5W Audio Amplifier+

A couple of years ago, my cheap guitar amplifier, after serving about three years, passed away... completely. So I decided to build my own amp, with some additional features like tone control, simple distortion and volume indication.

There are a lot of good and beneficial articles about audio circuits design, for any desired application. Each one has it's own point of view about sound processing: embedded digital systems, DIY power amps etc. I decided to build my own device, based on basic audio amplification principles, able to respond on my particular requirements.

Special thanks to the General Guitar Gadgets and the AMZ Stupidly Simple Tone Control 2 for their very helpful articles, by which the idea of tone control and pre-amplifier circuits design was inspired.

Block diagram defines how entire project operates, combining all the parts together.

Let's see every block separately:

Audio Input - 1/4" Jack input, here we plug in our audio instrument.

Pre-Amplifier - Combined audio filters with option to amplify the amplitude of an input signal before it reaches power amplification stage.

Distortion Circuit - Simple diode-clipping based circuit, since we have gain option at the pre-amplifier stage, we can adjust the gain of clipped signal.

Tone control circuit - Simple band-pass filter, used to change the characteristics of output frequencies.

Control Switch - Or as it will be called in further steps - "Bypass", is used to choose either of the audio input paths will enter the power amplifier circuitry: Clean audio instrument input signal or pre-amp processed signal.

5W Power amplifier - Audio power amplifier circuitry, designed in bridge tied load mode. We have two 2.5W LM380 ICs which act as combined differential amplifier with reversed polarity on one of the two outputs, thus we can retrieve doubled power from the circuit

Volume Indicator - Based on comparative method circuit, designed of basic peak detector, 4 comparators and 4 outputs, that are connected to the front panel LEDs.

Audio Output - 1/4" Jack differential output, notice that there is no ground net on the output. Here we connect 4-8 Ohm speaker, not to exceed our 5W limit

So, we got the abstract idea of this project . Let's build it!

Complete list of everything you need to build the project:

Parts

A. Common parts:

1. 1.5mm diameter wire
2. Plastic box - I used a 100x150mm plastic enclosure. Make sure it's easy to drill.
3. 0.3mm wire - for electrical connections.
4. 1 x Prototype board - single sided in preferable, in case there is a soldering mistake, it's easy to unsolder.
5. 16 x Shrinking tubes - protect unshielded wires.
6. 3 x Slider potentiometer knobs.
7. 4 x metallic screws - box enclosure
8. 14-18V DC Power supply, 0.9A or higher.

B. Front panel parts:

1. 1 x SPDT switch
2. 2 x SPST switch.
3. 2 x 1/4" Phone Jack - Guitar Input\Speaker Output.
4. 3 x Green LED.
5. 2 x Red LED.
6. 1 x Power Jack - 15V Power input.
7. 8 x Label sticker - Add description to the device.

C. Electrical Parts:

1. Resistors:

• • 2 x 1M
• • 12 x 10K
• • 5 x 1K
• • 1 x 220K
• • 1 x 100K
• • 2 x 470K
• • 1 x 10R - Power resistor
• • 1 x 2K2

2. Capacitors:
2.1 Non-polarized:

• • 1 x 1n
• • 1 x 10n
• • 1 x 50n
• • 1 x 50p
• • 1 x 22n
• • 1 x 470n
• • 1 x 100p
• • 1 x 100n

2.2 Polarized:

• • 2 x 10u
• • 3 x 330u

3. IC and other parts:

• • 1 x LM7812 - 12V Linear regulator.
• • 2 x LM380 - Audio 2.5W power amplifier.
• • 2 x LM324 - Low power quad operational amplifier.
• • 1 x LM741 - General purpose operational amplifier.
• • 1 x 1N4007 - General purpose diode.
• • 2 x 1N5817 - Signal diode
• • 3 x 100K Slider potentiometer - May be rotational - no difference.

