Intro: Bench Test Amplifier
This Instructable details the construction of a bench amplifier, suitable for testing audio circuits. It includes test clips to attacht the amp to the test circuit, batteries, volume control, a power switch, and a speaker. This expedites experiments with audio, as an amplifier need not be built for each setup.
Step 1: Materials and Tools
This project was built with an LM4861, a 1.1W, BTL speaker amplifier. It operates from direct battery voltages.
BILL OF MATERIALS
C1, C2 - 470nF (0.47uF) ceramic capacitor, 25V
C3 - 4.7uF tantalum capacitor, 10V
R1 - 4.7k, 1/4 W resistor
R2 - 180 ohm, 1/4W resistor
R3 - 25k potentiometer
SW1 - SPST (or SPDT) switch
D1 - Red LED
BT1 - battery holder, AA or AAA, 3 cells
U1 - LM4861, 1.1W apmplifier, 8-PDIP
LS1 - 8 ohm speaker
FR4 copper clad PCB board, plated at least one side
bottom half of an altoids tin
1" standoffs, internal 4-40 thread, 4 pieces
1/4" 4-40 screws, 4 pieces
1/4" 2-56 screws, 2 pieces
2-56 nuts, 2 pieces
magnifying glass / worklight
3/64", 1/8" machine bits
1.75" hole saw
soldering iron, solder
hot glue gun and sticks of glue
Step 2: Circuit Schematic
This is very close to the standard application circuit for the LM4861, changes primarily due to component availability.
Step 3: Board Layout and Preparation
Place the components on the board in a logical fashion. Do realize you will be marking the bottom of the PCB. In the below picture you can see the faint marks left by a ball point pen.
Note that because this is a scrap, there are existing holes. I use one for the potentiometer, but drill others as needed.
The switch and LED should have holes drilled to match their size. The #4 standoffs at the four corners of the board use 1/8" holes
I used a hole saw for the speaker, alternatively an array of smaller holes can be used
Step 4: Cutting the Component Traces (lands)
I use an exacto hobby knife to cut the required areas, providing electrically isolated pads on which to solder the IC and passive components.
Note that pins 1 and 7 of the LM4861 are connected to ground. They do not have pads cut, the remaining copper on the board becomes ground. This provides the best performace.
Cut the pads in the following manner:
# MAKE SURE TO CUT AWAY FROM YOURSELF!
Lightly score all the horizontal lines, holding the board in a comfortable position. Rotate the board 90 degrees clockwise, and lightly score the vertical lines. Rotate 90 degrees clockwise again, and cut a bit deeper. Repeat this for a few rotations, and you will notice a grrove about 0.02" wide (1mm). Use a magnifying glass or a jewler's loupe to verify each pad is isolated.
Once all are cut, I usually take the board to the sink for a scrubbing with Dawn and a plastic scrub sponge. The bristles of the scrubber help remove bits of dangling copper, and the detergent strips oils off the board, helping with solderability.
Step 5: Assembly
Attach the switch, LED, battery holder to the board
Using hot glue, attach the speaker to the board. Solder 6" wire leades to the speaker. Thread the leads through the hinge hole on the altoid tin bottm. Use hot glue to seal the tin to the PCB. This sealing helps the speaker function correctly by eliminating the back wave from the speaker.
Solder the components per the schematic. Again, any component connected to ground solders directly to the remaining copper.
Use a multimeter to verify each pad which is not ground is indeed not shorted to ground.
Step 6: Finish!
When finished, the bottom of the amp should look about like this.
Add the standoffs if you'd like