4Ω 8Ω 16Ω audio output

Discussion in 'General Electronics Chat' started by Willen, Mar 2, 2018.

  1. Willen

    Thread Starter Member

    Nov 13, 2015
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    Till now I have never seen any people around me who says the 4Ω the most powerful audio output in power amplifier. All think that the 16Ω is the most powerful output so they try to use 4 Ω output to protect their small speaker. As ohms law, I say the 16Ω output is the weakest output. They don't believe me actually.

    I also want to ask: some small speaker has 8Ω rating, some huge speaker has same rating too. So:
    - What's the difference between 4Ω and 16Ω output of an same audio power amp?
    - What's the difference between very small 8Ω speaker and very large 8Ω speaker?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. dendad

    Well-Known Member

    Feb 20, 2016
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    Basically, the 4Ω speaker is not the most powerful. Neither is 16Ω speaker. To get the most power, the speaker needs to be matched to the amplifier.
    4Ω is handy though, as it can be "more powerful" on low voltage amplifiers. The speakers are driven by the magnetic field generated by the current flowing through the voice coil. So current x turns is the main thing. If you have a 16Ω speaker on a 12V amp, it will be limited to a lot less current than another of 4Ω.
    Generally, a 4Ω speaker can be more "powerful" than a 16Ω on a given amplifier if the amplifier is capable of driving it to full current, but if the amp is designed for 16Ω, running a 4Ω speaker may damage the amp.
    The difference between a very small 8Ω speaker and a very large 8Ω is the power handling capability. If you have an 8Ω 1watt speaker you can drive it on a 5V powered amp, as so you can an 8Ω 100Watt speaker. But driving an 8Ω 1watt speaker on an amplifier that is capable of delivering the +/- 40V swing from it's output will generate smoke.
    Oh, and another thing , if you need to have long speaker cables, higher impedance speaker systems are better as the current is less so less of the signal is lost across the cables.
     
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  4. Willen

    Thread Starter Member

    Nov 13, 2015
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    Thank you both of you. Little diffetent: some power amp has huge impedance matching transformer and it has 4, 8, 16 ohms outputs. But some power amp has huge PNP or NPN set as final stage and do not has impedance matching transformer. In the case of such power amp, which determines its output impedance? Maybe there are a lots of chances of mismatching impedance and signal losses.
     
  5. takao21203

    AAC Fanatic!

    Apr 28, 2012
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    A mismatch can burn out the transistors or the whole Amp can become damaged, even if not, you dont get maximum power.

    Ohms are not mandatory for output power, depends how the Amp is configured. Higher Ohms simply needs more voltage for same effect and theres some limit or parts get very expensive, so they use lower Ohms for high Wattage.

    To some extent the speakers are also cheaper to make, older small speakers often had 50 to 100 Ohms. These are still around but specialized parts. Some even had several 100 Ohms such as old telephones, another approach was to use a small telephone transformer.

    Modern amplifier is D class often.

    Lower Ohms will not damage the Amp unless it is cranked up a lot.
     
  6. BobTPH

    Senior Member

    Jun 5, 2013
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    The output impedance of direct coupled transistor amps is a fraction of an Ohm. They only need to be "matched" to the speaker in the sense that you cannot use a speaker below the minimum impedance it is rated for, which is based on the current it can source and sink. Using a higher impedance speaker will work, but not produce as much power. Doubling the impedance of the speaker reduces the power by a factor of 4.

    Bob
     
  7. MrSoftware

    Senior Member

    Oct 29, 2013
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    The amplifier design determines how much power can be put into your speakers, the mechanical and heat dissipation limits of your speaker determine how much power it will take before it fails. It all works as a system. Some amplifiers can push more power into higher impedance loads, and some can push more power into lower impedance loads, it all depends on the amplifier design.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    The speaker power is V²/R so the power is reduced by a factor of 2 for a doubling of the speaker impedance and a given amp output voltage.

    Tube (valve) amps have a high impedance so they use a transformer to match this high impedance to the low speaker impedance for maximum power transfer.

    Transistor amps have a very low output impedance, so generally don't have an output transformer but they are optimized to give maximum power into a specific speaker impedance.
    Typically that's for 4 or 8 ohms, but some of the automobile subwoofer amps are optimized for 2 ohms.
     
  9. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Large output transformers usually are a part of a tube amplifier. Because even medium-power tube circuits were *very* expensive back in the 40's-70's, getting the most efficient conversion from the high voltage/low current tube circuit to the low impedance speaker was important. An impedance-matching transformer with multiple taps mad a big difference in how the amplifier performed. Generally speaking, output power was limited by the current rating of the tubes, and for any given output current, higher impedance load means higher power.

    Transistor amplifiers are the opposite. Power transistors that can move lots of current are relatively cheap, so the output power is limited by the amplifier's power supply voltage, not the output device's current. Almost all solid state amps do not have an output transformer, so the power amplifier circuit's output impedance is the impedance the speaker sees. Usually is is well below 1 ohm. In that situation and without a matching transformer, a higher impedance load means lower power.

    Tube amp: high impedance current source circuit drives transformer. Turns ratio determines output impedance.

    Transistor amp: low impedance voltage source drives the output directly.

    ak
     
  10. Audioguru

    Expert

    Dec 20, 2007
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    Hi Willen,
    Only amplifiers using old vacuum tubes and PA amplifiers driving many speakers use an output transformer. The vacuum tubes are high voltage, low current and therefore are a high impedance so they need a matching transformer to reduce the voltage and increase the current to drive a low impedance speaker. The PA amplifier uses the output transformer to stepup the voltage so that low current wires can be used to drive many speakers, each speaker having its own transformer to set how much power it uses.

