[stakarVN]

Hi all,
 
For a long time I wanted to use my 2x20 Watt Class A Musical Fidelity Amp for driving my headphones.  Because of oxidation of all switches/volumepot I desoldered these a long time ago.  It now only takes one input signal and is always full power (volume has been bypassed).  Then I saw this article by Rod Elliott of Elliott Sound Products, bless his heart.  It explains how to lower the amps output wattage using 3 resistors per channel, making it usable for headphones.  I would use my T-box headphone-amp as pre-amplifier for volume control.
 
I have my amp, and I have a Stax SRD-4 which includes a very handy switch to select between Earspeaker (Stax SR-30 Old But Good Earphones) and Loudspeaker (Which I shall use for my "Amphones")
 

 
I want to clarify that you don't need the Stax SRD-4.  I just use it as a housing and because the switch turns the headphone-output on and off.  You could just as easily directly connect the amps loudspeaker output to the resistors ( left and right positive signals) and jackplug (left and right negative signals), and put this in a housing of your choice.
 
So ...  I got myself 2 X (47, 120 an 180 Ohm) resistors and a 6.3mm jackplug.  I needed 47 and 180 Ohm resistors because the amp outputs 2 X 20 Watts of Pure Heaven.  Then I got soldering.
 

 
This is how it looks like when soldered onto the jackplug
 

 
and from another angle
 

 
This needed to get connected inside the SRD-4.  I used the loudspeaker-wiring to feed into the jackplug.  I desoldered these leads from the 4 terminals in the back.
 

 
I then removed the now unused terminals and used one of the holes to insert the jackplug.
 

 
This is everything connected
 

 
And then the final result
 

 
I do admit, I have crazy amounts of hummmmm on low volume... But I can live with it 

 Long live my Amphones !

 

[kchapdaily]

why so many resistors? why not just one complete value for each channel? also what kind of resistors did you use? these things could all contribute to the low volume hum

 

[samsquanch]

They look like half watt, it might be to avoid using wire wounds or other high wattage resistors that aren't the best in an audio path.  Spreads the voltage drop across multiple half watts as opposed to one 1 watt or 2 watt.

 

[stakarVN]

I didn't invest in expensive parts because I wanted to test this configuration first.  And the hum is coming from the amp, not from the resistors.

 

[kchapdaily]

ah gotcha that makes sense