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Originally Posted by cooperpwc /img/forum/go_quote.gif
Okay, so what is the practical implication of this +1.5vdc offset and purported lack of caps in signal path for driving headphones directly from the line out? Thanks!
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Right, well, it's like this.
Audio is AC, right. alternating current. But amps and whatnot run on DC.
The H120, having a battery pack, has a power supply of about 3 volts.
Since 3 volts isn't much to work with, the amp is run off of a single DC power supply.
This means that, on average, the output from the amp has about 1.5 volts of DC. Really, it's DC that swings up and down a whole lot, but it's an average of about 1.5 volts.
Capacitors don't pass DC. We won't get into the science, they just don't.
So, to protect the headphones from meltdown, the headphone jack has a capacitor in-line on it.
Because the capacitor and the load across it form a high-pass filter, and headphones are relatively low impedance, the capacitor has to be fairly large. Hopefully as large as 470uf.
line-input loads are typically very high impedance, tens of thousands of ohms, and so for the same frequency response, they can protect their input with a fairly small amount of capacitance - usually 1uf or less.
So, here's the deal: electrolytic capacitors are the most compact we've got, but they don't always do such a great job of passing an audio signal without distorting it. So, we prefer film capacitors in the signal path.
470uf of film caps is bigger than your fist. You won't see it in a portable device. But 1uf can be the size of an english pea.
So, working on the theory that film caps are better than electrolytics, and that a line-input device will have a protection capacitor pretty much always, the line-out on a device may as well not have a filter cap across it.
In general, fewer capacitors means less phase distortion, but you would be shocked at the number of cheap capacitors and jellybean chips that some of your favorite recordings have gone through before they got anywhere near the CD you bought.
Thousands. Even tens of thousands.
But hey, may as well take a few out of the path where you can.
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Originally Posted by epaludo
Didn't the line-out bypass the internal amp? So we are able to use a high quality external amp to improve sound quality.
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You'd hope so, but you'd be wrong 99.99% of the time.
That's the way it SHOULD be, but in truth nearly all of the compact consumer-grade DAC chips have a built-in amp and no line-out capability. I would be surprised if the H120 has separate dac and amp chips.
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| And the optical output (as in the Iriver H120) bypasses both internal amp and DAC, right? I think i read this somewhere ... |
Of course it does, it's digital, what need does it have for a DAC?
If it is right, does anyone know which mp3 players work in the same way?
