Audio Transformers

Linking signals between circuits, isolating and filtering using audio transformers
By Phillip Galey A typical electronic sound system incorporates a number of different kinds of audio transformers. The power supply uses an audio power transformer designed to minimize feedback through the ground, thus minimizing humming and other transients. Output audio transformers give you the impedance matching and isolation that is needed when connecting the audio output of the amplifiers to the speaker system.

You'll often want to use an isolation audio transformer between different major circuits (e.g., between a microphone and an input amplifier) to minimize noise as well as to impedance match the circuits. You can use digital audio amplifiers in applications where a wider range of frequencies is involved, e.g. to 192 kHz. If you are planning to purchase audio transformers, follow this blueprint:

1. For the power supply, shop for a power audio transformer.

2. Impedance match an audio amplifier's output to speakers using an output audio transformer.

3. Optimize microphone performance by employing a digital audio transformer.

4. Eliminate or reduce hum by using an isolation audio transformer.

Place an power audio transformer in the power supply of an audio system

Power audio transformers are necessary to isolate an audio device from the AC wall voltage. To avoid inserting hum into other parts of the circuitry, they are sometimes made with a magnetic or electrostatic shield. For optimal results, it's good to find one that has relatively low hysteresis so as to minimize harmonic distortion at lower frequencies and at low power levels.
Try:  Audio Electronics has dozens of audio power transformers to choose from, covering both 110 and 220 volts. Plitron carries numerous audio power transformers with multiple outputs in spun aluminum enclosures.

Use an audio output transformer to impedance match the output of an amplifier to speakers

The output audio transformer is required to isolate the output amplifier from the speakers, while providing an acceptable impedance match.
Try:  Edcore has a long list of audio output transformers, designed for use in the output of audio amplifiers. K&K Audio also carries a great selection of audio output transformers.

Opt for a digital audio transformer when a large frequency range and low jitter are required

Digital audio transformers offer superior performance characteristics over conventional transformers, especially at higher frequencies. Many sport a bandwidth of 192 MHz and extremely low coupling capacitance and super fast rise times. Surface mount units tend to have the best common mode rejection ration (CMRR) and noise rejection at high frequencies, while the through-hole type transformers will give you the best low-end bandwidth, e.g. down to 700 Hz.
Try:  Scientific Conversion claims to have the best performing digital audio transformers, as they are designed with low capacitance, low jitter, high interference rejection and fast rise time. Viteccorp makes a number of different models of digital audio transformer, made for installation onto a printed circuit board.

Utilize an audio isolation transformer between different circuits to stop hum

For linking two audio circuits with different grounds, you want to use an isolation audio transformer to prevent hum.
Try:  Op Amp Labs has audio isolation transformers, covering frequencies from 30 Hz to 15 kHz. Jensen also has a very impressive collection of audio isolation transformers.

• DC currents to an audio transformer cause the audio transformer to become magnetized (especially those with a steel core), resulting in measurable harmonic distortion. You can demagnetize an audio transformer by using an audio generator with a continuously variable output. Apply 20 Hz current to the transformer at low level, increase the intensity slowly until the transformer is deeply saturated and maintain it at that level for a few seconds. Then slowly decrease to intensity to zero. The transformer is now demagnetized.