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by Earle Eaton
Have there been times when you wished you had more headphone-drive capability from your tape recorder or compact disc player? If you are using digital recording and playback equipment, low-noise operation is especially important to you. Here is a high-quality stereo headphone amplifier that has a wide frequency response, exhibits low noise and distortion, can drive a pair of headphones of any impedance, protects against short circuits, and can easily be built in a couple of evenings (shown above). The circuit is based upon material described by Jung. A remote power supply is used to minimize the possibility of induced noise (Table 1). TABLE 1
Gain: 34 (31dB)
120mW (32 ohm load) 40mW (8 ohm load) Distortion <0.02% Signal-to-Noise Ratio: 80dB
Figure 1 - The Eaton headphone amplifier circuit.
Circuitry An audio input signal is fed into J1 (Fig. 1). Pl controls the amount of signal applied to the noninverting input of IC1-a low-noise, low-distortion operational amplifier which offers excellent linearity and high-drive capability. The output of IC1 drives transistors Q1 and Q2 - a complementary-symmetry Class AB emitter follower, which provides a high-current output with minimum loading of the op amp. Light-emitting diode D1 biases Q1 and Q2 at a quiescent current of approximately 30mA to avoid the possibility of any crossover distortion. Resistor R10 limits the amplifier output power to a good range of listening levels for head-phones with impedances between 8-600 ohms. However, you may wish to change R10 to meet your specific needs. The remote power supply provides ±15V at 170mA.
Construction
Figure 2 - Interior view of Eaton amplifier. The headphone amplifier is built in a 6"x 8" x 2" metal enclosure (Fig 2).
Figure 3 - Heatsink made of 0.03" aluminum. Use only the "U" portion and cut off the remainder. Alternatively, heatsinks may be fabricated as indicated. Transistors Q1 and Q2 are each heatsinked with a small aluminum "U" (Fig. 3). Except for the input and output jacks, power supply connector, volume control, D6 (power indicator LED) and R11, all of the amplifier circuitry is contained on the PC board (Fig. 4). Etched and drilled PC boards are available from the source given in the Parts List.
Figure 4 - Amplifier circuit pattern and stuffing guide. Drill holes and mount the three jacks and volume control in the enclosure. J1 must be insulated from teh enclosure to prevent ground loops. Drill the three PC board mounting holes to accommodate #4 screws. The leads from J1 to P1 and from P1 to R1 should be shielded cables. Mount all of the PC board components as shown in Fig. 4. Be careful to observe correct parts orientation and polarity, and don’t forget to install the jumper wire. (A socket is recommended for IC1.) Transistors Q1 and Q2 are soldered to the PC board so that their lead lengths are 3/8". Connect all of the interconnecting wires to the PC board before you mount it to the enclosure with 4-40 hardware and standoffs. Separate power supply leads are used for each amplifier on the PC board to prevent ground loops. Mount the PC board to the enclosure and then solder all of the interconnecting wires to the correct locations. (Note: the only chassis ground point for the amplifier is at the headphone-jack ground.) Mount R11 and D6 to a terminal strip, with the diode aimed through a hole in the front panel. Fasten the strip to the enclosure, and then wire R11 directly to Pin 3 of J4, and D6 to Pin 2 of J4.
Figure 7 - Interior view of power supply. Mount the remote power supply in a separate metal enclosure (Fig. 5). Drill holes and mount the power switch, fuseholder, line-cord anchor, and DIN connector. Attach a TO-220 aluminum heatsink to IC1 and IC2 using silicone compound. Drill the four PC board mounting holes to accommodate #4 hardware.
Figure 7 - Power supply circuit pattern and stuffing guide. Mount all of the PC board components as shown in Fig. 7. Be careful to observe correct parts orientation and polarity. Connect all of the interconnecting wires and power transformer leads to the PC board before you mount it to the enclosure. Mount the PC board to the enclosure with 4-40 hardware and standoffs, and then solder all the interconnecting leads to the correct locations. Solder first the line cord and then the T1 leads to SW1 and F1. Install the line fuse. Parts sources:
61 Water Street Mayville, NY 14757 716-753-3200
Transistors Q1 and Q2
PO Box 3047 Scottsdale, AZ 85271-3047 800-528-1417
Transformer and heatsinks
701 Brooks Avenue South Thief River Falls, MN 56701-2757 800-344-4539 Addendum
New Power Supply PC Board and stuffing guide. Dave Lockerman redesigned the layout for the power supply PCB to place greater separation between the AC line connections and the secondary connections (Audio Amateur, 3/94). The primary change is that the transformer is rotated 180-degrees from its original position. Mr. Eaton noted that the revised design was a definite improvement, even though he had not experienced any problems with the original PCB.
c. 1993, Audio Amateur Publications, Inc., P.O. Box 576, Peterborough, NH 03458, USA. All rights reserved. | |||||
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