Introduction: Bench Test Amplifier

Picture of 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

Picture of 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

MISC
FR4 copper clad PCB board, plated at least one side
bottom half of an altoids tin
clips leads
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
wire

TOOLS
hobby knife
soldering iron
magnifying glass / worklight
drill press
3/64", 1/8" machine bits
1.75" hole saw
soldering iron, solder
wire cutters
wire strippers
hot glue gun and sticks of glue

Step 2: Circuit Schematic

Picture of 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

Picture of 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)

Picture of 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

Picture of 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!

Picture of Finish!

When finished, the bottom of the amp should look about like this.

Add the standoffs if you'd like

Comments

AndrewW138 (author)2016-02-02

I made this little amp, had some SMD breakout boards with the chip. I replaced R1 with a little pot, if you put the pot to 0 ohms you can amplify very small signals and it becomes pretty damn loud!!

This is perfect for radio projects.

Sandisk1duo (author)2008-09-09

how loud is it?

Phantom5582 (author)Sandisk1duo2008-11-17

I want to know this as well and I want to know if you can make it more compact. I want to make one without the speaker and have it fit into a lined Altoids tin. I plan on using it to amp up the sound on my Zune and I want it to fit in the case I have. With the amp, I can let my Zune play at a lower volume and save battery power. By the way, how can one make it stereo?

Sandisk1duo (author)Phantom55822008-11-17

Make two of these, then wire on to the right audio signal, the other to the left

hexinverter (author)Sandisk1duo2010-07-25

Honestly, don't waste your time trying to make a decent sounding headphone amp out of these :/ This circuit is not designed with noise/etc in mind for true listening or crystal clear monitoring audio applications. It makes an excellent bench tester though! :) It would be a waste of money and time (although a valuable learning lesson I guess) to do it, since eBay has REAL headphone amps that are designed for headphones and portability and actually make music sound clearer for pretty darn cheap. What you're looking for is the "cMoy" pocket headphone amplifier (do a search on eBay....there are tonnes of different versions that people have made). I built a version of it called the "cMoyBB" (www.cmoybb.com) - it was very easy and its sound quality is mind blowing! (I know your post is years old, it's just food for thought!)

Phantom5582 (author)hexinverter2010-07-31

It's cool, and I never made it yet so no time wasted ^_^; I was going to go this route when I saw it, but I kind of need the kit and tools first. >_> http://www.youtube.com/watch?v=Z-RHKYjQVfE

Phantom5582 (author)Sandisk1duo2008-11-17

OK. Can both of them run off the same set of batteries? Is there a way to make the circuit smaller? And can the same volume control be used for both circuits?

To do a stereo device use two amp chips, same batteries OK, need a stereo potentiometer. Most surplus stores have these

Thanks for the help. I'm thinking about having two audio outputs that way I can have a set of speakers hooked in at the same time. I'll put a switch in so I can flip between the audio outputs. The biggest problem I face, other than getting the parts, is space. I have to fit two audio ports, two switches (one on/off and a three pole to flip between output ports), volume control and the led onto either the short side or a quarter of the front. I'm thinking about using a 9v battery instead of AAAs, What will I have to change to do that?

Piece of cake to fit in the tin without a speaker. In fact, could be built in a tin with the speaker if so desired. It should be easy if you have mad proto skittles

Not very loud. It was made for testing, not jamming.

pancho del rancho (author)2009-08-07

would a lm386 work

Yes, a bit. The LM386 operates from 4V to 18V, so would die out soon after the batteries became discharges. The newer Boomer amps swing closer tot he rails, AND have BTL (bridge tied load) outputs, so much more power from the lower voltage. If you want to use an LM386, change the device to 6 batteries, and C3 goes to 16V rating

oh okay couldnt i use the small 12 volt batteries and the new ones were can i get them and is it mono or stereo thx btw

Sure,that would work. Those batteries are expensive and die quick, maybe rechargeable 9V would be better. This is mono If you want a small portable stereo amp, conside a T amp setup

oh okay thx

The Speaker Guy (author)2009-08-07

Forget that comment about C3. You would want to meld the schematic from this to the standard schematic for the LM386.

LinuxH4x0r (author)2008-05-11

Nice job! I hate etchant!

digitaltripper (author)2008-05-11

Etchant...we don need no stinkin etchant...Great instructable. Great way to use the copper clad board!!

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