DIY bluetooth speaker

Ok i will have to do some research as far as speakers go.
I have a funny question about batteries. I have some bad batteries that will give me almost no mAh when discharged with 3A (fall under 2.8V to quickly) but give me about 1500mAh when i discharge them with 1A. Now before i begin, i know those batteries are dangerous, bad and should be discarded. But now i would like to verify if my calculations are ok. Now lets say we would like to use those batteries to power a 2x25W speaker and we put 5 cells in series to get 21V. Now if we drive the amp to theoretical maximum that would mean over 2A drawn right ? 21V * 2.4A = 50W. Ok so this wont work, cells will go under 2.8V to fast and BMS will disconect the batteries. Now lets continue, we put pairs of 2 cells in parallel and 5 pairs in series, so thats 10 cells together in 5S2P. So now we still need that 2.4A, but we have 2 cells that have to supply that, meaning each cell only has to produce 1.2A. And that means we can drive the speaker (until batteries ignite and burn the speaker that is). So did i calculate this correctly ?

The output power depends on the impedance of the speakers and how much clipping distortion you can survive.
Your TD7492P amplifier will overheat if it drives 4 ohm speakers with a 20V supply but is fine driving 8 ohm speakers at 18W per channel at low distortion.
The efficiency is 90% so an additional 4W is heat for a total power from the battery of 40W. Then the max RMS current is 40W/21V= 1.9W.

You never play music or speech continuously at full blast, the momentary loudest parts can be 36W at low distortion but the average power is much less, maybe 8W.
If you turn down the volume then the average power is much less than 8W.

I know all that but for the purpose of question i said that usage is 50W, lets not go into whether amp can do that or not. SO, 50W power, 21V, 5S2P batteries. Did i calculate correct about 1.2A per battery ?

With an output of 50W (with severe clipping distortion), the heating is 5.5W so the total power from the battery is 55.5W. At 21V then the RMS current is 55.5/21= 2.64A. Each battery in the paralleled pair will need to produce an average of 1.32A.

Great, thank you ! I just hate giving up on things and those batteries, i simply dont have the heart to throw them away when they have 1500mAh in them.
Btw i decided i will have 2 speakers, 1 smaller one and 1 bigger one. Because the place we go to, security guys keep saying our music is to loud. So there is no point having 2x25W speaker running it at small volume, because that speaker is heavy and bulky. So i plan to do a really small speaker, just a little bigger then a can of beer and i will use that neat 2x6W amp that is plenty powerful. I might just use 4 of those bad batteries in parallel.

I disconnected the 3" speaker in my clock radio and connected a two-way speaker with a 6.5" woofer, a 1" tweeter and a port. I needed to increase the values of a few coupling capacitors for good bass and now it sounds great. Its output is about 1W at low distortion but I never play it that loud.
I bought two of those Sylvania clock radios at an electronic products surplus store (Sayal) for $1.75CAN each.

I bought a couple of 10W speakers a while back, they are similar to my 30W speakers, actualy i think dimension and everything is the same, only that they are 10W and 4 ohms. Which is great cause that little amp needs 4 ohms. The speakers could be smaller but since i bulk bought them second hand very cheap, i need to use them.

I would like to add some things to the new speakers i make. Keep in mind, small dimension and low price is key here (otherwise like i said .. i can get JBL charge 4 for 95€ second hand and it will sound a lot better then my speakers).

So now i have a few questions, i will use numbering so it will be easier to answer:

1) Lets have a situation, i put my phone to 100% volume, there is no buttons on this amp to control volume. And lets say there is now distortion cause amp cant output enough power. Is there a way i can somehow limit this so that when volume is 100%, it is less loud and not distorted ? Maybe using a resistor or something else ?

2) Lets say i have a small speaker i made and now i would like to improve bass as much as i can. I make 2 holes and put passive radiator inside. Is there anything else that can be done or something else that has to be done or is this as much as i can do with limited finances ?

3) i saw a cheap 2$ board on ebay but i cant find it anymore. Anyway the way i saw it, it was like this. U connect amp speaker outputs to this board. And then this board to speakers. And this board has a knoble that allows you to set how much bass and treble you want. Would this be a viable option to improve bass or not ?

Bump ?

SentinelAeon said:

1) Lets have a situation, i put my phone to 100% volume, there is no buttons on this amp to control volume. And lets say there is now distortion cause amp cant output enough power. Is there a way i can somehow limit this so that when volume is 100%, it is less loud and not distorted ? Maybe using a resistor or something else ?

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Is the phone sending wireless Bluetooth audio to the amplifier? Then turn down the phone.

2) Lets say i have a small speaker i made and now i would like to improve bass as much as i can. I make 2 holes and put passive radiator inside. Is there anything else that can be done or something else that has to be done or is this as much as i can do with limited finances ?

Click to expand...

A cheap small speaker produces no bass.

3) i saw a cheap 2$ board on ebay but i cant find it anymore. Anyway the way i saw it, it was like this. U connect amp speaker outputs to this board. And then this board to speakers. And this board has a knob that allows you to set how much bass and treble you want. Would this be a viable option to improve bass or not ?

Click to expand...

Tone controls need extra amplifier power that would burn out your budget and burn out your cheap little speakers.

Parts Express online is a good place to find good speakers.

audioguru: Turning down the phone is 1 option but still it would be great knowing u can just crank it to 100% and there wont be that horrible distortion. A little distortion is fine, i even heard it last friday on a 300$ sony bluetooth speaker and i dont even think it was at 100%. So i am fine with that, what i am not fine is distortion so bad u can barely hear the song. And if there is a chance to prevent it with resistors or something, i would like to try.

Just add a dual-gang logarithmic pot between the bluetooth board and amp.

Use one half for each channel, with the anticlockwise end connected to the audio ground, clockwise end connected to bluetooth board out and wiper contact (middle, usually) connected to the amp in.

That's a standard "volume control" configuration. Either 47K or 10K should be a reasonable value.

eg.
**broken link removed**

Viewed from the back, though a dual gang will have two sets of contacts & you use one set for each channel:

That might be the problem since amp and bluetooth are on the same board :/

SentinelAeon said:

That might be the problem since amp and bluetooth are on the same board :/

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Doh... I'd lost track of exactly what amp etc. you were using...

ANY amplifier (including the amplifier in your phone) causes severe distortion (clipping) when its output level is trying to go higher than it can go. Simply turn down the input level so that the peaks are not clipping or use an amplifier that produces as much loudness as you want.

Your phone can also produce extreme clipping distortion at max volume so that you can turn up the volume of faint sounds.

I didnt know that about the phone .. that changes things. I thought i can put volume to 100% at phone and somehow block my speaker to only do 90% of what it can. But if 100% at speaker will produce distortion no matter what i do, no point.
Btw the reason i wanted to limit that is simple: sometimes other ppl connect to my speaker. And its kind of annoying explaining every single person that it should only put volume to 90%. Its easier for them to just connect and put volume all the way up.

Your system has too many volume controls. If the volume control for the amplifier is set too low then the phone volume is turned up too high then the phone produces distortion which the amplifier produces even if the amplifier output level is nowhere near max.

U misunderstood me. I have no control on amplifier right now to set volume. The only control for volume i have is my mobile phone.

Doesn't the Bluetooth/amplifier module have pushbuttons for its volume control?
Which of the 236 posts shows the Bluetooth/amplifier module?

**broken link removed**

Which of the two amplifiers are you using? It looks like one has pushbutton volume controls.