Audio Transformers

An audio transformer is a passive electromagnetic device that transfers electrical energy between two circuits through mutual induction, while providing electrical isolation, impedance transformation, and often desirable sonic coloration. Transformers are fundamental components in professional audio equipment, found in microphones, preamps, compressors, equalizers, and mixing consoles.

How Transformers Work

Basic Operating Principle

A transformer consists of two or more coils of wire (windings) wrapped around a magnetic core:

1. Primary winding - Receives the input audio signal
2. Magnetic core - Transfers energy between windings (typically iron, steel, or nickel alloys)
3. Secondary winding - Delivers the output audio signal

The process:

1. AC audio signal flows through the primary winding
2. This creates a fluctuating magnetic field in the core
3. The magnetic field induces a voltage in the secondary winding
4. The output voltage depends on the turns ratio between windings

Turns ratio example:

• Primary: 100 turns
• Secondary: 1000 turns
• Ratio: 1:10 (step-up transformer - increases voltage by 10x)
• This also changes impedance by the square of the turns ratio (1:100 impedance ratio)

Key Properties

Electrical isolation:

• No direct electrical connection between primary and secondary
• Prevents DC current and ground loops from passing through
• Protects equipment from voltage spikes

Impedance transformation:

• Transforms impedance by the square of the turns ratio
• Example: 1:10 turns ratio = 1:100 impedance ratio
• If primary sees 50Ω, secondary presents 5,000Ω

Signal balancing:

• Center-tapped transformers create balanced signals
• Converts unbalanced to balanced (or vice versa)
• Provides common-mode noise rejection

Types of Audio Transformers

Input Transformers

Location: At the input stage of preamps, consoles, and line-level equipment

Purpose:

• Convert microphone-level signals to line-level
• Provide impedance matching for optimal microphone loading
• Balance incoming signals
• Reject common-mode noise (hum, RF interference)

Examples:

• Chandler TG2 mic input transformers (EMI TG series design)
• Neve 1073 input transformer (Marinair)
• API 512c input transformer (API 2623)

Impedance characteristics:

• Typical mic input: 150Ω, 300Ω, 600Ω, 1.2kΩ, or 2.4kΩ (switchable)
• Loads the microphone's output transformer at specific impedance
• Affects frequency response and harmonic content

Output Transformers

Location: At the output stage of microphones, preamps, or line-level equipment

Purpose:

• Convert internal circuit impedance to standard output impedance
• Drive long cable runs without signal degradation
• Provide balanced output
• Add desirable harmonic distortion and "color"

In microphones specifically:

• Ribbon microphones - Always have step-up output transformers (ribbon impedance is extremely low, ~0.2Ω)
• Dynamic microphones - Some have transformers (e.g., Sennheiser MD 421), others are transformerless (e.g., Shure SM57/58)
• Tube condenser microphones - Always have output transformers (tubes require high voltage, transformer steps down to mic-level)
• Solid-state condensers - Can be transformer-coupled or transformerless

Typical output impedances:

• 150Ω (modern standard)
• 200Ω (vintage standard)
• 300Ω (common in ribbon mics and vintage dynamics)

Interstage Transformers

Location: Between gain stages within a piece of equipment

Purpose:

• Couple one amplifier stage to another
• Provide DC blocking between stages
• Create push-pull amplifier topologies
• Add harmonic saturation

Examples:

• Found in vintage tube compressors (Fairchild 670, Gates STA-Level)
• Used in some high-end mic preamps for additional coloration

Line Transformers

Location: At line-level inputs/outputs (insert points, mix bus, etc.)

Purpose:

• Impedance matching between equipment
• Convert between balanced and unbalanced
• Isolate equipment to prevent ground loops
• Add "iron" coloration to signals

Examples:

• Jensen transformers (popular retrofit for consoles and interfaces)
• Cinemag transformers (used in boutique gear)
• Used in "color boxes" like Radial JDX or passive summing mixers

Microphone Output Transformers

Why Microphones Use Output Transformers

Ribbon microphones:

• Ribbon element has extremely low impedance (~0.2Ω)
• Produces very low voltage output
• Step-up transformer (often 1:30 to 1:50 ratio) boosts voltage
• Transforms impedance to usable range (150-300Ω)
• Without transformer, signal would be too weak and noisy

Tube condenser microphones:

• Tube circuit operates at high voltage
• Output transformer steps voltage down to mic-level
• Transforms high impedance (10kΩ+) to low impedance (150-200Ω)
• Provides balanced output for long cable runs

Dynamic microphones (some models):

• Voice coil impedance may be too low for optimal cable driving
• Transformer provides impedance bump and balancing
• Can add desirable "weight" to the sound

Transformer-Coupled vs. Transformerless Microphones

Transformer-coupled microphones:

Advantages:

• Fuller, weightier low-end response
• Smooth, musical high-frequency rolloff
• Harmonic richness and "color"
• Better common-mode noise rejection
• Can handle higher SPL before distortion

