The EB-602/210 is a Single-Ended (SE) pure Class-A amplifier, capable of driving headphones between 32 and 600 Ohms. The PCB is 80x165mm; two identical but completely separate amplifiers are laid out on the PCB. The amplifiers need ±15 to ±24V regulated supplies at 160/100mA each. Feeding the amps from separate supplies is recommended.
The schematic is shown in Fig. 1. The topology is the same as my hybrid tube/MOSFET line amp, published in Glass-Audio 1/98 (See copy of article on the homepage). However, I have replaced the dual triode at the input with a dual JFET, the 2SK389. The JFET offers less noise, more gain, and better linearity than the ECC86 tube. It operates as a differential amplifier, with approx. 2mA in each of the JFETs. A constant current diode D1 supplies the source current to the differential amp. D1 is made up of two J508 2.4mA diodes in parallel. You can also use a single J511, which delivers 4.7mA.
The two drains, which produce out-of-phase signals, are converted to a single-ended signal using a current mirror composed of Q2, D2, and resistors R3/R4. Q3, a P-channel MOSFET in TO-220 package, is used in common-source mode as a Class-A single-ended second stage. Its drain resistor has been replaced with a second constant-current source, supplying the Class-A current of 100 or 160mA. The constant current source, which increases the gain and improves the linearity of the second stage, is made up of Q4, an N-channel MOSFET in TO-220 package, and its associated components. I am using the Hitachi 2SJ79 and 2SK216 for Q3 and Q4 respectively. The Toshiba 2SJ313 and 2SK2013 can also be used, but note that the pinout is different from the Hitachi. (GDS vs. GSD)

The amplifier can work with a ±15V to ±24V supply. The maximum dissipation allowed for Q3 and Q4 is 2.4W each, so the supply voltage determines the maximum current. At ±24V the current is 100mA and at ±15V its 160mA. Resistor R13 sets the current: it is 6R8 for 100mA and 3R9 for 160mA. Q3 and Q4 must be heatsinked. I am using the SK76-37.5 with 8 K/W thermal resistance. The temperature on the heatsinks is about 50 degrees centigrade, so proper ventilation is necessary.
The Open Loop gain of the amp is 67dB and the O.L. frequency response is 14kHz with 100-Ohm load. The feedback resistors R8 and R9 set the Closed Loop gain of the amp. Normal gain is 10x or 20dB. Changing R9 can change this gain. C.L. rise time is <0.2µs and the –3dB roll-off point is >900kHz. C.L. output impedance is <1Ohm (actually measured 0.3 Ohm). Equivalent input noise is 0.9µV! THD at 3V RMS/1kHz and ±24V supply, is 0.002% with no load, 0,0025% with 600 Ohm load, and 0.0058% with 100 Ohm load. At 1VRMS the THD is 0.0047% with 50-Ohm load and 0.008% with 32-Ohm load.
The amplifier is DC coupled and I have not included a capacitor at the input to block off a DC voltage that might come from the source. Should you have a source with DC at the output you need to put a 2.2uF film capacitor in series with the input.
Obviously, the maximum output power into different loads depends on the supply voltage and the available current from Q4. With ±24V and 100mA in the second stage, this is the power available at ≤ 0.1% THD:
600 Ohm: 240mW
300 Ohm: 418mW
100 Ohm: 250mW
50 Ohm: 135mW
32 Ohm: 87mW

With ±15V and 160mA the power into low-impedance loads increases:

600 Ohm: 106mW
300 Ohm: 200mW
100 Ohm: 462mW
50 Ohm: 300mW
32 Ohm: 219mW

The maximum power is limited by the available current at low load impedances and by the available voltage swing at high impedances. If your headphones are low impedance, you should operate the amplifier at ±15V with 160mA in the second stage, and if they are high impedance, use a ±24V supply with 100mA. Since high impedance headphones require less power than the low impedance ones, the ±15V operation will probably give more than enough power for ear shattering SPL over the whole impedance range.

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