Line input audio transformers are used to protect audio line inputs in studio

and live equipment from picking up external noise, caused by,

for instance, ground loops or cable noise pickup.

Line input transformers should have a high signal level capability, high no-load impedance and high Common Mode Rejection Ratio, CMRR.

Even short cables connecting two audio devices may sometimes pick up noise such as radio signals from cellphones or other transmitters around the setup. Such signals are however of common mode character, and are efficiently blocked by the Faraday shields in the input transformer.

The degree of the capability of the transformer to block such

signals is expressed as the transformer’s CMRR.

Ground loop noise can be induced when there is a direct (galvanic) connection between the signal source and receiver, and there is a ground level potential between them. This can occur, for example, when devices are connected to different wall outlets. It can also occur when lighting dimmers are connected to the same power distribution system as audio equipment, as dimmers have a tendency to make the electrical ground reference very noisy. But as the primary and secondary windings in an input transformer are galvanically isolated,

line input transformers eliminate ground loops.

A line input transformer will also take care of any balance-unbalance or unbalanced-balanced conversion needed if both balanced

and unbalanced equipment are used.

For Breakout PCB's, check here

Line output transformers with silicon iron C cores.

General properties

Compact

Low copper resistance

High level capability

For balance or unbalance drive

Lundahl silicon iron C cores

Driving circuitry for line output transformers should have low output impedance. In order to reach the very best performance in terms of low distortion and low line output impedance, we recommend that our output transformers are used with mixed feedback drive circuitry for negative output impedance and best THD performance.

In the book “OP AMP APPLICATIONS” (Analog Devices, edited by Walt Jung) the use of (op amps and) audio transformers in line input and output applications are discussed in depth. In particular the mixed feedback drive circuits for line output transformers are analyzed in great detail. We strongly recommend the book for anyone who is designing with op amps in general, and in particular if you are designing circuits for transformer line output applications.

The book is available on-line at https://www.analog.com/en/education/education-library/op-amp-applications-handbook.html. The mixed feedback circuitry is discussed in Section 6 (3.2Mb), page 6.71 and forward.

Sectioning of transformers are discussed in a separate paper.

Breakout pcb's for Lundahl audio transformers