[CJG888]

I have read, however, that it degrades the sound. Maybe you could buy one and improve the component quality.

 

[brunk]

I have read, however, that it degrades the sound. Maybe you could buy one and improve the component quality.

 
It does degrade the sound, but ever so slightly. You certainly can put in whatever resistors you wish, the box is very accessible.

 

[jaddie]

It's not an issue of component quality or selection, its a flaw in the concept.

 

[robrob]

brunk, thanks for the link to the HiFiMan resistor box. The specs say:
 
Input: Speaker Connector
output: 4 pin XLR
Resistors in parallel: 10 Ohm
Resistors in series: 25 Ohm
 
Anyone know how the resistors are actually connected to the headphone wires?

 

[jaddie]

  brunk, thanks for the link to the HiFiMan resistor box. The specs say:
 
Input: Speaker Connector
output: 4 pin XLR
Resistors in parallel: 10 Ohm
Resistors in series: 25 Ohm
 
Anyone know how the resistors are actually connected to the headphone wires?

 
Yeah, I saw the "specs" too.  Not much to go on there, but there's no magic involved.  I dug and found the pic below.
 

 
It's a 10 ohm load resistor on each channel of the amp, and a 25 ohm build-out resistor in series with each of the headphones.  So much wrong here.  First, the 10 ohm "load" looks to be a 2 watt resistor, so don't leave this connected if you plan to use speakers too, it's got to be speakers or headphones, never both, or you'll turn the termination resistors into SEDs  (that's Smoke Emitting Device).  Next, the 25 ohm build-out resistors, as mentioned, destroy whatever damping factor might once have existed, and if your cans have any sort of non-flat impedance curve, they will impress a significant change in frequency response.  You'd expect some sort of attenuation for this box to be worth the investment, to both protect your cans and reduce the residual noise of the amp.  Nope, not there.  With 32 ohm cans it's 5dB of loss, with 50 ohm cans, it works out to 3.5dB.  At the extreme, if you had 16 ohm IEMs, you'll get a whopping 8dB of attenuation.  Not nearly enough to make up for that 30dB of difference in amp power to required headphone power.  
 
So, it's basically a termination for your amp and a damping-factor and frequency response reducer.  All that for $100.  Sheesh.  It has a 4-pin XLR, so either get an adapter, make one, or re-connector your headphones.  
 
And that's what I mean by a flaw in concept.  

 

[robrob]

Thanks again jaddie.

 

[Kodhifi]

   
50K or 100K is, like I said, easily 250 to 500 TIMES too high.  If you want to play with series resistance, you'll need to be between 0-200 ohms.  Terminating any amp designed to drive speakers with even hundreds of ohms is completely pointless as far as it seeing a load.
 

I don't know what to tell you. They are easy to find off the shelf and they make great volume controls. For his purposes of having variable load and volume they should serve well. You don't have to crank them down to 50k and mute the audio. Pots are adjustable anywhere from almost no resistance, all the way down to almost full resistance. If I were going to build a direct box it's what I'd use because it's easy to find them at guitar stores and they have a proven track record. /shrug

 

[jaddie]

I agree they might be good for a DI box, but this is quite different. A DI doesn't have to worry about source impedance, and lots of attenuation too mic level is desirable, but that's not what you need for the headphone application.
 
 I guess I could go through the circuit analysis, probably not much point.  With a 50K pot adjustment will be touchy, source impedance excessive, getting channels to match difficult, etc.  Just the wrong value.  Assuming a linear taper, about a quarter turn puts you at a point with 10K one side of the wiper, 40K on the other, and with 30 ohm cans on the wiper to ground, 50dB of attenuation, and a source Z of 10K.  The desired source Z is less than 1 ohm.  
 
Are you sure about 50K?  5K would be more in range, the same pot rotation would be 30dB with 30 ohm cans, but with the rest of the problems still there. 
 
Yeah, you'll get sound through that, it's just not the best engineering.

 

[Kodhifi]

50k and 100k are definitely correct but the use can vary. Those pots can go in a guitar body to control the low voltage volume from the electric pickups going out to the amplifier, which is uaually 300ohm's or so. I think the idea is that at the lowest volume you're using the highest ohm which almost shorts the pickup so no noise gets through. I'm the first to admit I don't know why they use the parts they use, just that they work.
 
The setup I made, and it was many years ago, was a 25watt/channel car stereo I'd mounted in a wooden box the size of a conventional stereo chassis. I used a scavanged 1/4" trs jack for headphones on the front but doing it like that gave me full power to headphones and I destroyed some cheaper ones by burning the voice coils out. I ended up scavenging a volume control that was used in a guitar and mounted this internally. I remember it being a pain to wire up for stereo using a single volume control but not how I got past it.
 
I'm interested in how you would make a direct box since it sounds like you know electrical engineering better than I. This is you chance to educate.

 

[Kodhifi]

In that disassembled DI box It looks like they went tip to ground like I'd suggested for the resistor loading. Parallel has several advantages to serial when used for this purpose.

