In the dawning days of audio, transformers played a vital role in the functionality of first-generation all-tube based electronic circuitry. It was circa 1920 and radio broadcasts for the general populace had just begun, generating a rapid rise in the demand for broadcast audio systems, all of which needed transformers to function.
Later, as equipment for live sound reinforcement began to emerge, transformers again proved indispensible as the only means of matching microphone impedances to vacuum tube preamplifiers.
Transformers were also used as inter-stage devices in amplifiers, for line output drivers, and for matching a power amplifier’s output stage to the impedance of a loudspeaker voice coil – just as they are still used today in audiophile tube-based amplifiers and musical instrument amps.
Eventually, widespread use of transistor-based preamps and power amplifiers lessened the need for transformers, but as any electric guitarist will tell you, tubes “just sound better.”
However, transformers do much more than just impedance matching. They can differentially balance a microphone or line-level signal at the source, and then de-balance the same signal at the destination (or more properly stated, the “load”). In the process, electromagnetic interference (EMI), the cause of all-too-familiar hum and buzz, is cancelled out as a function of the transformer’s common-mode rejection ratio (CMRR or CMR).
Line-level transformer isolators. (click to enlarge)
The term “common mode” refers to any stray field that is common to both the plus and minus poles of a balanced line. Add the word “rejection” and it describes exactly what the transformer is doing: it phase-cancels the EMI because the poles are 180 degrees opposed to each other, thus rejecting any unwanted field induced in the cable.
This is pretty important stuff. When a line is not balanced, it becomes vulnerable to picking up all kinds of stray energy. Usually this takes the form of 60 Hz (or 50 Hz in Europe), along with related harmonics. The interference is induced in the cable from nearby alternating current (AC) power cables or from AC rectifiers in electrical devices.
Problems can also be caused by radio transmitters and other high-power devices that generate unwanted energy fields, such as diathermy machines and wood welders (yes, there really is such a thing as a wood welder). In the case of higher frequencies, the invasive energy may not be audible, but it can wreak havoc in a sensitive broadband mic preamp if the energy is not cancelled out by a precision-grade transformer (or by other means).
High-grade audio transformers, such as those manufactured by Lundahl in Sweden and Jensen in the U.S., exhibit high CMRR values across a broad spectrum of frequencies; low-grade transformers may help reduce hum and buzz a bit, but their CMRR is rarely sufficient to solve any significant problem. When it comes to hum and noise rejection, precision high-grade transformers are an invaluable insurance policy.