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If a K1000 runs off of speaker outputs, why not any headphones?

post #1 of 28
5/12/08 at 12:25am
Thread Starter 
That's basically my question.

I've searched the archives and run into a couple of explanations about headphones being the wrong impedance (sure, 30 ohms + rather than 4 or 8 for speakers), but then suggesting that resistors be used to -increase- the impedance (making it more unlike speakers).

The headphones in question are my ATH-M50's, which I've been thinking about modding anyway, so maybe I could rewire to balanced/dual mono as an experiment.

The amp is my new Nuforce Icon, which puts out 12 watts. I'm interested in trying the speaker outs on it because I think the main amp has some distortion-reduction circuitry that's not present in the headphone amp.

Sorry for the dumb question... just starting to try and understand this stuff!
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post #2 of 28
5/12/08 at 12:42am
You need to be careful doing that because speaker amps put out a lot more power than most headphones can handle. K1000s are often run off small power amps because they require huge amounts of power to function properly, but that amount of power would likely cook most headphones.

And yeah you can use resistors, that's how a lot of older integrated amps handle the headphone output.
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post #3 of 28
5/12/08 at 1:06am
I've heard that the Grado HP1000 was recommended to be run off speaker taps. There's no reason why you couldn't run a headphone off them, but you won't be able to turn the volume up much at all before the it becomes very very loud and destroys them, unless the headphones are very high impedance (~400ohm+), or the very low sensitivity like the K1000.
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post #4 of 28
5/12/08 at 1:15am
regardless of volume and other technical things, I noticed that because of the huge gain compared to headphone amps, there is a large amount of noise and hiss(gain hiss). I guess this depends on which headphones and which amplifier, but I think without changing the gain you will get alot of noise.
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post #5 of 28
5/12/08 at 1:24am
I'd like to know the differences as well. The first thing that comes to mind is the Voltage levels are probably much higher with speaker amps. Headphones need very little Voltage to be driven loudly. Adding large enough value resistors in series with the headphones will give a non-reactive load that will give a nice Voltage drop in relation to the voice coils.

Speaker amps (usually) have multiple gain stages. Each gain stage adds it's own noise and amplifies the noise of the previous gain stages. Headphones don't need lots of Voltage gain and in some cases no Voltage gain is needed at all. Example
So why add more noise when you don't need the extra Voltage?
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post #6 of 28
5/12/08 at 1:35am
Most headphones would easily be melted or otherwise destroyed by a good speaker amp.

Quote:
Originally Posted by enjoiflobees View Post
regardless of volume and other technical things, I noticed that because of the huge gain compared to headphone amps, there is a large amount of noise and hiss(gain hiss). I guess this depends on which headphones and which amplifier, but I think without changing the gain you will get alot of noise.
Oh god, you people actually believe that more gain necessarily means more noise?

No. Only if it's a terrible amp design with really poor noise rejection. Which in fairness would include the majority of consumer-grade speaker amps.

Quote:
Originally Posted by plaidplatypus View Post
I'd like to know the differences as well. The first thing that comes to mind is the Voltage levels are probably much higher with speaker amps. Headphones need very little Voltage to be driven loudly. Adding large enough value resistors in series with the headphones will give a non-reactive load that will give a nice Voltage drop in relation to the voice coils.
Quite common in vintage 2-channel speaker amps. My Rotel RA-820bx for example just pads the speaker output with a pair of 330-ohm resistors. In a lot of cases, it's advantageous to load the amp with a resistor as well - think of this as being like the 'class-a biasing' we sometimes do with headphone amps, but rotel didn't do that - in fact the rotel amp doesn't disable the speaker output just because you plugged in your headphones, either.

oddly enough, the headphone jack on my vintage rotel does a better job of driving 600-ohm cans than 80-ohm cans.
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post #7 of 28
5/12/08 at 1:50am
Thread Starter 
Quote:
Originally Posted by ericj View Post
Most headphones would easily be melted or otherwise destroyed by a good speaker amp.
Is there a reason, besides difficulty in keeping the volume control low enough?
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post #8 of 28
5/12/08 at 1:53am
Quote:
Originally Posted by pdennis View Post
Is there a reason, besides difficulty in keeping the volume control low enough?
Really the same reason. At a low enough volume, there's no problem, but one slip of the knob and it can all be over very quickly.
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post #9 of 28
5/12/08 at 2:02am
Quote:
Originally Posted by ericj View Post
Oh god, you people
"you people" ??? A tad condescending, eh?

