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Can someone help me with the (electrical) design of my bluetooth speaker? And also, how can I remove speaker shielding? Answered

Hello everyone, I would like to build my first Bluetooth speaker but I like some challenge since I study Electrical Engineering. That's why I looked up a lot of different designs and combined mostly these two: from Greatscottlab and that of Barry_L. I'm pretty sure my design will work, however I still like to have some advice on how to improve it.

For example, I'm not sure if it's a good idea to connect a capacitor like the Barry_L design, because if I switch between the 3.5 mm input and the bluetooth board, the cap still contains energy which can only be discharged with a resistor. But how big of a capacitor can I use or is necessarry to achieve a good bass? I calculated that in order to make the capacitor effectively discharged in 1 s that I need a resistor of 3.125 kOhm at least. But this only applies to the bluetooth module and not to the 3.5 mm jack amp.

These are the electrical components that matter most to this question:

Power Indicator
Protection Board 
Bluetooth Module 
PAM8406
Step up module 

(I'm also planning to add a NFC tag and two passive radiators)

Secondly I sort of have a problem with my speakers, the units are magnetically shielded. I found out that this is done so there is no interference with crt screens. This makes the speakers pretty heavy and also large. Do any of you know how to remove this shielding or a better place to find a way how? (google didn't serve me well)

Btw, I didn't got a response from one of the authors of the projects, so I think here might be a better place for advice.

Discussions

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Jeremyv30

2 years ago

Okay, so I just made a new design with only one step-up module. However, on ebay I stumbled across this FM radio module: http://www.ebay.com/itm/282109051777. I know I'm making my design more complicated, but why not? (Not a lot of smartphones have FM radio and spotify isn't really a replacement.)

Since the seller didn't really provide a good diagram of how to wire it up, I made some educated guesses and labeled all the pins I thought would be important.

Diagram.pngRadio (labeled).jpg
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Jeremyv30Jeremyv30

Answer 2 years ago

I like to note that I would like to switch between 3.5mm input and radio (since I think the FM radio still needs amplification). Also if possible I would like to switch off the FM radio if either the 3.5mm jack or the BT board is used. What would be the most simple way to achieve this?

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Downunder35m

2 years ago

The capacitor acts as a filter to avoid interference and unwanted noise on the speakers.
If you don't like the shielding get speakers without it, no realistic way to remove it without causing damage.

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Jeremyv30Downunder35m

Answer 2 years ago

The capacitor acts as a filter to avoid interference and unwanted noise on the speakers.

Then why is it attached to the step-up module? I can imagine that when the voltage across the step-up module drops, the capacitor can deliver more power if needed. But I don't get why the step-up module might cause interference.

If you don't like the shielding get speakers without it, no realistic way to remove it without causing damage.

Yeah, I guess you're right. I tried removing one of the speakers' shielding and found out that if I proceeded I would have broken the speaker. So I guess I need to make a good support for them if I'm installing them.

F9B4LRXIRPPNVPS.LARGE.jpg
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steveastroukJeremyv30

Answer 2 years ago

Its a supply bypass cap. I don't understand why you think it magically stops discharging when you disconnect the input - the load will continue to run on and discharge it very rapidly.

Switching power supplies are electrically noisy, and you often need to take heroic action to stop it getting into sensitive circuits, like amplifiers.

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Jeremyv30steveastrouk

Answer 2 years ago

Certainly true, in theory a capacitor can discharge infinitely. That's why I was thinking of connecting a resistor across the capacitor of the image.

I was thaught that you can calculate the time that the capacitor will be effectively recharged with this formula: 5*tau = R*C. If I were using the same cap as Barry_L did (1600uF) and wanted to discharge the cap (effectively ofc) in 1 s, then I will need a resistor of 3.125 kOhm.

However, do you have any suggestions how to improve it, or is my design too complicated (or hardly possible) to really make?

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steveastroukJeremyv30

Answer 2 years ago

Hardly, the capacitor will discharge almost instantaneously. Where do you think the energy would come from, for the capacitor to "discharge infinitely" ? You are overthinking this.

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Jeremyv30steveastrouk

Answer 2 years ago

"Hardly, the capacitor will discharge almost instantaneously."

Whoops, I guess I need to read better before I reply... Sorry for that.