Instruments

• Soldering Iron\Station
• Soldering tin, tip cleaner.
• Small plier
• Small cutter
• Electric screwdriver - May be used for cutting\drilling holes
• Hot glue gun
• Sharpened knife
• Ruler\Caliper - Taking measurements
• Rasp/File set - Cutting and grinding of plastic enclosure
• Philips screwdriver
• Multimeter
• Industrial alcohol

Testing Equipment (Optional)

• Oscilloscope
• External lab power supply
  
• Function Generator
• Two 1/4" PL to alligator cables.

5W Guitar Amplifier+ schematic may seem complicated at first sight, but is quite simple.

In order to make it easier to understand the circuit, schematic has been divided into sub-circuits, thus whole project becomes very clear. Important Note: Net names that are defined in the schematic pages also define connections between separate sub-circuits of the device (similar to Off-Page connectors).

1.Power Supply
Could be sufficient to power on the amplifier circuits directly from the 10-18V wall adapter, but I used a +12V LM7812 Regulator in order to improve power nets stability (VCC & GND).

As we can see, circuit is very simple, it has an +12V linear regulator, some bypass capacitors at input and output of the IC and a power-on indicator. Since it's a linear regulator, we should connect external power at least 1.25 ~ 3V bigger than the output voltage (+12V) because of IC dropout considerations, (seek for more information in LM7812 Datasheet)

• S1 - On/Off switch. It's a simple fixed position SPST switch, which "closes" the circuit, when in enable position.
• C1, C2, C3 - Bypass capacitors. Since voltage on cap cannot change relatively "fast" through time, noises that are generated on the power lines, cannot charge the capacitors because of their frequency, and potential on these lines remains stable.
• R1, D2 - Power indicator. It allows current flow through LED, thus this is our indication, whether device is enabled.

2. Pre-amp Circuit

The circuit may seem more complicated than the previous one, because of its large number of components. But it isn't. Let's see the description:

2.1. Input Cascade

Desired instrument has to be plugged to 1/4" mono input jack. GUITAR_IN net is connected to the select switch in order to conduct input signal straight to the power amplifier (We'll discuss it later). R16, C9 - Form an audio input filter, it is hard to explain it's affections on sound in technical way, (didn't have time to research voltage-frequency characteristics) but it makes input signal sound different.

• Since we have +12V single supply at the power, we have to to implement a bias circuit - input signal is delivered as AC, and has positive and negative parts.
• Task of C8 - to prevent DC voltage being held on the input line. So signal "crosses" the capacitor and "rides" on 6V bias voltage.
• R15 and R17 - provide additional Op-amp input resistance, since LM741 is based upon bipolar junction transistors, it's input resistance isn't infinite. So Op-amp would absorb less current from input cascade.

2.2 Amplification

After the signal is prepared and biased, amplifier appears as the next stage. since LM741 Op-amp is connected as a non-inverting amplifier we can define the gain:

[A] = Vout/Vin = (1 + Z2/Z1), where: Z2 = (ZC4 || R9), and Z1 = (R10 + ZC5).

ZC4 - Impedance of C4. ZC5 - Impedance of C5.

Common issue when designing a single supply amplifier, is when it comes to the point that "What are we actually want to amplify?" In simple terms, If input to the op-amp is biased and riding on the +6V DC voltage, according to the basic op-amp topology, total signal will be amplified by [A]:

Vout = (6V + Vin) * [A].

In order to amplify the AC signal only, C4 is placed as the electric potential guard. Its task is to charge to the +6V at the initial state and do not allow DC pass through gain-defined impedances (Z2 and Z1).
Desired output signal of the pre-amplifier has to be AC and not biased. So the task of C6 Cap is to remove DC offset, contained in the op-amp output voltage.

2.3 Distortion Circuit

Circuit is based on a very basic idea of clipping diodes. The first time when I saw implementation of the clipping diodes in op-amps, I could barely imagine how that thing operates. But, the idea is very simple: since only the AC signal is being amplified, voltage drop on the [Vout - V(-)] increases. Thus, when the op-amp output signal amplitude is greater than V(-) more than VD (Diode voltage drop) signal is clipped

Vout > V(-) + VD : Clipping region.

I've used classic 1N4007 Diode with ~0.7V voltage drop characteristics. May be substituted for any other distortion sound types. Also: signal is delivered as AC, so we need to place two diodes, for positive and negative wave parts.

