speaker ohms


Old 11-12-14, 05:05 PM
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speaker ohms

I am in the process of installing a stereo in my basement garage. I am unsure of the importance of matching the ohms rating from the receiver to the speaker. My receiver is an older surround sound system with 5 channels. My speakers are 3 way realistic floor model with an 8 ohm rating. I planed to use the "rear" channels of the receiver, as they provide the best overall sound. My question is that the rear channel is rated 16 ohms. What, if any harm can be done by mis-matching the ohm rating between the two? Should I try to find a different receiver?
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Old 11-12-14, 07:26 PM
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Two 8 ohm speakers = 16 ohms. As best as I can recall from my radiohead days, if the ohms of the speakers exceed the rating on your stereo, the speakers simply won't have enough power/volume.

You could also hook up a maximum of four 4 ohm speakers, since that would also = 16 ohms.

There is nothing wrong with having less ohms than it's maximum capacity either... its just that two 8 ohm speakers are almost always superior in sound than two or even four 4 ohm speakers. But sound quality will suffer if you exceed the maximum limit for what the channel is rated... 16 ohms.
Old 11-13-14, 04:52 AM
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Xsleeper, you may want to check your answer. It is a very bad thing for speaker impedance to be less than the rating of the amplifier. Impedance in ohms is the load the speakers present to the amp. The lower the number the heavier the load on the amp's output devices. Too much load can cause those devices to fail.

Also, two 8-ohm speakers wired in parallel will present 4 ohms. They will present 16 ohms only if they are wired in series. The rear speakers, therefore, must be wired in series.

Four 4-ohm speakers wired in parallel will total 1 ohm and will fry most amps, or at least cause the amp to go into protection. Four 4-ohm speakers wired in series will total 16 ohms. A series/parallel wiring scheme can also be used (2 in series; 2 in parallel) to get 4 ohms total load.
Old 11-13-14, 06:46 AM
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Rick, feel free to correct me as well, but I believe there are also line transformers (70 volt) used for longer runs. One could step up to 70 volts and then back down to whatever ohm rating they needed. We used these in large buildings for pa systems but I will assume they have higher quality options, cost unknown.

Old 11-13-14, 06:53 AM
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Wow, I was really off... thanks for correcting me Rick. Probably explains why my car stereos always sounded like crap!
Old 11-13-14, 07:03 AM
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Lol X.

I may be wrong as well, but it seemed like you could use higher impedance speakers if you had to, without a big issue, as long as you didn't run it full blast all the time? Something like that anyway.

Always best to match exactly of course.
Old 11-13-14, 07:39 AM
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I planed to use the "rear" channels of the receiver, as they provide the best overall sound.
The rear channel speakers don't always operate in all modes as they are considered surround fill channels.
The front channel outputs are higher wattage, a closer impedance match and should work in all modes.
Old 11-14-14, 03:05 AM
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Maximum power transfer occurs when the impedance (ohms) of the output stage of the amp matches that of the speakers. Running speakers below the output impedance of the amp can cause excessive load on the amp as Rick suggests. However impedance is not a linear thing in AC systems:

Speaker systems are rated for a nominal impedance, which will be typically 8 or 4 Ohms for hi-fi systems. Many of these claimed impedances are very misleading, since the actual impedance varies widely with frequency. There will be two impedance peaks at low frequencies, corresponding to the driver and enclosure resonance (the upper peak), with the vent and enclosure resonance providing the lower peak. Sealed enclosures will exhibit only the speaker/cabinet resonance peak, since there is no vent used.

Then there are impedance dips, where the actual impedance may fall to 1/2 (or less) the rated impedance. These are nearly always caused by the crossover networks, and can impose a significant reactive load on the power amplifier. It is very difficult to design the perfect passive crossover network, which is a very good reason to use a bi-amplified system with a good quality electronic crossover network, whose characteristics are far more easily controlled than any passive design.

The reactive load imposed on the amplifier by a speaker load causes far higher dissipation in the output transistors than the simple resistive load generally assumed during testing. At the extreme end, consider a load which is completely reactive (i.e. inductive or capacitive). The voltage and current are 90 degrees out of phase with each other, and no power is consumed by the load - even though there is voltage and current present (and measurable). Assuming a voltage of 20V and a current of 2A, the actual power is zero, so the amplifier must dissipate not only the normal internal losses inherent in all power amplifier designs, but the 40 Volt/Amps reflected back from the reactive load. (Volt/Amps - or VA - is roughly equivalent to Watts - but only when the load is resistive, implying that work is performed).

In reality, the reactance is always accompanied by some resistance, so the amount of power converted into work (moving the loudspeaker cone to create sound) will always be non-zero. An additional quantity of the supplied voltage and current are converted into heat (another form of work) due to resistive losses in the voice coil and crossover network. The reactive (also known in electrical engineering as the imaginary) component is reflected back into the output of the amplifier, where it must be absorbed and converted into heat.

It is the reflected power from the loudspeakers which is responsible for a great many amplifier failures. Because of the low efficiencies of most modern speaker systems, more power is needed from the amplifier. This means that the amp will have to dissipate more reflected power and this can lead to overheating (or internal "hot-spot" localised heating) which leads to the destruction of the output transistors. Some amplifier protection systems are sufficiently sophisticated that they can prevent this form of damage completely, but will generally provide an additional side-effect - the deterioration of sound quality. This is often noticed as a "grainy" or similarly described quality to the sound, and is difficult to eliminate when protection is used. A certain IC power amplifier I have tested has a very comprehensive protection circuit, which seems to work very well. However, as the amp reaches the point of clipping, the distortion component is multiplied tenfold by the protection circuit, with the result that what should be completely inaudible distortion becomes very audible indeed.

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