Indeed in the reality a capacitor cannot discharge infinitely, but my concerns where mostly to protect the amplifiers and that with the help of a resistor.

But back to basics... So what if I just ditched the second boost converter and buy a amp similar to the PAM8406 that can handle 6.5 V or just adjust the output of the step up to 5.0 V (which I think might be a better idea). Then put the switch behind the boost converter, will this solve a bit of the noise?

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-max-Jeremyv30

Answer 2 years ago

I think I know what you mean, a capacitor connected to a resistive load, yes, in theory the capacitor will never fully discharge, but in reality it will discharge to a negligible voltage almost instantly. So if even in theory the voltage and current never reach 0V, is that infinite energy?

NNOOOOOO!!!!!!!

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Lets first look at this thing called an "infinite sum." Did you know you can add an infinite number of numbers and still get a finite result? Imagine you take 1 + 0.1 + 0.01 + 0.001 + 0.0001 + ... forever. Do the math, you get:

1 + 0.1 = 1.1

result + 0.01 = 1.11

result + 0.001 = 1.111

...

= 1.1111111111.... repeating forever. But it's a finite number, not infinite! :)

You can see the same things happen with many types of infinite sums, another example is the geometric series or the power series.

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The speed at which the capacitor discharges (through a resistor), is proportional to the voltage across it, because the resistor draws less and less current with decreasing voltage. As it discharges, it discharges more slowly. Put another way, the rate of discharge is proportional to how far it has discharged. Ahh, THIS means we need some sort of math that allows us to work with things that keep dynamically changing over time. It's called calculus and differential equations!! :O to really fully understand what is happening. But even without that, I think you have the intuition of what's happening.

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Jeremyv30-max-

Answer 2 years ago

I didn't say anything about infinite power or energy. Since the capacitor in a RC circuit will discharge with an exponential equation and has the base e (for example v(t) = 16e^-0.125t), the graph will never reach zero and so the capacitor will discharge infinitely but 0 V will never be reached.

Hehe, my calculus book is a freaking bible...

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Downunder35mJeremyv30

Answer 2 years ago

I thought it was about the design and not some theoretical blah about capacitors that has no relevance to the problem...
To make it short: If you want good music you use a good amp and even better power supply.
This rules out boosters and such things, so why not put it together and try it?

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Jeremyv30Downunder35m

Answer 2 years ago

I thought it was about the design and not some theoretical blah about capacitors that has no relevance to the problem...

Certainly true!

This rules out boosters and such things, so why not put it together and try it?

Well first of all, it takes quite a lot of time to get these components. Secondly my mindset is not based around just doing it right away. In my study we first plan out a design, do some calculations and then start working on the real thing.

Since this project will cost me quite some money I like to make sure my plan has a high chance of working and so I kinda like my study's mindset. I know diy projects can cost quite some money (and I don't mind that), I just like to make sure my money will be used as efficiently as possible.

PS, I think that I will make a revision to my design tomorrow.

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steveastrouk-max-

Answer 2 years ago

The guy is studying electrical engineering, he must study calculus....

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-max-steveastrouk

Answer 2 years ago

I know many that studied it, didn't learn a thing and passed with A's. :/ But good point regardless.

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-max-Jeremyv30

Answer 2 years ago

If you want to learn more about capacitors, take a look at this video that I made. I try my best to explain them in an easy-to-follow way.

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steveastroukJeremyv30

Answer 2 years ago

The fewer sources of noise the better ( but the PAM8406 is ITSELF a switching amplifier, at least in one mode of operation. )

Sure, ditch the switcher if you can.

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Downunder35mJeremyv30

Answer 2 years ago

Step modules simply won't give you a clean DC unless you paid an arm and a leg for the thing.
Even in the good old days of transformers being the only power source we already used capacitors and choke coils to prevent unwanted noise reaching our speakers.
Check a decent audio amp and you often find filter cpas in the region of several thausand µF in use....

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Jeremyv30Downunder35m

Answer 2 years ago

Thanks, this explains a lot. Stupid me to forget that these step up modules are just transformers and that they will introduce a voltage ripple...

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Downunder35mJeremyv30

Answer 2 years ago

If you need a decent power supply for an amp to get good quality music you either need a proper power supply or make sure the buck converter won't produce too much unwanted noise.
And don't go for two, if anything then use one that is capable of driving all you need.