Quote:
Originally Posted by Greg Reid
Timely post Matt. I've been wondering about this for a while. I have several good 4ohm auto speakers around but 10ohm speakers seem a little less available.
I was wondering what the result would be if I wired the two 4 ohm speakers in series and used them with my factory AM unit? How could I make up the other two ohms?
I don't believe a resistor would be acceptable as ohms and impedance aren't exactly the same... Maybe an inductor or coil but I don't know how to calculate the impedance.
|
You are correct, DC resistance and impedance are not the same thing. When they say a speaker has say 4 ohms impedance, that is at a specific frequency, say 1,000 Hz. At 100 Hz it could be 20 ohms, or 2 ohms for that matter. What the Delco mono radios care about more than anything is the DC resistance of the speaker. But in general, lower impedance speakers have lower DC resistance.
The older Delco radios are DC coupled, and put out 1.5 volts DC to the speaker, even with no volume (assuming they are aligned correctly) so if you put a 4 ohm speaker, that is only 1.5 ohms DC on it, you have 1 amp of current flowing through it. That forces the amp in the radio to dissipate 13 watts or so without making any sound. It the bias is high the power goes up and the radio cooks itself.