I recently dug out of my box o' stuff a pair of seemingly decent 2.5ohm bookshelf speakers. I wired them to an old mp3 player I have and while they sound good at low volumes, I get lots of distortion at high volume. Given that they are only 2.5ohm, I'm assuming that this is because they are trying to draw more current than the poor mp3 player can produce.
Rather than buying one (or new speakers for that matter), I'd like to try and build an amplifier for these speakers. I was looking at using an IC such as the TDA7924 but like all of these chips, the advised load impedance is 4 ohms. It doesn't specifically state anywhere that the absolute minimum is 4-8 ohms, but has anyone had any experience with this sort of thing? Will it still work at a lower load impedance if I just make sure that the volume is limited? If not, will I be able to use this IC if I chuck a big 2 ohm power resistor in series (as much of a waste of electricity as that is)?
I'll probably get some anyway to play with but I am a poor student and I would prefer not to blow up several $12 chips. thanks!
Best Answer
If you don´t mind to lose stereo sound, you can wire up the speakers in series resulting in one load of 5 ohms.
I think that a 2 ohm resistor is not a good solution because a common output voltage will be arround 10 volts and with current of 5 amp you´ll need a resistor of 50 Watts, which is pretty expensive. Maybe buying an cheap tweeter will be better and improve high frequency sound quality.
Looking in the datasheet I found that this IC specifically (TDA7924) have an short-circuit and overload protection. Using a load of 2.5 ohms may not be a problem if you don´t exagerated in the volume.
Some other thoughts, how did you measured the speaker impedance? If it was only using an DC multimeter the number you got (2.5 ohms) is only the resistance of the speaker. The impedance also needs to consider the inductor. If the DC resistance is 2.5 ohms probably the total impedance is alredy 4 ohms, so you don´t need any modifications on the circuit. The speakers whose impedance are not 4 or 8 ohms are rarelly manufactured.