LM1875

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Hi !

I need a amplifier to replace the dead of my logitech X-230 refresh.

So 2.1 around 15Watts per head and 30Watts for the sub.

i saw this LM1875 2 1 Channel Subwoofer Amplifier Board KIT Without Heatsink AC15V 0 AC15V | eBay
which is perfect but I only have a transformer (45-0-45 5-0)

I already saw in datasheet that LM1875 can support a +-30V power supply.

With the board above, can I put my transformer with a regulator to +-30V ?

Or any other solution ?

Thank for the help and i'm sorry if my english is bad but i'm French.

Will.
 
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You cant get 22.5-0-22.5V AC easily without tapping secondary. LM3886 is also out of question as thermal dissipation(hot-spots) may become major issue above +/-35V DC. You can use 45-0-45V AC for making sub-woofer amp using discreet transistor design.
BTW theoretically 45-0-45V AC will give you +/-63.64V DC
 
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Hey

It's not possible to use just 45V and split him into 2 x 22,5V (+-22,5V ) ?

With this big transformer what can I do ? LM3886 ? Other?

Thanks !

You could 'sorta' do it by running only 1/2 the transformer i.e. one side and the center tap and adding two huge capacitors to create a false ground. Not sure if it would work or would be stable enough to keep the magic blue smoke in. :D

Do you want to do this as a labor of love?
STK4044V One Channel Audio Power Amplifier IC 100 Watt RMS IC Sanyo Original | eBay

It would cost ~$90 with shipping for three amps. You'd end up with a stripped down amp that still needs volume and tone controls. Assuming the transformer you have is the size of an arc welder and can power it. Of course you would be the only one on your block with a set up like this.
 
You could 'sorta' do it by running only 1/2 the transformer i.e. one side and the center tap and adding two huge capacitors to create a false ground. Not sure if it would work or would be stable enough to keep the magic blue smoke in. :D

Do you want to do this as a labor of love?
STK4044V One Channel Audio Power Amplifier IC 100 Watt RMS IC Sanyo Original | eBay

It would cost ~$90 with shipping for three amps. You'd end up with a stripped down amp that still needs volume and tone controls. Assuming the transformer you have is the size of an arc welder and can power it. Of course you would be the only one on your block with a set up like this.

It's too expansive just for my x-230 logitech kit, but the transformer was in a SU-G50 Technics with a chip like the STK4044V.

I think i'm just going to buy a transformer around 30$ but i want it to be evolutive. Maybe for a bigger diy project in the future.

Whats are your recommendations?

Thanks
 
Some pictures of the case and the transformer
 

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JSIXIS in another thread recommended p-8673 - transformers catalog
and I would have to agree it is a good general purpose transformer for just about anything audio.

Price is right, great brand, puts out 144 Watts which is enough to give you tinnitus, at the top end of specs for the LM1875 => maximum output, voltage at the bottom of most specs so you could probably get away with using 25V rated components in things like the split power supply.
 
I would just use a full wave bridge and a couple of large capacitors.
An externally hosted image should be here but it was not working when we last tested it.

Look at the upper left of this schematic in the section that says "Power input and regulation"

This was done for a TDA2030 so it is just a bit under what you would want with respect to the transformer. The 36 VAC center tapped transformer will output 18 VAC on either side of the center tap. 18 VAC rectified will give 25 VDC so in this set up you will get a -+25 VDC to power your chips.

The full wave bridge rectifier is good to go as specified but you may want to go with something a little different. For example a 100 V at ~10 amps would be a better choice IMHO.

The rest of the parts are OK if somewhat better then you need. You probably could get away with 25 V capacitors which are less expensive, 35 V is more then enough, 50 V is overkill. If you want to keep it simple, just use the 3300uF ones. If you want to play it safe, use all the capacitors called for and ones that are rated at 35V.
 
In general you will get ~1.4 times the AC voltage when it is rectified. You can think of a center tapped transformer almost like it is two transformers stacked on top of each other. That is a center tapped 25.2 VAC transformer is like having two 12.6 VAC transformers in one package.

So for the Stancor transformer it would be 18*1.4 or about 25 VDC. Wired as in the circuit above, it will generate -+25 VDC. The vendor recommended 30 VAC center tapped would produce +- 21 VDC. If you look close at the pictures you just posted and the amp you linked on eBay, you will notice the capacitors are rated at a maximum of 25V.

That is why the vendor recommended 36 VAC center tapped. Nice safety margin under what the parts are rated at.

The problem isn't the LM1875, it is the other parts used in the design. Usually vendors slightly under rate their componets, that is a capacitor rated at 25V would be good to 30V. It is a risk I would take to use that Stancor transformer but it is up to you of course.

If you want something safe, you could go with Inglot Electronics 51-122 | Alltronics

About the same price at 1/2 the current or 1/2 the Watts.

The second kit you posted I would have to say stay away from it. I just put together that exact kit and had huge problems with the parts supplied. If you look at the thread you will find there are a number of kits using the TDA2030 on eBay that are giving people lots of problems. I got mine working but only after substituting parts from jameco.com for the TDA2030. The consensus seems to be: There are counterfeit TDA2030 finding their way into the kits. I ended up replacing the TDA2030 with LM1875 from Jameco. The new chips cost as much as the amp! The good news is it did make a fine amp once the problems were sorted out.
 
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