1o. Stuff and solder a 1Ω resistor into Z14 and Y9, and a 0.22μF cap into Y5 and Y8. In back, solder Z14 to
Z15, and Y8 to Y9. Clip except for Y9 and Z15.
1p. Stuff and solder a 100μF cap into X12 (–) and X13 (+), and a 0.1μF cap into V9 and V13. Bend lead X12
up to V9 and solder it, then solder V13, X13, and Y13 together.
1q. Use separate jumpers to wire V9, Y5, AA8, AE10, and AG14
to the ground wire loop in back. Put a red 24-gauge jumper wire
through U3 from the component side and connect it to P3; pass
the other end through U13 and connect it to Y13. Put a green
jumper through S14 from the component side and connect it to
P13; pass the other end through AA12 and connect it to AA13.
For the right channel feedback, connect a 20K resistor (or a
22K and 220K) on the back between AB15 and Z15. Clip off
lead AB15.
2. TEST THE AMP
2a. For the power indicator light, stuff your favorite color LED into AC3 (anode, longer leg) and AD3 (
cathode). Bend and solder the cathode lead to AD1. Stuff an 830Ω, ½W resistor into Z3 and V3, then solder V3
to the ground wire loop, and Z3 to AC3.
2b. The circuit board is done now, and the only unconnected leads remaining should be I8 (left channel
input), E15 (left channel output), AD8 (right channel input), Z15 (right channel output), X1 and H1 (AC
input), and the ground at P6–P10.
To set up testing, hook up the transformer to the power supply as in Step 1g. Clamp a heat sink to the tab
of each LM1875 chip, making sure they don’t touch anything conductive. For dummy loads, clip-lead a
10Ω–25Ω, 10W resistor from E15 to ground, and a second resistor from Z15 to ground. Connect AD8 to I8.
Connect I8 to ground for the moment.
2c. Power up the circuit as in Step 1g. The power LED should come on. Measure the DC voltage across
each dummy load resistor. If it’s over 0.05 volts, power down and look for wiring errors.
2d. For signal testing, disconnect I8/AD8 from ground and connect
them to any signal generator source with a volume control. Power
the circuit back up and test across the load resistors, measuring
AC voltage this time. Briefly increase the volume of the input, and
look for the voltage to read up to about 9V AC output. But keep all
signal testing brief, to avoid overheating the chips. If you have an
oscilloscope, look for clipping at about 25V peak-to-peak. Reduce
the signal to 1V AC output on one channel, and then test the other
channel; it should also measure 1V AC. Switch the meter to DC and
confirm that both channels measure 0V DC. Try a speaker in place
of the load resistor to make sure the output signal sounds like
clean sine wave. If it all checks out, your board is done and working.
77 Make:
