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Originally Posted by jpelg /img/forum/go_quote.gif
Also, why is it so uncommon for headphone amps to have multiple source-switching capability? The easiest answer at the lower end is cost. However, as we approach or exceed the US$1k mark, this should not be an obstacle, and imo should be a standard feature at that price point.
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I think there are 2 things at play here.
In headphoneland, I dont think people are as concerned with multiple sources. Im seeing the same thing in a LOT of speaker-land too. Its CD or nothing for a lot of people. I think its kind of weird NOT to have a turntable and radio tuner (although I am radioless for the moment), but this is what I see.
There are some stumbling blocks with a GOOD source selector.
You have the one camp who takes alllll the wires in a big bundle from the back of the case to a selector in the front. This is probably the least expensive to build, but you have MANY MANY wires all physically in parallel which is not good for cross-talk.
Then you have the camp with relay-based switches mounted near the inputs. This is better, but there is some debate over whether relays have an effect on sound.
Next you have the folks with only a couple (usually 2 or 3) inputs, and a switch with the control in the back of the case. This gains the benefit of shorter wiring, BUT you have to reach behind the amp.
And lastly you have the group who mount a the switch in the back of the case with an extension to the front. No relay questions, shorter distance from input to switch, BUT requires more case work and a clear shot from the front to the back of the case.
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| The second side would from the premise that headphone amps have more in common with traditional preamps, particularly with respect to the levels of gain required to amp headphones, and particularly low-impedance cans. Of course, the issue here (at least one anyway) is current buffering. |
I think you mean gain required for HIGH impedance cans. Even with high impedance cans, people have had good success running off of a buffer DIRECTLY from a standard CD-player. If you have an RMS-meter, measure the voltage going to a HD-600 in actual use, Gain dosnt look so important anymore
Headphone amps do have more in common with a preamp as far as "signal purity" is concerned, but have the interesting requirement (for a preamp) of putting POWER into the load.
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| Which approach is better? If I am using efficient, low-impedance cans, is a quieter preamp setup with some high-current buffering to keep them well driven, the way to go? |
You know there is no definitive answer to this
I would suggest trying it out, and seeing if you like the results. If you can build a cmoy, just build it unity-gain with a unity-gain stable opamp.
It is actually a shame that nobody has built *just* a discrete buffer (even a simple mosfet emitter follower) as a commercial offering. Too many people think they need gain....
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| Also, what are the pros & cons of the various ways of adapting a preamp's output for driving headphones? |
If the output impedance of the preamp is low enough (and current capacity and caps are adequate) you can just plug the headphones in to the outputs. This keeps at least 1 active part out of the chain. EVERY active part adds distortions...
There is something of a stumbling block here. It is possible to have a VERY low output impedance that cant drive a headphone with some topologies. If the output stage is built to have an output impedance of say 12 ohms BUT a class-A bias of only 2mA it is possible that the preamp comes out of class-A, which at best screws up the sound, and worst damages the headphones. More often screwing up the sound.
If the output of the preamp cant drive the headphones directly this is where the additional active parts in a buffer can totally save the day.
