13v + Lm3875 = ?

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Is it alright to power that LM3875 chip with only 13v?

I just found a (BIG!) transformer from my old National XE550 equipment, though I haven't checked its output voltage yet. Loking at the label, the equipment can be powered by 13v worth of battery, so I assume that the transformer has an output voltage of 13v. The datasheet for the LM3875 states that the minimum voltage range is 20v. But, I think I've read here somewhere that someone has driven the chip below 20v. I just want to be sure. :)
 
Sensitivity is normally qouted in how many decibels you can create with 1 watt at a meter. I would say high efficiency designs would have figures of at least 90bd/W and hopeuflly more.... but this normaly comes at a sacrifice of power handleing with the exception of some PA equipment.

So I would bet your 160W speakers are not all that efficient.

I would say build the amp, give it a listrn with the tranny you have then get a proper one, you can even save a 10er or so a month, times speeds up as we age, and age we do. So in no time You can get what you need with some patience... I would save the tranny you have now for another project.

I have run LM3875 at with 11V trany for testing before, worked just fine. Sure it would not be a good idea to open up the volume...
 
Nordic said:

I would say build the amp, give it a listrn with the tranny you have then get a proper one, you can even save a 10er or so a month, times speeds up as we age, and age we do. So in no time You can get what you need with some patience... I would save the tranny you have now for another project.

I have run LM3875 at with 11V trany for testing before, worked just fine. Sure it would not be a good idea to open up the volume...

I"ll try the 13v transformer first, then if it wont work ( which I think is really going to happen :smash: ) then I guess I"ll have to save. Thanks for the replies everyone! :)
 
Just found out that the transformer has 4 wires for the secondaries. I dont know which is which but the 4 wires are being paired by 2. 2 wires are colored blue and has an output voltage of 16v while the other pair ( colored orange ) produces 27v. Got the voltage using my analog multitester. ;)

So, is this one alright to use for the LM3875 chip? Planning to use this for two LM3875 monoblocs.
 
AndrewT said:
Hi D,
what kind of transformer is it?
What size is it?
How heavy is it?
How thick are the copper leads? mains, 16V sec and 27V sec?

These will give us a clue or two.


Hello Andrew,

It's almost midnight here already. :xeye:
Though, as far as I can remember, the wires for the 27v are as thick as the wires for the mains, while the 16v is thinner. Approx. size would be about 3x2x2 and weighs more than 2 kilos.

I'll check it out again tomorrow.

Meanwhile, I have a problem in using this in Nuuk's PSU.

An externally hosted image should be here but it was not working when we last tested it.


There are 3 inputs there, AC, AC, and 0V. Where's 0V when there're only two leads for 27v ? After some research, I think I could connect the two secondaries in series so that I can have the 0v (is this correct?). But 43V is too much, I suppose.
 
Mabuhay Doomsweek. The above diagram is for a centre-tapped transformer and you are using dual secondaries.

You should ignore that 0V line on the input AND output of that diagram. After you have built your two rectifier bridges, you join the negative output of one bridge with the positive of the other to get your 0v line to the amp.

It should look like the first part of this circuit.

An externally hosted image should be here but it was not working when we last tested it.
 
Though, as far as I can remember, the wires for the 27v are as thick as the wires for the mains, while the 16v is thinner. Approx. size would be about 3x2x2 and weighs more than 2 kilos.

Hi Doomsweek, I don't think you will be able to use this transformer. Having different voltages and currents for the two rails can't be right?
I think you will need the secondaries to be exactly the same as each other...
 
is it an EI or a toroid?

I think it's an EI

What size is it?

Height 11.2 cm
Width 11.0 cm
Thickness 5 cm

How heavy is it?

2.35 Kg

How thick are the copper leads? mains, 16V sec and 27V sec?

Mains, 27v secondary diameter 2.5cm
16v secondary diameter 1.5cm

Mabuhay din Nuuk!

Like this?


I'm a little confused here. How much is the output voltage here? +/-27v or +/-54v?

Is it easy to modify the transformer so that both secondaries has a center tap?
 

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Hi,
your pic shows a toroid and your sizes seem more like a toroid.

The circuit shown in your pic will not work. Two rectifiers needs two secondary windings to give a dual polarity supply.
You can get a dual polarity supply from one rectifier with a centre tapped secondary, but your transformer appears to be single windings of different voltages and that is no good for power amplifier use.
 
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