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Dameo182
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21/07/2020 12:51 pm  

So im building my second bluetooth speaker, but being that i find it so confusing on the electronics side that i came here for help after watching 123 toids youtube videos. So my issue is that im building a small box that contains one 1.5" full range driver, the dayton DMA45-8, its 10 watts rms, but i need to be able to charge a phone from the amplifier board. The issue comes from the available amplifiers with built in bluetooth, i need it to have a built in usb too ( im assuming that would charge the phone) but the only ones i can find that have the usb are 2x5o watt amplifiers, which im sure is too much power for the driver i plan to use. The 2x15watt amps that ive seen dont have the usb, so what is the best way to power the driver and also be able to charge a phone? Id massively appreciate a fix to my newby issue 

Also, i read on another post that at 4ohm an amplifier gives 50 watts, but at 8ohms it gives around 25watts, can someone explain why that is please? 

 

Thankyou in advance for any advice

 


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TVOR-Ceasar
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21/07/2020 2:58 pm  

Here's a board that will do Bluetooth 4.2, has USB and a remote.

I understand the confusion with power and speaker ratings, etc. There's a lot that the manufacturers could do to clear up confusion, except that they want you to buy their stuff based on the maximums (psychology 101), as in you are getting so much for your $$.

I will go back to the other post and explain it there, since he asked first, then come back and link or explain further here. Suffice it to say that if you read the specs on the amp I linked, it says that it is 30W @ 4 ohm. Since Ohm is a unit of (in audio or AC) Impedance or Resistance (in DC), the higher the ohm rating, the less current will flow. So since the 8 ohm speaker is 2x's the 4 ohm speaker, only 1/2 the current will flow. So 1/2 the power rating of the amp - 30W @ 4 ohm, 15W @ 8 ohm.

Like I said, I'll delve deeper in the other thread, but at least link it here.

-Charlie


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123Toid
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21/07/2020 3:02 pm  

@dameo182

There's a couple ways to do this.  THe first is to buy a cheaper board and add a USB charge to it.  It is really cheap to do and I went over it in this video:

Another option would be to buy something like the 2 x 30 Dayton KAB board and the usb extension board

Finally, the last option would be to buy a board like you are talikng about and give it less power.  The basic mathematics behind wattage is volt x amperage = wattage.  There isa loss due to the efficiency of the amplifier, but let's not get too confused yet.  Most class D amplifiers are rated at 4ohm.  This is harder for the amp to push, but it will give you more power. With that in mind, an 8ohm load is easier for the amplifier to push.  The easiest way to think about this, is an 8 ohm speaker will receiver about half the power of a 4ohm rated amplifier.  

Let's put this in practical terms.  If you have a 2x30 watt amplifier it will not produce 2x30 unless you give it enough power.  Let's assume you want to give it full wattage.  In this case that would be 60 watts (2x30=60).  Let's also assume you want to use a 12v power supply.  Now we will want to use our formula from above.

watts=volts x amps

60watts= 12volts x amps 

60w/12v=5amps

So in order for the amplifier to even put out 60watts you will need a power supply that is at least 12v and 5amps.  If it is smaller, the amplifier will not be cabable of producing full power.

Now in your case, if you were to use a 2x50x amplifier and you only want a total of 20watts out of it.  Just buy a power supply that will work with the amplifier that will produce about 20watts.  Something like a 12v 2a for example (24watts). However, keep in mind if you plan on using it off battery, you would have to limit the amps, which in that case, would be better to go with a different option.


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Dameo182
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21/07/2020 6:57 pm  

@tvor-ceasar

 

Thankyou for that explanation, i understand amplifiers now way more than i did ten minutes ago, probably saved me from making a clumsy mistake down the line, also thankyou for finding that amplifier, ill look into getting that if the cost of shipping is reasonable as i live in the uk, which i forgot to mention earlier. It can be very hard to find quality products like that here as everything is chinese, i usually get my parts from sound imports but they dont have that particular board


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Dameo182
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21/07/2020 7:17 pm  

@123toid

 

Thankyou for giving me so many options to fix this issue, i actually watched that video while trying to find an answer, but i wasnt sure i could do that with any of the amplifiers i could find online, i dont really have the knowledge to do it if the amplifier was in any way different to the one in the video. The idea of using the power lead to decrease the wattage though is genius, id considered adding a potentiometer between the board and the driver and turning it down, but since i have to purchase a 12v power lead anyway it means i can use a more powerful board and not have to worry about damaging the driver, id prefer to use a matching board like the one linked in the post above, but if that turns out to be hard for me to get delivered then this is a perfect fix, thankyou for that 

 

Ill post pictures of the speaker once ive completed it, would be good to get the forums opinion on it to see how i did 


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123Toid
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21/07/2020 7:24 pm  

@dameo182

Absolutely.  And if you ever want to try something new, like the first option, jhsut buy the parts and we can help walk you through it.  We're here to help further your knowledge.  I can't wait to see the speaker.  When are you planning on starting?


