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There are 4, 6, 8 ohm speakers with this input impedance, but no 4, 6, 8 ohm amplifiers with this output impedance. 
It is customary for the connection of audio devices to talk of the source and load impedance or output and input impedance. If we hear something of an "8 ohm amplifier", then we think usually of the 8 ohm loudspeakers. 
Here are people asking for help in the forums. They seem to be told
something wrong about the actual "impedance" of the power amplifiers and
about the speakers and the connections: How to hook a 4 ohm speaker to a 8 ohm amp? Will 4 ohm speakers work with an 8 ohm amplifier? Must I connect 8 ohm speakers to 8 ohm amp? Connecting 4 ohm speakers to an 8 ohm receiver 4 ohm output into 8 ohm loudspeakers 8 ohm speaker to 4 ohm channel? 8 ohm speakers 4 ohm amp 4 ohm amplifier with 2 or 8 ohm subwoofer 4 ohm speaker on 8 ohm amp? 8 ohm speakers  4 ohm amp? Can 4 ohm speakers cause an 8 ohm amp to fail Can you use 4 ohm speaker on a 8 ohm amp? 4 ohm sub to a 6 to 8 ohm amp Will 16 ohm speakers work with an 8 ohm amplifier 4 ohm speaker on 8 ohm amp? 4 ohms speakers on a 8 ohms amplifier? Matching speakers to amp (4 ohm to 8 ohm)? Can I use 8 ohms amp to 6 ohms speakers? Can a 8 ohm speaker damage 6 ohm amp? Where can I find audio receivers that have an actual impedance of 4 ohms? Do we have to connect an output impedace from an amp with exactly an input impedace of the speaker? Oh no, never! All the questioners think with a great determination that the speaker amplifier must have a large output impedance of 4, 6, 8, or even 16 ohms. ... and here comes the a disappointment: 
You can only find power amplifiers with an actual
output impedance of less than 0.1 ohm. Z_{out} < 0.1 Ω. Please read this here: "8 Ohm Output" and "150 Ohm Input" − What is that? There are no 4 ohm or 8 ohm speaker amplifiers with this high output impedance. There is really no matching in audio technology. It's a myth when you are told that you need matching of the amplifier and the loudspeaker. Really wrong is: Z_{out} = Z_{in}. Audio technology needs bridging: Source Z_{out} < load Z_{in}. 
An AV (audio/video) receiver has essentially zero output impedance (0 ohms) so you do not have to match the impedance of your amplifier to the speakers. The amplifier does not expect to "see" speakers of given impedance and you can connect loudspeakers with different input impedances (8 ohms, 6 ohms, 4 ohms) to an receiver with no negative effects so long as the impedance of any of your speakers does n't go below 4 ohms. If speaker impedance is too low the weak power supply of the amplifier gets problems. Too much current will run through the AV receiver's output transistors, causing the receiver to overheat and shut down. If you get 4ohm speakers, make sure your AV receiver is able to drive them easily. 
The nominal input impedance (Zload) of a loudspeaker can
be measured to be 8 ohms. The nominal output impedance (Zsource) of a power amplifier can never be measured to be 8 ohms. The real output impedance (Zsource) of a power amplifier is usually less than 0.1 O, but this is rarely specified. Instead it is "hidden" within the damping factor parameter. 
In audio technology even for speaker amplifiers (power amplifiers), we use only
voltage bridging. The frequent call for "real" matching Z_{source} = Z_{load} is really wrong. There is no speaker impedance matching. We got speaker impedance bridging. 
The seldom specified source resistance Z_{source} is hidden in the damping factor D_{F}. A damping factor of at least D_{F} = 100 is entirely in order. Loudspeaker and amplifier connections have a source impedance (amplifier) of Z_{output} less than 0.04 ohms, with a load impedance (loudspeaker) of Z_{input} greater than 4 ohms. Impedance matching (power matching) Z_{1} = Z_{2} on the other hand, is only used in telecommunications and RF technology applied to antennas. 
Notice: There are really no 4 ohm amplifiers and there are no 8 ohm amplifiers for the 4 ohm or 8 ohm loudspeakers. 
In audio technology, even with power amplifiers and speakers we use voltage bridging Z_{out} << Z_{in}. The frequent call for "real" impedance matching Z_{out} = Z_{in} is wrong. 
There is no impedance matching Z_{out} = Z_{in} between power amplifier and loudspeakers. 