2.4 Tone Control

The idea is based on Stupidly Wonderful Tone Control 2 - The circuit is fully passive, operation of the circuit is described in attached article - see the link above.

3. Volume Indicator

Volume indicator, as it was mentioned before, is based on the input signals amplitude comparison with fixed voltage "points" divided into groups, so every peak "point" voltage, when is exceeded, appropriate LED is enabled. Let's see, how device can measure amplitude of the audio signal:

3.1 Peak Detector

First part of the volume indicator circuit. Main target of the peak detector is to provide approximate constant voltage appropriate to amplitude of the input signal.

AMP_IN net is connected to power amplifier's input, so we can measure the input signals amplitude from any input - either "BYPASS" or "PRE-AMP" mode. LM324 is connected as non-inverting type amplifier. Like the LM741, LM324 also powered by a single supply (+12V), but we don't need bias circuit here; negative side of the input signals wave is neglected, signals amplitude is derived from the positive side of the wave. When signal is amplified, at the output of U4D we get only positive voltage part. The input signal amplitude may be very small, especially when device is in the BYPASS mode, so there is need for some amplification. In this case, U4D is connected as a non-inverting amplifier, its gain [Ax] defined as follows:

[Ax] = (1 + R23/R21) = (1 + 100K/10K) = 11

U4C receives the amplified half-wave input signal, and acts as pure peak detector: Since there is a voltage drop on the diode D8 (~0.7 in our case), op-amp makes effort to force V(-) become equal to V(+). Thus, V(+) will be equal to V(-) and output pin of U4C achieves following state:

V(U4C output) + V(Diode voltage drop) = V(U4C inverting in [-]) = V(U4C non-inverting pin [+]).

Capacitor is charged by U4C output current. Since there is almost no "significant current" at the input pins (See LM324 datasheet for current leakage at the input stage), only way for capacitor to discharge - via R19: When U4C output amplitude decreases, capacitor voltage drops to the new lower value, being discharged with current drive through R19. C10 and R19 value were chosen empirically - I merely conducted an experiment, at which values PK_OUT line will show us relatively "stable" voltage. We need almost-DC voltage on PK_OUT, so cut-off frequency shoud be low. The calculation:

f(-3dB) = 1 / (2*pi*R19*C10) ~ 0.048 [Hz] ~ DC area.

As noticed before, non ideal op-amp has non-infinite input resistance, and we do want to decrease current that will flow into indicator circuit, so R20 adds resistance to the IN(+) of LM324 op-amp.

3.2 Voltage Comparison Circuit
I have a large amount of LM324 in my stock, so I used it also on the comparison circuit.
Prepared input signal on the PK_OUT line is tied to the set of op-amps, which are used as a comparators. Each comparator has fixed voltage "point" reference so PK_OUT value is compared to all comparator reference points. As we can see, There are a simple voltage divider network, which defines specific voltage for each comparator: Vr(0)..(3). I recommend using of 10K or greater on the voltage divider network, in order to reduce current drained from the comparators reference voltage output... And the calculations:

Vr(3) (U2D) = VREF
Vr(2) (U2C) = VREF * (R7+R8+R13)/(R7+R3+R8+R13) = 0.75 * VREF
Vr(1) (U2B) = VREF * (R8+R13)/(R7+R3+R8+R13) = 0.5 * VREF
Vr(0) (U2A) = VREF * R13/(R7+R3+R8+R13) = 0.25 * VREF

3.3 LED Outputs
There are total of 4 LEDs of volume indicator output. each one has a resistor connected in series to the LED, so its current is limited to the desired level. Since when one comparator switches to the "ON" state, output voltage switches to the 12V. I wanted every LED operate in same brightness so current able to flow through every LED is the same. The calculations:

I(D) = [V(OUT) - V(LED)] / Rx ~ (12V - 2V)/1000 = 0.012 = 12 [mA]

4. Control Switch
Control switch allows us to choose signal to be amplified at the output stage. Switch's upper pin is tied straight from the guitar input jack - The switch will be called "BYPASS". Second pin is connected to the pre-amp circuit output, so we can choose whether apply any of the effects on the signal (Distortion, tone change, etc.). Switch's throw pin is tied to the input stage of power amplifier.