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Dameo182
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21/07/2020 8:54 pm  

@123toid

 

Thankyou, I appreciate that, since i heard you mention this forum in one of your videos its felt like a weight has been lifted, due to the fact trying to find answers to specific problems online gets really frustrating. So to be able to ask a question and get solid detailed advice on the matter is a great thing. Ill hopefully be  starting it in a couple of weeks, it really depends on the time it takes to get the parts i need, ive already worked out the box size on winisd with a passive radiator, so now that i know what im doing with the amp im good to go


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Dameo182
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23/07/2020 2:51 pm  

So, i changed my mind on the passive radiator, the box was so small after i had worked out the dimensions that it just wouldnt work for my project, so ive gone with two of those drivers instead of one, and done a ported design, winisd says the f3 on that is 90 hz, with a box size of .019ft and tuned to 103hz, at this point in my experience i dont know if all that is good or bad for those little drivers. I checked the air volocity and its at 16, but the port has to be a 20mm tube at a length of 97mm, i plan to flare the end of the port but this is where im a bit unsure. What do people use when the port diameter is bigger or smaller than you can buy online? Im thinking i will use a piece of 20mm steel tube that i can get from work, but another thing is how would the steel affect the sound, or will it make no difference? 


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TVOR-Ceasar
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28/07/2020 1:14 pm  

I wanted to let you know that I went deeper into amplifier power on this thread. You may have already seen it, but I said I'd link it for you, so here it is.

-Charlie


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TVOR-Ceasar
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28/07/2020 2:11 pm  

You could use steel, but it may actually ring unless you externally dampen the pipe. I've never used it in this manner, so I could be mistaken. (I do like my wind chimes though, especially the Gregorian Monk's chimes, so deep and resonant... 😀 )

What you could do is use a slot port (rectangular) like you would see on The DINAS. Basic geometry gives the 20 mm tube an opening area of 314.16 sq. mm, so a reasonable opening, say 10 mm x 31.5 mm or so at 97 mm long will work.

 

-Charlie


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Dameo182
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28/07/2020 3:55 pm  

@tvor-ceasar

 

Yes i saw your other post, i learned a lot from that so thankyou for taking the time to explain it all, and as for using the steel, ive decided against it and your above comment has just reinforced my dicision lol. I actually used a slot port on my first build that worked out good, i was trying find an easier way to port this one though as the box is so small i imagine it being very fiddly to do a slot. So i think im going to find a way to use wood by maybe drilling out a dowel or make multiple holes in thick flat sheet and glue them together to make the length, i havent really decided yet. 

I do have another dumb question that i hope you can advice me with though, i watched 123 toids video on the baffle step, as my first speaker is a little bright on the trebble, because of that video i know how to do that now. But im unsure on the measurement of the baffle, in my speaker i have two drivers side by side, two seperate boxes to make one big one, and in the centre of the baffle i have two vertical slot ports, one for each box. Im confused about which measurement to take, is it half the width of the overall baffle (one box for one driver) or the total width of the baffle that i need to work out the value of the componants? Its the two drivers and the slots that have thrown me a bit 

 

 

 


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TVOR-Ceasar
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30/07/2020 6:31 pm  

This article should help clear up any questions you may have. 

-Charlie


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Dameo182
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31/07/2020 3:57 pm  

@tvor-ceasar

 

Thanks, it does help me to understand it in general, but it doesnt quite answer my question, maybe im overthinking it lol, but it still uses one driver and one box as an example, i still have no idea how two drivers in one box would be measured, as the baffle is double the size as it would be for one. Does the same equation work for that? My baffle is roughly 300mm wide. So do i use 300mmx8÷102, or would i use 150mmx8÷102?

 

I actually cant believe how confused i am by this simple thing 

 


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TVOR-Ceasar
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31/07/2020 4:52 pm  

You want the width of the enclosure - the smallest dimension where the drivers are mounted. Plus, it wouldn't hurt to have any other dimensions that would help place the driver(s) on that face.

If you can attach a quick sketch with the dimensions on there, I think that would be the best way to understand your particular situation. Nothing fancy, MS Paint, pic of a paper/pencil sketch, no need to be precise or to scale. Just a quick illustration and some numbers to nail it down.

-Charlie


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Dameo182
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31/07/2020 5:31 pm  

@tvor-ceasar

 

Ok thankyou for taking the time, ill make a sketch and try to upload it for you 


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