[Chris J]

  No, I haven't.   Unless you are measuring the voltage out of the amplifier with no load connected.  Which makes no sense.  So, if you measure the output voltage with the load attached, what I said above is exactly true.  Ohms law doesn't lie.

 
Try again!
Think!
If you assume the output impedance is (I don't know what it is, it's just an assumption to prove that even a fairly high output impedance doesn't make a huge difference) then calculating backwards, open circuit output voltage is 6.71 Volts.
 
If you had a real world amplifier and you wanted to measure the output impedance, you would measure the output voltage loaded and unloaded,  i.e. you would want to measure two points. 
Ohms law may not lie, but you can't ignore the output impedance.

 

[Chris J]

  No, I haven't.   Unless you are measuring the voltage out of the amplifier with no load connected (how would you know your load wasn't driving your amp into current clipping?)  So, if you measure the output voltage with the load attached, what I said above is exactly true.  Ohms law doesn't lie.

 
read this:
 
http://electronics.stackexchange.com/questions/6917/measuring-output-impedance

 

[Chris J]

  No, I haven't.   Unless you are measuring the voltage out of the amplifier with no load connected (how would you know your load wasn't driving your amp into current clipping?)  So, if you measure the output voltage with the load attached, what I said above is exactly true.  Ohms law doesn't lie.

 
How would you know your load wasn't driving the amp into current clipping?
Use a THD analyzer
or
use an oscilloscope.
 
Stay away from current clipping, measure the output impedance at 1/2 full output voltage (for example).

 

[potterma]

I'm measuring the output with the load attached.  The voltage divider has already been accomplished INSIDE the ampl
 
You are measuring with no load attached and measuring the effect of loading the amplifier with the parallel combination of the resistor and the headphone.
 
So, I concede you are correct for your measurement scenario.  Which is an important consideration in the broader application, and of course the output impedance of the amp has to be considered.
 
I was looking at the specific case of measuring the output with the load attached.  And you're right, if the power specification for the amp is stated at or near the unloaded voltage output limit, then you have to care about the drop across the output impedance of the amp.
 
BUT, If my initial condition is that I have a bit of headroom over the amp power spec and I can drive x volts into some load, if I change the load and adjust the input signal to the gain stage such that the output voltage is again x volts, then the power to the headphones remains constant.

 

[Rossliew]

Is it recommended to use the 8 ohm taps of the speakers out to connect the adaptor to the headphones? Is it OK to use the 4 ohm taps instead? I have a Hifiman HE-adaptor which i'm keen to use and try it via the 4 ohm taps but worried it might either blow the amp or the headphones...any advice is most appreciated!

 

[Chris J]

Is it recommended to use the 8 ohm taps of the speakers out to connect the adaptor to the headphones? Is it OK to use the 4 ohm taps instead? I have a Hifiman HE-adaptor which i'm keen to use and try it via the 4 ohm taps but worried it might either blow the amp or the headphones...any advice is most appreciated!

Sounds like you are using a tube amp with two output taps?
4 Ohms and 8 Ohms?
The 4 Ohm taps with output less voltage.
When you first plug your headphones in, turn the volume down to Zero.
Just turn the volume up very gradually and you should be OK.

 

[Chris J]

I'm measuring the output with the load attached.  The voltage divider has already been accomplished INSIDE the ampl

You are measuring with no load attached and measuring the effect of loading the amplifier with the parallel combination of the resistor and the headphone.

So, I concede you are correct for your measurement scenario.  Which is an important consideration in the broader application, and of course the output impedance of the amp has to be considered.

I was looking at the specific case of measuring the output with the load attached.  And you're right, if the power specification for the amp is stated at or near the unloaded voltage output limit, then you have to care about the drop across the output impedance of the amp.

BUT, If my initial condition is that I have a bit of headroom over the amp power spec and I can drive x volts into some load, if I change the load and adjust the input signal to the gain stage such that the output voltage is again x volts, then the power to the headphones remains constant.

I was assuming output power was measured terminated into load.

