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Old 15th July 2009, 03:36 AM   #1
jestah is offline jestah  New Zealand
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Question Boombox on wheels needs an experts trouble shooting skills!

Hi

My name is Jestah and i have been trying to learn enough about electronics to not kill my self while i attempt to build a decent sounding skateboard mounted sound system...

http://www.youtube.com/watch?v=n9YNcEbYBho

this was one of the first attempts, nothing flash just some speakers i found on the side of the road with a small tape deck strapped to the top. This then lead to my first speaker box built and a car head deck but i have since learnt that the tape deck uses way to much battery for its output so i have looked into build my own D-class amp.

I started with a kit that used two TDA2004 chips bridged to get 15w of power (loud enough but a little too much distortion at high volumes) and will be moving to a tripath based amp soon (thinking of the 100w from sure electronics so i can run a very clean sound at low power)

I have been running the kit from a 7a 12v SLA but can see that i would like to be able to also plug it into the wall and play at a fixed V (as the battery drains the sound quality falls) but also charge the battery

I am still getting my head around this but here is what i have attemted so far:

I was thinking of putting a regulated 14.4v supply into the box that the battery would feed the battery but am not sure about what to do when its full as most chargers switch to trickle

In the mean time since i have broken my ankle i decided to just try and hook up a transformer so i dont have to swap batteries out (which when charged sound good, no noise or hum with no input). I hooked up a (marked as 9v 3A secondary so im assuming around 27va) not huge the but kit suggests any psu that can cope with more than 2A. This went to a bridge rectifier, 2.5a fast blow fuse on the negative rail then two 50v 4700uf caps in parallel (both neg legs connected to the neg out put of the secondary and the neg input to the amp).

The out put after the bridge reads around 13.4v and the sound with no input is almost DEAD quite on left but on the right it has a hum ( i think this is the right term as its a lower sound but im tone deff too :P)

I have guessed its a ground loop but unsure when to earth of the amp to the secondary side as the instructions say each chip (two in total run bridged) should not be grounded. Also there was only two wires on the secondary side of the transformer so im assuming its just a single winding ....

any way as you can see im very lost and would really appreciate help learning a good trouble shooting procedure as well as filling some of the gaps in my knowledge as im VERY excited about building amps and speakers! I have almost finished my first cnc router and would like to build some carbon speaker housings that create ultra low distortion but need to get this amp running ok before i can commit to a more expensive kit (thinking a 41hz for my room, sure for the sound system and a chip amp for my dad!).

oh and i have searched and found a few things but did not feel that i understood enough about what i was doing to make it safe for me to finish this project and know that my solution was one that is safe, wont catch on fire or become a shock hazzard.

thanks in advance
Jestah
Uglie Carnie Longboards
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Old 15th July 2009, 10:21 AM   #2
col is offline col  Australia
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Hi Jestah,

Thats a way cool youtube vid

I guess you have already come across the Boominator thread? Most of your questions would have already been answered in there:

The Boominator - another stab at the ultimate party machine

Think... 7aH SLAs, Tripath TA2020 (either one of Arjen's boards or 41hz amp6, or one off Eb*y, Lepai doesn't like battery power over 13.2v), and integrated charger. Maybe a solar panel even.


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Old 15th July 2009, 02:03 PM   #3
jestah is offline jestah  New Zealand
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Read the boominator thread and got alot of good info (i plan on the next version to have a lot of the principles of that build but for now i just want to get this kit (that i can post the schematic for if that helps or if someone is willing to tell me where i could upgrade parts) working on a toroidal power supply with out the battery as this boombox is being retired to become a kitchen sound box and will not need the hassle of batteries any more...

I have read a few more threads but still none that explains where my ground loop is forming (i am assuming that its a ground issue as battery power is sounding fine)

I think it may have something to do with how im wiring my transformer as most diagrams i have seen have a central tap that is wired to earth where my transformer only has a single pair of wires coming out.
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Old 15th July 2009, 11:34 PM   #4
col is offline col  Australia
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maybe upload some pics.

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Old 16th July 2009, 12:12 AM   #5
jestah is offline jestah  New Zealand
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Default photo

Here is the schematic:

Click the image to open in full size.

and here is set up so far:

Click the image to open in full size.

As you can see i dont know enough about grounding as i have the earth from the wall floating out there connected to nothing when im guessing it should be tied to some point in the amp circuit. At a guess i'm assuming the negative input? This is where my knowledge is lacking as part of me thinks this is safe but then again part of me also thinks that this could result in me with a very stand-on-end hair doo!

My limited understanding is saying that the transformer provides isolation from the mains and that this should not be affected by connecting the wall ground (is this called the safety ground?) to some point but also think that earth is different to the negative supply rail and they should not be mixed....

Im very aware that a little knowledge is the fastest way to get dead and am so please to gain the skills to be able to move away from battery supplies!
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Old 16th July 2009, 01:53 AM   #6
col is offline col  Australia
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do you have a close up photo of the rectifier/filter caps PCB side?

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Old 16th July 2009, 02:44 AM   #7
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There's a lot of good sense in the electrical code. If you violate it and some damage ensues (life or property) then you will be liable.

The safety ground or mains earth/ground must be connected to all exposed metal parts. This includes the heatsinks, 0V reference, signal ground and any chassis metalwork. Technically, metal volume knobs must be grounded too. You will have to insulate various parts (output transistors etc) to be able to do this without blowing a fuse.

It's hard to see but does that IEC mains connector have fuses? If so, they must be an appropriate value so that they do provide protection. If not, the primary mains circuit, at minimum, must be fused.

Obviously, the set up you show violates code and must be flagged as such. Not to say that none of us have ever done something like that before.

Be safe.
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Old 16th July 2009, 04:33 PM   #8
jestah is offline jestah  New Zealand
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so after a bit more Google i think i have found some possible start points as to why im getting my hum, these are all guesses but im keen to give it a shot in the dark just to see how close i get...

So because its a low sound that only is present when connected to the ac line i think its a ground loop from not making a central grounding point.

Are there 4 different grounds that need to come together all at the same point? Ac ground, Signal ground ( from the rts connector i use to plug in my ipod) and Circuit ground and lastly grounding the case and all metal that can come in contact with some one. Where are the best points to take the circuit ground from and is it best to keep signal ground as short as possible?

I have seen a lot of amp cases that have the transformer surrounded by "walls" of metal to encase it, is this to reduce interference from the transformer? I have also heard of people who epoxy them to make them quieter...
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Old 16th July 2009, 04:33 PM   #9
jestah is offline jestah  New Zealand
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Yes that set up is a bit scary, I realized that i did not have the safe ability to make this power supply the moment I flipped the switch....

BUT im really keen to learn how to safely build a psu as it would really open up alot of projects that i had been putting off. Are there any good starting points to get the basic skills needed to keep my self safe while working on and then enjoying my efforts?

I have fused both ac lines with 2.5a slow blow fuses and have a 3a fast blow on the ground side of the dc line (before the caps). Should i have fused the positive side?
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Old 16th July 2009, 05:43 PM   #10
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Like other class-B chipamps the TDA2004 is notoriously difficult to ground properly. A star ground is needed, and basically all ground planes should have exactly the same resistance to within 0.1% accuracy to avoid problems.

Better post in the chipamp section to get advice on your build as class-d/t amps have very little to do with class a/b amps.
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