EK038 Audio amplifier 1W mono
Part no: 41036664
DIY soldering kit for making a one watt single channel (mono) audio amplifier. The kit contains everything necessary to build a simple amplifier, and is based on the LM4861 amplifier chip. Multiple connectors are included, which can be combined in different ways for various uses. The audio signal may be connected via a 3.5mm connector (mini jack), a directly soldered cable, a pin header or screw terminals. Speakers and power supply can also be soldered directly, connected via pin headers or screw terminals. The pin header may be mounted upwards, for example for use with an Arduino Uno (with male-female jumper wires) or downwards for use with breadboads or prototyping board. Four mounting holes for machine screws (M2.5) can be used to mount the amplifier onto e.g. a chassis, or other printed circuit boards.
Bill of materials
| Reference | Part description | Qty. | Part no. | Note |
|---|---|---|---|---|
| - | EK038 1W mono amp PCB | 1 | - | |
| U1 | LM4861, amplifier IC | 1 | 41035600 | Note orientation |
| - (U1) | Socket, DIP8 | 1 | 40410008 | Orient as U1 |
| R1 | Resistor 10kohm | 1 | 40811410 | |
| R2, R3 | Resistor 220kohm | 2 | 40811522 | |
| C1-C3 | Capacitor, 100nF (marked 104) | 3 | 41003038 | |
| C5 | Capacitor, 220pF (marked 221) | 1 | 40500022 | |
| C4, C6 | Capacitor, 100uF | 2 | 40520017 | Polarised (+/-) |
| RV1 | Potentiometer, 100kohm linear (marked 104) | 1 | 41017263 | |
| J4 | Audio jack | 1 | 41001189 | Switching jack, see Connections heading |
| J1-J3 | Pin header 2.54mm 1x8 | 1 | 41036643 | Optional, see Connections heading |
| J1-J3 | Screw terminal 3.5mm 2-pole | 3 | 41016487 | Optional, see Connections heading |
Assembly instructions
Read the assembly instructions before starting assembly.
Interactive assembly guide ↗
Tools
This kit can be assembled with basic tools.
The following tools are required:
- Soldering station or soldering iron (at least 30W)
- Lead-free solder
- Side cutters (for cutting component leads)
Recommended extra tools:
- Wire stripper (for stripping wires to speakers and from power supply)
- Needle-nose pliers (for bending component leads)
- Solder sucker (for desoldering components after mistakes)
- Multimeter (for troubleshooting and component identification)
In addition to these tools, a flat workspace and good lighting are necessary. A solder fume extractor and/or good ventilation is recommended – solder fumes are hazardous with repeated exposure.
Before starting assembly, check that the PCB is free from damage and scratches. Check that you have all the components to be assembled using the bill of materials! All components are to be mounted on the top side of the board – the one with component designations printed (R1, U1, C1, etc.). The components can be soldered from either side.
Place one component, and then bend both leads slightly apart to prevent it from falling out. You can choose between soldering the components into place one by one or placing all of them (at least, the ones that can be held in place by bent component leads) before soldering, according to preference. Placing many components in one go may help with finding (and fixing) mistakes.
The resistors and ceramic capacitors are unpolarised, and thus can be placed in either direction. The electrolytic capacitors however must be mounted in the correct orientation. The long lead goes in the hole by the plus sign (+). The negative lead goes in the other hole. The negative lead is indicated by an arrow with minus signs (usually white).
Step by step
The easiest way is to begin with placing the lowest components first, e.g. the resistors. The kit contains two different resistance values, 10kohm and 220kohm. Refer to the component list above to identify them.
When the resistors are mounted, do the the socket for the amplifier. The socket is just a holder for the amplifier and has no polarity, but it is best to orient it correctly anyway. Otherwise, the risk that you mount the amplifier in backwards is increased. The semicircular mark (notch) on the socket should correspond to the one in the silkscreen on the PCB, towards the reference U1.
Continue with the lowest components, e.g. the small ceramic capacitors. There are two values, 100nF and 220pF. These have different sizes and different markings. Refer to the component list for identification.
Headers and screw terminals
The pin headers and screw terminals' leads can't be bent to stay in place. You can hold them in place with e.g. kapton tape, masking tape or blu-tac, or place the board upside down with the component held in place by some object. Solder one pin, then make sure that the connector is flush against the board. Then, do the rest of the pins.
If using the pin header for all the connections, two pins need to be removed (see image below). You can push them out with, e.g. a side cutter or needle-nose pliers. Do not cut off the pins, as it may damage the pin header or the side cutter.
If you want to use two or more screw terminals next to each other, assemble them into a unit before placing and soldering.
Potentiometer
Save the potentiometer until last. The supplied potentiometer (100K linear) can be replaced with a standard 16mm potentiometer (not included).
General tips
Do one last inspection when all the components are placed and wnsure that they are in the right place and facing in the right direction. If so, it's time to solder them. When a component is soldered, cut off its leads. Cut as close to the board as possible without cutting the soldering point itself. Be careful so that the component leads don’t fly away into your eyes. If you hold a finger lightly on the lead when you cut it off, it is retained.
