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Originally Posted by Uncle Erik /img/forum/go_quote.gif
There have been such products. I believe that Eddie Current offered one for a bit as well as Antique Sound Labs. I think people didn't quite understand them and they didn't sell many.
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I can relate to that.
Some years back I was making a ground isolation unit, which was essentially a nice box with a pair of high quality input transformers in it.
Even though my website explained in great detail what it did, I'd still get phone calls and EMails from people asking "What's it do?"
*sigh*
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| If you want to experiment with variable resistance, get a decade box. Well, you'd need two of them for stereo. |
And four for balanced stereo.
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| Another solution is using an autoformer. It's a specialized transformer that matches impedance. |
Not quite. An autoformer is an autoformer, not a specialized transformer that matches impedance.
An autoformer functions just like a transformer, except that it uses one winding, instead of two (in essence, it's just an inductor with one or more taps). And just like a transformer, it has no intrinsic impedance of its own, but rather, it reflects impedances as a function of the square of the turns ratio. And just like a transformer, it can step up the signal or step down the signal as a function of the turns ratio.
The only fundamental difference between an autoformer and and a transformer is that they're a bit simpler to make as they only have one winding, and because they only have one winding they can't provide the ground isolation that a transformer can.
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| There's one on the market called the Zero that's marketed towards speakers. The problem with those for headphones is that headphone amps don't put out much power. All transformers eat power when you run a signal through them, so something like the Zero would sharply cut what gets through to headphones. Speakers aren't a big deal since a power amp will give you, say, 50W. A little loss isn't that noticeable. However, when you put, for example, 350mW into one, you are really going to notice the loss of power. |
A properly designed autoformer or transformer won't eat up much power at all. While there are some losses due to the core, the main power loss mechanism in an autoformer or transformer is the winding resistances. But if the transformer's designed properly (basically using sufficiently large gauge wire to keep the winding resistance to a minimum) the power loss will be negligible.
In any case, you wouldn't want to use an autoformer or a transformer for the purpose of increasing the output impedance of an amplifier. Because to do so means the autoformer or transformer needs to function as a step-up.
Let's take PJ's case as an example.
I believe the Phoenix specs its output impedance as 3 ohms. And let's assume that PJ wants to step that up to the 100 ohms that he's using now.
As mentioned previously, an autoformer or transformer reflects imedances as the square of the turns ratio. 100 divided by 3 is 33.33. The square root of 33.33 is 5.77. So in order to turn that 3 ohms into 100 ohms, you'd need a step up autoformer or transformer with a turns ratio of 1:5.77.
Not so bad in and of itself.
However the problem arises when you consider the fact that autoformers and transformers don't reflect impedances in just one direction, but bi-directionally.
Looking from the output of the headphone amp toward the headphones, you have a step up function. But looking back the other way, from the headphones to the headphone amp, you have a step down function, and instead of increasing the impedance by the square of the turns ratio, you decrease the impedance by the square of the turns ratio.
So instead of the Phoenix seeing PJ's 300 ohm Sennheisers as 300 ohms, it will see them as 300 ohms divided by 33.33, or just 9 ohms.
So no, an autoformer or transformer wouldn't be the thing to use for increasing the output impedance of a headphone amp. Best thing for that is good ol' simple resistance.
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| Finally, you could try adapting a balun for headphone use. Baluns are commonly used to match impedance between a radio transmitter and an antenna. Hams use them all the time to maximize output and minimize reflected energy for their broadcasts. I don't know if the technology would lend itself to audiophilia, but it certainly works in radio. |
The technology of the balun is the same as that of an autoformer or transformer. That's because that's what most every balun is, an autoformer or transformer. Only that they're designed for use at radio frequencies.
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