HIGH-END POWER AMP-High-class watts （4）

Published:2011/8/21 21:11:00 Author:Phyllis From:SeekIC

By Ton Giesbets

IC1 is a fast dual opamp (type AD827). Here ICla is configured as a difference amplifier that compares the two signals at its inputs. R8 and R9 attenuate the loudspeaker signal by a factor of two (10.5÷2). This means that IClb only has to provide a gain of 3.5 (10.5 ÷ 2 ÷ 1.5). The gain of ICla can be adjusted using PI to compensate for variations in component values. In theory, PI should be set to 210Ω .

The output signal from ICla is fed to the input of IC2, a dual comparator that is used here as a window comparator. The threshold levels are +0.5 V and -0.5 V, which means that the output voltage must differ from the expected value by more than 1V (10.5 times the input voltage) to generate a warning signal. It hardly needs saying that a large difference is most likely a sign of clipping, which occurs when the amplifier is overdriven. Reducing the threshold levels would excessively increase the sensitivity of the circuit to noise and other forms of HF interference, so the LED would tend to light up before the amplifier was actually being overdriven.

The combined open-collector outputs of the comparators charge capacitor C5. Here R15 provides current limiting. The voltage on C5 is used to cause LED Dl to shine quite visibly even if overdrive only occurs during short peaks. After the comparator outputs have returned to the high-impedance state, the LED will continue to be supplied with current via constant-current source Tl. This causes the energy stored in the capacitor to be used as effectively as possible, so the LED will shine with constant brightness as long as possible. The current through the JFET is set to approximately 2 mA by R14. Using a low LED current allows a fairly small capacitor to be used for C5, which saves space. However, this requires using a low-current LED that shines brightly with a current of 2 mA (an example is given in the components list).

The input ground is kept strictly separate from the remainder of the circuit on the circuit board, in order to avoid ground loops. The input signal ground is connected to a star point on the amplifier circuit board. The input signal and the ground connection of the input socket on the indicator board go from here to the amplifier board. Resistor Rl ensures that the input terminal of CI is always connected to ground. R2 is necessary to provide the bias current for the input of IClb. As R2 is connected in parallel with the input, we decided to tie it to the input ground of the amplifier board. Drawing the bias current for IClb from that source does not have any negative effects. The advantage of placing the input capacitor on the indicator board is that it allows the signals to be compared from DC to more than 20 kHz. This avoids any measurement errors due to the lower roll-off frequency.

The supply voltage for the indicator circuit is taken from the main power supply and stabilised using Zener diodes, since the value is not critical. Allowance has been made for a fairly high load on the supply and associated voltage slumps. For this reason, the Zeners are operated at relatively high currents, which means that the associated series resistors (R16, R17, R33 and R34) must be 1-watt types. To save space, we selected especially small types (Vishay BCcomponents PR01 series).

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