Hey everyone. First post on the message. I bought an amplifier for my speakers and it’s rated minimum 4 ohms. However, the speakers I have are 3.2 ohms. Should I be worried about damaging my amp? It’s a Lepai LP-2020A+. The speakers are rated for like, 8 watts max and the amp is rated for 20 watts into 4 ohms, I believe.
I wouldn’t worry about it. It’s not DC resistance, it’s inductive reactance, which varies with frequency anyway. I can’t imagine 3.2 verses 4 ohms would be an issue.
The output power of the amp (20W) could damage the 8W speakers if you crank it to 11 though.
You know, my friend told me that the straight dope would be the best place to ask random questions like this, but I didn’t expect a reply in 10 minutes. Thank you much! Awesome!
Yeah, I figure I should be okay, if I just back off the volume knob if I hear clipping, right?
Our resident expert on this kind of stuff is engineer_comp_geek. Maybe he’ll wander in soon.
Yup, the nominal impedance of a speaker is typically a shade higher than the DC resistance. A 3.2 ohm DC resistance is going to be spot on 4 ohms nominal.
The actual nature of impedance speakers and amplifiers causes more worry than is usually justified.
Typical modern amplifiers are voltage sources, and are simply deliver that voltage to whatever impedance they are connected to - which means that they eventually run out of the ability to deliver current into the speaker. It is this limitation of current delivery that is the dominant determinant of the power rating of the amplifier. The Lepali is one of a legion of Tripath T amps based on the ever popular 2020 chip. It is class D. It operates as pretty much a voltage source as well. It also has reasonable internal protection circuits, and will safely shutdown if presented with too low a load impedance, or is driven to thermal limits.
In general an amplifier is limited by a mixture of the safe operating area of the output devices - which places a hard limit on current delivery, thermal limits, the voltage swing the output stage is capable of - which is limited by the power supply design, and average (not peak) power delivery is also limited by the current delivery capacity of the power supply. Lower speaker impedances means higher current delivery, and for the same power supply voltage more power is delivered to the speakers. The 2020 chip was really designed for automotive use, and is designed to run directly off a nominal 13.5 volt supply (ie a car battery.) The power ratings are, as usual, rather optimistic, in that they quote maximum power at 10% distortion, which is reasonably horrid. Like most amplifiers these amps have a knee in the distortion performance, and actually deliver respectable distortion up to a point a bit before where it all goes horrid. In reality, with a 13.5 volt supply the 2020 chip will deliver about 6.5 watts into 8 ohms and about 12 watts into 4 ohms. Because of the higher currents needed when driving 4 ohms the chip is a little less efficient, but like all class D chips, is pretty efficient, getting around 70% efficiency. Which is why it gets away with such a little box and small heatsinking.
As a very good approximation, you will be fine with the setup you have. Speaker power ratings are a whole separate can of worms. Unless you do something stupid you won’t damage either amp or speakers. Just don’t turn it up to the point it is obviously really distorted.
People often refer to 3.2 ohm speakers as 4 ohm speakers, and a lot of the times the difference is just the DC resistance vs. the AC impedance. I can’t imagine that an amplifier rated for 4 ohm speakers wouldn’t be able to handle a 3.2 ohm speaker.
The wattage concerns me though. Ideally you want your speakers to be rated a bit above your amplifier. With 8 watt speakers on a 20 watt amplifier, the amplifier can easily supply enough power to blow the speakers. As was previously mentioned, if you just make sure you don’t turn the amp up so loud that the speakers start to distort you won’t cause damage, but you are one slip of the volume knob away from disaster and something like a static pop could produce enough power to blow the speakers.
It would be a good idea to put some fuses in series with your speakers. If I did my math right the max current on those 8 watt speakers should be about 1.4 amps. Fuses aren’t exactly high tolerance devices, so you’d probably want to go with something like a 1.25 amp fuse (they’ll blow at a bit over 6.25 watts at 4 ohms), which is a nice commonly available size (you can probably get it at ye ol local Radio Shack, along with some handy dandy inline fuse holders you can hook into your speaker wires). If you keep blowing fuses then you need to either use a lower volume or buy bigger speakers.
Tangentially related, my understanding is that under-powering a speaker is more harmful to it than over-powering it. Over-powered, the signal going to the speaker is still smooth, and I imagine most speakers are rated with some margin for safety. Running the speaker over-powered for an extended period, the coil will run hot, which could cause it to warp, and that’s when the speaker fails. Running the speaker with an under-powered amp turned up, even though you’re feeding it less wattage than it’s rated for, the amp will likely send a clipped signal, which, as I understand it, is more detrimental to the speaker.
If I’m misinformed, I’m sure someone will set me straight in no time… 
Underpowering a speaker is fine. An underpowered amp running within its limits is no issue at all. The potential issue is the constant distortion of an underpowered amp running cranked up into distortion. The speaker sees what are effectively square waves, which aren’t the nicest for it. Won’t cause instant damage or anything, though, just not best in the long run.
What Gorsnak said, with one addition. A clipped signal can fry tweeters pretty quickly due to their thin voice coil wires and lack of thermal mass to dissipate the heat.
The reason this happens is because when the signal is unclipped, speakers generally “see” the RMS power of the signal, which is dependent on the signal’s crest factor. As the signal becomes more and more clipped (approaching a square wave) the RMS power approaches the peak power (crest factor approaches 1,) and overheats the speaker’s voice coil.