No matching of speaker impedance (load) to amplifier  No loudspeaker load impedance matching  Matching is a myth. We got speaker impedance bridging Z_{source} < Z_{load}. (Z_{out} < Z_{in}) That is the case even with valve or tube amplifiers. 
In sound engineering there is no Power matching or Impedance matching. In audio we use only Voltage bridging or high Impedance bridging. 
A loudspeaker with a nominal impedance of Ra = 8 ohms gets this value of the input resistance actually by the loudspeaker manufacturer. An amplifier with a nominal impedance of Ri = 0.01 ohm resistor does not get this value of the output resistance by the amplifier manufacturer. They are talking of incorrect 8 ohms? Why is this the case? 
The output impedance of the amplifier is always a tenth or a hundredth of the value of the loudspeaker impedance. The value of the output impedance of the amplifier Z_{source} is always hidden in the damping factor D_{F} and can easily be calculated: Z_{load } is the impedance of the loudspeaker. The cable resistance (impedance) is a part of the output impedance of the amplifier. Because of the high voltage of the source, cable screening (shielding) is not necessary in this case. The typical continuous query in the forums: "Can I use 4 ohm speakers to a 6 ohms amplifier?" (See reply under "Notice".) These issues show that there must be storytellers underway. How can we get rid of the nonsense of 2 to 16 ohms amplifiers? The output impedance of an amplifier Z_{out} is always smaller than 0.5 ohm. Z_{out} << Z_{in}. Most people are happy to speak of an 8 ohm amplifier, if they think not correctly that the amplifier output should be connected to "8 ohm speakers". Dealers like to use exactly these unclear expressions. In reality, the speaker impedance curve of a loudspeaker is never a straight line; see: Typical loudspeaker impedance curves vs. frequency 
Impedances behave like an unloaded voltage divider. Z_{load} = Z_{1}and Z_{source} = Z_{2}.
Signal loss in dB: 20 log [Z_{1} / (Z_{1} + Z_{2})] Damping factor: D_{F} = Z_{1} / Z_{2} Source impedance in ohms: Z_{2} = Z_{1} / D_{F} 
Calculation of theoutput impedance Z_{out} from the damping factor DF
Please enter two values, the third value will be calculated.
Z_{1} = load impedance and Z_{2} = source impedance.
Calculation of the damping of impedance bridging or power matching an interface connecting Zout and Zin Calculations: voltage divider (potentiometer)  damping pad  loaded and open circuit (unloaded) 
Please tell your friends: There are really no 4 or 8 ohms amplifiers to match the speakers.
The damping factor is usually given as numerical value, but also as decibels.
Simply enter the value to the left or the right side.
The calculator works in both directions of the ↔ sign. 
Calculation of the damping factor.
How do I calculate the damping factor D_{F} for example, at 1 kHz, if neither the impedance of the source Z_{2} nor the impedance of the load Z_{1} is known? Allow the source to send out of a 1 kHz sine tone and measure the resulting voltageV_{0} at the output without any load. Then measure at this point the voltage V_{L}, when the load is applied. The damping factor is: 
The impact of input impedance and output impedance
of studio gear for bridging in audio engineering:
Measurement of input impedance and output impedance Calculations: voltage divider or potentiometer − Loaded and open circuit (unloaded) Bridging (voltage) or matching (power) − Interface connecting Zout and Zin impedance Voltage bridging or impedance bridging − Zout < Zin Cable length, cable capacitance, and treble loss (Attenuation, cutoff frequency) 
Fortunately, there are no amplifiers with an output impedance of
4ohm or 8ohm which have to fit to speakers with these values. We have no power matching, we have voltage bridging, whereby the power amplifier often has an output impedance of only one hundredth of the speaker's input impedance. At power amplifiers for musicians usually you can read at the output plugs: 4 ohms to 8 ohms − to tell the user that only a 4ohm speaker or an 8ohm speaker has to be used and does not give the "correct" output impedance value, which is around 0.1 ohms. This is often not known by users. 

There are no stupid questions  or perhaps: Can I operate an 8 ohm speaker with a 4ohm amplifier? While there are speakers with a nominal input impedance of 8 ohm, there is no amplifier on the market with a nominal output impedance of 4 ohms. All speaker amplifiers have an output impedance of less than 0.1 ohm; see: http://hyperphysics.phyastr.gsu.edu/hbase/audio/imped.html#c2 Citation: Modern audio amplifiers are active control devices, and the impedance matching of the amplifier to the loudspeaker is since 1965 no longer considered best practice. That means: Z_{out} << Z_{in}. 
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