[Jon Salem]

So if I want to connect a 1971 factory radio expecting 8 ohms to a newer amplifier connected to 4 ohm speakers, can I splice in a four ohm resistor in series to get the nominally correct resistance? And that’s avoid frying my old radio.

Originally Posted by LASJayhawk

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.