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Quick OTL / transformer coupled question - Page 2

post #16 of 21
11/6/09 at 1:31pm
Quote:
Originally Posted by Hz_joe View Post
Winding resistance not 16! multiplied by a factor of 16 is AC impedance.
When used as a 1:4 step up, the primary's winding resistance will also be multiplied by a factor of 16.

The total output impedance as seen by the headphones will be:

((Rsource + Rpri) x 16) + Rsec

Where Rsource is the output impedance of the amplifier that's driving the transformer, Rpri is the transformer's primary winding resistance and Rsec is the transformer's secondary winding resistance.

Quote:
May be you can found Hints on commercial products. It series a 100 Ohm or high resistor at output, Why ?
Most commercial headphone amplifiers have very low output impedances. And most users seem to prefer driving their headphones from low impedance sources. Though some do prefer higher output impedances and have added resistance to increase output impedance.

And I'm not saying no one should use a transformer. I LOVE transformers. All I'm saying is that it would be useful to know the winding resistances so people can assess whether or not using a transformer, particularly as a step up, will work for them.

se
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post #17 of 21
11/6/09 at 2:05pm
Quote:
Originally Posted by zigo View Post
Very interesting thread!

By sure OTL's, usually, improve their performances proportionally with the increase of the headphone's impedance.
@ Hz_joe
Then the amplifier "see" your transformer like an high impedance "charge", and the output impedance to the headphone is correct? 4:1, it means that the ampli, for example, see a 240Ohms charge and the output is 60OHMs?
Please, the transformer attenuates the power?
An ideal transformer does not attenuate power. What gets attenuated (or amplified depending on whether you're talking step up or step down) is voltage.

Power in and power out remain the same due to the transformer's reflecting impedances by the square of the turns ratio.

Ignoring the transformer's winding resistances, if your amplifier's output impedance is 100 ohms and your headphone impedance is 600 ohms, then when used as a 4:1 step down, the amplifier's 100 ohm output impedance gets reflected to the headphones as 6.25 ohms, and the 600 ohm headphones get reflected to the amplifier as 9,600 ohms.

When used as a 1:4 step up, the amplifier's 100 ohm output impedance gets reflected to the headphones as 1,600 ohms, and the 600 ohm headphones get reflected to the amplifier as 37.5 ohms.

Quote:
Is it possible do the same, or something close, with a resistor?
Not really, no.

se
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post #18 of 21
11/6/09 at 2:15pm
Quote:
Originally Posted by Koyaan I. Sqatsi View Post
When used as a 1:4 step up, the primary's winding resistance will also be multiplied by a factor of 16.

The total output impedance as seen by the headphones will be:

((Rsource + Rpri) x 16) + Rsec

Where Rsource is the output impedance of the amplifier that's driving the transformer, Rpri is the transformer's primary winding resistance and Rsec is the transformer's secondary winding resistance.



Most commercial headphone amplifiers have very low output impedances. And most users seem to prefer driving their headphones from low impedance sources. Though some do prefer higher output impedances and have added resistance to increase output impedance.

And I'm not saying no one should use a transformer. I LOVE transformers. All I'm saying is that it would be useful to know the winding resistances so people can assess whether or not using a transformer, particularly as a step up, will work for them.

se
If you say AC impedance is correct but I misunderstood you say DC resistance ! Actually DC resistance is not a fix factor. It is depend on wire gauge !
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post #19 of 21
11/6/09 at 2:20pm
Quote:
Originally Posted by Koyaan I. Sqatsi View Post
When used as a 1:4 step up, the primary's winding resistance will also be multiplied by a factor of 16.

The total output impedance as seen by the headphones will be:

((Rsource + Rpri) x 16) + Rsec

Where Rsource is the output impedance of the amplifier that's driving the transformer, Rpri is the transformer's primary winding resistance and Rsec is the transformer's secondary winding resistance.



Most commercial headphone amplifiers have very low output impedances. And most users seem to prefer driving their headphones from low impedance sources. Though some do prefer higher output impedances and have added resistance to increase output impedance.

And I'm not saying no one should use a transformer. I LOVE transformers. All I'm saying is that it would be useful to know the winding resistances so people can assess whether or not using a transformer, particularly as a step up, will work for them.

se
No ! 100 Ohm resistor is not add by end users but Ex-Factory. I can remember Solo or Lemm has a 100 Ohm resistor at output.
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post #20 of 21
11/6/09 at 2:32pm
Quote:
Originally Posted by Hz_joe View Post
If you say AC impedance is correct but I misunderstood you say DC resistance !
I did mean DC resistance, as in the DC resistance of the transformer's primary and secondary windings. These resistances get reflected by the transformer by the square of the turns ratio and contribute to the impedances seen at the input and output of the transformer.

For example, if you're using the transformer as a 1:4 step up, and the DC resistance of the primary is 5 ohms, then that 5 ohms will contribute 80 ohms to the output impedance seen across the transformer's secondary.

Quote:
Actually DC resistance is not a fix factor. It is depend on wire gauge !
Yes, it depends on wire gauge and the number of turns, but it will be a fixed factor for a specific transformer.

se
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post #21 of 21
11/6/09 at 2:40pm
Quote:
Originally Posted by Hz_joe View Post
No ! 100 Ohm resistor is not add by end users but Ex-Factory.
Some users HAVE added their own resistance to their headphone amps because their headphone amps had very low output impedances and they wanted to have a higher output impedance for driving their headphones.

Quote:
I can remember Solo or Lemm has a 100 Ohm resistor at output.
Sure. Some manufacturers do make headphone amps with higher impedance outputs, often using resistors to do so. But not all of them. Most of the commercial headphone amps I've surveyed have had output impedances of 25 ohms or less.

se
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