Step-Up Transformers Explained: What They Do and When You Need One
A step-up transformer (often called an SUT) is a passive device used with low-output moving coil cartridges to increase signal voltage before it reaches a phono stage. While simple in concept, step-up transformers affect gain, loading, noise, and overall sound character. This guide explains how step-up transformers work, when they make sense, and how they compare to active MC phono stages.
What is a step-up transformer?
A step-up transformer is a passive device that increases the voltage output of a low-output moving coil cartridge so it can be used with a standard moving magnet phono input.
Unlike active electronics, a transformer uses magnetic induction rather than amplification circuits.
Why step-up transformers exist
Low-output MC cartridges typically produce between 0.1mV and 0.5mV—too little for most MM phono stages. A step-up transformer raises this voltage to MM-compatible levels without adding active gain.
How a step-up transformer works
An SUT uses two coils of wire: a primary coil connected to the cartridge, and a secondary coil connected to the phono stage.
The ratio between these coils determines how much the signal voltage is increased.
Common transformer ratios
- 1:10 (10x voltage increase)
- 1:15 (15x voltage increase)
- 1:20 (20x voltage increase)
Gain and output voltage
Step-up transformers increase voltage but do not amplify current. This means gain is fixed by the transformer ratio.
For example, a 0.3mV cartridge through a 1:10 transformer produces 3mV—ideal for most MM phono stages.
How step-up transformers affect cartridge loading
Loading with a transformer works differently than with an active phono stage.
The effective load seen by the cartridge is calculated by dividing the phono stage’s input impedance (usually 47kΩ) by the square of the transformer ratio.
Example loading calculation
- 47kΩ ÷ (10 × 10) = 470Ω
- 47kΩ ÷ (20 × 20) = 117Ω
This means loading is fixed by design unless additional resistors are used.
Advantages of step-up transformers
- No active noise or electronic amplification
- Excellent signal-to-noise performance
- Natural, dynamic sound presentation
- Ideal match for very low-output MC cartridges
Disadvantages of step-up transformers
- Fixed gain and loading
- Less flexible than active MC stages
- Can be expensive when well-designed
- Must be matched carefully to cartridge output
Step-up transformer vs active MC phono stage
Both approaches can sound excellent. The choice depends on system needs and listening preferences.
| Step-Up Transformer | Active MC Stage |
|---|---|
| Passive, no electronic gain | Uses active amplification |
| Fixed loading and gain | Adjustable loading and gain |
| Very low noise floor | More flexible cartridge matching |
When a step-up transformer makes sense
- You use a low-output MC cartridge
- Your MM phono stage is high quality
- You prefer simplicity and passive signal paths
- You don’t need frequent cartridge changes
When an active MC stage may be better
- You want adjustable loading and gain
- You swap cartridges often
- Your cartridge output is borderline MC/MM
- You prefer convenience and flexibility
Common mistakes with step-up transformers
- Using the wrong ratio for cartridge output
- Ignoring loading interactions
- Assuming higher ratios always sound better
- Using poor-quality cabling or grounding
Final thoughts
Step-up transformers are not outdated technology—they are precision tools. When properly matched, they can deliver exceptional clarity, dynamics, and musical realism from moving coil cartridges.