Disadvantages:

• More expensive to manufacture
• Heavier and larger
• Frequency response affected by core saturation
• Phase response affected at frequency extremes
• Susceptible to electromagnetic interference if poorly shielded

Examples:

• Neumann U87 (transformer-coupled)
• AKG C414 series (transformer-coupled)
• All ribbon microphones
• All tube condensers

Transformerless microphones:

Advantages:

• Extended, flat frequency response
• Better transient response (no transformer "smearing")
• Lower noise floor
• Lighter weight, more compact
• Less expensive to manufacture
• Immune to electromagnetic interference

Disadvantages:

• Can sound "thin" or "clinical" compared to transformer designs
• Less harmonic complexity
• May require more careful preamp matching
• Lower maximum SPL handling in some designs

Examples:

• Neumann TLM series (TLM = TransformerLess Microphone)
• Most small-diaphragm condensers (Neumann KM 184, DPA, Earthworks)
• Modern budget condensers
• Shure SM57/SM58 dynamics

Impedance Matching and Microphone Loading

What is Impedance Matching?

Impedance is the opposition to alternating current (AC) flow, measured in ohms (Ω). In audio, we deal with:

• Output impedance - The resistance the source (microphone) presents to the load
• Input impedance - The resistance the destination (preamp) presents to the source

Historical "matched" impedance:

• In vintage systems, output and input impedances were equal (e.g., 600Ω microphone into 600Ω preamp input)
• This maximizes power transfer
• Problem: Causes loading effects that alter frequency response and increase noise

Modern "bridging" impedance:

• Input impedance is 5-10x higher than output impedance
• Example: 200Ω microphone into 1.2kΩ-2.4kΩ preamp input
• Minimizes loading effects
• Provides flatter frequency response
• This is the current standard

Preamp Input Impedance Controls (e.g., Chandler TG2)

Many high-end preamps offer switchable input impedance to change how they load the microphone's output transformer. This is NOT "impedance matching" in the traditional sense - it's deliberate transformer loading to shape tone.

On the Chandler TG2:

• Typical settings: 300Ω, 600Ω, 1.2kΩ, 2.4kΩ (check manual for exact values)
• Lower impedance (300Ω) - Heavier loading on microphone transformer
• Higher impedance (2.4kΩ) - Lighter loading on microphone transformer

The 300Ω setting - when to use it:

Best for microphones with output transformers:

• Ribbon microphones (Royer, Coles, AEA, Beyerdynamic)
• Vintage dynamics with transformers (Sennheiser MD 421, EV RE20)
• Transformer-coupled condensers (Neumann U87, AKG C414, Telefunken)
• Tube condensers (all have output transformers)

Why 300Ω works well:

1. Loads the output transformer closer to its design impedance
2. Fuller low-end response - Loading increases bass thickness
3. Smoother high-end - Reduces potential harshness or peaks
4. More harmonic saturation - Transformer core works harder, adds coloration
5. "Vintage" character - Similar to how classic consoles loaded microphones

Sonic effect of 300Ω loading:

• Warmer, rounder tone
• Enhanced low-mids and bass
• Slightly rolled-off high frequencies
• More "analog" character
• Can reduce output level slightly (more transformer losses)

Higher impedance settings (1.2kΩ - 2.4kΩ) - when to use:

Best for transformerless microphones:

• Neumann TLM series
• Modern small-diaphragm condensers
• Transformerless dynamics (SM57, SM58, SM7B)

Why higher impedance works well:

• Minimal loading on the microphone's output stage
• Flatter frequency response
• Extended high-frequency response
• Lower noise floor
• More "neutral" or "transparent" sound

Sonic effect of high impedance loading:

• Brighter, more open tone
• Extended top end
• Tighter, leaner bass
• More "modern" character
• Slightly higher output level

Practical Example: Ribbon Mic on Guitar Amp

Setup:

• Royer R-122 ribbon microphone on guitar cabinet
• Chandler TG2 preamp

300Ω setting:

• Fuller, thicker guitar tone
• More "body" in the low-mids
• Smooth, musical high end
• Classic rock sound
• Recommended starting point for ribbons

1.2kΩ setting:

• Brighter, more articulate guitar tone
• Extended high-frequency detail
• Leaner, tighter bass response
• More "modern" or "hi-fi" sound

Neither is "wrong" - it's a creative choice based on the desired sound.