Quote:
actually believe that more gain necessarily means more noise?
It's not a function of the amps gain, but the speakers efficiency. The noise output of the amp is fixed. If the speakers (or in this case, headphones) are insanely efficient, the noise is more likely to be audible.

Moreover, with the efficient speakers you need to crank down the input signal (the music). The noise is therefore louder relative to the music than it would be with inefficient speakers.
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post #10 of 28
5/12/08 at 2:10am
Thread Starter 
If my amp is rated 12W @ 8 ohms, and my headphones have a 38 ohm impedance, then nominally I'll max out at a little less than 3 watts, right?

My headphones are rated at a maximum of 1.6 watts. Seems like it could be doable. What am I missing?
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post #11 of 28
5/12/08 at 3:35am
I believe the above are some of the reasons that Antique Sound Labs (ASL) released their UHC, and more recently the UHC Signature. This is what they dub an amplifier to headphone transformer, essentially to iron out the power and/or impedence mismatches.
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post #12 of 28
5/12/08 at 3:40am
Thread Starter 
Quote:
Originally Posted by xenithon View Post
I believe the above are some of the reasons that Antique Sound Labs (ASL) released their UHC, and more recently the UHC Signature. This is what they dub an amplifier to headphone transformer, essentially to iron out the power and/or impedence mismatches.
It's a nice idea, but they aren't exactly giving it away.
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post #13 of 28
5/12/08 at 4:27am
Quote:
Originally Posted by pdennis View Post
If my amp is rated 12W @ 8 ohms, and my headphones have a 38 ohm impedance, then nominally I'll max out at a little less than 3 watts, right?

My headphones are rated at a maximum of 1.6 watts. Seems like it could be doable. What am I missing?
Could be workable. A few points:

1: the voltage output to load impedance ratio might not be linear. Output into 38 ohms might be more than 3 watts. Sometimes it's linear, sometimes it's not. I have power amplifiers here that are rated 25 watts RMS into 8 ohms and 35 watts RMS into 4 ohms. I suspect that the power output into 16 ohms may be more than 12.5 watts.

2: If there's going to be a TRS plug involved, find out whether the amp's output is single-ended or not. If the output is differential (aka push-pull, etc), then the 'ground' side of the signal isn't so much ground as it is negative signal, and connecting the two negatives together could damage the amp. Notably, the T-Amps have differential output, and it is not safe to tie the negative outputs of a T-amp together.

3: You should probably still pad the output from the amp with a resistor. Whether you decide to do it in parallel (so that the resistor is getting some fraction of the power output) or in series. If i were trying it, i would put a pair of 5-watt 33-ohm resistors in parallel with the 38-ohm headphones. The amp will see an impedance of, what, like 18 ohms, and the resistors will absorb a little more than half the amp's power output. the ancient pioneer jb-21 actually puts 8-ohm resistors across the speaker terminals and then puts a 110-ohm ressitor in series with the headphones, which turns out to be wasting too much audio power as heat for really hard to drive headphones to benefit from it.
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post #14 of 28
5/12/08 at 4:39am
I've been running an HD650 off the speaker outputs of a Dared VP-20. It sounds awesome this way, largely due to the VP-20 being a push-pull amp and giving the HD650 most of the benefits of balanced drive. Still, the amp does clip a bit during dynamic passages which is why I had to give the rig up. From talks with more knowledgeable members I believe that this is simply due to the inadequacy of the design of the VP-20, and the idea of driving a headphone with a speaker amp is not invalid. I wasn't using any resistors of any sort; the gain was too high but still workable.

I don't have any other quality speaker amplification on hand so I can't really comment further.
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post #15 of 28
5/12/08 at 4:44am
Quote:
Originally Posted by catscratch View Post
I've been running an HD650 off the speaker outputs of a Dared VP-20. It sounds awesome this way, largely due to the VP-20 being a push-pull amp and giving the HD650 most of the benefits of balanced drive. Still, the amp does clip a bit during dynamic passages which is why I had to give the rig up. From talks with more knowledgeable members I believe that this is simply due to the inadequacy of the design of the VP-20, and the idea of driving a headphone with a speaker amp is not invalid. I wasn't using any resistors of any sort; the gain was too high but still workable.

I don't have any other quality speaker amplification on hand so I can't really comment further.
that seems like such a crazy thing to do,
i wonder how it sounds compared to much more expensive balanced offerings
Going to any meets anytime soon?
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