Roger jelbert's web site... back to homepage


Gardners MU7524 - MU7525 - MU7529 audio matching transformers

In spite of a lot of Googling I couldn't find any real details of these transformers. So if you've arrived here it's probably because you've also been searching on MU7525 etc rather than mic splitters.. I wanted to make use of some I had, in a three channel mic. splitter I was building for our band,. One thing is clear, these are high quality transformers and when used as microphone input transformers must have a very high signal handling ability. Thay are massive compared to the tiny things that are used inside mic bodies.

The "MU" in the type number indicates a mu metal case.

All three transformer types have a 600 ohm centre tapped winding with the 600 ohm winding on pins 6 & 8 with the CT on pin 7.

In the case of the MU7525 this is designated as the primary on the top of the can. On the 7424 and 7529 this winding is designated as the secondary.

This puzzles me as such mentions of all these transformers that I did find mentioned on the internet refererred to then as input transformers. They do seem to work OK in either direction so it's not clear to me why the types are labelled differently.

Well that's how I've used them in my mic splitter and they are all interchangeable albeit with different ratios of course.

Each type has two other windings, designated as secondary on the 7525 and primary on the 7524 and 7529.

In each case the two windings are identical and are as follows:

MU7524 - 2 X 150ohms. When connected in series = 600ohms which means the turns ratio = 1:1

MU7525 - 2 X 300ohms. When connected in series = 1200 ohms which means the turns ratio = 1:2 ( + 6dbv)

MU7529 - 2 X 50ohms. When connected in series = 200ohms which means the turns ratio = 1: .6 (point 6) (- 4.5dbv)

In all cases the two identical windings are on pins 1 & 3 and 2 & 4. The pins are counted anti clockwise around the base from the key.

Pin 5 I believe to be the internal shield between windings.

The windings are presented in phase order. ie. If the "phase +" is wired to pin 8, "phase +" will appear on pins 4 & 3.

To connect the two secondary (primary!) windings in series connect pins 2 & 3 together.

If you found this while searching for Gardners transformers info, I hope this will be useful. If you find a mistake, or can add more info, please email me.

Nov 2016..I have found this data sheet.


Three channel mic splitter with Gardner transformers.