Thursday, September 22, 2011

Bogen T725 transformer

I have found a little gem of a transformer for my radio projects: The Bogen T725 audio transformer.  I basically gives you the ability to match 8 ohm loads to a high(er) impedance source from about 150 ohms to about 40k ohms.  Specs below courtesy of where you can find lots of ideas on how to use these devices.
Bogen T725 Schematic
I was able to locate a number of these little dudes online ( and picked them up for future projects.  In the table below, all values in the primary are relative to the black tap (blk).

     Color      Resistance     Inductance       XL @ 300 hz      Rounded Value
   White  (WH)  1424.3 ohms      24     H       45.239k  ohms      40k    ohms 
   Gray   (GRY)  886.4 ohms      12.04  H       22.694k  ohms      20k    ohms
   Violet (VIO)  516.5 ohms       6.06  H       11.423k  ohms      10k    ohms   
   Blue   (BLU)  260.1 ohms       3.04  H        5.730k  ohms       5k    ohms  
   Green  (GRN)   81.8 ohms       1.565 H        2.950k  ohms       2.5k  ohms    
   Yellow (YEL)   56   ohms         787 mH       1.483k  ohms       1.2k  ohms
   Orange (OR)    38.2 ohms         398 mH         750.2 ohms         600 ohms
   Red    (RED)   26   ohms         197 mH         371.3 ohms         300 ohms
   Brown  (BRN)   18.2 ohms          98 mH         184.7 ohms         150 ohms

   Pink to Pink    0.5 ohms        5.23 mH          9.86 ohms           8 ohms 

I plan to use one of these to get the B+ voltage from my National SW-3 off of the headphones and to provide an impedance match to a more typical 8 ohm speaker or headphone.

I also found a nice transformer P-T31 at Antique Electronic Supply that does 5k to 8 ohm impedance.  These were about twice the price and no taps on the primary.


  1. How to match an 8 ohm load to a 4 ohm amplifier output?

    1. You would need a different transformer for this task. This one is 8 ohm output and anything from 40K ohm to 150 ohm input. Sorry about that.