Will a 25 watt, 4 ohm resistor in line with a 4 ohm speaker safely replace a 8 ohm speaker?
The original speaker is labeled 8 Ω, NOM. 10W, MAX. 20W. The diameter is 3½ inches. Apparently 4Ω speakers in this size are readily available, but 8Ω ones are not. I could splice in the resistor mentioned, as I gather it’s important to maintain 8 Ω impedance.
Is there anything I need to be aware of?
Is the sound quality and volume likely to be reasonable?
One speaker is shot (open, not fixable) but the other works. Any point in only replacing one, or should I have a matched set?
yes, but you’ll need a 12 watt resistor, which is big and bulky.
it’s probably not that important. car amplifiers are capable of driving lower than 4 ohm impedances; in most cases if a speaker that small is 8 ohms it’s actually to limit the power the speaker has to handle; it’s a lot easier for a small voice coil like that to dissipate 10 watts than it is 20.
just put in a 4 ohm speaker, it’ll be fine.
impossible to say w/o knowing the difference in sensitivity and frequency response between the two speakers.
if the goal is to just make it work, only one should need to be replaced. but if one failed, the other may not be far behind.
Over the years, I have run into comparable situations many times. What usually happens is that someone wants to put two speakers in series (instead of a resistor in series with a single speaker). The result is a simple example of a “voltage divider.” With two speakers in series, the driving voltage from the amplifier will be split evenly between the speakers. For example, if you have a transient that reaches 12 volts, one speaker will get 6 volts and the other speaker will get 6 volts. This is bad. It won’t hurt the speakers, but they will not perform to their capabilities.
I have greatly simplified this example, but you should know that it takes voltage and current to produce watts (power). Half the voltage and…well, you can guess. In the situation you describe (resistor with speaker in series), at least half your amplifier power would do nothing but heat up the resistor.
With that said, I don’t really think you will have any problem putting the 4-ohm speaker in, as long as you don’t drive the amplifier too harshly. I hope it has protection circuitry.
I would get the specs for the radio/amplifier before attempting to connect a 4 ohm speaker to it. There are some that can’t handle less than 8 ohms.
If you use a resistor, be careful how you place it physically. That resistor is going to be putting off a lot of waste heat, especially at higher volumes. The resistor is also going to be wasting half of your radio amplifier’s power as heat, making the speaker that much quieter. it will definitely be at a noticeably different volume than the other speaker if you don’t replace them as a matched set.
You may have to dig a bit, but I would try to find another 8 ohm in that size. I would also replace them as pairs. In addition to the above issue, that will avoid mismatched sound levels due to different speaker designs.
then chances are it’s a bog-standard BTL power amp IC driving it, along the lines of the ST TDA7561. most of these are fine down to even 2 ohm speakers. just get whatever 4 or 8 ohm speakers you can find, you won’t hurt anything.
It is rare to find an 8 ohm car speaker. The problem you have in cars is that they are voltage limited. A cheap car sound system runs the amplifiers off the 12 volt power supply. Power is V[sup]2[/sup]/R. Halving the impedance of the speaker doubles the power you can feed it from a limited power rail. More modern car amplifiers, or high power amplifiers include switch mode power supplies or various hybrid switching system to get much more power from the available supply voltage into the speakers. But they also often use speakers with impedances down to 1 Ohm. Also worth noting that power ratings for most car sound systems is subject to the whim of the imagination of the marketeer.
That said, it looks as if you have the absolute bare minimum sound system, and probably don’t care much about the sound.
The power limit for a speaker is a mix of thermal limit of heating of the voice coil, and excursion limit of the voice coil. Drive it too far and it hits to pole piece and is killed on the spot. Drive it too loud for too long and the voice coil cooks. Amplifier maximum safe power limits are thermal and current. They die if they get too hot, and die if they source too much current. The only problem you have with this is if you have a speaker with a very low impedance and you drive it to full volume. You might exceed one of these limits. Might. Mostly it isn’t going to happen. If you were in the habit of running the sound system to its limits you would almost certainly have been in the market for an upgraded system.
From a technical point of view, there is an important difference between an 8 ohm speaker driven from an amplifier and a 4 ohm speaker with a resistor driven from the same amplifier. The bass response will change. Speaker drivers are, in the main, designed to be driven from a voltage source. A bass driver has its bass rolloff set by a mix of mechanical and electrical parameters that yield a simple resonant system. Speakers designs that are placed in a box also include the properties of the air volume in the box in the final system design. Simple car speakers are designed to be installed anywhere, and are designed for “infinite baffle” alignment. This means they assume no important effect from the air volume. A critical part of the system parameters comes from the damping of the cone motion by the electro-magnetic motor dissipating energy into the voice coil resistance. Adding a resistor in series over doubles this resistance, and thus halves the electro-mechanical damping. Does this matter for you? Probably not at all. But I’ve added it for completeness. In general, replacing an 8 ohm speaker with a four ohm and a resistor is not going to work properly. But in a very basic car setup it will be a wash.
Personally I would go out and buy a reasonable quality basic pair of speakers and replace both. You will almost certainly get a pleasant upgrade in sound for little money. That, versus having random power resistors hanging about in your car, and the extra time and messing about to do it, makes it an easy choice.
8 ohm car speakers aren’t that rare. Tweeters and smaller mid ranges frequently are 8 ohms because they are not operated at high power levels, are filtered (high passed) which reduces the applied power even further, so the higher impedance reduces the thermal load the small voice coil has to dissipate.
Good points above, I’ll just add that you should probably replace them both. Most car speakers are sold in pairs anyway, and you could always hit somewhere like Solen if you want to get something more exotic.
If the amplifiers are solid state they can drive down to 2 ohms. What will make a difference is that the lower the ohm the harder the amp work, meaning that you can overdrive the amp. Just watch what you do with the volume control and you’ll be ok.
