About the article

OTL Headphone Amplifier with Ecc82 (12AU7)

An ‘iron-free’ valve amplifier

OTL Headphone Amplifier with Ecc82 (12AU7)
A headphone amplifier with outstanding sound quality can be built using readily available modern components. Omitting the output transformer, avoiding overall negative feedback and using good coupling capacitors guarantees a highly linear frequency characteristic, while certain construction methods yield low channel crosstalk. Valve experts know that even power valves that can deliver several hundred milliampères of anode current can never drive a loudspeaker with an impedance of 8 ?, since the internal resistance of a valve is several kiloohms. The two impedances are thus almost always matched using a transformer. In principle, there isn’t any objection to using this form of impedance conversion, but every transformer tends to degrade the sound quality. Keeping this effect to a minimum requires very careful and complicated coil winding techniques, good-quality transformer iron and large core cross-sections.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Component list
R1,R3,R12,R14 = 1MOhm
R2,R13 = 470Ohm
R4,R15 = 82kOhm
R5,R16 = 1kOhm8
R6,R17 = 330Ohm
R7,R18 = 4kOhm7
R8,R19 = 10kOhm
R9,R20 = 33kOhm
R10,R11,R21,R22 = 2kOhm2
C1,C2,C7,C8 = 1µ F 400V (MKP4 or MKS4, 250V)
C3,C9 = 47µ F 63V radial
C4,C10 = 470µ F 63V radial
C5,C11 = 10µ F 400V radial, e.g., Panasonic ECA2GHG100n (Farnell # 219-9320)
C6,C12 = 47µ F 400V radial (e.g., Conrad Electronics # 475858)
V1,V2 = ECC82 or 12UA7, with Noval (9-way) socket (Chelmer Valve Corp.)
JP1 = 2-way PCB pinheader with jumper
K1 = 2-way PCB terminal block, 5mm lead pitch
K2 = 2- way PCB terminal block, 7.5mm lead pitch
PCB, available from The PCBShop
(Power supply)
R1,R2 = 100kOhm
C1-C4 = 47nF ceramic
C5-C8 = 47nF 275 VAC, Class X2, lead pitch 15mm
C9 = 2200µ F 25V radial
C10,C11 = 220nF
C12 = 10µ F 63V radial
C13 = 10nF 400V, lead pitch 7.5mm or 10mm
C14 = 47µ F 400V radial (BC Components # 22215266109, Farnell # 322-7984)
(BC Components # 22215266479, Farnell # 322-8009)
Panasonic ECA2GHG470 (e.g., Farnell # 319-9356)
D1-D4 = 1N4002
D5-D8 = 1N4007
D9 = LED red, low current
IC1 = LM2940CT-12 with heatsink, Fischer type SK104 (50.8 mm)
JP1 = 2-way PCB pinheader with jumper
K1,K3 = 2-way PCB terminal block, lead pitch 7.5mm
K2 = 2- way PCB terminal block, lead pitch 5mm
F1 = fuse, 100mAT (slow), with PCB mount holder
TR1 = mains transformer 12V / 16VA (ERA BV054-5383.0K (Conrad Electronics # 506575)
TR2 = mains transformer 12V /10VA (ERA BV048-5383.0H (Conrad Electronics # 506478)
PCB, available from The PCBShop
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