used to reset the flip-flop (13 V operation) or by D5 and D7 (6.3 V with). Only after a certain interval does formed by IC3.B and IC3.C. The output operation). If the voltage is not present,

3/2005 - elektor electronics 31 Figure 4. The amplifier seen from below.

the relay drops out immediately. cycle must be repeated. This ensures Construction Diodes D1 and D9 ensure that capaci- that the output is once again muted, The monoblock amplifier comprises a tors C10 and D11 discharge quickly. If ensuring that the unwanted effects total of three printed circuit boards and power is applied again, the whole mentioned earlier are avoided. several wound components fitted into

A valve is a valve is a valve...

Parameter Symbol EL34 EL156 Units In some cases quantities have been rounded or guideline values Heater voltage Uf 6.3 6.3 V shown, since not all data books Heater current If 1.5 1.9 A exactly agree on all the figures. Maximum anode voltage Uamax 800 800 V Values shown after an oblique are at maximum drive. Maximum cathode current Ikmax 150 180 mA

Maximum anode power dissipation PVmax 25/27,5 40 W

Maximum screen grid power dissipation Pvg2 8 8/12 W The EL34 and EL156 are genuine audio power pentodes rather than Screen grid current I 11/22 5/25 mA g2 beam power tetrodes like the 6L6, Maximum screen grid voltage Ug2max 425 450 KT88 or 6550. They are similar V in class AB operation 425 350 to one another, but not identical in all respects. The EL156 Grid bias voltage U –39 –24 V g1 requires approximately 27 % AC grid driving voltage Ug1AC 23 18 V more heater power, while offering Transconductance S 11 13 mA/V a maximum cathode current about 20 % higher. Its maximum anode R Ω Internal resistance i 15 20 k power dissipation is about 60 % Maximum grid leak resistor Rg1max 700 100 kΩ higher than that of the EL34. The EL156 also features a higher internal resistance, higher transconductance, higher current and slightly lower grid bias voltage. Maximum drive can also be obtained using a smaller grid

32 elektor electronics - 3/2005 Figure 5. The comforting glow of an EL156 in action.

an enclosure as shown in Figure 4. Our finish. The use of aluminium provides ground connections must be brought prototype amplifier was housed in a shielding from magnetic interference together at the amplifier board and seamlessly-welded nickel-plated alu- which mostly originates in the fields bonded to the enclosure at a single minium enclosure polished to a glossy produced by the transformers. All the point using a bolt and solder tag. If this

driving voltage. Because of these characteristics, greater gain can be achieved using the EL156 than with the EL34, and as a consequence we only need a double triode in the driver stage, despite our high output power.

For both valves a number of details must be observed in high power operation. At higher supply voltages the screen grid voltage must be fixed at a given maximum value. It is also in gen- eral necessary to provide a fixed grid bias voltage. The maximum permitted grid leak resistor is considerably lower for the EL156 than for the EL34. The maximum value of the grid leak resistor is specified in the data sheet for each valve. In theory a valve can be driven without dissipating power, but in practice a small grid current flows which must be drained away. Account must be taken of this when designing the driver circuit. The EL156 can only be driven efficiently when the anode voltage is sufficiently high: power has its price! In a triode circuit using class AB push-pull operation dissipation can reach 30 W. The screen grid voltage for the EL156 in high power class AB push-pull operation must be at least 350 V; for the EL34 at least 400 V is required.

3/2005 - elektor electronics 33 On the test bench

Measured characteristics (all measurements taken with an 8 Ω load) Parameter Conditions Value Units

Input sensitivity 90 W, 1 % THD+N 1.4 Veff 20 Hz 4 Input impedance 1 kHz 9 kΩ 20 kHz 1.08 Sine wave output power 1% THD+N 90 W Bandwidth –3 dB, 1 W 41 kHz Slew rate 10 µs step 5 V/µs at 1 W, 88 dB Signal-to-noise ratio bandwith = 22 Hz to 22 kHz 102 BA 1 kHz 0.12 1 W Harmonic distortion and noise 20 kHz 0.21 % over 80 kHz bandwidth 1 kHz 0.6 50 W 20 kHz 1.43 1 W 0.5 Intermodulation distortion 50 Hz : 7 kHz = 4:1 % 50 W 2.6

3.15 kHz square and 1 W 0.064 Dynamic intermodulation distortion % 15 kHz sine wave 50 W 0.33 1 kHz 2.9 Damping factor – 20 kHz 2.3

10 10 A B 5 5

2 2 1

1 0.5 % % 0.5 0.2

0.1 0.2 0.05

0.1 0.02 0.06 0.01 20 50 100 200 500 1k 2k 5k 10k 20k 1m 2m 5m 10m 20m 50m 100m 200m 500m 1 2 5 10 20 50 100 300 Hz 030334 - E W 030334 - B

Figure A shows the total harmonic distortion plus noise (THD+N) as a function of frequency when the amplifier is driven at 1W and at 50 W. The measurement was carried out using a bandwidth of 80 kHz. As is to be expected from a valve amplifier, the distortion increases as the core in the output transformer approaches saturation. This is not a particular disadvantage as the human ear is insensitive to low frequencies and does not find higher distortion levels unpleasant.

