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Impedance Matching ....

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hesam_m

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Hi
I want to connect one amplifier's output that has 4-ohm output impedance, to one load that has only 0.5-ohm impedance. I should match these two different impedances. I know two methods :

1 - Using a load with one 3.5 to 4 ohm resistor in series with amplifier output.
2 - Using a impedance matching transformer.

the first method is easier, and I can build a transformer if second method is best, but I want to know whitch of them is better.

Do you know any other method?

Regards
 
You first need to be FAR more specific about EXACTLY what you're trying to do.

4 ohms is a common speaker impedance, but transistor amplifiers have a MUCH lower output impedance than that - adding a 3.5 ohm resistor in series with a 0.5 ohm load means the resistor dissipates 7/8 of the power, and the load only 1/8.
 
You first need to be FAR more specific about EXACTLY what you're trying to do.

4 ohms is a common speaker impedance, but transistor amplifiers have a MUCH lower output impedance than that - adding a 3.5 ohm resistor in series with a 0.5 ohm load means the resistor dissipates 7/8 of the power, and the load only 1/8.

I want to transmit the amplifier power to one coil that has only 0.5 ohm impedance, for some physical experiments. this is my purpose.
Therefore the matching transformer is better ?
 
You can't measure a speaker with a multimeter - that measures resistance, NOT impedance.

Again, tell us EXACTLY what you're trying to do.

Yes, you say right, its resistance is 0.5 ohm but when we check the speaker "resistance" with multimeter, it's also 4-ohm as they write on it.
 
When you measure the speaker with a multimeter, it will measure something less than 4 ohms (in your case it is about 0.5 ohms).
The value of "4 ohms" is determined in a TECHNICAL WAY. It is not a measurement with a multimeter.
It is determined by connecting the speaker to an oscillator and measuring the current into the speaker at a certain frequency, when a certain voltage is present. They NEVER measure the speaker. It is a value worked out from a formula. That's how they get a value of "4 ohms."

If you have an amplifier with an output of "4 ohms" it will connect DIRECTLY to a speaker that has "4 ohms" written on it.
 
What is the inductance of this "coil" and what frequency(s) are you using?
 
Do you need to feed the full power of the amplifier to the 0.5 ohm coil?
 
Yes, I want to transmit the all power to 0.5 ohm load as 4 ohm load. I don't know the L value of the coil. I know only its resistance. maybe I can measure it with L meter. frequency is in audio frequency range and it is not one fixed frequency.
 
Speaker impedance is nominally the ohmic resistance of the coil, inductance and capacitance aren't really taken into account on standard coil+magnet designs. Every speaker I have ever tested on a meter on DC resistance has been within a few percent of it's listed 'impedance' value. I think audio 'impedance' is generally measured at 1khz, but there is no true standard. This is one of the reasons why equalizers exist. Even given a static speaker the enclosure will change it's impedance over a wide frequency range.
In the case of speakers this can all be pretty well generalized, but .5ohms is definitely not a standard speaker. If reactance can be taken out of the picture, you need an 8 to 1 step-down transformer rated for whatever power you're transferring. If your destination load is too reactive and changes impedance dramatically over the frequency ranges your run it at you could very well destroy your amplifier though, so I'll say it again in case you didn't hear Nigel the first three times he said it.

WHAT exactly are you trying to drive at .5 ohms?
 
Get a 10R resistor or 1 ohm resistor and measure the value to make sure your multimeter is reading correctly.

The lowest resistance speaker I had some 20 years ago was 1.5 ohm or 1.8ohm. These were designed to get maximum power into a speaker when used in a 12v car radio.
 
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A half-decent 8 ohm speaker resonates in a sealed enclosure at about 40Hz when its impedance is about 20 ohms. Then its impedance is the lowest at about 400Hz.
Above 400Hz the impedance rises because of the inductance.

Most 8 ohm speakers have an impedance of 8 ohms at 400Hz and a DC resistance of 7 ohms.
 
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