My step son & family have bought a second home and are furnishing it with 2nd hand stuff. My neighbor donated a Sony receiver/surround sound without speakers. I found speakers at our dump's "swap shop". Trouble is the amp uses 3 ohm speakers and the ones that I found are 8 ohm.
The speakers will need 1.4 times the voltage to output the same power, but levels are non-linear between the dial & the output, so I'm not sure if I should expect the amp to have enough power (voltage). In other words, if a level of "5" would be normal for 3 ohm speakers, would a level of "10" be enough to drive 8 ohm ones?
Fidelity is not a big concern - this is for a TV system. But if distortion is going to be major, I could match impedance by adding a 5 ohm resistor in parallel with each speaker. But then the power demand on the amp would be that much greater.
Not real "high fidelity", but I have paralleled two 8 Ohm speakers to get 4 Ohm loads... and the power is not wasted :-) ...Jim Thompson
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How loud do you need it to be? I'd just try it. The power and impedance ratings on amps and speakers are almost meaningless, and speaker efficiencies are all over the place anyhow. And you do want this to be cheap.
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You're assuming that the 3-ohm and 8-ohm speakers would have the same acoustic efficiency. That's not necessarily true. Speaker efficiencies vary all over the place, depending on the driver design, size of enclosure, and type of enclosure.
Almost certainly yes. Since audio preamps/receivers usually use an "audio taper" volume control (which, as you say, has a non-linear response curve), a setting of "5" is not a "half power" setting... it's more likely a "half perceived loudness" (or even less). Instead of being 3 dB below full (half power), or even 6 dB below full (half voltage, or 1/4 power) it's likely to be 20 dB or more below full.
In other words, even if the speaker efficiencies are identical, and the 8-ohm speaker is delivering only about 40% of the power of the
3-ohm speaker at any given volume-control level, you won't have to turn the volume control up very far at all to compensate for this... maybe to 6 or perhaps 7, but almost certainly not up as far as 10.
No reason at all to do that.
For one thing... the difference in speaker impedances won't cause "distortion"... just decreased loudness at any given volume control setting (perhaps). All you need to do to compensate, is turn the volume control up a bit.
For a second thing... you do not need to "match" impedances, in an audio amplification situation like this... the amp-to-speaker relationship is *not* one of matched impedances (either identical or conjugate). It's not like matching impedances on a transmission line. The actual output impedance of the amplifier is quite low (probably less than an ohm). Transistor audio power amplifiers are usually perfectly happy driving impedances that are "too high". They aren't happy driving impedances that are "too low".
Adding resistors in parallel with the speakers will simply waste power, as heat in the resistors. The amplifier's output transistors will run hotter than they need to, also... you'd actually be
*reducing* the amp's ability to deliver the necessary amount of power to the speakers, by loading them down in this way.
Go ahead and do it. Try it - just hook up the speakers and receiver with cables, with everything sitting in your living room, before you install everything.
Odds are, it'll work just fine. The quality of the sound you get will very probably depend mostly on whether the speakers you have salvaged are decent, and will have very little to do with the fact that they are "8 ohm nominal" impedance.
If the situation were reversed... if the amp was rated "8 ohm speakers only" and the salvaged speakers were 3 or 4 ohm... then it might or might not be safe to do it. The amp might distort on loud passages, or (at worst) might overheat or shut down or be damaged. An amp designed to work only with 8-ohm-or-above speakers, is often one which was cheaply made and has some significant design compromises (wimpy power supply, output transistors, and/or heat-sink) and simply can't drive a more difficult (low-impedance) load.
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Most likely, "3 ohm" refers to the minimum allowed speaker impedance, going any lower could permanently damage the amplifier. Thus any 4-8 ohm speaker would be suitable for this amplifier.
A surround sound amplifier might be specified as 5x100 W, while the power supply might be able to deliver perhaps 150 W constantly :-). The 100 W output power might have been measured with one channel driven at a time and even then only for a short time (and then the number is multiplied by 5).
In practice, the power supply voltage will sag, when multiple channels are driven at the same time, thus reducing the output available from each channel.
By using 8 ohm speakers instead of 4 ohm speakers, the speaker will draw less current and the power supply voltage will not sag as much, delivering a slightly higher output voltage to the speaker, before distortion starts.
With a stabilized power supply, the audio output level would drop by 3 dB when going from 4 to 8 ohm (assuming same speaker efficiency), however, with a badly regulated power supply, the loss in SPL might be just 1-2 dB.
A better crude way is to use two 8 ohm speakers in parallel; that would give more volume per "unit" of drive due to more speaker area. If you used 3 ohm speakers instead, "the power demand on the amp would be that much greater" also.
Last item first: No need to "match impedance." The output impedance of a semiconductor amp is quite low, much lower than the impedance it has to drive. Distortion at low frequencies should actually be less for higher impedance speakers. (Note: the impedance of a dynamic loudspeaker is not uniform. At the resonance of the woofer, it rises to a peak, then is more or less flat until the woofer's voice coil inductive reactance takes over. Crossovers, midrange drivers and tweeters has less effect, because the power necessary to move those diaphragms is less.
Second to last: Speaker acoustic output will depend on its efficiency, which along with the impedance depends on the magnetic field in the motor gap, number of turns on the voice coil in and near the gap, moving mass of the diaphragm, voice coil, and some amount of the surround, spider, and leads. Here, a higher impedance should correspond to a higher number of turns, so efficiency should be closer than you would think even if the moving mass were the same.
From last to first: If all other things were equal, these would be quieter than a 3 ohm version. Will these speakers be used as the surround sound speakers? Efficiency should be less important than if they were the primary speakers. I'd take them home, with an eye to using them for surround sound anyways if they weren't loud enough cranked full. The price is right.
Twice power is hardly noticeable. Yes, the bass will tend to be 4 times the power due to efficiency increase of two in phase speakers. That's noticeable.
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