Tech Talk
Transformer Basics
Colby Buddelmeyer
Director, Engineering, Mark Levinson
Intro:
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force (EMF) across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits.
Origins and history of the transformer:
The foundational principle of transformers—electromagnetic induction—was discovered by Michael Faraday in 1831. Faraday demonstrated that a changing magnetic field could induce a voltage in a nearby coil, laying the groundwork for all transformer technologies. While Faraday’s work provided the theory, practical transformers began to take shape in the 1880s. In 1885, Hungarian engineers developed the first closed-core transformer for power applications. Around the same time, William Stanley, working for Westinghouse Electric, designed and patented a more practical transformer for commercial use.
Photo courtesy of: https://edisontechcenter.org
Basic Transformer Construction:
A transformer typically consists of two coils of wire (windings) wound around a magnetic core. The primary winding connects to the input circuit, and the secondary winding connects to the output circuit. The number of turns in the windings determines the voltage and impedance transformation ratios. Here is more detail for each sub-component:
- Core
- Material: Typically made of a laminated soft magnetic material (like silicon steel or ferrite) to minimize eddy current losses.
- Shape: Often a toroidal, E-I core, or C-core design to efficiently couple the magnetic flux.
- Function: Provides a medium for magnetic flux transfer between the windings.
- Windings
- Primary Winding: Connected to the input signal source.
- Secondary Winding: Connected to the load or receiving circuit.
- Wire: Made of enameled copper wire to provide good conductivity and insulation.
- Turns Ratio: Determines the voltage and current transformation ratio (e.g., 1:1 for isolation, or step-up/step-down for impedance matching).
- Insulation
- Purpose: Electrically insulates the windings from each other and from the core.
- Material: Often uses layers of insulating tape, varnish, or other dielectric materials.
- Shielding
- Material: A conductive shield (such as a foil wrap or metal casing) around the transformer.
- Purpose: Reduces electromagnetic interference (EMI) from external sources and prevents signal leakage.
- Encapsulation
- Purpose: Protects the transformer from environmental factors like moisture and mechanical damage.
- Material: Epoxy resin or plastic housing.
Use of Transformers in Mark Levinson Products:
Mark Levinson designs utilize two types of transformers including power transformers and chokes. The power transformers are of the toroidal type to minimize stray flux. Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. Design and material selection are critical to ensure electrical stability and prevent losses. Extreme care is taken to minimize any transfer of vibration from the transformer into the chassis. Chokes are specifically designed to block high-frequency electrical noise (like ripple current) while allowing the desired low-frequency AC power to pass through, effectively filtering the power supply coming from the transformer and providing a cleaner output.
Key Functions of Power Transformers in Solid-State Amplifiers:
In a solid-state audio amplifier, a power transformer is a key component that takes the AC power from the wall outlet and converts it into the appropriate voltage levels needed to power the different circuits within the amplifier. Unlike tube amplifiers, solid-state amplifiers usually only have a single power transformer as the output stage is directly driven by transistors, eliminating the need for a separate output transformer. The power transformer will have multiple secondary windings to provide different voltage levels for various stages of the amplifier, such as the pre-amplifier, driver stage, and output stage.
Key Specifications:
Key specifications for a power transformer include rated power (MVA), primary and secondary voltage levels, frequency (50Hz or 60Hz), number of phases (single or three-phase), cooling method, winding connection group, impedance voltage, no-load loss, load loss, tap changer type (on-load or off-load), and installation type (indoor or outdoor).
Conclusion:
In conclusion, transformers in solid-state audio amplifiers are indispensable for power management and filtering, where they provide essential functions such as voltage transformation, isolation, and noise reduction. By enabling efficient power delivery and acting as chokes to smooth out fluctuations, transformers contribute to the amplifier’s stability and overall performance. While modern designs often minimize or eliminate inductors in the audio signal path to preserve signal purity, the use of transformers in the power stage ensures clean, consistent energy for optimal operation. As audio technology progresses, transformers will remain a vital component in power and filtering applications, supporting the performance and reliability of solid-state amplifiers.