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).
Headphone Amplifier Performance Specifications
Gain: 34 (31dB)
RMS Output Power: 80mW (600 ohm load)
(R10 = 100 ohms)
160mW (100 ohm load)
120mW (32 ohm load)
40mW (8 ohm load)
Frequency Response: 10Hz-50kHz +0, -2dB
Signal-to-Noise Ratio: 80dB
Figure 1 – The Eaton headphone amplifier circuit.
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.
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.
61 Water Street
Mayville, NY 14757
Transistors Q1 and Q2
PO Box 3047
Scottsdale, AZ 85271-3047
Transformer and heatsinks
701 Brooks Avenue South
Thief River Falls, MN 56701-2757
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.
7/10/2004: Rainer Böttchers created this version of the Eaton amp with a new opamp (OPA2132) and output transistors (TIP120/125). The amp is powerful enough to drive the AKG K1000 headphones (120 ohms impedance and a rated power handling of 1W). Turn-on thumps are blocked with a 3-second delay relay at the output.
The original Eaton used the NE5532 chip. Substituting the TL072C produced no difference in sound quality. The OPA2132 had better treble response than the NE5532 or the TL072C. The new TIP output darlington transistors are easier to obtain than the MJEs, but have a higher Vbe(on) spec: MJE243 Vbe(on) = 1.5V and the TIP120 Vbe = 2.5V.
Biasing the TIP darlington transistors with an LED (2V), the output quiescent current is 2.5mA, which is on the low side for class AB operation. The author did try other options, such as a Vbe multiplier, to replace the LED, but stayed with the LED in the end because there was no noticable distortion even on low-level signals. Any distortion due to the output stage was corrected via feedback to the opamp. The emitter resistors R8/R9 were changed to 3.3 ohms.
The circuits in the pictures were mounted on a Veroboard prototyping board, and the author has created a printed circuit layout for it. The pcb layout is shown below. The Eagle pcb files can be downloaded here.
The author says that the amp will drive any dynamic headphone with ease. With the AKG K1000, the volume is “really loud enough when driving the Eaton with a +10db source. With -4db sources I would select a gain of 50 or a little bit more. I never tried it with a CD player or walkman, only with mixer outputs or sound cards.” A full writeup of his modification can be found here.
10/31/2005: Rainer Böttchers converted his Eaton-type amp (see above) into a stereo 15W/15W amplifier to power monitor loudspeakers (JBL Control 1C). The switch over the headphone jack toggles between the front-panel headphone jack and the back-panel headphone jack. When connecting speakers, he uses a break-out box (having speaker terminals and a headphone plug) connected to the rear jack.
On the amplifier board, he changed the opamp back to the NE5532, because it can handle the higher power supply voltage. The feedback resistor went from 33K ohms to 47K ohms to increase the overall gain of the amp. The emitter resistors R8/R9 were upgraded to 9W. The four TIP transistors were mounted on a single 7K/W heatsink.
At first, the author tried an unregulated 2x21V @1.2A power supply. The amp was quiet with headphones, but there was a slight hum when the speakers were connected. After he traced the hum to the power supply, he added 7818 and 7918 voltage regulators, which outputted a stable 2 x 18V, and the hum disappeared. The power supply circuit is fairly basic: bridge rectifier, 2 x 4700uF filter capacitors, LM78/79 regulators and 0.1uF + 470uF caps for output blocking. The regulators must be mounted on standard U-type heatsinks.
c. 1993, Audio Amateur Publications, Inc., P.O. Box 576, Peterborough, NH 03458, USA. All rights reserved.
From Audio Amateur (now Audio Electronics), Issue 3/1993, pp. 20-21, 24, 26-27. (Republished with permission.)