Joined: 04 Nov 2003 Posts: 886 Location: SF Bay Area
Posted: Wed May 19, 2004 11:00 pm Post subject: Impedance
Impedance is a sort of reactive or interactive resistance. It is the load that is placed on the driving device, in our case the load that the speaker places on the amplifier. We use the static resistance measurement of "ohms" to describe it but in fact impedance varies depending on what we are trying to get the load to do.
Imagine we have Randy Johnson, a machine pretty well tuned to throwing a baseball a fixed distance with some degree of accuracy
If we were to change the load by taking away the baseball and give him a shot put instead, we would pretty much throw off the whole thing. The extra weight would upset the mechanics of his motion and strain his muscles. Conversely, if we gave him a pebble, he would probably throw his arm out and have wild aim. His pitching technique expects to accelerate the mass of a baseball, not a cue ball or pebble. If we asked Randy to gently toss each load underhanded for 10 feet, he could probably do each pretty much the same. If we asked him to put them over the plate at 98mph, there would be a big difference in how well he could adapt to the way each load reacts to using a pitching motion.
The switch on the back of your amplifier selects different sections of the output transformer. Each one tuned to expect a particular load. If the actual load presented by the speaker is higher than the amplifier expects, the load dampens the signal like the extra weight of the shot put, keeping it from performing to potential. If the load of the speaker is less than expected there is nothing for the amp to press against as it were and the flow of electricty exceeds what the amp was designed for. In literal terms, excessive current flows.
What does it mean to you? Fender amps are historically tolerant of impedance mismatches although some other amps are considerably less so. The effects are the same, just to varying degrees. Overloading an amp with too high a load will range from a tightening up of the sound, to damping it so much it sounds muffled and you lose a lot of volume. Underloading an amp will loosen up the sound (which might be a good thing to some folks) but the excessive current flowing though the output section can overstress things, most notably the output transformer. The smell of melting transformer wax is well known to folks who played loud in the sixties and seventies. With a solid state amp, lowering the load will just make the rig louder, until the transistors can't pass any more current and they give up, leaving you in silence.
Speaker impedances can be generally thought of as following the same rules as resistors; series sums and parallel divides. If your Twin has two 8 ohm speakers wired in parallel (both are wired together the same way to the jack) then you have 4 ohms. Running the swith on the 8 or 16 ohm setting risks burning out something. If you have two 16 ohm speakers wired in parallel, you have 8 ohms. Running the switch in the 16 ohm position again put the output at risk. Running it at the 4 ohm position overloads the amp and can lose volume and top end response. If two 8 ohm speakers are run in series (like a loop from one terminal of the jack to each terminal of the speakers in sequence and back to the other terminal of the jack) then you have a 16 ohm load. Running the amp switch in 4 or 8 ohms will again overload the amp and muffle things.
You may not notice these differences at low volumes (remember Randy tossing underhanded for 10 feet) but dime the amp in a loud gig and you'll see. Hope this helps. _________________ There are no such things as wrong notes, there's only the look on your face.
My Stuff: www.stevekirbymusic.com
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