HIGH-END POWER AMP-High-class watts （8）

Published:2011/8/22 22:02:00 Author:Phyllis From:SeekIC

By Ton Giesbets

Alignment

After everything has been assembled and the wiring has been completed, you come to the moment when you switch on the amplifier for the first time. As a precautionary measure, connect 33 il, 5 W resistors in series with the power supply lines before switching on the amplifier, just in case something has been fitted incorrectly. Also first turn PI fully to the left (anticlockwise) to set the quiescent current to a minimum. After this, you can switch on the power.

To set the quiescent current, you can measure the voltage between the emitter of T14 or T15 and R36 (which should be easily accessible with a measurement probe). Adjust the voltage to 22 mV. If you can do this for both output stages and the settings remain stable, you can switch off the amplifier and remove the resistors from the supply leads.

The easiest way to adjust the overdrive indicator is to use an oscilloscope, but you can certainly manage without one. After the quiescent current has been set, drive the amplifier to just below its maximum output amplitude using a 1-kHz sine-wave signal. This is best done using an 8-D load resistor with a power rating of at least 30 W, but it can also be done without a load. Measure the signals at the outputs of ICla and IC3a, and adjust them to a minimum. If you do not have access to a scope, you can use a multimeter. The simplest method is to set the presets to 210 Q before soldering them to the circuit board.

Let there be sound!

Once all this has been done, the amplifier can do what it’s supposed to do: amplify audio signals in a very refined manner. The initial sound impressions from this amplifier are clearly reminiscent of its predecessors with the same basic design, such as the Elektor Electronics Power Amp, HEXFET Power Amp and IGBT Power Amp. The choice of components and the compact, doublesided circuit board layout appear to bear fruit here in the form of an especially transparent, unstrained reproduction with a very spacious, well-detailed stereo image. This amplifier demands a pair of outstanding loudspeakers, preferably relatively large types (since they usually have better efficiency than compact speakers). After all, the output power is rather modest, but that shouldn’t present a problem for anyone outside the ranks of hard-rock, grunge and heavy metal fans. The accurate indicators are quite helpful in setting the maximum volume, and this amplifier shows once again how little power is actually necessary for outstanding stereo reproduction at a respectable sound level.

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