Transformer Saturation and Harmonic Distortion

How Transformers Add "Color"

Transformers are non-linear devices - they don't reproduce the input signal perfectly. As signal level increases, the magnetic core begins to saturate, causing:

1. Harmonic distortion - Adds harmonics not present in the original signal
2. Compression - Gentle limiting of peaks as core saturates
3. Frequency response shifts - Low and high frequencies affected differently

Harmonic content:

• Even harmonics (2nd, 4th, 6th) - Musical, "warm" character
• Odd harmonics (3rd, 5th, 7th) - Can add "edge" or "grit"
• High-quality audio transformers produce mostly even harmonics

Why this sounds good:

• Adds richness and dimension to sounds
• Makes signals feel more "analog" or "vintage"
• Natural compression smooths transients
• Can make thin sources sound fuller

Controlling Saturation

In microphones:

• Fixed - determined by transformer design and SPL
• High SPL sources (drums, guitar amps) may drive mic transformer into saturation

In preamps (like Chandler TG2):

• Input drive control - Pushing input drive saturates the input transformer
• Impedance switch - Lower impedance = more transformer loading = more saturation potential
• Combination of both controls provides wide range of coloration

Sweet spots:

• Light saturation: Warmth and fullness without obvious distortion
• Medium saturation: Musical harmonic richness, gentle compression
• Heavy saturation: Obvious distortion, lo-fi character (e.g., John Lennon's "Polythene Pam" vocal)

Common Transformer Manufacturers

Microphone Transformers

• Jensen - Classic American designs, warm character
• Cinemag - Modern boutique transformers, excellent specs
• Lundahl - Swedish manufacturer, hi-fi performance with musicality
• Sowter - British manufacturer, vintage reproductions
• OEP (Carnhill) - Used in many modern microphones
• Haufe - German manufacturer, vintage Neumann specs

Preamp/Console Transformers

• Marinair - Vintage Neve transformers (1073, 1081, etc.)
• Carnhill/St. Ives - Modern Neve-style transformers
• API 2623 - API 500-series input transformers
• Cinemag CMMI-8 - Popular retrofit for API-style preamps
• Jensen JT-11 - Popular line input transformer

Transformer-Coupled Equipment in Studio A

Based on your Studio A gear (see Chandler Limited TG2):

Preamps with input transformers:

• Chandler TG2 - EMI TG series transformer design
• Likely: Neve 1073LB (if you have it) - Marinair-style transformers
• Likely: API 512V (if you have it) - API 2623 input transformer

Microphones with output transformers:

• All ribbon mics: Royer R-122, Royer SF-24, Coles 4038, Beyerdynamic M130
• Tube condensers: Neumann M147, Neumann M149, Warm Audio WA-CX12, Avantone CV-12, Brauner Valvet
• Transformer-coupled condensers: Neumann U-87, AKG C414 series, Telefunken models
• Some dynamics: Sennheiser MD 421, Sennheiser MD 441, EV RE20

Transformerless microphones:

• Neumann TLM 103, TLM 103 D
• Most small-diaphragm condensers (Neumann KM 184, DPA, Earthworks)
• Shure SM57, SM58, SM7B

Practical Workflow: Using the TG2 Impedance Switch

Step 1: Identify microphone type

• Check if microphone has output transformer (see lists above)

Step 2: Starting point

• Transformer-coupled mic → Start with 300Ω setting
• Transformerless mic → Start with highest impedance setting (1.2kΩ or 2.4kΩ)

Step 3: Listen and adjust

• Too dark/muffled? → Increase impedance (less loading)
• Too bright/thin? → Decrease impedance (more loading)
• Need more vintage color? → Decrease impedance
• Need more clarity/detail? → Increase impedance

Step 4: Combine with input drive

• Low impedance + high input drive = maximum transformer saturation
• High impedance + low input drive = cleanest, most transparent sound

Example workflow (vocal with Neumann U87):

1. Start: 300Ω impedance, input drive at position 5 (minimum)
2. Listen: Full, warm, but maybe too dark for modern pop vocal
3. Adjust: Increase to 1.2kΩ impedance
4. Listen: Brighter, more present, but losing some body
5. Adjust: Push input drive to position 7-8
6. Result: Extended highs from higher impedance, added warmth/body from input drive saturation

Related Concepts

• Impedance - Understanding electrical resistance in AC circuits
• Ohm's Law - Relationship between voltage, current, and resistance
• Balanced vs. Unbalanced Signals - How transformers create balanced outputs
• Harmonic Distortion - How transformers add musical color
• Microphone Preamps - Where input transformers live
• Ribbon Microphones - Always require step-up output transformers
• Chandler Limited TG2 - Example of transformer-coupled preamp with impedance switching

Links

Jensen Transformers: Understanding Audio Transformers

• URL: https://www.jensen-transformers.com/understanding-audio-transformers/↗
• Summary: Technical primer on transformer theory, impedance matching, and common-mode rejection from a leading transformer manufacturer.
• Related: Audio Electronics, Balanced Audio, Common-Mode Rejection

Lundahl Transformers: Transformer Loading and Frequency Response

• URL: https://www.lundahltransformers.com/↗
• Summary: Detailed technical documentation on how transformer loading affects frequency response, with measurements and graphs.
• Related: Frequency Response, Impedance Matching, Microphone Specifications