What is an audio transformer?
As the name suggests, audio transformers are used in electronic audio circuits for signal coupling/isolation and impedance matching of amplifiers and speakers.
Like any other transformer, the audio transformer is also a passive device. This means that the power in the primary windings is the same as in the secondary windings.
Primary function of audio transformer
The primary functions of an audio transformer are :
- Isolation.
- Step up/down voltage.
- Impedance matching.
Impedance matching
If \(Z_p\) is the primary winding impedance, \(Z_s\) is the secondary winding impedance, \((N_p/N_s)\) is the transformer turns ratio, and \((V_p/V_s)\) is the transformers voltage ratio, then we have following relation among them :
$$\cfrac{Z_p}{Z_s} = \left(\cfrac{N_p}{N_s}\right)^2 = \left(\cfrac{V_p}{V_s}\right)^2$$
Example :
An audio transformer with a turn ratio of 3.16:1 is to be used to match the output of a power amplifier to a loudspeaker. Suppose the amplifier’s output impedance is 50\(\Omega{}\). Next, calculate the speaker’s nominal impedance required for maximum power transfer.
$$\cfrac{Z_p}{Z_s} = \left(\cfrac{N_p}{N_s}\right)^2 = \cfrac{3.16^2}{1} \simeq{} 10$$
$$\implies{} Z_s = \cfrac{Z_p}{10} = 5$$
With the help of the transformer, this power amplifier can drive 5\(\Omega{}\) of speaker load.
Multitap Audio transformers
Multitap audio transformers have multiple primary or secondary windings/connections (called “taps”). These offer different electrical impedances/step-up/down ratios in a single construction which save space.
Size and weight of Audio transformers
The size of an audio transformer depends on the following parameters :
- Frequency range.
- Current drive/power.
With lower frequency, to maintain the same impedance, the self/mutual inductance of the transformer has to be increased. Since self-inductance is directly proportional to size, size has to be increased to support low frequency.
The power that can delivered to a load depends on the thickness of the copper windings and the material of the core. To support high currents, it is necessary to increase the thickness of the copper windings.
100V audio line for PA systems
100V line audio is a standard solution that offers a simple interface for multiple speakers distributed over long distances, e.g.:
- Restaurants, Pubs, and Bars.
- Places of worship, schools, and colleges.
- Warehouses, factories, Offices.
- Supermarkets, shopping centers.
- Stadiums, Horse yards, etc.
This 100V line is also called “High Impedance” or “Constant Voltage.”
The total wattage of all the loudspeakers added together must NOT be higher than the rated wattage of your amplifier.