    A modern amplifier has power transistors using high currents and lots of negative feedback so its output level does not change with or without a load. Then its output impedance is extremely low, many are 0.04 ohms or less. The power rating of the transistors and power supply determine how much current it can supply which determines the lowest speaker impedance it can drive.

    If the power supply produces 40V DC then an amplifier powered from it will have a maximum output of 36V peak-to-peak which is 36/2.828= 12.7V RMS which produces a power into an 8 ohm speaker of (12.7V squared/8 ohms= 20W. If the amplifier is linear then it heats with about 15W then the total power the power supply must produce is 35W. If you use a 16 ohm speaker then its current is about half so the amplifier works fine but Ohm's Law says the 16 ohm speaker gets only half the power that an 8 ohm speaker gets. If you use a 4 ohm speaker then it uses double the current that an 8 ohm speaker uses so the power supply and probably the amplifier output transistors are seriously overloaded.
    If the amplifier is designed to drive a 4 ohm speaker then its power supply and amplifier output transistors maximum currents must be twice as much as if it drives an 8 ohm speaker.

    A car amplifier has a 14V power supply from its 12V battery so its output would be 11.5V peak-to-peak which is 4V RMS. Then the power in an 8 ohm speaker is (4V squared)/8 ohms= 2W which is not enough in a noisy car. Then a 4 ohm speaker would take about 3.9V RMS from the amplifier and produce (3.9V squared/4 ohms)= 3.8W which is still not enough.
    How do you get more power? Provide more voltage.
    How do you provide more voltage? Use two amplifiers with the speaker bridged between them. Then the speaker gets almost double the voltage and also uses almost double the current so the power in the 4 ohm speaker is about 3.8W x 1.9 x 1.9= 13.7W.

    I had a car that used bridged amplifiers driving 2 ohm speakers. Then the power in each speaker was about 24W.
    I am talking about real Watts, not fake Whats that are advertised.
     
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  11. MrChips

    Moderator

    Oct 2, 2009
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    There are car amplifiers that can output 100W and more.
    How do they do this from a 12V battery? They use a switching power supply to boost the power supply to ±40V or more.
     
  12. Audioguru

    Expert

    Dec 20, 2007
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    No. Ohm's Law says it reduces the power by a factor of 2.
     
  13. Audioguru

    Expert

    Dec 20, 2007
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    An ordinary car radio is rated at 200W (200 Whats, not 200 Watts). It has 4 channels so each channel is 50W. The real 14 Watts per channel is 21W when it is turned up so that its sound is very distorted and 21W continuously has a peak power of 42W. Then they add a little more power because the battery is over-charging.
     
  14. dendad

    Well-Known Member

    Feb 20, 2016
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    And a lot of amplifiers nowadays are not rated at RMS power but Advertising Division Power that has a multiplication factor determined by the number of LEDs and chrome knobs ;)
     
  15. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Technically and pedantically, Watt's Law.

    ak
     
  16. Audioguru

    Expert

    Dec 20, 2007
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    I was told that the rated power of a speaker or amplifier has the age of the salesman's grandmother multiplying the true number.
     
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  17. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Many years ago 3.2R was most common, but 8R became more or less the standard.

    4R is very common in car audio because you can get nearly 5W from the limited voltage headroom - a BTL amplifier gets somewhere close to 17W.

    16R is fairly common on low power gadgets and seems to be favourite for Walkman style ear buds.
     
  18. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    Tube stuff: Impeadance matching. use the right tap for the best power. Don;t run a tube amp without spaeakers connected.

    Solid state stuff: Modern solid state amps have output Z in the fraction of an ohm. They have both a voltage and a current limitation which determines the load limitation.

    There were 40 ohm voice coils in the days of automotive tube amps and cars started out a 6V systems,

    That said, you then had 12 V DC to play with. To get higher power you can reduce the speaker impeadance to a point. A simple way of getting higher power is to use a bridge amplifier. This EFFECTIVELY gets you +-12 Volt supply to work with and with 4 ohms a decent amount of power.

    To go higher, you need to lower the speaker impedances or increase the voltage. So, switching power supplies took over again with a common 4 ohm load.

    Now the sub-woofers, The need for high power and low frequencies. So 2 ohm might even be common. Putting the amp in the bx (powered subwoffer) is also common.

    Earbuds were addressed above. 3.2 ohms was used for tube stuff too.

    You can parallel and series combinations up till what the amplifier can drive. Its current and voltage limitations.

    I missed all of this when I built the Leach Amp in the 80's. A 3 A supply can only get me 3*3*8 or 72 W; yet the transistors are capable of 30 A. I can have higher peak power because of capacitors. 10,000 uF per rail.
    I did run an 18 A secondary and the amp sounded really good all around, but the transformer made too much noise. The amp when described in the article said it was used for electrostatic speakers. Current is really low for these speakers. They are huge, so they are loud,

    The impeadance of the speaker is all over the map, so 400 Hz is used and the rated Z is called a nominal impeadance, The resistance of the speaker is lower than the nominal number.

    A Positive voltage pushes the cone outward. A 1.5 V battery can be used to test speakers.

    To sum things up, I had my amp (72 W) against a Macintosh tube amp that may have been under 10 W with horn speakers. The tube amp drove the horns better with less bass. The Leach did a really good job with the bass and an OK job with the horns. The music used was classical.

    I like dome tweeters and Folk music. No comparison was done with that. So, the horns reproduced the violins very well.

    The SPL level at 1m for a given power output is what you really need to look at.
     
  19. BobTPH

    Senior Member

    Jun 5, 2013
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    Right, I was thinking about the relation between voltage and power, which is a square.

    Bob
     
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