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In Reply to: RE: Need help calculating power output for headphone circuit... posted by soulbrass on June 07, 2017 at 07:20:49
It looks like you will be getting the following for headphone output...
Based on the rated output of 12 watts into 6 ohms, that calculates to 8.5 volts over the 6 ohms load:
Power = (V*V)/R or (8.5*8.5)/6 = 72/6 = 12 watts
For the same voltage output into 502 ohms (or the 470ohm resistor plus a 32 ohm headphone load) you get 143.5 milliwatts.
For a 300 ohm headphone (that would be 470 + 300 = 770 ohm load) you get 0.0935 millwatts.
Assuming my calculations are correct, you are not going to get much output for headphones. IMO, the 470 ohm resistor should be changed to a lower value to be able to drive a good set of headphones.
To be practical, the 470 ohm resistor should be no higher than 47 ohms. With that value you would get 0.911 milliwatts for 32 ohm headphones and 207 milliwatts to drive a 300 ohm headphone. Those are much more realistic power output values for decent headphone listening.
If you are going to go with a nice set of planar magnetic headphones you will likely need even more power. Doubling the output power from the headphone jack can be done by halving the proposed internal resistor from 47 ohms to 28.5 ohms (or something close to that value).
Ed
We don't shush around here!
Life is analog...digital is just samples thereof
Follow Ups:
Thank you for the calculations - appreciated. The math is over my head...but the calculated outputs seem a little low based on the Six Moon review. The SQ-N100 reportedly drove several top-tier headphones (Audio-Technica WHT-5000, Sennheiser HD800, AKG K702, Grado PS-1000) with ease and bested several well-regarded headphone amps.
Are there other variables that I'm missing? It's entirely possible that I'm asking a question whose answer I don't have the background to fully understand.
Can be viewed from different perspectives. If you tend to listen at low volume levels, low output headphone amps will serve the purpose well but if you tend to listen at higher volume levels, the lower output is an issue.
A 470 ohm resistor in line is a bit high but, from a design point, you don't know what headphones were used to test the output or what type of volume restriction was used in its design. A headphone output jack could very well be designed with safety in mind and the power output limited so high output levels can be avoided (and human hearing protected).
People could also plug in in-ear monitors, which are very sensitive, and the 470 ohm resistor makes perfect sense with these type of headphones. A user could also choose Audeze LCD-3 headphones and the output would not be sufficient with those headphones.
If you drop that resistor to 47 ohms, like I suggested, the output of approximately 1 watt can be very high power for many headphones. If you own the aforementioned Audeze headphones, 1 watt may still be a little low powered.
It is a matter of what is enough. I own a Schiit Lyr 2 headphone amp and it is capable of 6 watts/ch at 32 ohms and that is a lot. The output circuit has a sensing circuit to prevent over powering headphones but the amp is capable of lots of power with every headphone that I've tried. The owners manual makes a statement about taking care with volume settings due to the high power.
I have several vintage pieces of audio gear that uses a load reducing resistor like your amp uses and I don't recall any of them using a value as high as 470 ohms. All of these vintage amps have much higher power output than 12 watts/ch.
As a result, I was a little surprised by the 470 ohm rating for the power reducing resistor. Considering the fact that its capable of 12 watts into 6 ohms, dropping the output with a 470 ohm resistor is a bit of overkill but like I already said you do not know what they were designing for.
Have you tried any headphones with this amp? What type of volume level are you used to listening to?
Ed
We don't shush around here!
Life is analog...digital is just samples thereof
I wouldn't consider myself a head-banger...maybe occasionally ; ) The majority of my listening is orchestral, small group jazz, and 70s funk and rock. On the whole - very moderate volumes.
I have Shure SRH-1540 headphones and Shure SE535 earphones...both very easy to drive. I chose them because they perform well directly out of my iPhone, iPad and MacBook or paired with a DragonFly Red. I travel quite a bit - my "road kit". On the road, TIDAL is my music source.
I've tried both 'phones with the Luxman. Definitely an improvement over the DragonFly (although it's hard to separate the DAC and headphone amp variables since I'm using a different source with the Luxman). My perception of "better" is based on improved fullness with both. The SRH-1540 volume is very comfortable at ~9 o'clock, fuller and louder at 10 o'clock, and uncomfortably loud at 11 o'clock; the SE535, a little less.
I attended a CanJam headphone conference earlier this year and decided I'd like to own an open-backed headphone for home use...either 1) Focal Elear, 2) Sennheiser HD800/800S, or 3) Audeze LCD-X or LCD-3. Since I intend to pair them with the Luxman, this thread is background to [hopefully] an informed purchase. Ultimately, since the Luxman is small, it will be possible to take it somewhere and actually audition all three cans before I purchase...not sure where, though.
Someone else suggested connecting headphones directly to the speaker taps. Thoughts (relative to my 3 possible purchases)?
I appreciate the feedback...thanks.
You definitely want resistors in line to drop the output a bit. You may not need the resistors for the HD-800 headphones but I wouldn't try it on the other headphones.You also have the problem where you do not want to hook the two negative speaker outputs together. You want a resistor between the negative speaker output and the common negative hookup on the headphone. I believe most amplifiers would balk at shorting the negative speakers outputs together.
Single ended headphones have individual hot connections for the left and right sides but share a common negative hookup. You don't want to try hooking that to your speaker outputs. You may get away with that on some amps but I don't think you want to fry anything. Before you hook the grounds together make sure it won't damage anything.
Ed
We don't shush around here!
Life is analog...digital is just samples thereof
Edits: 07/12/17
" I believe most amplifiers would balk at shorting the negative speakers outputs together."
So far I have seen very few that the individual outputs are balanced to ground. I happen to have one, and DIY amps with 0-4-16R output taps have grounded the 4R and used the secondary for some cathode fb. With SS amps, if you want to bridge them, you count on the negative already being tied together and connect only to the positives.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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