Hi,
I have been auditioning a cheap, but very nice sounding Chinese made chip amp board with a variety of different speakers.
With a pair of 15 inch 15 Ohm vintage "Monkey Coffins" I get get some serious volume and the little heatsink only gets a couple of degrees over ambient.
Same volume with 8 Ohm speakers the heatsink is warm and with 4 Ohm speakers the heatsink gets bloody hot.
The only reason I can imagine for the development of 4 Ohm speakers is that the extra current is required for cheaper magnets with weaker magnetic fields?
I have been auditioning a cheap, but very nice sounding Chinese made chip amp board with a variety of different speakers.
With a pair of 15 inch 15 Ohm vintage "Monkey Coffins" I get get some serious volume and the little heatsink only gets a couple of degrees over ambient.
Same volume with 8 Ohm speakers the heatsink is warm and with 4 Ohm speakers the heatsink gets bloody hot.
The only reason I can imagine for the development of 4 Ohm speakers is that the extra current is required for cheaper magnets with weaker magnetic fields?
you are comparing speakers with not only different (decreasing) resistance, but with different, most likely decreasing efficiency too
to answer your question, why 4 ohm? they produce low efficiency speaker at 4 ohms, so it actually makes some sound, if they would make it 16 ohms, it would be quiet like headphones...
power is cheap...and you chip amp is weak
to answer your question, why 4 ohm? they produce low efficiency speaker at 4 ohms, so it actually makes some sound, if they would make it 16 ohms, it would be quiet like headphones...
power is cheap...and you chip amp is weak
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The DC resistance or AC complex impedance of a loudspeaker's voice coil does not necessarily have any relation to its sensitivity / efficiency.
Indeed, if higher impedance drivers, such as in your 15ohm vintage monkey coffins, have both more voice coil in the gap and higher magnetic flux density at that point, they may well be substantially more efficient than modern / lower impedance drivers. Keep in mind that your drivers may harken back to an era when amplifier power was much more expensive per watt than these days.
Why low impedance these days? At least one reason would relate to what Adason noted - power is cheap these days, and within limits of their SOA most SS amps can deliver substantially more power into lower impedance loads (albeit with more distortion)
It's only one of numerous factors that are part of the calculus of compromise with which a driver / system engineer wrestles.
Indeed, if higher impedance drivers, such as in your 15ohm vintage monkey coffins, have both more voice coil in the gap and higher magnetic flux density at that point, they may well be substantially more efficient than modern / lower impedance drivers. Keep in mind that your drivers may harken back to an era when amplifier power was much more expensive per watt than these days.
Why low impedance these days? At least one reason would relate to what Adason noted - power is cheap these days, and within limits of their SOA most SS amps can deliver substantially more power into lower impedance loads (albeit with more distortion)
It's only one of numerous factors that are part of the calculus of compromise with which a driver / system engineer wrestles.
Because efficient speakers are the size of monkey coffins and people want the size of Bose cubes.
@ radiosmuck
4 Ohm Amplifiers, for the same power output as an 8 Ohm Amp, don't need as high a voltage power supply to achieve the same wattage. So "can" be more effecient, depending on the speakers used !
That's why vehicle Amplifiers were designed for 4 Ohms, initially. In recent times some are now capable of 2 - 1 Ohms, an naturally increased power outputs.
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@ Cal Weldon
Hi, the reason 4 Ohms is often used in PA systems, is to save $ on extra Amps, plus the weight advantages that go with it.
4 Ohm Amplifiers, for the same power output as an 8 Ohm Amp, don't need as high a voltage power supply to achieve the same wattage. So "can" be more effecient, depending on the speakers used !
That's why vehicle Amplifiers were designed for 4 Ohms, initially. In recent times some are now capable of 2 - 1 Ohms, an naturally increased power outputs.
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@ Cal Weldon
Hi, the reason 4 Ohms is often used in PA systems, is to save $ on extra Amps, plus the weight advantages that go with it.
Vehicle amplifiers and portable PA equipment need to run on low voltage batteries and high current, thus 4 Ohm speakers are the choice if you want to make a lot of noise and get noticed.
I seem to remember now that the recommended impedance for the TDA7247 is 8 Ohm.
I would still like to know why 4 Ohm speakers are in home hi-fi?
I don't have technical background many of you have, but perhaps the reason is as simple as the frequency response from a given driver. Recently I have purchased large quantities of ScanSpeak drivers in both 4 and 8 ohm configurations, because while they are physically the same, the different impedance produces different frequency responses.
I'm not sure this applies to my 15 Ohm speakers. Taking the forums test http://www.diyaudio.com/forums/mult...much-voltage-power-do-your-speakers-need.html.
The 15 Ohm Tannoys only draw 0.4v which calculates out to 0.24 watts RMS at my usual serious listening volume. Perhaps I should try the test out on the 4 Ohm speakers I mentioned?
Speaker efficiency has almost nothing to do with coil impedance. Heavy coned drivers eg polypropylene neeed 100W+ drive. To generate >100W at 15R would require power rails of about +/-70V. This is beyond most IC processes and require high voltage (slow) power transistors. Generating 100W at 4R can be done by a couple of paralleled chip amps
Because low efficiency means heavy moving mass and weak magnets.
This not only decreases efficiency, it also improve deep bass abilities.
This does not necessarily have any relation to driver impedance. There
is some minor relation though.
Many PA woofers come in a choice of either 4, 8 or 16Ohm.
The latter two are usually paralleled to present a 4 or 2 Ohm load to the amp.
4Ohm drivers are handy for 4 driver bass bins in serial/parallel configuration.
All depends on the layout of the rig.
Practically all PA amps list their power rating at 4 Ohm as this is the most common load they'll experience.
Weight and space are a very serious financial consideration when it comes to large touring rigs. A high-powered amp that is stable into a 2 Ohm load pays for itself in no time.
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