Are there differences in the way speaker amps and headphones amps are designed besides intended power output?
In other words if I connected my headphones to the speaker terminals, is there any particular reason why this would sound bad or would be bad for the amp?
In other words if I connected my headphones to the speaker terminals, is there any particular reason why this would sound bad or would be bad for the amp?
You would probably find a typical power amp connected directly to headphones would produce a lot of audible noise (hiss/hum). One reason for this is that a power amp might have a voltage gain of 20 to 40 or more, a headphone amp is usually little more than a line level buffer stage with at most a gain of say five, often less.
Should work fine, but there could be more noise or hum audible from the speaker amplifier. You could try a series resistor in each channel around 100 Ohms,
check the manufacturer's literature.
Would this mean that low powered tube amps would be viable for headphone use? Or even low powered SS amps like the First Watt amps by Nelson Pass?
Yes, the Pass amps would be good, especially ones with low gain. You may have some hum from some tube amps with phones.
Some amplifiers, both tube and ss, will require 4 wire balanced conections to phones, with no common ground.
Wouldn't that increase the output impedance to 200 ohms and cause a lot of distortion?
Hmmm... if the basic noise/hum level was low then a possibly yes.
This is exactly how I see it too. Low gain is key and the hum... well if you can detect any hum/hiss/noise with your ear up to the speaker then imagine how that will be with headphones directly coupled to the amp output.
All things are possible... it just might not be as straightforward as you think.
Actually the difference is even greater than that if you compare apples to apples and include all the stages after volume control.
Typical pre + power amp / integrated amp gain = 40..46 dB (100-200x)
Typical headphone amp gain = 6-16 dB (2x-6.5x), extremes <0-32 dB (<1x-40x)
Headphone amp gain levels vary a lot more since headphone sensitivity varies a lot more than speaker sensitivity as well. Hence why selectable gain is fairly common in headphone amps but rarely ever seen in speaker gear.
Some headphone amps actually make use of speaker amp ICs like the LM1876, but circuitry has to be adapted a bit in order to keep things stable at reduced gain.
The HiFiMAN HE6, absolutely among the most inefficient current production headphones, still only need a gain of 4.2 with a 2Vrms source to reach sound levels of 115dB. I have no idea what a similar extreme in speaker-land would be, but I'm sure you are right in that the differences are on orders of magnitudes, or more.
I don't know how you did that calculation but you are wrong. Assuming I read the spec right of 83.5dB/1V for the HE6, you need a gain of 25.5dB (18x) with a 2Vrms source to reach 115dB.
It's absolutely possible that I have my arithmetic wrong, but the specs I use say 83.5dB/mW (and an impedance of 50Ω). I don't remember where I got those numbers from, so they might also be wrong.
As Mooly points out, unless your headphones are very high impedance it'll be rather too noisy to directly connect them to a speaker amp. However if you were to use a step down transformer (say 10 or 20:1) then you could expect subjective performance substantially better than from the average headphone amp.
I see what happened. If its 83.5dB/mW then that would be equivalent to 96.6 dB/1V. Which would mean my iPod could drive the HE6. 😛
Most headphones don't need any gain at all, I would really like to see a headamp designed with this in mind.
Right here:
http://www.diyaudio.com/forums/head...igh-performance-headphone-amplifier-pcbs.html
opc's unity gain "wire" headamp. 🙂
Thx I never read the construction notes 😉 To bad that the Wire is too complicated for me to build.
And what about a discrete amp, with the same design idea?
And what about a discrete amp, with the same design idea?
burgunder - You can also do a unity-gain build with the O2 headphone amp, which is all through-hole parts plus has the power supply on the board and has a case. Just bypass the gain stage and run the signal directly to the pot in the middle. The pot then feeds the output unity-gain current buffers. I have the instructions posted here in the 1st half of the 3rd paragraph. I see a typo though reading through it again, R22 also needs to be removed. After those resistors and capacitors are out, with U1 removed, jumper U1 pins 5 to 7 and 3 to 1.
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