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Old 15th November 2010, 11:02 PM   #1
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Default Impedence/Resitance/Ohms

Ohms is a measure of resistance. I think. The higher the ohms, the more the resistance. Right?

The resistance I measure across the two conductors of my speakers is just over 4 ohms. What if I hook them up to the "8 Ohm" connector on my amplifier? The amplifier will be expecting more resistance, right?
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Old 15th November 2010, 11:05 PM   #2
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Also, is this how one would measure the proper rating for a speaker? By putting an Ohmmeter across the connectors?
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Old 15th November 2010, 11:11 PM   #3
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Quote:
Originally Posted by Rosinante View Post
Ohms is a measure of resistance. I think. The higher the ohms, the more the resistance. Right?
correct on both counts

Quote:
Originally Posted by Rosinante View Post
The resistance I measure across the two conductors of my speakers is just over 4 ohms. What if I hook them up to the "8 Ohm" connector on my amplifier? The amplifier will be expecting more resistance, right?
Depends on the amplifier. In almost all cases, you won't notice a huge difference. Like anything ymmv
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Old 15th November 2010, 11:25 PM   #4
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An ohmmeter measures the DC resistance. The proper way to measure a speaker impedance is with an impedance bridge, since speaker impedance varies with frequency. Look at the data sheet parameters of some drivers. You'll see Re (DC voice coil resistance) and a nominal impedance. Most 8 ohm nominal speakers will measure around 6 Ohms DC, and 4 ohm nominal speakers a bit over 3. Nominal impedance may not mean a lot.

Sounds like your speakers are probably 6 ohm nominal impedance. Is your amp solid state or tube? Using the right tap matters more with tubes. If you have a solid state amp with multiple outputs it probably doesn't matter, but if using a single pair of speakers use the 8 ohm terminals. Do not add another pair of speakers on that same channel.

Solid state amps will generally just deliver more current to a lower impedance load. If the load is too low you may run into heat or protection circuit issues. You should have no problems with a 6 ohm nominal impedance. Many "8 ohm" speakers drop down close to 4 ohms at some frequencies.
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Old 15th November 2010, 11:29 PM   #5
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Thanks, guys. I am using a tube amp that has both "8 Ohm" and "4 Ohm" speaker outputs. Again, my speakers seem to measure just over 4 Ohms. Like......4.7.
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Old 15th November 2010, 11:36 PM   #6
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Use the 8 ohm taps. Probably a better match. If you don't like the sound, try the 4 ohm tap.
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Old 16th November 2010, 12:13 AM   #7
sreten is offline sreten  United Kingdom
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Hi,

the issues are complicated. Your just measuring the DC resistance
of your bass units, not the nominal overall impesance. the taps on
valve amplifers can behave very diferently on dioffrent models.

Simply try both and go with the one that sounds best, usually 8 ohm.

rgds, sreten.
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Old 16th November 2010, 12:33 AM   #8
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Quote:
Originally Posted by Rosinante View Post
Also, is this how one would measure the proper rating for a speaker? By putting an Ohmmeter across the connectors?
Don't confuse A/C with D/C. And don't mix measurements of resistance with impedance, coils, speakers drivers.
Passive Crossover Network Design
Inductors - Introduction
T-S Parameter Calculator
Quote:
"nominal impedance is usually taken as an average value over the usable frequency range"
Measuring Loudspeaker Driver Parameters
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Old 16th November 2010, 07:33 PM   #9
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Originally Posted by sreten View Post
Hi, Your (sic) just measuring the DC resistance
of your bass units
No. What the Ohmeter is measuring is the net DC resistance of EVERYTHING connected to the binding posts.

Rosinante, using the 8-Ohm taps will damp the speaker system's bass driver less well than will using the 4-Ohm taps. As a result, you should hear slightly tighter bass from the 4-Ohm taps than from the 8s. Also, the overall level--but NOT the maximum power--will be higher from the 8s than 4s.
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Old 16th November 2010, 10:30 PM   #10
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It's interesting that a speaker rated 8 Ohms may actually measure that value at DC. The speaker is a specialized form of motor, so when it produces motion, it draws power from the source. This raises the impedance, since the dc resistance is the lowest it can be. So an efficient speaker will have an impedance quite a bit higher than its resistance.

It's a complex subject and can't be resolved in a few posts. Or maybe at all. Head for the books and find out what some engineers think about this subject.
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