Figure B shows distortion as a function of drive level. The 200 distortion rises from about 50 mW onwards, being dominat- B 100 ed by harmonic components. The measurement was taken using a bandwidth from 22 Hz to 22 kHz in order to show 50 more clearly the effect of harmonic distortion at low power levels. At 90 W the amplifier starts to clip.

20 W 10 Figure C shows the maximum power as a function of frequency for a fixed distortion (here 1 %). The bandwidth used 5 for the distortion measurement was 80 kHz. The maximum power starts to fall off towards the upper and lower ends of

2 the amplifier’s frequency range. At the upper end of the frequency range the situation is not too bad, since generally less

20 50 100 200 500 1k 2k 5k 10k 20k power is required here anyway. Things are different below Hz 030334 - C 40 Hz, where deep tones often demand a lot of power.

34 elektor electronics - 3/2005 +0 +6 DE-10 +4 -20 +2 -30 -0 -40 -2 -50 -4 -60 -6 d -70 d B B -8 r -80 r -10 A -90 A -12 -100 -14 -110 -16 -120 -18 -130 -140 -20 -150 -22 -160 -24 20 50 100 200 500 1k 2k 5k 10k 20k 10 20 50 100 200 500 1k 2k 5k 10k 20k 50k 100k Hz 030334 - D Hz 030334 - E

A spectrum analysis of the distortion to a 1 kHz sine wave signal at an output power of 1 W is shown in Figure D. Almost all the distortion is accounted for by the second harmonic at –58.3 dB. The third harmonic lies at –80 dB, and all the remaining harmonics, along with mains hum, are below –90 dB. The hum component is mainly due to the magnetic field of the transformers (50 Hz component); otherwise the 100 Hz component would be expected to be much more significant.

Finally, Figure E shows the effect of the input circuit to the amplifier, where a 4.7 kΩ resistor and a 2.2 nF capacitor are effectively connected in parallel across the secondary side of the input transformer. If the output impedance of the preamplifier is greater than 50 Ω, the input impedance of the power amplifier has a clear effect. The upper curve shows the normal frequency response with an impedance of 20 Ω, while at 600 Ω the frequency response falls off markedly at both the upper and lower end of the frequency range.

is not done, the enclosure will act as If voltage can be adjusted over a range wiring is correct and that the earthing an antenna and the amplifier will hum. of two to three volts, but the value of is sound. If the amplifier goes into The high voltages used mean that the 12.6 V cannot be reached, then resis- large-amplitude oscillation as soon as enclosure must be earthed. Input and tor R10 or R12 will need to be it is switched on, which can be seen output connections must be isolated changed. Next fit the valves. Shortly, on the oscilloscope as a distorted from the enclosure, or else stray earth the heater filaments should start to squarewave with a frequency of currents may arise. glow, as shown in Figure 5. approximately 100 Hz, and heard as a Cable with a cross-section of at least hum in the transformer and output 0.5 mm2 should be used between the We can now proceed to test the circuit valves, switch off immediately. The relay contacts and the loudspeaker with the high voltage present. It is effect indicates that the output trans- outputs. Thinner cables have too high a absolutely essential that a load resis- former has been connected with the resistance, with the result that rumble tor rated to at least 150 W must wrong polarity, and anode 1 must be can be heard faintly through the loud- always be connected. Do not forget to interchanged with anode 2, turning speakers during the warm-up phase. switch off the unit before fitting the positive feedback into negative feed- The heater connections need cable high voltage supply fuse! back. As long as the circuit is switched with a cross-section of 1.5 mm2, the off immediately there should be no earth connections cable with a cross- Turn the unit back on, and connect an damage to the valves or other compo- section of 0.75 mm2, the high voltage oscilloscope across the load resistor to nents. connections cable with a cross-section act as an output monitor. Once the (030334-1) of 0.5 mm2, and finally the auxiliary warm-up phase is complete the quies- supply connections cable with a cross- cent current of the output valves can section of 0.25 mm2. be set. Measure the voltage drop Operation is relatively straightfor- across each cathode resistor, R20 and ward. First check again that all the R21, using a multimeter. Alternately components are mounted correctly adjust the currents through V2 and V3 Note and that the wiring is correct. Next for a voltage drop of 450 mV, which cor- For reasons of space, printed circuit check the auxiliary voltages, leaving responds to 45 mA per valve. Next con- board layouts, component mounting out the fuse in the high voltage supply nect a signal generator producing a plans and parts lists are only available for now. When mains power is applied 1 kHz sinewave to the input, and grad- in electronic form on the Internet, down- the negative grid bias voltage should ually increase the amplitude of the sig- loadable from immediately be present on the valve nal. Observe the output signal on the bases, and can be adjusted using the oscilloscope. It should increase in www.elektor-electronics.co.uk. trimmer potentiometers. For the amplitude without distortion or spuri- Ready-made unpopulated printed circuit moment, set the voltage to its maxi- ous oscillation until the point where it boards and kits can be obtained from mum negative value. Next check the starts to clip. If the amplifier does have the author heater voltage and adjust it to 12.6 V. a tendency to oscillate, check that the ([email protected]).

3/2005 - elektor electronics 35