Wiring circuit

This schema is derived from the Portable Headphone Amplifier featuring an NPN/PNP compound pair emitter follower output stage. An improved output driving capability is gained by making this a push-pull Class-A arrangement. Output power can reach 427mW RMS into a 32 Ohm load at a fixed standing current of 100mA. The single voltage gain stage allows the easy implementation of a shunt-feedback schemary giving excellent frequency stability.

Class-A Headphone Amplifier Circuit diagram

The above mentioned shunt-feedback configuration also allows the easy addition of frequency dependent networks in order to obtain an useful, unobtrusive, switchable Tilt control (optional). When SW1 is set in the first position a gentle, shelving bass lift and treble cut is obtained. The central position of SW1 allows a flat frequency response, whereas the third position of this switch enables a shelving treble lift and bass cut.

P1 : 22K Dual gang Log Potentiometer 
R1 : 15K 
R2 : 220K
R3 : 100K
R4 : 33K 
R5 : 68K 
R6 : 50K 
R7 : 10K 
R8,R9 : 47K 
R10,R11 : 2R2 
R12 : 4K7
R13 : 4R7
R14 : 1K2
R15,R18 : 330K
R16 : 680K
R17,R19 : 220K
R20,R21 : 22K
C1,C2,C3,C4 : 10µF/25V 
C5,C7 : 220µF/25V
C6,C11 : 100nF
C8 : 2200µF/25V
C9,C12 : 1nF
C10 : 470pF
C13 : 15nF
D1 : LED
D2,D3 : 1N4002 
Q1,Q2 : BC550C 
Q3 : BC560C 
Q4 : BD136 
Q5 : BD135 
IC1 : 7815
T1 : 15CT/5VA Mains transformer
SW1 : 4 poles 3 ways rotary Switch 
SW2 : SPST slide or toggle Switch

Mention valve amplifiers and many designers go depressive instantly over the thought of a suitable output transformer. The part will be in the history books forever as esoteric, bulky and expensive because, it says, it is designed and manufactured for a specific valve constellation and output power. There exist thick books on valve output transformers, as well as gurus lecturing on them and winding them by hand. However, with some concessions to distortion (but keeping a lot of money in your pocket) a schema configuration known as SRPP (series regulated push-pull) allows a low-power valve amplifier to be built that does not require the infamous output transformer. SRPP is normally used for preamplifier stages only, employing two triodes in what looks like a cascade arrangement.

 SRPP Headphone Amplifier Circuit Diagram

Here we propose the use of two EL84 (6BQ5) power pentodes in triode SRPP configuration. The reasons for using the EL84 (6CA5) are mainly that it’s cheap, widely available and forgiving of the odd overload condition. Here, two of these valves are SRPP’d into an amplifier that’s sure to reproduce that ‘warm thermionic sound’ so much in demand these days.

Before describing the schema operation, it must be mentioned that construction of this schema must not be attempted unless you have experience in working with valves at high voltages, or can rely on the advice and assistance of an ‘old hand’. As a safety measure, two anti-series connected zener diodes are f it ted at the amplifier output. These devices protect the output (i.e. your head-phones and ears) against possibly dangerous voltages at switch-on,or when output capacitor C3 breaks down.

The power supply is dimensioned for two channels, i.e. a stereo version of the amplifier. The values in brackets are for Elektor readers on 120 VAC power. Note the doubled values of fuses F1 and F3 in the AC primary diagram. The PSU is a conventional design, possibly with the exception of the 6.3 V heater voltage being raised to a level of about +80 V through voltage divider R7-R8. This is done to prevent exceeding the maximum cathode-heater voltage specified for the EL84 (6CA5). R6 is a bleeder resistor emptying the reservoir capacitors C8 and C9 in a quick but control-led manner when the amplifier is switched off. Rectifier diodes D3–D6 each have an anti-rattle capacitor across them.

In the amplifier, assuming the valves used have roughly the same emission, the half-voltage level of about +145 V exists at the junction of the anode of V1 and the control grid of V2. The SRPP is no exception to the rule that high quality, (preferably) new capacitors are essential not just for reproducibility and sound fidelity, but also for safety.