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I'm working in a project with the goal of make a portable bluetooth speakers without distortion. When I try to turn up the volume, the speakers start to make a distorsion, like when rhe radio can't get the correct frequency.p

In the project, I use a 3×2 PAM8403 amplifier in a tiny module, 1× 7805 and a 2N60B transistor (I asked in another forum for this problem, and they answered I have to use the transistor if I want to get more amperes and remove the audio distortion) and two speakers (4Ω, 5-8W). The amp is powered by 5 volts generated by the 7805 from a 9V battery.

I searched around the web by 3 months. Different answers and any of them works.

I hope some of you could help me removing the distortion definitely, I've been working on this project since the past summer.

Schematic Schematic of the project, done in 120s.

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    \$\begingroup\$ We can't help you unless you provide a lot more detail about your existing circuit and the nature of the distortion you're experiencing. \$\endgroup\$
    – Dave Tweed
    Dec 23 '15 at 12:33
  • \$\begingroup\$ People will ask for a schematic of your actual circuit. Also, what kind of distortion are you experiencing? Do you have a oscilloscope to see the output? Is your 9V battery a regular "PP3" one? \$\endgroup\$ Dec 23 '15 at 12:35
  • \$\begingroup\$ And provide a link to the PAM8403 amplifier or give the basic specification in the post. You are asking us to do the work that you should have shown in your post - hence your downvotes for a poor question. \$\endgroup\$
    – Transistor
    Dec 23 '15 at 12:43
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    \$\begingroup\$ "When I try to turn up the volume, the speakers start to make a distorsion, like when rhe radio can't get the correct frequency". How loud can it get before becoming distorted, and what exactly does the distortion sound like? These questions may be impossible to answer in words alone. Can you video the circuit in operation and upload it to YouTube? Also what speakers are you using? Post photos of speakers and the complete setup. \$\endgroup\$ Dec 23 '15 at 16:12
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    \$\begingroup\$ The transistor in your power supply is doing nothing. The 7805 IN pin should be connected to the point where the transistor's base meets the 10R resistor. \$\endgroup\$
    – brhans
    Dec 29 '15 at 15:45
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If you read the PAM8403 datasheet, it tells you on the first line of the 'Features' section that it promises terrible distortion.

3W Output at 10% THD with a 4 Ω load and 5V power supply

It seems that it is working to specification.

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  • \$\begingroup\$ Now I know distorsion is generated by the AMP (I don't understand english perfectly, sorry), there's some amplifier in a IC (3x2) that doesn't generate distorsion? I searched everywhere, I didn't found it. \$\endgroup\$
    – spund3
    Dec 23 '15 at 13:03
  • \$\begingroup\$ @spund3 distortionless amplifiers DO NOT exist. \$\endgroup\$
    – Andy aka
    Dec 23 '15 at 14:51
  • \$\begingroup\$ @Andyaka And what about Xiaomi Speakers? No distorsion, 3x2W. \$\endgroup\$
    – spund3
    Dec 23 '15 at 14:52
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    \$\begingroup\$ @spund3 distortionless speakers DO NOT exist. If they say they do then you are talking to or reading something that a salesman or marketing man said, \$\endgroup\$
    – Andy aka
    Dec 23 '15 at 14:54
  • \$\begingroup\$ There are no such things as distortionless wires if you dig deep enough. \$\endgroup\$
    – Andy aka
    Dec 23 '15 at 14:55
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If you are powering that thing from a typical 9Volt battery, then of course you are going to have distortion.

3W Output at 10% THD with a 4 Ω load and 5V power supply

A 9Volt battery cannot supply any really large amount of current. They are typically used for loads under 100mA.

This guy ran some tests.

If you put a heavy load (more than 100mA) on a 9Volt battery, the voltage drops.
If you really load it, the voltage will drop so far that your 7805 can't regulate the voltage any more - the supply voltage to the amplifier will drop and you will get really horrible distortion.

To put out 3Watts at 5volts you are going to need 600mA of current.

See the chart for 500mA? At that current level, the voltage drops almost immediately to less than 8 Volts, and very soon is down to less than 7 Volts. The 7805 needs at least 2Volts between in and output, so when your battery gets down to less than 7 Volts it won't work right.

enter image description here

You need a different power source if you are trying to get 3Watts out of that amplifier. A 9Volt radio battery will NOT do it. You would do better to put several (4) AA batteries in series then use a low drop out linear regulator, or use several AA batteries in parallel then use a boost regulator to get 5Volts for the amplifier.


Added after comment about having tried other voltages and power sources:

The data sheet for the PAM8403 has a chart showing distortion as a function of power output and input voltage.
For 5Volts and 3Watt, you get 10% distortion.
For 3.3Volts and 1.5Watt you get 10% distortion.
If you put a heavier load on the amplifier, the distortion goes up. To lower the distortion with that amp, you need to use the highest voltage you can and stay below 2Watts of output power. At five volts you will need to use a speaker with higher than 4Ohm impedance.

Another thing you may be missing is the gain. The PAM8403 includes upto 24dB of gain. If you are using maximum gain (which you would be if there is no resistor between the bluetooth output and the amplifier input) then you will be amplifying the level by a factor of more than 10. Assuming a line level output from the bluetooth module, you will get really bad distortion playing at a normal volume. Test by putting a resistor in series with the inputs to the PAM8403.

If the problem is the gain, then you can set the gain by changing the input resistor. The total gain is 2*(142000/R) where R is the value of your series resistor.
Since you don't currently have an R at all, the amplifier is just running wild, amplifying for all it is worth - and distorting like crazy.

To get a gain of 1, use a series resistor of about 284KOhm (next standard size is 287K, which will do just fine.)

You also need to make sure to do all the stuff around it to make it work right

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  • \$\begingroup\$ Before using the 9V battery, I used a 3'7V and then a DC-DC (3'7 to 5V), and I got distorsion also. Do you mean using that? \$\endgroup\$
    – spund3
    Dec 23 '15 at 13:23
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That isn't distortion, that is noise caused by a ground-loop. Like others mentioned, a 9V is not capable to deliver a huge amount of current. Use an USB powerbank or such to provide power. Besides 9V is a way to high to power the amp (and also I guess the BT-module), the amp accepts 6V at a maximum, better is to use 5V.

The problem of the noise is a cause of using the same power source and they share the same ground and the BT-module processor creates a differential on the ground plane (switching noise) and also it is a class-D amp that is also switching at a specific frequency. Because the millivolt audio output/input is also on the same ground, that is the noise you will notice.

There are several things you can do:

  • Use two batteries, one for the amp and one for the BT-module

  • Use filter/buffer capacitors as close as is possible to the power input on the modules, for example 220uF on the BT-module and 1000uF on the amp module.

  • Use an isolated DC/DC converter on the BT-module such as the mornsun b0505s-1w

The isolated DC/DC converter is king to reduce/eliminate ground-loop noise.

Hopes it helps.

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