111c isolation transformers

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have come across a pair of these Western Electric transformers and wanted to experiment with making an isolation unit for my source devices.

Many of you have seen the schematic configuration recommended for 600:600 use, but I have also looked at some Japanese brand that uses WECO 111Cs with resistors between the individual coils and a single capacitor (poly ?) I have attached the original and modified schematics (surmised from a decent quality image of the wiring) for review.

What purpose do these additional components serve? I can see the color code on the resistors, but need to determine the value of the capacitor and opinions if polar/non-polar.

Thanks in advance!


  • 111c.jpg
    30.4 KB · Views: 199
  • A625_I3.jpg
    33 KB · Views: 179
I do not understand why someone would but a 3K resistor in series with the primary on a 600 ohm transformer. Also the 10k resistor on the secondary does not make sense unless they wanted a grid stopper. If so, put the grid stopper closer to the grid.

The capacitor to ground for the primary make some sense if you still avoid a ground loop, but stop some RF from leaking in.
Thanks daqvin, I thought maybe I was missing something here. It looks like most users simply tie the input/output grounds to the "G" lug on the transformer and some, tie in the shield as well. If I wanted to provide a chassis ground (say from a turntable) what in your opinion would be the best point to use?
Back a half century ago, those transformers were used to provide quality audio from distant remote locations back to the radio broadcast studio(and from the studio to the transmitter site). The telephone company would install the temporary line and do the equalization. Those R's and C's may have had something to do with the EQ.
The resistor in series with the primary would cause a loss of bandwidth on both sides, which would be desired for telephone use. Not so desired for audio use.

In most uses, you would tie the shield to the signal ground of your local device. Tying the shield through a capacitor should just let the core float at what voltage it wants.

For a turntable ground, you want to go directly to signal ground in a way that does not allow noise current into the input. Attaching at the power supply ground would be best.
Joined 2004
Paid Member
The transformers are generally used into matched 600 ohm impedances and any EQ was applied with separate networks.

I use a number of these for various purposes. Western Electric calls them Repeater Coils, they're actually large toroids. They were used in permanent installations for studio to transmitter links on ATT long lines.

They do much better on both distortion and frequency response than WE claimed for them.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.