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DIY bluetooth speaker

There is no buzz if i use batteries. But now i am using it with DC power adapter to serve as my main computer's amplifier. I am using 18 ohm resistors in series with my speaker to reduce loudness :)


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But now i am using it with DC power adapter to serve as my main computer's amplifier.
It's almost certainly ground noise; commonly that is due to some AC leakage from the PSU, influencing the ground on the amp relative to the PC ground, and that adds in to the audio fed to the amp.

Or if the PSU is also grounded, you get a "hum loop" - similar effect, but with inductive pickup influencing the ground voltages.
I have something to report regarding the problem with my bluetooth speaker using the button. I seem to have found a simple workaround that will solve the issue, at least for now (if ur interested in the issue, i described it in the attached image). Btw in the meanwhile, i noticed that even other buttons produce the same problem. Like i said, the issue described in an attached image.

The solution:
So what i did was attach a simple 100W 1.5Ohm resistor in series with the power going into the amplifier. This reduces the voltage coming into the amplifier. The difference is small, 25.5V on my bms, 25V on my amplifier. Thats a 2% difference. Does that mean that i lose 2% of loudness and 2% of battery life ? Because if it is, i can certanly live with that, if it solves the issue.


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I am still absolutely stunned how there can be such an issue. My BMS is rated for something like 6A if i remember correctly. 25.2V and 6A thats like 130W. The amplifier only has caps if im seeing correctly, each rated like 300uF. A fart can charge that capacitors. So the fact that obviously some kind of protection kicks in is amazing. The cells i am using arent new, they arent great, but mind you, they are rated at 10A and i am using the same cells in my 4S electric drill, granted i am using different bms with much higher amps, something like 40A, but just saying that if it was cells fault, they wouldn't work in a drill. Also the way the protection is triggered ... if its over amp protection, the speaker should return to normal after a while, no ? Why do i need to connect the charger in order for it to work again.


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The BMS is designed to protect the battery from too much current when charging but not for charging the large amplifier capacitors. The 1.5 ohms resistor reduces current and voltage to the battery and amplifier.

The reduction of current and voltage by the resistor also reduces the maximum output power and maximum heating of the amplifier.
Could you verify my numbers ? If resistors drops voltage from 25.5V to 25V, thats 0.5V difference which is about 2%. Does that mean 2% less loudness and battery life, since 2% of "power" is used to heat the resistor ?


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Your amplifier will sound "odd" if you add a 1.5 ohms resistor in series with its positive power input.

When your bridged amplifier has a 25.5V supply and 8 ohm stereo loads then its maximum undistorted output is about 35W per channel or 70W stereo. The class-D heating is about 10W so the power from the supply is 80W.

The power supply current is about 80W/25.5V= 3.1A. 3.1A in the 1.5 ohms resistor is 4.7V then the max undistorted output power is more than 23W per channel (46W stereo) since the resistor voltage drop will be less than the 4.7V calculated. Power wasted in the resistor is less than 4.7V x 3.1A= 14.6W.

You will not notice the difference in the small loss of maximum volume and the small increase in the time of a battery charge is used.
I am using 4 ohm woofer and 8 ohm tweeter. But i am not sure i understand your calculation. I measured voltage on bms output and it is reading 25.5V. Then i measured the voltage on amplifier input and it reads 25.0V. Which leads me to belive 0.5V is lost on the resistor. What am i seeing wrong ?

Why will the amplifier sound odd if i add resistor on positive power input ? What if i add it on negative ?


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Your amplifier output power into 4 ohms with a 25.5V supply is probably 64W per channel with low distortion of 128W stereo. The heating is probably 25W then the power supply current is (128 + 25)/25.5= 6A.
the 1.5 ohm resistor drops 9V at 6A making the power supply be only 16.5V producing an output from the amplifier of 25W per channel or 50W stereo plus heating of maybe 10W.

The output power is less then the voltage loss in the resistor is less than 9V so the output power is higher than this calculation.

