Building an L-Pad for Sensitive Low-Impednace Headphones

[boulos]

In this thread, I'll describe the process I used to build an L-Pad for my 32Ohm headphones to use with the S.E.X.  For reference, the headphones I'm using are AudioTechnica ATH-AD700 Open-air dynamic headphones (32Ohm,  98 dB/mW at 1 kHz, 5Hz - 30kHz), which are relatively cheap.  The reasons I needed the L-pad are because I had a very small usable range on the volume pot with these headphones (if 0 signal is 6 o'clock, I could barely go to 7); and there was a slight but bothersome hum at 0 signal. The headphones never sounded any good, and before I purchased more expensive ones, I posted on the forum and received advice from PB and PJ which resulted in this mini-project (thank you! :-)  Note that my amp has the 8Ohm tap connected, and that I built an external L-Pad because I wanted to maintain the ability to disable the speakers when listening to headphones.

Here is the original thread: http://bottlehead.com/smf/index.php?topic=7185.0
Attached is the completed device I built.

The improvement to the sound of my headphones was very noticeable. Now I could the volume pot up to ~10 o'clock (depending on the recording). The hum is almost entirely gone, and the headphones finally sound good!

Next, I will list the parts.

[boulos]

Parts and Spec sheets

I used the following parts:

- 2x Vishay CPF3 100Ohm 3W 0.5% resistors. [CPF3100R00DKE14] ~$2.50
- 2x Vishay CPF3 33Ohm 3W 1% resistors. [CPF333R000FKB14] ~$2.50
- Switchcraft phone plug. [297] ~$5
- Switchcraft 3 conductor 1/4" shielded jack. [CN12B] ~$15

For wire, I used the wire from my headphones.  I cut it, connected the Switchcraft plug to the headphones, and connected the original wire+plug to the shielded jack housing the resistors. (This should explain the previous attachment showing final assembly if anyone was wondering.)  The ncie thing about my headphone wire is that it is the exact same diameter as the opening in the back of the jack (0.125 inch).

The CPF resistors have a small size given their power rating, and are low noise. CPF2 (the 2Watt version) are as small as 'regular' 1/2W resistors, and the CPF3 are slightly bigger.  Nonetheless, the difficultly I found in building this L-Pad comes from the size of the 3W resistors. If using the 2W version, the build would be easier, but for 32Ohm headphones PJ recommended 2W as minimum so I went for the 3W resistors.

Attached are the spec sheets for CPF resistors, the headphone plug and the jack (this is for the CN12A--2 conductor version, not CN12B but the sizes should be the same), and a picture I took of the parts.

[boulos]

Instructions:

Mark the jack terminals:
Insert the plug into the jack.
The eyelet (solder terminal) for the tip is the one opposite to the jack terminal touching the plug tip.
The eyelet for the ring is the one opposite to the jack terminal touching the plug ring (the slightly shorter terminal).
The third eyelet is the one for the sleeve (ground).
You may need to bend the eyelets for the tip and the ring in so that they don’t touch the casing.

Prepare the resistors: This step is needed so that the resistors sit low in the jack and prevents them from touching the metal enclosure. If you're going for 2W resistors, this step is not needed. Do this for each of the four resistors only on one of the leads:
- Bend the lead close to resistor body (call this the y-direction).
- Bend the lead again at 2mm away in the z-direction.
- Trim this lead slightly.
The resistors should look like Picture 1.

Unfortunately I don't have pictures at the various stages below since I made a mistake initially and it shows in the pictures I had taken.  Hopefully Pictures 2 and 3 of the final correct build are sufficiently illustrative. Please refer to these pictures when reading the instructions below. The schematic for each channel is also attached.

Tip resistors:
- Attach and solder the prepared lead of a 100Ohm resistor to the eyelet of the tip terminal.
- Attach the prepared lead of a 33Ohm resistor to the sleeve terminal (ground). Do not solder.
- Bend the free lead of the 33Ohm resistor close to its body toward the 100Ohm resistor, and wrap it around the 100Ohm resistor's free lead close to the 100Ohm resistor's body.  Solder this connection taking care not to damage the 100Ohm resistor.
- Bend the free lead of the 100Ohm resistor toward the ring terminal making sure not to touch it.  Use a 1cm wire insulation to cover the part of the lead that passes above the terminal. Then make a small hook at the end of it and trim the lead.

Ring resistors:
Repeat the above steps for the ring resistors making sure the 100Ohm resistor lead is bent toward the tip terminal in the last step.

Sleeve terminal:
Make two hooks at the ends of a 1.5cm bare wire. Attach one hook to the sleeve terminal where the two 33Ohm resistors attach and solder this joint. This wire allows all the headphone wires to be the same length when trimming them.

This completes the jack setup.

[boulos]

Plug and wire installation:

- Prepare the wire as shown in Picture 4.
- Insert the metal enclosure into the wire.
- Connect (a) one wire to the 100Ohm tip resistor's free lead. It should go in the hook you created at the end of the lead. I connected the white.
- Connect (b) another wire to the 100Ohm ring resistor's free lead. It should go in the hook you created at the end of the lead. I connected the red.
- Connect (c) the last wire to the extension connected to the sleeve terminal (ground). I connected the blue.
- Do not close the case yet to allow for the resistance check.

Plug:
- Insert the plug casing in the wire.
- Make the three wires the following lengths: Sleeve (c) 1", Tip (a) 3/4", Ring (b) 1/2" and trim each 1/4".
- Lift the insulating plastic attached to the sleeve terminal. Connect wire (c) to it from the back. See Picture 5.
- Return the insulating plastic to its original position. Connect wire (a) to the tip terminal (the shorter one), and connect wire (b) to the ring terminal (the longer one).
- Bend the ends of the sleeve terminal around the wire. See Picture 6.

Resistance check:
- On the plug:
  Ring to sleeve: 33Ohm.
  Tip to sleeve: 33Ohm.
- Between the plug and the jack:
  Ring on plug to ring terminal in jack: 100Ohm.
  Tip on plug to tip terminal in jack: 100Ohm

Finally, gently close the jack case.  I didn't close it all the way (there are holes that fit into indentations). It will be very difficult to open it later if you close it tightly. Also, I was worried the resistors would hit the back of the case.  After closing the case, repeat the plug resistance checks.  Pictures 7 and 8 show the final assembly from the back, and shows it attached to the amp.

Plug in and enjoy! 

[feeench]

Thank you for this. I have to do the same thing and I like this solution the best. You made my work a lot easier!

Cheers

[boulos]

You're welcome   Please let me know if you run into any issues following the instructions so that I may update/clarify them.
Thanks!

[Grainger49]

That is an excellent write up and perfect pictures to supplement the wording.  Just what some fellow Bottleheads will need to be happy with two divergent pairs of headphones.