# A newbie looking for help

#### jayy111

Joined Jan 9, 2007
1
Hi All,

Wonder if anybody can help a total newbie...

I recently took apart an old computer speaker system, 2 sats and a sub as I was thinking about trying to build myself a small amp, I managed to remove a TDA7375 amp plus a load of other components, there was also a transformer.

I've looked all other the web for some ideas on how to get a very basic working circuit and can't find anything that I understand.

With regards to the transformer it says on it output AV 12.5/3A and there are two output wires both yellow, I have a multimeter for testing but am not sure how to test the transformer and I dont want to blow myself up, so I thought I would ask knowledgeable people for some advice

Any help would be great
thanks
jayy

#### rootboy

Joined Jan 4, 2007
13
You've really got two questions there, building the small amp, and testing a transformer.

As for the amp, that's easy. Find the datasheet for the TDA7375 and look for the application circuits (page 4 of the PDF file).

http://www.audiosound.info/katlisty/TDA7375.pdf

The examples in the datasheet will give you everything that you need to build a simple amp.

As for the transformer, without a part number, you really have to be careful.

I'm assuming (keep in mind, "ass-u-me") that the primary is wound for 120 volts AC and the secondary is wound for 12.5 volts AC (capable of delivering a 3 amp load from the looks of it).

Since all this transformer is intended to do is to convert a higher voltage to a lower voltage, then it stands to reason that the primary side, having 10x the number of turns than the secondary side has will read higher in resistance. The secondary side not only has less turns, the windings are usually larger, thus decreasing the resistance even more.

So first off, for this transformer to be of the type that "transforms" the voltage from a higher voltage to a lower one, then it has to have at least four leads. Where are the other two leads?

Second off, if you want to safely test an unknown transformer, then start with a low, low AC voltage to begin with.

Try to find yourself an AC source in the range of a few volts or so (it has to be AC). This way if you wire it up backwards, when you apply the voltage to the Xformer the output voltage will (should?) still be in a safe range.

Keep in mind that wiring up an unknown transformer can be risky. If you are not sure then just don't do it. I would rather not have to explain my choice of advice to your next-of-kin.