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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 V_be(on) spec: MJE243 V_be(on) = 1.5V and the TIP120 V_be = 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.

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