5. LM380 Based Bridge Power Amplifier:

The idea of choosing this particular IC amp, came from literally nowhere. I just wanted to build my guitar amp as fast as I can, and it was the only IC I had at the time.

The schematic diagram is very simple:

Switch "throw" pin is connected to the potentiometer, it's opposite pin connected to the ground and the wiper pin is connected to the LM380 ICs inputs. Notice, that 100K potentiometer will be used as volume controller. the LM380 ICs are connected in a Bridge-Tied Load mode. Let's understand the idea:

Sound signal with a very little amplitude is amplified by both ICs, but second LM380 acts as Inverting amplifier - thus at the outputs of the two ICs we have outr amplified signal but with opposite polarity. By connecting load to these outputs, load's differential voltage doubles, thus we have double maximum power on the load. Proof:

P(differential) = 2*V*I = 2*P(Single) = 2*2.5 = 5W

The differential output (J3 Symbol at the schematics) is the audio output from an amplifier. I have used 1/4" Phono jack, so I can simply connect my output speaker straight to the amp.

6. Terminal block map:

Terminal blocks are used as interface between front/back panels and soldered circuit board. Notice that not all the nets are present on the TB map: For example, power indicator nets are combined with back panel components, so there is only need to conduct VCC and GND nets from back panel to circuit board. Probably, the best way to arrange connections, is to make TB per each component, and complexity of assembling process would drop drastically.

(Bypass capacitors and output filter recommendations. are well described in the datasheet)

Probably, soldering is the most entertaining part in projects like this, especially when it comes to the audio circuits.

Foremost, it is crucial to think through the soldering process, and work out components placement. For me, it was much easier to divide whole circuit into groups, and solder it by parts:

Power amp > Pre-amplifier > Volume control > Power supply > Terminal blocks > Cutting the board.

Let's describe the steps:

1. LM380 Power amplifier circuits: It's the last but not the least in the schematics, but first one that I soldered. After completing this step, there will be possibility to check circuit operation. practically - solder a couple of pairs of wires, and hear what we have at the output.

Important: Reserve some space for output filter resistor. it has much bigger geometrical volume than other resistors in the circuits.

2. Pre-amplifier circuits: Circuit with greatest number of components in the project. At this step, it is strongly recommended to place components without soldering them yet, and check the most convenient way to solder interconnections between them.

3. Volume control circuit: It is easy to imagine where every component should be placed at that step, since we have single IC of quad operational amplifiers. LEDs are present at the front panel, so there is need to solder a terminal block throughput for each LED (How we connect front panel with the board is explained in the next section). Voltage bias supply (4.5V, see in the schematics the U4A part) is connected to the U2 (Voltage comparators) so don't forget to connect between the two ICs.

4. Power supply: the last circuit that will be present on the board. I didn't solder it at the first place - I just soldered two wires to the power pins (VCC and GND) and supplied power from the external power supply, in order to check initial amp operation. The bypass capacitors should be placed close to the LM7812 IC.

5. Terminal blocks: The interface between front and back panels and freshly soldered board. TBs should be soldered around desired board area. After soldering all the connection wires, it is recommended to make a map of TBs connections, so when we get to the assembly step, we can easily understand the wiring (See example of the TB map in the schematics step).

Box enclosure preparing was a tough process for me. Well, I had a limited instruments set, so the only way to make it look somehow good, before any cutting job make sure your circuit will be fit according to enclosure volume, with some additional depth, for cables and interconnecting wires - there will be plenty of them.

First of all, mark the zones that will be cut out, I strongly advice to draw a "cross" on each hole area - that will be a drill reference for each cut.

The drill list:
1. 2 x Circular 1/4" Audio I/O holes
2. 2 x Circular SPST switches holes
3. 4 x Circular LED volume indication holes
4. 1 x Circular LED power indication hole
5. 1 x Circular SPDT switch hole
6. 3 x Rectangular slider potentiometer cutout regions
7. 1 x Rectangular power jack input hole

A. Holes for LEDs are good to start with. I just measured diameters for each bit and LED. If sizes match well, probably there will be no need to add glue at the LED areas.

