There is little doubt that the headphone amplifier circuit described here belongs in the so-called high end class of audio equipment, and is, therefore, perfect for incorporation in, or adding to, the Top-of-the-range Preamplifier described in (1), although it is also suitable as an autonomous, high quality, unit.
The circuit diagram of the headphone amplifier circuit appears in picture. The unit is based on Type OP-50 power operational amplifiers, whose technical features are summarized in Table 1. Clearly, everything feasible has been done by the manufacturers, Precision Monolithics Inc., to ensure optimum operation of the device, and it is with this in mind that the remainder of the amplifier was designed.
Both supply rails to the amplifier ICs are adequately decoupled and filtered with a small series resistor, (R4-R5) and a combination of an electrolytic and a solid capacitor (C4-C2 and C5-C3). With reference to the upper of the two identical channels, preset P2
enables compensating the (small) offset voltage at the output of the OP-50, while C1-R3 forms a compensation circuit to minimize overshoot for a given closed-loop voltage amplification, Avcl. In the present application, Avcl is about 6, since
When it is intended to alter the amplification, R2 should be left at 20 kQ. Also observe that the indicated values for R3 and C1 are valid when Avcl is between 5 and 20, while R3 = 3.3 kQ and C1 = 1 nF when Avcl is between 20 and 50. No R-C compensation is required when Avcl is greater than 50.
The +15 V supply for the headphone amplifier is a relatively extensive circuit based on a precision regulator Type LM325, which features excellent noise suppression whilst ensuring smooth and simultaneously rising output voltages at power on. Mains-borne interference and clicks from Si are suppressed in varistor R9 and high- voltage capacitor C19. The four diodes in rectifier bridge B1 are bypassed with rattle suppression capacitors to ensure minimum noise on the supply rails to the
The headphone amplifier circuit can function optimally only if great care is taken both in the choice of the components and in the construction on PCB Type 87512, details of which are shown in Fig. 2. As already stated, the headphone amplifier circuit is suitable for building into the Top-of-the-Range Preamplifier. This makes it possible to feed the ± 15 V regulator from the raw voltage across C9 (+ ) and C10 (— ) of the existing ± 18.5 V supply, while the inputs of the volume control of the headphone amplifier are driven direct from the outputs of IC4 (R) and KY (L).
Opamps IC1 and IC2 should be soldered direct onto the PCB, and are preferably fitted with a DIL-type heatsink. Provision has been made to screen the amplifiers and the supply on the board by means of two sheets of brass or tin plate, which are mounted vertically onto the dotted lines, and secured with three soldering pins each. Series regulators T1 and T2 can do without a heat-sink. When the board is complete, its underside should be thoroughly cleaned with a brush dipped into white spirit or alcohol to remove any residual resin. Next, the track side is sealed with a suitable plastic spray.
When possible, use insulated sockets for the stereo input and output of the amplifier. At the input side, few problems are expected to arise when using goldplated phono sockets mounted onto a separate ABS or epoxy plate. When a good quality, insulated, 6.3 mm, stereo headphone socket proves unobtainable, the nearest alternative is a non-insulated type, whose common tag is connected direct to the ground point on the PCB, between C17 and C18 to effect central earthing.
Mains transformer Tn is preferably a toroidal type fitted behind a metal screen to ensure minimum hum and other interference picked up by the amplifier inputs. Presets P2 and P2′ are trimmed for minimum offset voltage at the respective amplifier output— this is likely to require a very sensitive DMM. The headphone amplifier can be terminated in 100 Q to 1 kQ, and is therefore perfect for use as a high-quality line driver also. The outputs are short-circuit resistant.
Finally, a brief summary of the amplifier’s expected performance at Vo = 6 Vrms and Avcl— 6; Total harmonic distortion: 0.0025% (100 Hz); 0.003% (1 kHz); 0.011% (10 kHz). Signal-to-noise ratio: ^80 dB. Response flatness: ±0.4 dB from 10 Hz to 20 kHz.
Headphone Amplifier Circuit Parts list:
R2;R 2 ‘ = 20K0F
R3;R3′ = 560R
R4;R4′;R5;R5′;Re;R7 = 2R2
R8 = 820R; 0.5 W
R9 = SIOV S10 K250 varistor (Siemens; ElectroValue (0784) 33603).
R10 = 2M2
P1 = 25K logarithmic stereo potentiometer.
P2;P2’ = 100K multiturn preset.
C2;C2′;C3;C3′ = 220n
C4;C4′;C5;C5′ , =470uF/16 V; radial
C6;C7;C8;C9 = 22n
C10;C12= 1000u/25 V; radial
C11;C13;C15;C16 = 100nF
C14= 1u; 16 V; tantalum
C19 = 22n; 250 VAC
D1. . .D6 incl. = 1N4001
D7 = LED red
IC1;IC2 = OP-50 (Precision Monolithics Inc.) +
IC3 = LM325
T1;T2 = BD241
F1 = 250 mA delayed action fuse plus panel mount holder.
Tr1= 2×15 V; 15 VA (~2×0.50 A) toroidal mains transformer, e.g. ILP Type 0301 3.
DIL-14 heat-sink for ICi and IC2.
Mains entrance socket.
S1 = SPST miniature mains switch.
Stereo 6.3 mm headphone socket, preferably insulated.
2 off phone input sockets.
Suitable metal enclosure.
* See text
(1) Top-of-the-Range Preamplifier. Elektor Electronics, November and December 1 986, January 1987.
(2) Linear and Conversion Applications Handbook (1986). Precision Monolithics Incorporated.