Make sure not to use too much solder. Heat both the component lead and PCB pad, and wait until the solder flows nicely.
It is easy to accidentally short two connections when soldering. This needs to be fixed before proceeding with the assembly. To break such short circuits, simply melt the solder bridge and pull it sideways with the solder tip. This may need to be repeated several times until the bridge is gone. If the solder becomes sticky and doesn't move, you can add some fresh solder, or use flux. If needed, remove excess solder with the solder sucker and start over.
When all components are soldered, place the amplifier in the socket. To do this, the legs need to be bent inwards slightly so that they are at a right angle. The easiest way is to lay the legs of the IC flat against a table and gently push on the IC so that all legs are bent equally. Do this on both rows of legs. It is important that the amplifier is mounted in the right orientation, with the notch in the same direction as on the printed circuit board (facing the reference U1). If you placed backwards and powered on the chip will be destroyed.
Connections
EK038 can be assembled in several ways. You can use the minijack connector, pin header or screw terminals, or different combinations of these. Another option is to solder wires directly to the PCB. If used as a test amplifier, you may e.g. want to solder the power supply and speaker cables to the board, but use a screw terminal for the signal input.
The circuit has six different connections: signal in, power supply and ground as well as speaker outputs (negative and positive terminals).
| Name | Function | Note |
|---|---|---|
| IN | signal input | grounded by switch in mini jack |
| GND | ground for signal input | |
| V+ | power supply input | +2.5 to +5V DC |
| GND | ground for power supply | |
| spkr - | negative speaker output | not commoned with GND |
| spkr + | positive speaker output |
The signal input is connected to both the pin header/screw terminal and the audio jack (tip). Note that the audio jack has an internal switch, which grounds the signal input if no plug is inserted. Thus, if the mini jack is placed, the other connectoin (on the pin header/screw terminal) will only function if a mini-telephone plug is connected. If both are used, it can be used to e.g. send a signal onwards to another board (or vice versa). If you don't mount the audio jack, the signal input will work as usual.
The speaker output is differential, which means that no large output capacitor is required. It also means that the negative speaker output is not common with the amplifier ground, unlike on many other single supply audio amplifiers.
Power supply
The amplifier operates on +2.5 to +5V (DC), 0.5A or more is recommended. Suitable power supplies are e.g. three AA or AAA batteries (providing 4.5V), power banks (with a cut and stripped USB cable: red is +, black is ground) or an old 5V wall wart. The circuit has no protection against reverse polarity, so double check before connecting the supply voltage. There is also no protection against overvoltage, so always measure if unsure about a wall wart. If the amplifier receives more than the aboslute maximum of 6V (which is itself not recommended) or supplied with negative voltage (reversed polarity) then it will quickly be destroyed.
Fault finding
In the unlikely event that something doesn't work when everything is assembled, follow this troubleshooting guide:
- Check that all components are in the correct place.
- Check that the electrolytic capacitors and the amplifier circuit are mounted in the correct orientation. Compare with the pictures in the assembly guide.
- Check that all components are properly soldered, that you haven't forgotten a lead somewhere.
- Check that there are no short circuits (solder between component leads).
- Check that the power supply, signal and speaker are both connected and working correctly.
- Check that no leads of the IC are bent underneath the body. All the legs need to fit into the socket.
The amplifier works, but pulses when I turn up the volume.
- This is likely due to a too small power supply (and loud signal). More current output is needed.
More information
Schematic
Dimensions
Modifications
If you don't want to use the potentiometer (e.g. controlling the volume elsewhere), you must bypass it with a cut-off component lead. Connect the right hole of the potentiometer (the one nearest the reference RV1) with the middle one.
If using with an electric guitar, contact microphone or other devices with high output impedance, some component values need to be changed:
- Replace potentiometer with a 1M linear pot (or logarithmic, in which case R3 may be omitted)
- Replace R1 and R3 with resistors around with approx. 470kohms to 1Mohm resistance
- Replace R2 with a high-value resistor – the higher the resistance, the more gain. 10Mohm (with 1M at R1) provides a 10x gain. For maximum distortion, R3 can be left open (no resistor).
About the circuit
EK038 is based on the amplifier IC LM4861. The gain is set as with op amp inverting amplifiers: gain = Rfb/Rin. Rin (the input resistor) is R1, while Rfb (the feedback resistor) is R2. With the values 220Kohm and 10Kohm, the gain is thus 220/10 = 22, or around 26dB. Thus, if you want more gain, you can replace one of the resistors, or connect one parallel to R1 (giving a lower resistance).
Errata
The plated mounting hole (and the two mounting holes for the potentiometer) are not connected to ground in version 1 of the board (labeled batch 93960 on the back). This doesn't affect the functionality, but may be confusing trying to use it as a ground reference when measuring. You can solder a component lead from the mounting hole to an appropriate ground point, e.g. the outermost oval hole on the audio jack.