B. 1/4" Audio I/O holes are much bigger than LED's, so if you don't have any large diameter bit, cone drill bit will solve the problem. Before drilling, there is need to measure 1/4" jack diameter and determine maximum drill depth. In order to avoid exceeding jack diameter, draw a circular line on the cone bit, that can be seen, so mistake probability drops. If you did exceed jack diameter, it can be repaired with a hot glue - will be described in the "Assembling" section.

C. Let's get to the hardest part - rectangular cut areas. First of all, it is recommended to mark the areas for slider potentiometers, with respect to knob sizes. I used sequential method:
1. Drill some holes with diameter equal to the rectangle width.
2. Cut plastic residues between holes.
3. Grind the areas, until it looks something like rectangles.

And that's it. Any additional drill for general purpose I/O or custom power input is up to you.

This is the final step in our project, all the hard stuff is done let's focus on what we have to do:

Front Panel: Contains 3 slider potentiometers four LEDs for volume indicator and Distortion On/Off switch.

• Potentiometers - must be placed at the fixed positions, each wiper should have it's own moving space. After the placement is done, potentiometers should be glued with the hot glue.
• Distortion On/Off switch - that I used has its own side plastic clamps so there is no need to hot-glue it. An insertion should be sufficient.
• LEDs - Before we proceed to place them on the front panel we have to prepare them: solder colored wires and masking. Solder the black wires to the cathode and the red wire to the anode of each LED, so it is easy to understand the front panel connections. After soldering, place a shrinking tube on all the solder joints, warm it up with a blower or a lighter.

Back panel: Contains power connector, 1/4" female audio I/O interface, power-on indicator and power On/Off switch.

• Power Connector - Circular shape of connector is strongly recommended - there will be no need in sharpening the slot. After slot behind the back panel is ready, glue the connector with the hot glue.
• 1/4" Audio I/O interface - After holes of the appropriate diameter are done, insert connectors, if you are lucky and you have screw-nut for each connector, use it. If you don't, glue it with the hot-glue.
• Power-On indicator - LED should pass the same procedure as the front panel LEDs.
• Power On/Off switch - Same switch type as the one on the front panel. I made a little mistake, and drilled excessive diameter. So, I used hot glue to solve my problem.

Wires & Nets: After All the parts are roady, there is need to solder the wires to each net that is present on the front/back panels. Since we have TB for each wire coming from the top/bottom case, it is recommended to place small stickers with names of nets they are connected to - In the case of the fault, it will be much easier to troubleshoot in the future.

Label stickers: To make it look more cool, I've added labels on each element on both panels (Turns out I had label-printing device by BROTHER at home).

Knobs: Design contains slider-pot knobs, something from the eBay.

Speaker set: Because the output signal comes from the 1/4" female jack connector, I soldered 1/4" male jack to the speaker wires (See the image above).

After everything is completed, connect ALL the wires from back and front panel to the TBs of circuit board, adjust circuit board to the fixed position and close the device. After all the screws on the corners are fastened, we can officially declare that our project is finished.

When the project is done, testing it in a lab conditions is a pretty good idea.

Input: Audio input is connected to the function generator, with an audio frequency range capability : 20Hz-20KHz. Sine waveform and a "small" amplitudes range 20mV to 700mV.

Power supply: If you don't want to use your DC wall adapter yet, for the testing step, external power supply with DC current indication may be very useful - in the case of the fault and troubleshooting as well.

Output: Audio output is connected to 8 Ohm speaker with minimum 5W power availability. If you want to connect with oscilloscope to the output, don't forget that device's output is differential, and if your oscilloscope tied to earth, it may cause short circuit on one of the device's outputs. In this case, connect oscilloscope's ground to the device ground, take 2 probes and subtract second channel from the first (Math between channels as defined in the scope).

During the testing process, it is much easier to troubleshoot the problem and understand the origins of it. (Bypass caps, bad solder joints, etc.)

Hope you find this instructable useful,

thanks for reading!