UNDERSTANDING SPECIFICATIONS

a)      Power Levels

Transformer windings must be able to handle maximum current and voltage during the operations. Accordingly, wire of the windings must be chosen.

Maximum power level specified on the datasheet of an audio transformer refers to the maximum power that can be delivered to the load. Power levels are specified at particular frequency, typically at 1 KHz.

Power level depends upon the current handling capacity of the wire used for the windings-primary and secondary.

b)      Frequency Response

Frequency response indicates that voltage output levels at all the frequencies in this frequency range do not vary more than the given limits (typically ±3 dB, ±1 dB, ±and 0.5 dB).

The upper frequency falls off 3 dB when the normalised impedance equals leakage inductive reactance.

This suggests use of higher primary inductance for lower frequency operation. This translates to requirement of larger core. This will make difficult to obtain higher frequency limit.

As high frequency response is governed by the leakage inductances, it could be improved by reducing the number of turns (leakage inductance is directly proportional to the number of turns). But since number of turns affects lower frequency limit, only option to improve high frequency response is to interleave the primary and secondary windings.

c)      Total Harmonic Distortion

Total Harmonic Distortion is primarily a function of operating flux density (at the lowest operating frequency) in the core. Distortion due to this cause falls off rapidly with increase in frequency. Reducing the flux density reduces the distortion. This is a function of the magnetic material used for the core.

The distortion is higher at low levels due to magnetic hysteresis and at high levels due to magnetic saturation

Distortion is specified in the datasheets in terms of percentage at a specific frequency, typically 1 KHz, and at rated power levels.

d)     Insulation Resistance

Windings of the transformer are properly insulated to ensure that the current flow along the coiled conductors. The insulation is characterised by Insulation resistance. Typical value of insulation resistance is 10 MW.

e)      Impedance- primary & Secondary

Impedance ratio of primary and secondary is chosen as per the application. Primary and secondary impedances are separately mentioned in the datasheets. If CT is suffixed to the impedance value, it implies that that winding is center tapped.

f)       Rated DC Current

Rated DC current (IDC) is the amount of direct current that can be passed through the transformer winding without causing any damage. The DC current level is based upon the temperature rise at the maximum rated ambient temperature.  The rated current informs about the transformer's ability to minimize the power losses in the winding by having a low DC resistance. 

g)      Insertion Loss

It is the measure of the power available out of the secondary Vs power input to the transformer. It depends upon the losses in the transformer; loss due to DC resistance of the windings is the most obvious and hence should be kept to the minimum possible.

h)      3 dB Bandwidth

The range of frequencies over which the insertion loss is less than 3 dB with respect to mid-band insertion loss is called 3 dB bandwidth.

i)        DC Resistance

Direct Current resistance is the resistance of the transformer winding measured with the help of DC current. This number should be kept to the minimum possible.

Apart from these, datasheet of an audio transformer mentions about the core used, turns ratio, wire dimensions, weight, and temperature range and may be few more parameters.