Transformer core choice by application: For audio-frequency coupling (AF transformers) and low-frequency power conversion (mains transformers), which core type is most commonly used due to high permeability and low magnetizing current?

Introduction / Context:
Transformer core selection is driven by frequency. Audio transformers (roughly 20 Hz to 20 kHz) and grid-power transformers (50/60 Hz) require materials with high permeability to concentrate flux and reduce magnetizing current and size. The right choice improves efficiency, bandwidth, and reduces distortion and heating.

Given Data / Assumptions:

• Application: audio coupling and low-frequency power.
• Goal: low magnetizing current, acceptable losses, economical construction.
• We compare air, iron/steel laminations, and ferrite cores.

Concept / Approach:
Laminated silicon-steel (commonly called iron-core) offers very high permeability and manageable losses at audio and mains frequencies. Air-cores lack permeability, making them impractically large. Ferrites excel at higher frequencies (tens of kHz to MHz) but have different loss characteristics at low frequency and are uncommon for bulk 50/60 Hz power unless specially designed and large.

Step-by-Step Solution:

Verification / Alternative check:
Commercial mains transformers (EI, UI, toroidal) and classic audio output transformers all use laminated iron or grain-oriented steel, confirming the standard practice.

Why Other Options Are Wrong:

• Air-core: Requires massive turns and current; inefficient at low frequency.
• Ferrite-core: Optimized for high-frequency SMPS; not typical for 50/60 Hz power transformers.
• All of the above: Not true; only iron-core fits the general case stated.
• Powdered-iron core: Used in RF inductors/filters, not typical for AF or mains transformers.

Common Pitfalls:
Assuming ferrite is always better; ignoring that core losses and permeability vary strongly with frequency.

Final Answer:
iron-core