Power specification (I.e. 5 Watts @ 8 Ohms) would be measured with the amp terminated into an 8 Ohm load.
The power spec at full output should be measured and should read(for example):
5 Watts @ 8 Ohms, 1kHz, THD = 0.1 %
Or describe output power with a set of curves.
There are a few reasons why output power from a power amp does not double as the load impedance is halved, the effect of output impedance is one of them.

You are missing my point:
Changing the parallel resistor from 10 to 15 Ohms changed the output (assuming input is fixed) into the headphones only slightly.
Again,
Read my posts again.
Assuming input is a fixed voltage
10 Ohm parallel resistor. 368 milliWatts
15 Ohm parallel resistor: 380 milliWatts
As I was saying: the difference is trivial. To put it another way, it wouldn't be audible. OTOH, but it is there.

Basically, I was debating Terja's calculations.
He thought that output would be 5 Watts into both an 8 Ohm and a 9.17 Ohm load. His calculations were erroneous.

I don't know what else to say.
You can bet that a 5 Watt tube amp uses an unregulated power supply.
The output tubes have finite output impedance.
The output transformer has finite out impedance.

After working for many years as an Electrical Power Engineer, etc, etc, there is a chance that I may actually know what I am talking about.....

 

[wuwhere]

  Is it recommended to use the 8 ohm taps of the speakers out to connect the adaptor to the headphones? Is it OK to use the 4 ohm taps instead? I have a Hifiman HE-adaptor which i'm keen to use and try it via the 4 ohm taps but worried it might either blow the amp or the headphones...any advice is most appreciated!

 
If it is a tube amp, it should be fine. I'm using the 4 ohm on my tube amp with the 10 ohm resistors in parallel with the HE-6.

 

[Armaegis]

After working for many years as an Electrical Power Engineer, etc, etc, there is a chance that I may actually know what I am talking about.....

 
Pfft, what some call years and years of experience, others will call witchcraft

 
And really, at the end of the day 300ish mW is going to give you plenty of juice for an Audeze. I don't know why we're all tossing numbers around when the guy could have just plugged the headphones in and listened.

 

[Rossliew]

Sounds like you are using a tube amp with two output taps?
4 Ohms and 8 Ohms?
The 4 Ohm taps with output less voltage.
When you first plug your headphones in, turn the volume down to Zero.
Just turn the volume up very gradually and you should be OK.

 
Yes, using a Chinese made generic tube amp with +/- 5W of output power..

   
If it is a tube amp, it should be fine. I'm using the 4 ohm on my tube amp with the 10 ohm resistors in parallel with the HE-6.

 
Thanks for the feedback guys, shall try it out and see how it fares vis a vis the 8 ohm taps.

 

[Terja]

<snip ........

Basically, I was debating Terja's calculations.
He thought that output would be 5 Watts into both an 8 Ohm and a 9.17 Ohm load. His calculations were erroneous.

...... snip>

 
Thanks for setting me straight; basically I am learning this stuff myself, thanks to Head-fi and members like you.

 

[davidsh]

Please, this is quite uninteresting. Stop beating a dead horse. 

 

[kokushu]

So how do you goes about using a headphone through a speaker amp.  Do you need a special adapter or something?  Or do you just use the headphone jack and it just give you more power?

 

[Chris J]

Pfft, what some call years and years of experience, others will call witchcraft

And really, at the end of the day 300ish mW is going to give you plenty of juice for an Audeze. I don't know why we're all tossing numbers around when the guy could have just plugged the headphones in and listened.

No it isn't witchcraft, read my profile.
It's alchemy.
There's a difference.
At the end of the day, you're right, try the headphones with amp and see if you like it.
300 mW is enough to blow yer brains out.

Thanks for setting me straight; basically I am learning this stuff myself, thanks to Head-fi and members like you.

No problem, it's not a personal attack

Please, this is quite uninteresting. Stop beating a dead horse. 

Go read another thread if you don't like this one.
This is called the speaker amps for headphones thread.

So how do you goes about using a headphone through a speaker amp.  Do you need a special adapter or something?  Or do you just use the headphone jack and it just give you more power?

I've driven headphones straight off the speaker connections on 250 Watt/channel amp.
You have to try it and see with your own amp to see if it suits your taste.

 

[davidsh]

I was referring to determining the power output of the 5W tube amp.