Audio transformers are remarkably reliable components, but small details in installation, connection and measurement can have a significant impact on performance.
In this first part of our Transformer Insights & Best Practices series, we highlight three practical areas that are worth paying extra attention to when working with Lundahl transformers and chokes.
1. Do Not Trim the Pins
A few times each year, we are contacted by customers who have discovered what appears to be an open winding. This can be surprising, as every Lundahl transformer is fully tested before leaving our factory. In many of these cases, the problem turns out to have occurred after installation.
One common cause is that the transformer pins have been trimmed or shortened after the transformer has been soldered onto a PCB.
Why can trimming the pins cause a problem?
The pins used on many Lundahl transformers are not solid metal pins. They are small tubes.
The winding wire from inside the transformer is connected inside the pin itself. This construction creates a secure electrical connection, but it also means that cutting the pin can potentially damage the internal connection between the winding wire and the terminal. The result can be an intermittent connection — or in some cases what appears to be an open winding.
Best practice
Do not trim the transformer pins after soldering.
If pin length is important for the mechanical design of your PCB or enclosure, we recommend taking this into account during the product design stage rather than modifying the transformer pins afterwards. Small mechanical details can have a surprisingly large impact on reliability.
2. Be Careful With Unused Windings
Many Lundahl transformers are built using an electromechanical structure we refer to as a:
Dual-coil, single-core structure.
In simple terms, the transformer contains two winding coils that share one common magnetic core. In our chokes, each coil contains one winding. In our transformers, however, each coil normally contains sections belonging to both the primary and secondary sides.
This construction offers several important advantages.
Reduced magnetic interference
The dual-coil structure helps minimize external magnetic fields generated by the transformer. It also provides good immunity to magnetically induced interference from nearby components such as:
- mains transformers
- electric motors
- other magnetic components
This can be especially valuable in sensitive audio circuits where low noise and signal integrity are important. However, there is an important detail.
Both coils need to work together
For the structure to perform as intended, both coils should normally be driven and loaded symmetrically. This is not always obvious when looking only at the electrical schematic. Leaving winding sections unused may alter the electrical behaviour of the transformer and can, in some configurations, result in reduced high-frequency response.
Best practice
Always follow the recommended connection instructions in the datasheet for the specific transformer model. If your application requires a different connection, do not assume that an apparently logical wiring configuration will necessarily provide optimum performance.
Contact us. Our technical team may be able to recommend a more suitable connection for your application.
3. Magnetization of the Core
Another question we occasionally receive concerns permanent magnetization of transformer cores. Different core materials behave differently when exposed to DC current. Understanding those differences can help prevent unnecessary distortion.
C-cores and laminated cores
In our experience, the C-cores used in Lundahl transformers do not normally become permanently magnetized. Permanent magnetization is also extremely rare in our laminated-core transformers. This remains true even when they have been exposed to small DC currents, for example during resistance measurements of the windings.
There is, however, one important exception.
Un-cut amorphous cores require extra care
Transformers using our un-cut amorphous core are considerably more sensitive to DC current. Even very small DC currents can magnetize the core material. And the source of that current can be surprisingly ordinary. A simple resistance measurement with an ohm-meter, for example, may introduce enough DC current to magnetize the core. The transformer may still appear electrically functional, but the magnetized core can lead to increased distortion. In some cases, that distortion may even become audible.
Before connecting the ohm-meter
Ask yourself one question:
Is there another way to confirm what I am looking for?
If the goal is simply to determine whether a winding is intact, consider whether another measurement method can provide the answer without introducing DC into the transformer.
What if the transformer has already been magnetized?
Fortunately, magnetization does not necessarily mean that the transformer is damaged. Instructions for demagnetizing Lundahl transformers can be found in the Application Notes section of our website.