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SE Hybrid Tube/MOSFET Headphone Amplifier |
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The EB-804/421 is a Single-Ended (SE) pure Class-A amplifier, capable of driving headphones between 32 and 600 Ohms. The PCB for one amp is 90x80mm. The amplifiers need ±15 to ±24V regulated supplies at 160/100mA and 6.3VDC at 350mA for the tube heater. Feeding the amps from separate supplies is recommended.
The schematic is shown in Fig. 1. The topology is the same as the hybrid tube/MOSFET line amp, published in Glass-Audio 1/98 (See copy of article on the homepage). Q1 is a double triode. It operates as a differential amplifier, with approx. 2mA in each of the triodes. A constant current diode D1 supplies the source current to the differential amp. D1 is made up of two J508 or E-202 diodes in parallel. You can also use a single J511, which delivers 4.7mA.
The two anodes, 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 pin-out is different from the Hitachi. (GDS vs. GSD)
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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 55 degrees C, so proper ventilation is obligatory.
The input tube requires a 6.3V/350mA heater supply. Use a well regulated/low ripple supply for this. It is recommended to ground the negative side of the heater supply to the PGND on the PCB.
Some notes on the linearity of the amplifier. The input tube dominates the overall distortion characteristics of the amplifier. Tubes of different manufacturers produce different amount on distortion. I have tested the ECC86 from Telefunken and Ultron, ECC88 from AEG and E88CC/6922 from Tungsram and Russian Military. The difference in THD can be 6-10dB! The Russian 6922 produced the lowest THD. We are shipping the kits with these tubes. Nevertheless I recommend that you try different types of tubes, and select the one you like best.
Note also that the tube can pick up hum from mains fields. Again, tubes from different manufacturers show different sensitivity to these fields. It would help to use a shielded tube socket, however, it is difficult to find one for PCB mounting.
Finally, it is a good idea to switch on the heater before you apply the +/- supply to the amplifier. This has nothing to do with cathode stripping, but with the DC operation of the amplifier. As long as the heater is not on, the input does not function even if you apply the +/- supply. Consequently the DC feedback loop is inactive and the output is not sitting at zero Volt. Only after the heater is on can the output stabilize to zero Volt.
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. output impedance is 15 Ohm. Equivalent input noise depends on the tube used and is 1.2-1.5µV! At 0.1% THD the amp delivers 0.5V into 32 Ohm and 7V into 600 Ohm with +/-24V supply and approx. 90mA in the second stage.
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, the amp delivers >100mW into 32 Ohm and >250mW into 600 Ohm at 1% THD. With ±15V and 160mA the power into low-impedance loads increases: 300mW into 32 Ohm at 1% THD.
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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