PC Speaker Amplifier

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Introduction: PC Speaker Amplifier

This is small power (Less than 10Watt) transistor amplifier using LM386 and TIP41/42.

Although output power is not much impressive, still it can well serve as an amplifier for PC speaker and MP3 player.

When living in a packed apartment together, a half output power from this amplifier easily produce complains by my family.

Anyway, it can drive 8ohm and 4ohm speaker with maximum 12V power supply.

I got the original schematics from the website (http://www.bristolwatch.com/radio/lm386_power_amp.htm, Lm386 Audio Amplifier Adding Push-Pull Output Stage).

As the circuit not using dual polarity (+/-) power supply, overall circuit complexity is not much high and compact size (15cm(W) x 10cm(D) x 5cm(H)) of chassis can be used as shown in the picture above.

I had made several amplifiers with original schematics and the one of them shown in the picture above is the final version which applied slight modifications from the original.

Step 1: Previous Amplifier Version

This is old version of amplifier made according to the original schematics.

It use TIP31/32 transistors as push-pull output stage..

I’m using common LM7812 voltage regulator circuit and 220V(in)/15V(out) wall adaptor as power supply because the amplifier circuit requires less than 1A current during normal operation.

It’s quite satisfying as output level is adequate enough to drive any of 8ohm or 4ohm speakers I have.

Sound quality is also reasonable when comparing with commercial audio amplifier I used before.

But it seems a high pitch noise is coming out when hearing closely to a speaker.

Maybe LM386 amplifier IC seems to produce high frequency of hissing noise together with normal amplified audio signal.

Therefore, this amplifier is not frequently used as hearing several hours usually make me uncomfortable due to the high pitch sound coming out from a speaker.

And sometime RF (radio frequency) burst pickup happen when motorcycle is passing by near my apartment with big noise.

***

I had searched internet to reduce high pitched hissing and occasional RF pickup altogether.

The schematic below is the outcome which is applied with some modifications recommended in several web-pages.

Step 2: Circuit Schematics

As I’m not good at analogue electronics, a scientific explanation is not possible for the modifications I made in the schematics above.

But result is quite satisfying when I’m listening MP3 playing and hearing audio output of videos for several hours with modified circuit of amplifier.

As audio quality is very subjective matter according to a personal viewpoint, the remedying actions above will not be suitable to somebody.

But anyway no more RF pickup and also high pitches of troublesome noise is disappeared finally.

The reasoning of the addition and removing of electronic components are as follows.

***

- Applying low (100uF) and high (0.1uf) frequency bypassing capacitors are recommended to the power supply line of LM386 to remove noise inlet to the amplifier IC

- Reducing gain of LM386 (make open pin 1 and 8 to fix gain as default 20(26dB)) help to remove high frequency noise is also recommended in other web-pages.

- And finally add one more ceramic capacitor (0.1uF capacitor which is numbered as 3 in the schematic above) to LM386 output is claimed for removing any high pitch noise altogether as the ceramic capacitor is acting as low pass filter

***

All above recommendations I found in web-pages are applied and tested one by one to produce the final schematic shown in the picture above.

Firstly, I don’t think the one more add-on of ceramic capacitor (number 3 portion in the schematics) to LM386 output as good idea.

Because possibly the capacitor may remove some of useful high frequency audio signal from the speaker output is very reasonable suspicion for anybody.

But the capacitor addition becomes quite effective solution to remove RF pickup and high pitched hissing sound from audio output in the end.

Step 3: Wiring Drawing

As stereo output is required, two amplifier circuits are positioned and wired on universal PCB board.

When comparing schematics and wiring diagram together, you can see each wiring depicted in the schematics is matched with wiring pattern in the drawing above.

Similar size of each electronic component is depicted, located and wired together with other components in the wiring drawing.

To reduce overall wiring length, not oblong and slanted wiring pattern is used.

And orange colored lines are wired and connected on the top side of PCB.

