# 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]]