Need help calculating headphone amp power output...

[soulbrass]

Hi,

Not sure if here is the correct venue, but here goes:

I own a Luxman SQ-N100 tube integrated that I purchased to anchor my office system. Rumor has it that it is also an exceptional headphone amp. In order to make some informed headphone choices, I'm trying to understand what the power output would be for headphones of different ohm ratings.

I have little to no understanding of electronic circuits...given the info I've included below, is it possible to calculate headphone power output? ...say for 20, 36, 46, 62, 80, 100, 300 and 600 ohms? I realize that sensitivity comes into play, too, but those ratings are usually available in published specs.

If there is general formula I can apply, I'm happy to do the math. Thanks!

JJ
==
This is a quote from the Luxman's SixMoons.com review describing the headphone circuit:

"Headfi fanciers will be excited to learn that their ¼" socket taps directly into the SQ-N100's main valve output stage. For ear speaker purposes, the output voltage simply couples through a pair of 470-ohm load resistors to attenuate its strength."

These are the Luxman specs:

Rated output: 12W + 12W (6Ω), 10W + 10W (8Ω, 4Ω)
Input sensitivity: LINE: 150mV, PHONO (MM): 2.2mV
Input impedance 47KΩ
Frequency response 20Hz - 50KHz (within -3 dB)
Total harmonic distortion 0.3% or less (1 kHz rated output)
S/N ratio 90dB or more
Input LINE: 3, PHONO (MM): 1
Output SPEAKERS: 1
Functions Remote control applied (sound volume), head phone output
Tone control (bypass enabled), AC inlet
AC outlet X 2 (non-interactive, total 200W max.)
Circuiting system Mullard type UL connections
Vacuum tubes used ECC83 X 1, ECC82 X 2, EL84 X 4
Power consumption 95 W (Electrical Appliance and Material Safety Law),
71 W (no signal)
Power supply AC 100V (50/60Hz)
Accessories Remote controller, power cable
Dimensions 297W X 162H X 210 (258) D mm
(I/O terminals in brackets, including operating knobs)
Weight 11.5kg

 

[4krow]

Despite the fact that I do not have a formula on hand, I find it impressive to see that headphone amp attenuated right from the main output. One possibility is that more power could be drawn by replacing the 470 ohm resistors with a lesser value. It should also be noted that if this headphone output is designed for say 600 ohm headphones back in the day, then that may have a bearing on the anticipated output. Again, it is reasonable to assume that the resistors can be changed to allow for needed power increase.

 

[MindsMirror]

A power supply or amplifier with X Ohms output impedance is able to deliver maximum power to a load that is also X Ohms.

Given that the amp without the 470 Ohm resistors is rated for max power at 6 Ohms, you can estimate that it has an output impedance of around 6. Using Ohms law equations (P=V^2/R), 12W into 6 Ohms equates to 8.5V across the load, or 17V across the 6 Ohm load + the 6 Ohm output impedance. Now that I've calculated that voltage, I can use it to find the power delivered to the headphones. If you have 470 Ohm headphones and 470 Ohm output impedance, the headphones will receive 8.5V and (using Ohms law again) 154mW.
A few more examples, a 32 Ohm headphone would receive 32/(32+470) * 17V = 1.08V, 37mW.
A 600 Ohm headphone would receive 9.5V, 151mW.

Of course this is not a theoretically ideal amplifier so this might not be totally accurate.

Normally for hi-fi amps you want the load impedance at least 8 times higher than the output impedance, not for maximum power deliver but for efficiency and damping factor. That's not really practical in this case. I would try to use it with higher impedance headphones, around 300 Ohms or more. You could modify the output resistors or plug into the speaker terminals instead, but the resistors might be there for more than just limiting power. They will also help suppress noise when using low impedance, high sensitivity headphones.