The power supply circuit diagram is shown in Figure 1. The ground symbols on this diagram represent the central ground point in the amplifer. The power supply uses a transformer with a center tapped secondary and a bridge rectifier to produce bipolar supply voltages. The approximate dc voltage on each side of the power supply is given by the ac rms secondary transformer rating divided by 1.414 minus 0.5. The recommended transformers have a secondary rating of 80 V AC rms. With these transformers, the power supply puts out about +58 V and -58 V. With 12,000 uF filter capacitors, the energy stored in the power supply is about 40 joules. This is enough energy to lift a 10 pound dog almost 3 feet off the floor.
|Figure 1. Power supply circuit diagram.|
On the circuit boards, C21 through C24 decouple the power supply rails at the points that they connect to the boards. R32, R33, and C13 through C16 form low-pass filters to prevent ripple on the power supply rails from reaching the low-level input stages. C2 through C4 provide AC ground references for the bases of Q5 and Q6. Each decoupling capacitor consists of two capacitors in parallel, a 100 uF electrolytic and a 0.1 uF film. The film capacitors improve the high frequency characteristics of the electrolytic capacitors.
I have seen some circuits in which the values of the decoupling capacitors are far too small. When I first started designing the Low TIM amplifier, I used a single 0.1 uF capacitor to decouple each power supply rail at the points where they connect to the circuit boards, i.e. in place of C21 through C24. I used that value because I had seen it used in so many other circuits. In taking measurements on the amplifier, I found a very large AC signal would appear on the power supply rails when I tested the amplifier at high frequencies. At 1 MHz, the AC signal on the rails had the same amplitude as the amplifier output signal. When I removed the 0.1 uF decoupling capacitors, the AC signals on the rails would almost disappear. Replacing the 0.1 uF capacitors with 10 uF capacitors eliminated the problem. I settled on 100 uF capacitors to be on the safe side.
I was describing this experience to a former student who had worked as a repair technician for a company that did factory repair service for several audio equipment manufacturers. He told me that whenever they received a particular KLH receiver for repair, the first step was to remove the 0.1 uF power supply decoupling capacitors because they were causing the amplifier sections of the receivers to fail. What was happening was the inductance of the wires that ran from the circuit boards back to the power supplies and the 0.1 uF capacitors formed high-Q resonant circuits on the power supply rails. This was causing the amplifiers to oscillate and fail. Rather than removing the capacitors, a far better solution is to make them larger. With the 100 uF values that I used, I found no evidence of resonance effects on the rails.
The circuit board has two ground leads, both of which connect to the central power supply ground. One lead grounds the signal reference points for the diff amp input stage. The other grounds the power supply decoupling capacitors and provides a ground reference for the protection circuit. R51 connects the two ground leads together on the circuit board. This resistor is small enough to look like a signal short circuit between the two grounds but large enough to force the currents in the two grounds to flow to central ground through the separate wires. This helps to prevent hum induced by power supply ripple currents in the ground system.
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