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  ExtremA, a reference class-A DIY amplifier. 
  Feb 22, 2007, 07:30am EST 

Scalability and construction

By: Sander Sassen

Obviously not everybody is willing to put up with an amplifier dissipating about 250W to enable 100W output in an 8-ohm load. In many cases about 50W would be sufficient, in which case the bias currents for the output stage and the supply voltages can be scaled accordingly. Suppose we would like to have 50W into a 6-ohm load the peak current will be 4.1A, which results in a bias current of 2.05A. As mentioned the bias current is set by the emitter resistors of the output transistors. The voltage drop across these four parallel resistors is 0.65V, hence they would need to be 0.32-ohm in value, or rather 1.2-ohm per resistor. The output voltage would need to be 25V, to be able to supply that voltage we need a little more than half that per supply rail, hence 14V would be sufficient. Both output stages will now dissipate 2x2.05Ax2x14V=115W, about half that of the 100W configuration.

Amplifier and powersupply PCBs

Fig 4. Unpopulated amplifier and powersupply PCBs.

Two PCBs are available to facilitate construction of this amplifier, one amplifier PCB and one power supply PCB. One power supply as described is meant to be used with a single amplifier, thus a pair of amplifiers would require two amplifier and two power supply PCBs. Construction is straightforward with all parts being normal through-hole components. Populating the PCBs starts with the smallest components, the diodes, followed by the resistors, capacitors and transistors. The BD139/BD140, 2SC2922/2SA1216 transistors are not soldered in at this stage. Once they are mounted onto the heat sink and the PCB has been fully installed, the leads of these transistors may be soldered. Working the other way round would stress the leads and packages to the breaking point when tightening the screws and/or thermal stress occurs under operation. Obviously all transistors mounted on the heatsink need to be electrically isolated from one another, we prefer using glassfiber reinforced thermal pads for this purpose that can be cut to the required shape. Take notice that the heatsink requirements are quite steep, for 100W/8-ohm a heatsink rated at 0.2K/W is a minimum requirement. Potentiometer P1 should be set halfway prior to soldering it in place and should be adjusted for minimum offset across the attached load after the amplifier has been allowed to warm up.

Populated amplifier PCB

Fig 5. Populated amplifier PCB, minus output transistors.

Populated powersupply PCB

Fig 6. Powersupply PCB, with 132.000uF of buffer capacity.

Amplifier with output transistors and heatsink

Fig 7. Amplifier PCB with heatsink and output transistors.

The power supply uses two 200V/35A bridge rectifiers which should be attached to the heatsink as well, to provide sufficient cooling. The four buffer capacitors are BHC ALC10 series or equivalent, of either 22.000 of 33.000uF in size, for a total of 88.000 or 132.000uF. The transformer for the input stage should be about 25VA with 2x25V secondaries whereas the transformer for the output stage should be scaled according to the desired output power. For 100W/8-ohm a 300 or 500VA toroidal transformer with 2x18V or 2x22V secondaries is sufficient. Total cost of a single amplifier, including transformers and heatsink will be around $700 or 500-euros.

1. Introduction
2. Transistors
3. Concept and design
4. Scalability and construction
5. Measurements
6. Copyrights, Updates etc.

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