 

[jaddie]

Just so you know, we're talking about three different applications here.  The headphone/speaker amp adapter, the direct box, and a guitar.  They really are not very closely related at all, and the parts and values choices you make for one will not be optimal for the others.  
 
You can't put in a volume control for a stereo headphone set with only one mono pot without creating quite a few performance issues.  
 
I have to say, I completely admire someone who in this day has the interest to experiment with audio and electronics.  Sorry you burned up a pair of headphones, but you did learn what you were supposed to by doing it, and will never do that again.  
 
So, to be clear, a Direct Box (DI Box...stands for Direct Input), is a device for coupling an instrument to a recording console directly without the use of a microphone.  I can connect to a low level guitar pickup, or the output of an amplifier with a speaker on it.  It provides a means of level adjustment, isolation and balancing, and sometimes mild response shaping.
Here are some examples:
For very high levels to mic levels:
http://www.jensen-transformers.com/as/as008.pdf
 
From guitar pickup to mic level:
http://www.jensen-transformers.com/as/as066.pdf
 
From instrument (synthesizer) to mic level:
http://www.jensen-transformers.com/as/as007.pdf
 
A volume control built into a guitar body is quite different.  The pickup, if passive, has an output impedance of 6K - 17K about, and works best if very lightly loaded, so pot you'd use would be 200K - 500K, and the output of the pot is taken from the wiper to ground.  It feeds a guitar amp which has an input impedance of 1Meg ohm.  But if you have an "active" pickup, the output impedance is quite low, so a lower value pot would be more appropriate. 
 
Then we come to a headphone adapter for coupling headphones to higher power amplifiers, like the Hifiman HE Adapter.  Pretty much everything up to this line was slightly off-topic. 
Now, if you're asking how I would build this kind of thing, I'd need to back way up first and ask why the need.  Driving highly sensitive headphones (which is NOT what the HE Adapter does!) with a 40W tube amp is kind of dumb to begin with.  It's like, you can water your flowers with a fire hose, but it may not be the best idea. There are way better watering devices that will get the flowers wet without blasting them across the yard, and without a lot of trouble trying to force a fire hose to water flowers without blasting them away.  See, it's a misapplication of technology.  Yes, you can get it to work, but it will never work well, and never give the optimum results, and may burn something up in the process.  
 
But if you want to do this...
 
The tube amp probably needs a load.  It expects a speaker, and we won't have that, so the first thing is a load resistor.  The HE Adapter uses a 10 ohm, 2 watt resistor, which is under-rated for the job, but works fine if the amp volume is kept low. One mistake, though, and it's a goner, so it should be at least 5 watts, if not 10 watts, and should be 8 ohms.  A 10 watt 8 ohm resistor (2 of them) will cost you enough to discourage the project, but moving on anyway.  Next we need 20 - 30dB of loss, but also the headphone should see a fairly low impedance across its drivers.  So, we can fake this, sort of, by placing a pair 1 ohm, 1 watt resistors across the headphone drivers. Now, we just need to pick the right resistor between the loaded amp output and the 1 ohm resistor to drop level 20 or 30 dB, and that works out to 25 ohms, and it will have to be a 2 - 5 watt resistor.
 
The entire thing looks like this:

 
That would be fairly burn-out proof (unless somebody cranks the volume full up with a sine wave and lets it just cook), will provide the needed attenuation and reasonably low Z drive for the headphones.  It's NOT the same as driving them from a good low output impedance headphone amp!  It only provide a bit of damping for the driver.  But the entire idea is a huge compromise.  For headphones of different efficiency, you can play with the 25 ohm resistor value, being carful to note that is you reduce it (for more drive to the headphones), you'll also dump more power into the 1 ohm resistor too, so it may have to get bumped up to a 2 or 5 watt unit.
 
It's also drawn for a single-ended stereo amp.  Anything using a bridged output won't work, but wouldn't anyway unless you had headphones wired for "balanced" cable and connectors.

 

[robrob]

Interesting stuff, thanks again jaddie.

 

[Sonido]

I had the same question as I'm thinking about trying a tube speaker amp with my HE-500. I saw this for the HE-6:
 
http://www.ttvjaudio.com/HiFiMan_HE_6_and_Sophia_Baby_Amplifier_Combo_p/ttvjpkg007.htm
 
It looks like they don't even use the adapter in that setup. Any idea if this amp can just handle headphones, and maybe the HE-500 as well? Here are the specs to the amp:
 
http://www.sophiaelectric.com/pages/amp/ampbaby.htm
http://www.sophiaelectric.com/pages/amp/specification_babyamp.htm
 
Thanks.

 

[robrob]

The Sophia Baby Amp is a push-pull tube amp and the included speaker out to headphone adapter (pictured below) looks like a single ended TRS jack. I didn't think you could combine the R- and L- lines of a push-pull amp to create a single ended output.
 

 
 
An unrelated question: How do receivers and amps normally implement a headphone jack? Do they have a separate headphone amp or an adapter circuit?

 

[Synthax]

Dear Collegues,
But how about using 2A3 3,8W SET with 60ohm ortho headphones?