Your voltage drop of only 0.5V in the 1.5 ohm resistor has a current of 0.5V/1.5 ohms= 0.33A so the amplifier is far from full power in both channels into 4 ohms.

Maybe your test frequency was the speaker low frequency resonance where it is 40 ohms instead of 4 ohms?
Ok so using a resistor is not a solution. Since i cant seem to understand why it happens, my only solution then is to use another BMS. Currently i only have 4S with balance or 5S without balance. Its really a shame cause i really loved that 6S. But with this amplifiers it just causes problems.
What if i use a really small rated resistor ? Like 0.1 ohm ? I mean, wiress add some resistance too, wouldnt that mean it will also add noise ?

This BMS has 8 amps allowed. I have no idea how 2 small capacitors can trigger any kind of protection.

Do you have any idea how i could use any other common element to prevent this protection from triggering ?
Not sure if anyone is reading this or caring, but i will write anyway. I couldn't give it a rest. I now replaced every single thing in the speaker to make sure nothing is faulty. And i found another amplifier that triggers this thing even more often - not 1/3 time but rather like 4/5 time. It has 4 capacitors instead of 2. My theory is that those capacitors draw so much current quickly, that it pulls cells under 2.8 or 2.4. Dont know how since those are tiny 470uF capacitors. I actualy tried connecting BMS to capacitors of different values and i couldn't trigger the protection. I have no idea what those amplifiers have on them that is triggering the protection. Btw i tested the BMS and it handles continuus 8 amps without triggering. How the hell are those tiny capacitors pulling more than 8A is beyond me.

Good news is that i seem to have temporary solved the issue. I took a piece of thick heating wire and cut it to get exactly 0.2 Ohms. And that seems to have solved it, no matter what i do i cant get it to trigger the protection. And i took your equations for the loss and if i calculated it correctly i will be losing somewhere between 1.5 and 3W, which is nothing. I hope you can tell me some other solutions and theories because this is really interesting. Another way i solved it btw is that i put a capacitor directly on bms output. Sadly that would mean battery pack constantly losing energy even when its off. But it really led me to belive that sudden spark draws cells under 2.8 or 2.4 and that triggers the protection. I would need some kind of element that would limit the whole thing to like 6 amps. Inductor ? o_O


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A very poor quality or very old 18650 Li-Ion battery cell voltage drops to 2.5V with a current surge of only 8A.
The BMS is made to charge a battery and its protection shuts it down when a defective battery conducts more than 8A to prevent a fire. It is normal for many amplifiers to have a huge power supply capacitor that takes a massive current surge when charging.

Your idea of adding a large capacitor to the output of a BMS is good because it provides high current when the capacitors in the amplifier charge. Any half-decent capacitor has a very low leakage current and take over a year to discharge the battery.
Alright, i just put a 6S battery pack with bms and a 800uF capacitor on its output to test to see how much it loses battery charge over time. If its not much, then a solution is here.

The 0.2ohm solution that i made, as opposed to capacitor solution, why is it bad ? I would really like to know more :) Because even wires add some resistance, but we still use them :)


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A resistance between a power supply and a high power amplifier causes the voltage to jump up and down with changes in current that produces weird sounds that should not be there.
Some car radios use "super capacitors" at the amplifier to prevent it.
Ok, you convinced me. I will use capacitors, i will test them and see how much it affects my battery drainage. If they indeed hold charge for 1 year ... that would mean, once a year batteries have to charge a capacitor, which is like nothing.

I am still not sure how amplifier can draw that much current. It has 2 470uF small capacitors, its bluetooth amplifier board, small one. I tried with 2 big 470uf and 2 big 680uF capacitors in parallel and i couldn't get bms protection to kick in. And remember, i tried with really big capacitors from power suply unit, the 200V 680uF ones. The ones on bluetooth amplifiers are tiny. How.


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Since adding capacitors to the BMS output did not trigger its shutdown then maybe it is the amplifier or your battery that is no good.

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