Figure 7.6 Heath AA2015 and other similar models. Except for twenty components this diagram is the same as figure 7.5. The connections of the added parts will be at the very end of the main description. These additional parts add over current protection to the amplifier. The amplifier begins with a PNP differential pair with a PNP transistor current source in their emitters. The input goes through a 33 k ohm resistor to ground. The input also goes to the base of the left hand transistor in the differential pair, Q1. The emitter of q1 goes to the emitter of Q2. The two emitters go to the collector of the current source, Q3. The base of Q3 goes to the anode of a 3.6 volt zener diode. The cathode of the zener goes to plus 39 volts. The emitter of Q3 goes through a 2.2 k ohm resistor to plus 39 volts. The anode of the zener goes through a 15 k ohm resistor to minus 39 volts. The collector of Q2 goes to minus 39 volts. We will come back to the base later. The collector of Q1 goes through an 820 ohm resistor to minus 39 volts. The collector goes through a 33 pf cap to its base, which is also the input terminal. The base also goes through a 180 pf cap to ground. The collector of Q1 connects to the base of Q4, an N P N BJT. The emitter goes through a 2.7 ohm resistor to minus 39 volts. There is a 47 pf cap from the base to the collector of Q4. The collector of Q4 goes through the parallel combination of a 0.05 microfarad cap and a 68 ohm resistor to the cathode of the first of three seriesed diodes. The anode of the third diode goes to the collector of Q5, the predriver current source. The emitter goes through a 100 ohm resistor to plus 39 volts. The base goes through an 820 ohm resistor to plus 39 volts. The base also goes through a 2.2 k ohm resistor to the anode of the 3.6 volt zener mentioned earlier. The junction of the 0.05 microfarad cap, the 68 ohm resistor, and the cathode of the first of the three diodes goes to the base of Q6, a PNP transistor. There is a 200 pf cap connected between the base and collector of Q6. The junction of the collector of Q5 and the anode of the upper diode connects to the base of Q7, an NPN. There is a 200 pf cap connected between the base and collector of Q7. The collector goes to plus 39 volts. The emitter of Q7 goes to the base of q9, an NPN. The base of Q9 goes through a 75 ohm resistor to the amplifier midpoint. The collector of Q6 goes through a 75 ohm resistor to minus 39 volts. The collector also goes to the base of Q8, an NPN. The emitter goes to the cathode of a diode. The anode goes to the collector of Q8. There is a 39 ohm resistor in parallel with the diode. The collector of Q9 goes to plus 39 volts. The emitter of Q9 goes through a 0.33 ohm 5 watt resistor to the amplifier midpoint. The amplifier mid point goes through another 0.33 ohm 5 watt resistor to the collector of Q8. The emitter goes to minus 39 volts. The amplifier mid point goes through a 22 k ohm resistor to the base of Q2. This base also goes through a 470 ohm resistor then a 250 microfarad capacitor in series to ground. Plus 39 volts connects to the cathode of a diode. The anode connects to the amplifier mid point. The amplifier mid point goes to the cathode of a diode. The anode goes to minus 39 volts. The amplifier mid point goes through a 3 microhenry coil to the hot speaker output. There is a 10 ohm resistor in parallel with the coil. The hot speaker output goes through a 0.1 microfarad capacitor then a 20 ohm 2 watt resistor to ground. The cold speaker output is grounded.

Added components. Negative half.

The over current protection circuit is totally symmetrical. Both halves will be described. The collector of Q8 connects to one end of a 68 ohm resistor in parallel with a 0.01 u f capacitor. The other end of this parallel combination goes to the amplifier midpoint through the series combination of a 15 k ohm resistor then a 250 u f capacitor. The junction of the parallel R and C and the 15 k ohm resistor goes to one end of a 1 k ohm resistor. The other end of the 1k goes to one end of a 4.7 k ohm resistor. The other end of the 4.7 k goes to the other side of the limiting circuit and will be picked up when that half is described. The junction of the 1 k and 4.7 k connects to the cathode of a diode, D4. The anode is grounded. The junction of the parallel R C, the 15 k, and the 1 k, all connect to the base of Q10, a PNP BJT. The base also goes to one end of a 0.02 u f capacitor. The other end of this capacitor goes to the other side of the limiting circuit and will be picked up when that half is described. The base also goes through a 0.01 u f capacitor to the collector. The collector goes to the anode of a diode, D2. The cathode connects to the base of Q6. The emitter of Q10 goes to the amplifier midpoint.

Added components. Positive half.

The emitter of Q9 connects to one end of a 68 ohm resistor in parallel with a 0.01 u f capacitor. The other end of this parallel combination goes to the amplifier midpoint through the series combination of a 15 k ohm resistor then a 250 u f capacitor. The junction of the parallel R C and the 15 k ohm resistor goes to one end of a 1 k ohm resistor. The other end of the 1k goes to one end of a 4.7 k ohm resistor. The other end of the 4.7 k goes to the other side of the limiting circuit and has already been described. The junction of the 1 k and 4.7 k connects to the anode of a diode, D3. The cathode is grounded. The junction of the parallel R C, the 15 k, and the 1 k, all connect to the base of Q11, an NPN BJT. The base also goes to one end of a 0.02 u f capacitor. The other end of this capacitor goes to the other side of the limiting circuit and has been described. The base also goes through a 0.01 u f capacitor to the collector. The collector goes to the cathode of a diode, D1. The anode connects to the base of Q7. The emitter of Q11 goes to the amplifier midpoint. End verbal description.
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