My latest article / "knoweledge-drop" for the Traditional car owner: Resistance. Impedance. Ohms. What they are and one way those terms relate to the old-car world...Please read it and let me know what you think! https://redirad.com/blogs/news/why-resistance-it-futile-and-impedance-is-important
They glossed over a few important things.... First thing to realize is speaker impedance values as given are 'nominal', i.e. that's an average value, not the actual speaker coil resistance as measured with an Ohm meter. Actual impedance when operating can vary by 100% or more in both directions depending on the frequency make-up of the signal. If a speaker is a simple full-range speaker (not a 2 or 3 way) with no cross-overs present in the system, you'll have inductive reactance along with the resistance; the reactance is what changes. Add cross-overs (capacitors) to the circuit and you now also have capacitive reactance. The interesting part is these can cancel each other out; large capacitor banks are commonly used in industrial installations with lots of electric motors or other inductive loads to correct 'power factor' as utilities generally have a pretty heavy surcharge on 'reactive power'. If you've ever seen a VAR meter along with a Watthour meter at a electric service, that's what the VAR measures; 'Volt Amps Reactive'. As used in audio systems, capacitors are there to 'steer' the various frequencies to the 'right' speakers and may or may not do anything to correct power factor. But back to speaker selection. Yes, you don't want to use a too-low impedance speaker as that will put power demands on the amplifier that it's not designed for. At best you'll get 'clipping' in your output (where the waveform gets 'chopped off' when it can't furnish the needed power) with bad sound, overheating and damage to the amp can easily follow. Unable to find a speaker with the right impedance? In the example given, a 10 Ohm speaker is OEM. Generally, 20% either way will be close enough, so a 8 Ohm should work, with a caveat explained below. Most speakers these days are rated at either 4 or 8 Ohms, but if you connect two 4 Ohm speakers in series, you'll get your 8 Ohm load. The caveat? Most 'vintage' speakers use relatively small magnet structures compared to 'modern' speakers. This makes a difference, as magnet strength is directly related the power needed to reproduce sound waves. A larger magnet will allow increased frequency range, as well as more precise response, but your power requirements will go up, even at the same impedance. So just because you matched impedance, that doesn't means your amplifier is 'safe'.
This is true with solid state amplification, but not with vacuum tube technology. With solid state a rule of thumb is it's OK to apply a higher impedance than the amp is rated for, no damage to the amp will occur, the amp just won't produce as much power as it is rated for. But you don't want to go with a lower impedance, as damage can occur to the power transistors. But with tube technology, and there are tube car radios out there, the opposite is true. Going with a lower impedance load really won't harm the tubes at all (though they may wear out quicker as they will be working harder), but too high of impedance can. Too much impedance can cause very high flyback voltage through the output transformer back to the power tube sockets causing arcing to occur at the sockets. It can cause failure of the tubes, and under some conditions it can take out some other components, such as the output transformer. Overall it's best to match the impedance of the load to what the amp is designed to operate with, but there is usually some tolerance either way. With a solid state amp, it's always safer to apply a higher impedance load if you cannot match what the amp wants to see; and with a tube amp it's always safer to apply a lower impedance load if you cannot match what the amp wants to see.
Steve, I intentionally kept the "voice" (technical scope) of this article "high-level". You added some excellent detail to the discussion - Thank you.
Old joke: New boss wants to test electrician's knowledge New boss: What's an ohm? Electrician: It's an Englishman's castle. You don't catch me that easy you bum.
The Jalopy Journal