Meanwhile other red/green colored lines are wired and connected at the back(soldering) side of PCB.

Step 4: Parts

I can’t depict and explain each component one by one in the picture above.

But the most of note worthy components are described in the picture.

The detail BOM (Bill of Materials) is described in the list below. (Cost of only important component is written. But the cost information is provided just as indicative)

***

- LM386 amplifier IC x 2 (About 1$)

- TIP41 (NPN transistor) x 2, TIP42 (PNP transistor) x 2 (About 1.2$ for each)

- 1N4148 diode x 4 for biasing transistors as Class AB

- LM7812 voltage regulator (Amplifier power supply)

- ALPS blue velvet 20K potentiometer (Volume control, dual 20K VR included, 10$)

- 1000uF electrolytic capacitor x 2 for filtering DC from audio output

- 100uF electrolytic capacitor x 2 for bypassing low frequency noise from power line

- 10uF electrolytic capacitor x 2 for bypassing power with LM386 IC

- 2.2uF electrolytic capacitor x 2 for coupling audio input to amplifier circuit

- 0.1uF ceramic capacitor x 6 for power filtering and high frequency noise suppression

- 0.33uF film capacitor x 1 for LM7812 regulator noise filtering

- 0.047uF film capacitor x 2 for output stabilizing (Zobel network)

- 2.2ohm 1/2W resistor x 4 for transistor loading

- 1K 1/4W resistor x 2 for transistor biasing

- 10ohm x 2 for output stabilizing with Zobel network

- Speaker cable wiring terminal block (4 Pins, 3$)

- 3.5mm stereo audio input socket

- Circular power inlet socket for 15V wall mount power supply adaptor

- Universal PCB board about 15cm (W) x 10cm (D)

- Acrylic board x 4 (15cm (W) x 10cm (D) x 5mm/3mm (H))

- Metal supporter M3 size (bolt/nut) 3.5cm x 4

- 2 wire cables (rating 5V and more than 2A)

***

Transistor matching is recommended in the web-page where the original schematic is posted.

For better audio quality, usually transistor matching is required for supporting identical physical characteristics of NPN/PNP transistors.

But as matching process is a little bit tricky, I will not mention details in this story.

Step 5: Wiring and Soldering

Tin wires (AWG 24 size) are used for making wiring patterns as depicted in the schematics and wiring drawing.

Several jumper cables are used due to the mistakes made during soldering.

As soldering method is explain in other instructable (https://www.instructables.com/circuits/raspberry-pi/projects/recent/), I will not describe details in this story.

But basically wiring and soldering is performed according to the details as shown in the wiring drawing.

As seen in the picture above, various cables are connected to the amplifier including such as stereo audio cable, 2 wire speaker cables and 15V power supply cable.

Step 6: Playing and Further Development

As amplifier making is finished, let start to listening some music with it.

The speaker shown in the picture above is Scandyna MicroPod SE which was bought about 10 years ago.

Now audio cable connection model is changed into Bluetooth and still the same shape of model seems available for buying.

Personally I’m supposing the speaker’s technical specification and performance is more important than amplifier for audio quality.

The speaker’s technical specification is as follows.

***

- Applications Hi-Fi stereo, AV-Home Theatre systems

- Amplifier requirement 10 - 100 watt

- Nominal impedance 4 Ω

- Frequency Response 65-20.000 Hz (± 3dB)

***

I described the usage of this amplifier for PC speaker.

But it can be interfaced with diverse audio sources for playback music or video.

You can view the video of the amplifier operating in the following link.

*** <Video Link>

https://drive.google.com/file/d/131MuCqJzu-P7cf5pM...

***

As recording is performed by smart-phone, audio quality is not much recognizable.

Anyway I’m using this amplifier as basic device for playback any multi-media contents with PC , Raspberry Pi server, smart-phone and so on..

As an extension of this project, some add-on functional will be integrated to this amplifier.

Thanks for reading.

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