Ohm My Gosh

November 1, 2009

0.gp1009_gearshopI recently replaced both speakers in my 2x10 cabinet, and when I measure the resistance with my digital multi-meter, I get a reading of 11.2Ω with the two speakers wired in series. Each speaker measured 5.6Ω individually, so I know I wired it correctly. How should I set the impedance selector on my amp: 16Ω or 8Ω? In your opinion, which would sound the best? —Jake, via email

You are confusing DC resistance with impedance. Impedance is the sum of everything that impedes the flow of electrons. That would include DC resistance, AC inductive reactance, and AC capacitive reactance. When you measure the DC resistance with a multi-meter, you are not measuring the full impedance, but only the DC resistance. Your ohmmeter function doesn’t measure capacitive reactance or inductive reactance. If it did, you would probably have a little over 1Ω of each – depending on the actual frequency of the signal voltage. If you were to add that to the 5.6Ωof DC resistance, you would get about 8Ωof total impedance on each speaker. Wiring the two speakers in series would yield 16Ω, so you would set the impedance selector on your amp to that setting.

You are smart to run the two speakers in series to get the 16Ω load. When a transformer is made, it is wound for the largest impedance— the other impedance taps are just a smaller percentage of the whole transformer. So, for example, on a 16Ω transformer the 8Ω tap would be placed at 70.7 percent of the full winding. If there were a 4Ω tap, it would be placed at 50 percent of the full winding. So if you were using the 4Ω tap into a 4Ω speaker, you would only be using half of your transformer! Think of an eight-cylinder engine with four of the spark plugs disconnected. It may run, but it won’t be optimum. Using the entire transformer winding means you’re getting full performance from the transformer. —Gerald Weber, Kendrick Amplifiers

I just bought a new amplifier and when I play through it with my Strat, I notice that the amp hums really loudly when the Strat’s 5-way switch is put into either position 1, 3, or 5, yet seems to have very little noise when played in position 2 or 4. What would make the amp have more noise in those positions? —Ted, via email

This is not an amplifier problem. A Stratocaster has single-coil pickups, which produce more hum than dual-coil humbucking pickups. Evidently, your middle pickup is reversewound, so when you go to position 2 or 4 on the 5-way selector switch, you are mixing the middle pickup with either the neck pickup (nonreverse wound) or the bridge pickup (also nonreverse wound). So the hum induced in the middle pickup will be 180 degrees out of phase with either the neck or bridge pickup when the 5-way selector is set in position 2 or 4. This creates a humbucking effect where the in-phase hum from one pickup cancels the out-of-phase hum from the other pickup, resulting in nearly zero hum. When you switch to positions 1, 3, or 5, you are only using one pickup, so there’s nothing to “buck” the hum. This is all perfectly normal and is just the way a Stratocaster works. Also, consider your perspective with regards to the environment and ambient noise. If you are listening to your amp in a quiet living room, the slightest bit of hum is really noticeable. Take the same guitar and amp rig to a club gig and you would be hard pressed to notice any hum at all. —Gerald Weber, Kendrick Amplifiers

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