Project 18


Project 18 is another power amplifier project that uses an LME49811 as the input stage. The output stage consists of TO220-packaged power MOSFETs. You can think of Project 18 as being Project 17’s little brother! The schematic diagram is similar to Project 17. The power output is about 100Wrms at 8Ω load with a ±50VDC power supply. Higher power output can be obtained with a 4Ω load.

A couple of SMDs were combined with the rest of the through-hole components to simplify copper trace runs on the PCB layout. A word of warning, these SMDs are challenging to populate into the PCB. If you plan to build this project, the PCB layout as well as Project18 BOM are provided for free by request, as is, no warranty.

              Project18 LME49811 input stage schematic diagram.                                               Project18 output stage schematic diagram.

Project18 Silkscreen Parts Placement Layer.



Here comes the fun part. Install a shunt at the input and do not connect any load at the output of the amplifier under test. If a Variac is available, initially power the amplifier at ±24VDC. The DC offset voltage is about 1.3mV, as shown in the picture on the left. Adjust VR1 to obtain about 40mA across the F+ fuse terminals after a warm-up period of about 10 minutes. The DMM in the middle of both pictures will be measuring the voltage drop across the source resistors R9 and R16. Other than those readings, switch OFF the power supply immediately and check the PCB for assembly errors. Otherwise proceed with the next step.

Gradually increase the voltage to about ±50VDC while observing the AC ammeter and wattmeter readings on the Variac. Any sudden change or increase in DMM or Variac meter’s reading, switch OFF the power supply immediately and check for PCB assembly errors. If no problems were encountered, proceed with music test. The picture on the right is showing the DMM readings when the amplifier under test is applied with a 1KHz 2Vp-p sine wave input signal and an 8Ω dummy load. The DC offset voltage is 22.8mV, the voltage drop across R9 and R16 is 44.6mV and the current at F+ fuse terminals is 450mA.



The next step is optional, but if you have the equipment to perform the test, proceed with power output measurement. NEVER touch the amplifier’s heat sink or dummy load; they will get extremely hot during this test. The power output was measured using an HP3312A Function Generator set at about 1KHz 6.16Vp-p sine wave output and connected directly to the input of the amplifier under test as well as CH1 test probe of the oscilloscope. An 8Ω/400Watt dummy load was connected at the output of the amplifier under test as well as CH2 probe of the Tektronix TDS520D 500MHz DPO.

The amplifier produces a 29.1Vrms across the 8Ω/400W dummy load or a power output of about 106Wrms, as shown in the TEK hardcopy picture. The horizontal scale was adjusted at 200uS to verify for any visible clipping on its peaks. Enjoy!