Is it safe to drive my 38 ohm HE-500 headphones with my 40 watt per channel all tube amp? The amp has 4 and 8 ohm speaker taps so I'd use the 8 ohm tap and of course I wouldn't need to turn the volume up much but I don't want to hurt my amp. Thanks in advance.
 
Rob

robrob said:

  Is it safe to drive my 38 ohm HE-500 headphones with my 40 watt per channel all tube amp? The amp has 4 and 8 ohm speaker taps so I'd use the 8 ohm tap and of course I wouldn't need to turn the volume up much but I don't want to hurt my amp. Thanks in advance.
 
Rob

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Depending on the design, some tube amps didn't like operating without a load, they would sometimes break into oscillation destroying their output transformers.  38 ohms is like no load to that amp, so you might consider a big honkin' 8 ohm resistor in parallel with the headphones.  40 watts is about 38 watts too much, so you'll probably have very touchy volume control, and a good possibility of a fair bit of noise in your cans.  
 
Seems like not the best choice, overall, but you can make it work if you really have to.  If I told you that you should build up a resistive pad to better match the amp and headphones, would that freak you out? 

I don't need to run the HE-500s with the tube speaker amp but I'm curious what it will sound like. My primary headphone amp is a solid state Schiit Mjolnir.

I would guess: noisy, otherwise probably fine.

It wouldn't sound good and you'd put stress on the cans from the load not matching the output to keep it short.

chris_himself said:

  It wouldn't sound good and you'd put stress on the cans from the load not matching the output to keep it short.

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Actually, the cans would be stressed because of the high output possible only.  Load matching would stress the amp, not the headphones.  Headphones (in fact most transducers) work fine with an impedance mismatch so long as the driving device is lower, which it is in this case.  The problem here is that many tube amps expect a certain load to establish the correct negative feedback from the output transformer.  If there's too light a load, the things can become high power oscillators.  Many tube amps included a warning in the manual to not operate them without a speaker connected. It's not always a problem, it depends on where the negative feedback is taken in the circuit, whether the transformer is part of it or not.  They took a feedback sample from the transformer in an attempt to servo-out the nonlinearities in the transformer.
 
But, if your tube amp is unconditionally stable and won't oscillate with a light load, the only problem is noise.  Speaker sensitivity is specified at a reference of 1W@1 meter.  Headphones are specified at 1mW, and that's a 30dB difference.  A 40 watt amp with noise at -80dB below full output has it's noise at .4mW, which is only 4dB below the headphone reference level of 1mW.  If your headphones had a rated sensitivity of 90dB@ 1mW, then the quiescent noise when connected to that tube amp would be at 86dB...that's a pretty loud din.  
 
Noise is the problem.  If you could get the whole thing to work, frequency response would be fine.

robrob said:

  Is it safe to drive my 38 ohm HE-500 headphones with my 40 watt per channel all tube amp? The amp has 4 and 8 ohm speaker taps so I'd use the 8 ohm tap and of course I wouldn't need to turn the volume up much but I don't want to hurt my amp. Thanks in advance.
 
Rob

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Heya,
 
I drive my HE500 with a 50 watt solid state amplifier. It's completely safe.
 
If your tube amp is truly a tube amp, I would actually say it probably won't be ideal, as true tubes are better at higher voltage (for higher work loads) and have less current throughput.

What amp are we talking here?
 
Very best,

A typical speaker load is 4-8 ohms and your headphones are much higher than that so you are not in danger of running "without a load". I have driven cheap headphones with a car stereo back in my tinkering days and I found the power to be very clean and low distortion however it can be deceptive and dangerous if you're not careful. By deceptive I mean that because of the lack of distortion and the smooth power delivery you may be listening at a much higher level than is safe for your hearing and not realize it. There are other dangers such as pops from the power line, like when a refrigerator compressor kicks on, ceiling fans, CFL light bulbs etc.
 
What you can do to increase your safety level is what stereo manufacturers already do, add some resisters to your headphone connection. They do it internally, you would do it where you screw the wires in or at the headphone jack between tip/ring/ground on the female jack connector. Try a few different resistors until you find something that works well with the 500's. Figure 50k to 100k for starters of course it varies wildly depending upon how you wire the resistors. I find tip to ground and ring to ground to work well.

malveaux said:

   
Heya,
 
I drive my HE500 with a 50 watt solid state amplifier. It's completely safe.
 
If your tube amp is truly a tube amp, I would actually say it probably won't be ideal, as true tubes are better at higher voltage (for higher work loads) and have less current throughput.

What amp are we talking here?
 
Very best,

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Actually, the current in this system is LOAD dependant, not AMP dependant.  The general statement that tubes are better at high voltages is meaningless, because tube amps have output transformers, which puts output voltages and currents right back down where they are supposed to be.
 
Might be nice if you check your electronics before posting this kind of thing...

kodhifi said:

  A typical speaker load is 4-8 ohms and your headphones are much higher than that so you are not in danger of running "without a load". I have driven cheap headphones with a car stereo back in my tinkering days and I found the power to be very clean and low distortion however it can be deceptive and dangerous if you're not careful. 

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Yes, you're right, high power amps can drive headphones, but there actually is a possible issue here with headphones not fully loading a TUBE amp.  SS amps really don't care how you load them.  I've explained the problem in an above post.  For a tube amp with 8 ohm output taps, 32 ohms is pretty much unloaded.
Quote:

kodhifi said:

  What you can do to increase your safety level is what stereo manufacturers already do, add some resisters to your headphone connection. They do it internally, you would do it where you screw the wires in or at the headphone jack between tip/ring/ground on the female jack connector. Try a few different resistors until you find something that works well with the 500's. Figure 50k to 100k for starters of course it varies wildly depending upon how you wire the resistors. I find tip to ground and ring to ground to work well.

