Everything you wanted to know about wideband, low-frequency transformers:
PDF contains multiple circuits - scroll to find this circuit.

External Isolation Transformer Box: designed for Mackie and other Mixers Eliminates RFI Problems  

Grounding & Shielding: describes basic audio transformer uses like balanced in/out and signal splitters  

High CMRR Balanced Interface: Uses for Crystal18 or20 Bit A/D Converters.  This is a PDF file  

High Output Line Driver for Single Ended Power Supply: PDF file  

IC Mic Preamp uses Jensen tw1in Servo topology:

Interconnection of Balanced & Unbalanced Equipment: 4 page booklet in PDF format  

Internal Modification to Mic Inputs of the Mackie 1604 Mixer Eliminates RFI Problems: adding transformer isolation increases common mode rejection and eliminates RF interference, PDF document 

Isolation Transformer Passes millihertz Signals: 08/04/94 EDN-Design Ideas 

IT 4 Four Channel Isolation Transformer unit: audio signal isolation transformer, product datasheet with circuit diagram in PDF format 

IT one inline 600 / 600 Ohm Line Transformer: audio signal isolation transformer, product datasheet with circuit diagram in PDF format  

Jensen Direct Box Transformers: data sheets have example circuit diagrams  

Jensen Transformers Application Schematics: includes many microphone preamplifier related   

JP 11P 1 Input Transformer Improvement for Electronically balanced Input:

JS 11SPP 6M 600 Ohm Line Isolator with Line termination Switch: for used with long 600 ohm lines that need termination 

JS 11SSP 6M 150 Ohm Line to 600 Ohm Load application: isolation transformer with impedance conversion  

JT 10B D Used in Line Level 2 Way Splitter / Isolator: active line level splitter circuit with input and output isolation  

JT 10KB D & JT 6110K B "Pro" to "Consumer" Interfaces: from +4dBu balanced output to -10dbV consumer equipment input 

JT 10KB D & JT DB E Provide "J Box" Consumer to Studio Interfaces: from -10dBV unbalanced to150 ohm balanced microphone level -45dBu signal and from +4dBu balanced signal to -10dBV unbalanced signal  

JT 10KB D Isolates & Converts Speaker Drive to Line Level:

JT 11 BM High Level Differential IC Output Stage with DC Servo:

JT 11 BM High Level High Current Differential Output Stage with DC Servo:

JT 11 DM Basic Hum Stopper Transformer Isolation Box: simple isolator for 600 ohm balanced audio lines  

JT 11 DM Converts Unbalanced Output to Balanced:

JT 11 FL Isolates & Converts Guitar Amp Speaker Out to Line Output:

JT 11 FL provides Isolated Line Output from Guitar Amplifier:

JT 11 YZPC in Unbalanced Long Line Driver / Isolator:

JT 11B 1 + "Phantom" Buffer Provide Balanced Guitar Output:

JT 11P 1 as "Phase Splitter" with Symmetrical Outputs:

JT 11P 1 Balanced Tube Line Input Stage:

JT 11P 1 Conversion of Unbalanced Input to Balanced:

JT 11P 1 in 2 Way Passive "Guitar Splitter": from one guitar to2- guitar amplifiers 

JT 11P 1 in 4 Way Active "Guitar Splitter": from one guitar to up to4 guitar amplifiers  

JT 11P 1 Unbalanced Automotive or Consumer Application: isolation transformer box for unbalanced consumer audio line signals 

JT 11P 1 used as Unbalanced Polarity Inverter: passive transformer based audio signal inverter circuit 

JT 11SSP 6M 600 Ohm Line Isolation Application: isolation transformer for 600 ohm balanced audio line  

JT 11SSP 6M as Input Transformer for Crystal CS5367 A / D Converter: transformer balanced passive input circuit for CS5367 A/D converter, PDF file 

JT 11SSP 6M Bi-Directional Lo Z Long Line Application: this circuit isolated and impedance matches less than100 ohm source to long cable line and that to over10 kohm input  

JT 11SSP 6M Passive Stereo Matrix for M S Mic Technique:

JT 11SSP 6M Universal Patchbay Isolation Application: for use with professional low impedance sources and high impedance loads  

JT 123 BL High Level Low Impedance Differential Output Stage with DC Servo:

JT 123 S Line Driver with Feedback Around the Transformer:

JT 13K6 C in Simple 1 IC Stage Mic Preamp: 15KB PDF  

JT 16 A Improves Performance of SSM2017 Mic Preamp IC:

JT 6110K B at Input Stage for Crystal a to D Converters: high performance transformer balanced input stage for Crystal A/D converters, PDF file  

JT 6110K B Isolates "70 Volt" Line & Converts to Line Level:

JT DB E "Direct Box" for very High Level Signals: takes high level line signal and outputs balanced microphone level signal  

JT DB E Converts Consumer Stereo Inputs to Mono Balanced Mic Output:

JT DB E Full featured "Direct Box" Application Schematic:

JT DB E Standard Musical instrument or Synthesizer "Direct Box":

JT MB C "Real" Mic Inputs & Phantom Power for Sony DAT:

JT MB C in Low Noise, High Isolation Line Receiver for Low Z Sources:

JT MB C Isolates both Mic & Line Inputs of inexpensive Mixer:

JT MB C Mic Input Isolation for Mackie Mixers:

JT MB C Used to "Phantom" Power the Realistic PZM Mic:

JT MB D Converts 600 Ohm Unbalanced Mic to 150 Ohm Balanced Mic Line:

JT MB D Passive Stereo Matrix for M S Mic Technique:

Method sets voltage in multiple-output converters: 02/01/01 EDN-Design Ideas / PDF contains multiple circuits - scroll to find this circuit.

Mixed Feedback Drive  for Audio Output Transformers: Technical paper from Lundahl Transformers in PDF format 

Power Supplies & Transformers: Basic structures of power supplies  

Step Down Rectifier makes a Simple DC Power Supply: 04/09/98 EDN-Design Ideas  /  A simple and useful nonisolated rectifier features voltage step-down operation, acceptable Class A line-current harmonics, inherent short-circuit protection, and, optionally, a regulated output

Theory & Construction of Mic "Splitters": 5 page booklet in PDF format 

Transformer Coupled Splitter: Active amplified transformer isolated signal splitter that enables hum-free connection of1-guitar to more than1-amp as well as having a direct output  

Transformers for Audio Interfacing: Discussion on using audio transformers in equipment interconnections, read also Line-level transformers in High-End Audio