HIGH-END POWER AMP-High-class watts （2）

Published:2011/8/21 20:43:00 Author:Phyllis From:SeekIC

By Ton Giesbets

As can clearly be seen, all of the operating points in the amplifier are directly dependent on these two current sources, so we gave particular attention to their design. The reference voltages for the current sources are provided by flat LEDs, which in turn are operated at reasonably constant currents provided by a pair of simple JFET current sources. As a result, the current sources built around T3 and T4 are practically immune to power-supply ripple and supply voltage fluctuations. Unfortunately, JFETs have a rather large tolerance range, so you should measure the voltages at the points shown on the schematic diagram and compare them with the indicated values. A deviation of more than 20% is not acceptable.

The power output stages are wired as emitter followers using a standard design with two complementary sets of transistors in Darlington configuration (T12/T14 and T13/T15). Resistors R26 and R27 are placed at the inputs of the emitter followers to linearise the input impedance of the current amplification stage and reduce the effects of various parasitic capacitances. Although this reduces the open-loop gain, it also considerably increases the bandwidth. This allows less negative feedback to be used, which from an audiophile perspective can be regarded as better for reproduction quality.

T11 is used to set the quiescent current. Here we use the same type of transistor as the NPN driver (T12). This makes assembly easier because these transistors have a plastic package that can be fitted to the heat sink without using an insulator, which is also true of T13- The quiescent current bias transistor is located on the circuit board between the two output-stage transistors (T14 and T15), which do have to be fitted with mica insulating washers, in order to achieve maximum thermal coupling. Driver transistors T12 and T13 are located on either side of this set of three transistors. The quiescent current of the output transistors is set to 100 mA, which is sufficient.

Integrator IC1 compensates for the input bias current, as well as any off-sets that may arise due to inequalities in the various DC settings, by injecting a current into the input of the amplifier. The bias current is primarily due to differences in the amplification factors of the complementary dual transistors and the currents from the two current sources, as well as imbalances in the transistors of the following stages (differences in amplification factors and base-emitter voltages). A bias current of approximately 8 uA can be compensated using the indicated component values. R48 and C20 determine the integration time constant, and C19 decouples any effects from IG1 (except for bias current correction).

Rl and CI at the input of the amplifier form a low-pass filter that prevents the amplifier from being overdriven if excessively steep signal edges are present at the input. To keep the resulting offset voltage small, input resistor R2 has a lower value than usual (10 kl> in this case). The input sensitivity of this final amplifier has also been made relatively low to reduce the likelihood of overdriving. Its gain is thus set to approximately 10.5 using R17 and R18 (but don’t forget the effect of Rl).

For IC1 to do its job properly, the input must be AC coupled. With the value chosen for the input resistor, the capacitance necessary to yield a sufficiently low roll-off frequency can only be provided by a capacitor with rather large dimensions, so we decided to put it on the printed circuit board for the overdrive indicator circuit. That allows the circuit board for the final amplifier to remain relatively small and increases the distance between the mains transformers and the amplifier circuit board. An additional benefit is that the overdrive indicator can directly access the input signal after the input capacitor, which eliminates the need for an extra tap. The frequency compensation for the amplifier, which largely determines its open-loop characteristics, is provided by C2/R9 and C3/R12. HF decoupling is provided by the C10/R19 network. The amplifier output is switched by a 16-A relay. The type specified in the components list has an industry-standard pinout and can be replaced by other equivalent types.

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