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TRRS to TRS adapter
 
HiFiMAN HM-901 in balanced mode used with the balanced phone TRRS plug.
This topic will DIY adapter for convert that balanced phone to 3.5mm single ended in compact size.

 

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This is the structure of balanced TRRS plug that used with HM-901.

 

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The main problem >>> I can't find the high quality female TRRS jack.(at July 2013)
Now they are  TRS Oyaide  and  TRS Rean(Neutrik) jack.(that I think they are good enough)
 

 
I choose to modify the Rean(Neutrik) because its body shorter...can total put into Viablue body.

 

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Survey internal structure of Rean(Neutrik) jack.

 

 

 

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The position when plugged with TRRS plug.
3 pins that contact to TRRS.
 
Pin for T...tip.

 
Pin for R1...ring

 
Pin for R2...ring

 
*** no any pin to contact with S...sleeve .

 

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If the body of TRRS plug made from metal.
S...sleeve can contact with the body of Rean(Neutrik) jack.
 
The right- signal can flow pass body to body.

 

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But in some conditions.
 
- the metal plug has a gap between them with the position at very centrical.
- the plug's body made from plastic with the position at very centrical.
>>> no any part of the S...sleeve can contact with the TRS jack.

 

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For this reason I will insert the fourth pin for contact with S...sleeve.

 
In the picture, The largest area is above the left signal pin.(blue arrows)

 
This area is the safety zone that can insert the fourth pin.
>>> not damage or cantact with another pins.
 
When look at the outer of normal jack.
I will make a hole at blue arrow.(in a line above the left signal pin)

 

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Make a hole by drilling.
 
*** Try to drill just beneath the rim of the TRS jack.(about 2mm.)
For ensure that the fourth pin can contact with S...sleeve
and not below to contact with the internal left signal pin.

 

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The fourth pin I get it from the first Rean(Neutrik) that I had opened it to survey the internal structure.
One jack has 2 long pins and 1 short pin.
Thus pin 1 , 2 , 3 and the fourth pin come from the same material for the same sound quality.

 

 

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Bend the fourth pin.

 
 
Test by put on the jack's body >>> head of the pin into the hole.

 

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Prepare for solding.
 
Mark the length of the Viablue body.

 
Cut and bend all legs of the Rean(Neutrik) jack.

 
Cut the ground leg of the Viablue plug.

 
 
Measure the lenght again.
*** the left signal pin of Viablue touch to the base of Rean(Neutrik) jack.

      This is the shortest length that I can do.

 

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Solder the Rean(Neutrik) jack to the Viablue plug.

 
Solder the fourth pin to the Rean(Neutrik) body.

***Solder at the end only.
Leave the body and the head of the fourth pin free.
 
The fourth pin act as the spring when plug and unplug.
-if solder at distal >>> the spring will soft.
-if solder involve to the mid part of pin >>> the spring will hard.
 
The softer spring dose not damage the TRRS plug of the earphone in long term usage.

 

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The fourth pin for S...sleeve when look inside the jack.

 

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Test the fourth pin.
 
This is in the normal position.

 
When the jack has been plugged in.
The fourth pin will spring out.

 
When the jack has been unplugged >>> the fourth pin will return to the normal position.

As in the first picture.