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# Powerline transformer to audio PA?

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#### HMX

##### Member level 2
I need for some measurements make 230V vith variable frequency of 50-60Hz. So, I connected audio PA with two TDA1514 in bridge and powerline transformer 230/12V connected in reverse order to amplifier output. But problem is that amplifier get quickly very hot even vithout load and worse when use bigger (some 30VA) transformer. How can I compensate this PA or what I need to do, so that PA don't heating so much. Can anyone help?

Your problem is that you have to low inductance in the 12V winding,it is almost like a short circuit to the TDA1514:s
Run TDA:s at as high voltage as possible ie. +-30V and use a 230/60V transformer and your inductance will be a lot higher.
230/12=19/1 voltage ratio=367/1 impedance ratio. Lets say primary inductance is 3H=1000ohm at 50Hz, 1000/367=2.7ohm in the 12V winding + the transformed load =HOT!!!
The 230/60V transformer will have about 68ohms, now you will not waste so much energi in the transformer it selves.

Re the inductance: as per definition the current and voltage in a "perfect" inductance is 90 degrees apart, so the real power (which is V*I*cos(phi) should in theory be zero.

I have another theory why your PA might get so hot; in case you have a bridge-coupled PA you may have an offset output voltage, which in that case sees only the very low-resistance of the copper winding itself. This may cause a DC current of an appreciable size.

Try to connect a large capacitor in series with the winding and you get rid of the DC-component mentioned. If using electrolytic capacitors you may connect them in series back-to-back (+ to + sides) and in so doing get the equivalent of a bipolar capacitor of half the capacitance (provided the two capacitors are equal otherwise). Or use a bipolar type, such that are intended for passive filters in loudspeakers. Be sure though to use a sufficiently large capacitance, otherwise you will get some voltage drop over it. And watch out for resonance phenomenons (perhaps you want it, perhaps not).

/Pim

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