Click to expand...

Adding a series resistor does tame the high output problem, but it also destroys the damping factor.  Headphones and speakers need a source impedance that is quite low to keep mechanical resonances in check by damping their back-EMF through the low source Z of the power amp.  Adding a resistor in series takes the source impedance of an amp, normally a few ohms, and bumps it up to that plus the value of the resistor.
 
Speaking of resistor values, adding a 50K or 100K resistor will result in pretty much no audio at all, the values are off by several orders of magnitude.  Typical home stereo build-out resistors were 100 ohms or so, which is still a rather poor compromise as it blows the damping factor away, and only results in less than 10dB of actual attenuation.  The build out resistors need to be in series with the headphone.  Adding 50K or 100K from tip and ring to ground does absolutely nothing.  Even adding a 100 ohm resistor from tip and ring to ground does absolutely nothing. 

Thanks jaddie, I had read of problems with driving headphones with tube amps that's why I asked and you've done a great job explaining the danger. It was just a curiosity thing so I'll just forgo the experiment and keep my amp (Yaqin MC-13S) safe. I have driven several pair of balanced headphones from a Sony solid state receiver's speaker taps and wanted to compare the sq to the tube amp.
 
Thanks everyone for the help,
 
Rob

jaddie said:

  Adding a series resistor does tame the high output problem, but it also destroys the damping factor.  Headphones and speakers need a source impedance that is quite low to keep mechanical resonances in check by damping their back-EMF through the low source Z of the power amp.  Adding a resistor in series takes the source impedance of an amp, normally a few ohms, and bumps it up to that plus the value of the resistor.
 
Speaking of resistor values, adding a 50K or 100K resistor will result in pretty much no audio at all, the values are off by several orders of magnitude.  Typical home stereo build-out resistors were 100 ohms or so, which is still a rather poor compromise as it blows the damping factor away, and only results in less than 10dB of actual attenuation.  The build out resistors need to be in series with the headphone.  Adding 50K or 100K from tip and ring to ground does absolutely nothing.  Even adding a 100 ohm resistor from tip and ring to ground does absolutely nothing. 

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The joys of getting interrupted by life when typing.
 
50k to 100k POTENTIOMETER resistor. The idea is to quickly change the response until he finds something that works. I don't know about tube output stages for headphones. On a guitar amp you just tap off the transformer. /shrug
 
Going series or parallel gives different characteristics. I have had good luck going parallel IE tip/ring to ground when I last tried to drive headphones with a 25watt car stereo when I used to tinker with stuff like that.

kodhifi said:

  The joys of getting interrupted by life when typing.
 
50k to 100k POTENTIOMETER resistor. The idea is to quickly change the response until he finds something that works. I don't know about tube output stages for headphones. On a guitar amp you just tap off the transformer. /shrug
 
Going series or parallel gives different characteristics. I have had good luck going parallel IE tip/ring to ground when I last tried to drive headphones with a 25watt car stereo when I used to tinker with stuff like that.

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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.
 
If you connect the pot so the resistive element is across the amp outputs, then take the headphone output from wiper to ground, that's fine, but, again, your pot values are 100s of times too high.  You need to be in the 100 - 200 ohm range.  In the range where it will actually sort of work, you've blown away whatever damping factor you had, and again, you won't load the amp with anything like it was designed for.  Tube amps are nothing like car stereos.   
 
Sorry to be blunt, but the whole idea makes no sense at all, unless you must want to get some sound, any sound, out of your headphones and don't care about levels, noise, and amps and pots burning up.
 
We live in a world of fantastic headphone amps for $129, and decent ones well under $100.  There are 20wpc stereo amps, good ones, for $30, and with any of that you'll have none of these issues.  If you want a tube to drive your headphones, invest in a tube headphone amp.   The reality is, tubes aren't great at all. Neither are transistors or ICs.  It's the circuits they're in that makes them great.  A mis-applied circuit, or a badly designed one will always be far worse than a cheap and simple, but properly designed and applied one of any topology.  
 
Just to make sure you know I'm not just talking out of class here, I have actually connected headphones to a 35 watt tube (6L6, class AB push-pull) PA amp.  Yes, I got sound, no it wasn't good, and I did burn parts up before I realized my approach was wrong.  That was a hand-full of decades ago, but when I turned up to volume,  I'll never forget the smell of burning pot...er...potentiometer...yeah, that's what I meant to say.  No, really.  

Much of the value of your amp lies in the output transformers. If you blow these, it may not be economically viable to repair the amp...

Personally, I wouldn't risk it (I have managed to resist hooking up my HE-500s to my Opera Consonance Ref. 5.5!).

robrob said:

  Is it safe to drive my 38 ohm HE-500 headphones with my 40 watt per channel all tube amp? The amp has 4 and 8 ohm speaker taps so I'd use the 8 ohm tap and of course I wouldn't need to turn the volume up much but I don't want to hurt my amp. Thanks in advance.
 
Rob

Click to expand...

 
If you don't feel like fiddling with resistors and stuff, HifiMan has made a resistor box for this very purpose - to protect tube amps, and bring the HP load down to ~8ohms.
 
HifiMan HE Adapter