Figure 7.4 Schematic Diagram of the Heath A R-15. Except for three components this diagram is the same as figure 7.3. 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. One side of the input is grounded. The other side goes through a 10 microfarad capacitor then a 10 k ohm resistor to the base of Q1. We will return to this point two more times. The emitter of Q1 is grounded. The collector goes through a 33 k ohm resistor to plus 27 volts. The collector also goes to ground through a 270 pf capacitor. The collector also goes to the base of Q2. The emitter goes through a 330 ohm than a 1.8 k ohm resistor to ground. There is a 250 microfarad capacitor in parallel with the 1.8 k ohm. The junction of the 330 ohm and 1.8 k ohm goes to one end of a 270 k ohm resistor. The other end of this resistor goes to the base of Q1. The base also goes through a 33 k ohm resistor to ground. The collector of Q2 goes through a 3.3 k ohm resistor to plus 27 volts. The collector also goes through a 250 microfarad capacitor then a 1 k ohm resistor to the base of Q3. The base also goes through a 2.2 k ohm resistor to ground. The base also goes through a 33 k ohm resistor to a point will call the amplifier midpoint. The emitter goes through a 270 ohm resistor to ground. The collector goes through a 330 pf capacitor to ground. The collector also goes to the base of Q4, a PNP. Note, this is the only PNP transistor in the circuit. All others are NPN. The collector also goes through 3 diodes all in series, then through a 4.7 k ohm then a 1 k ohm resistor to plus 85 volts. There is a 50 microfarad cap connected from the amplifier mid point to the junction of the 1 k and 4.7 k ohm resistors. The junction of the 4.7 k ohm resistor and the anode of the upper diode goes to the base of Q5. The collector goes to plus 85 volts. The emitter goes through a 100 ohm resistor to the amplifier mid point. The emitter also goes to the base of Q7. The collector of Q4 goes through a 100 ohm resistor to ground. The collector also goes to the base of Q6. The emitter of Q4 goes through a 4.7 ohm resistor to the collector of Q6. The collector of Q7 goes to plus 85 volts. The emitter goes through a 0.67 ohm 5 watt resistor to the amplifier mid point. The amplifier mid point goes through another 0.67 ohm 5 watt resistor to the collector of Q6. The emitter goes to ground. The amplifier mid point goes to one end of a 2.2 microfarad capacitor. The other end of this capacitor goes through the parallel combination of a 330 k ohm resistor and a 2.7 pf capacitor to the base of Q1. The amplifier mid point goes to the positive end of a 4000 microfarad capacitor. The other end goes through am 8.8 microhenry coil to the hot speaker output. The amplifier mid point goes through a 0.001 microfarad capacitor to the hot speaker output. The hot speaker output goes through a 0.1 microfarad capacitor then an 11 ohm 5 watt resistor to ground. The cold speaker output goes to ground. Added components. The connection which is in common to the collector of Q3 and the base of Q4 goes to the cathode of a zener diode. The anode of this diode goes to the anode of another zener diode. The cathode of the second zener goes to a 100 u f nonpolar electrolytic capacitor. The other end of the capacitor goes to the amplifier midpoint. The voltage of the zener diodes is not known. End verbal description.
Go back.