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Audio Transformer

Written By: 

Preeti Jain

Amazed by the electronic music of your favourite band? Mesmerized by the way suddenly an electric guitar goes as loud as the singer? Like most of know, there is an expert recording engineer present in the studio to make sure that music instruments and the voices make a perfect blend. A critical device that helps in creating this blend is an audio transformer. Works just like any other conventional transformer; audio transformers are designed to work at audio frequencies, i.e. between 20Hz to 20 KHz. Interestingly, the initial need to design to such transformer was to enhance long distancevoice communication. Now serving essentially for professional musicians, audio transformers stand as one electronic-audio component. This article will cover technical and applicative features of audio transformers explaining how they are different from the conventional counterparts.

 
Typical specifications of an audio transformer (42TM028) are
• Maximum output: 200mW
• Frequency response: ±2dB 300HZ-3, .4KHZ @ 1 kHz 0dB
• Insulation resistance: >10MW @ 100VDC
• Impedance ratio: Pri 600WCT, Sec1 600W, Sec2 600W
• Impedance variation: ±10% @ 1 kHz, .25V
• Primary wire: ø.08mm x 2 turns 320 ref.
• Secondary wire: ø.08mm x 2 turns 640 ref.
• DC resistance: Pri 78W ±15%, Sec1 (6-7) 65W ±15%, Sec2 (4-5) 60W ±15%
• Core type: EI-19
• Heat tolerance: +80°~+90°C
• Wave solderable
• Waxing: vacuum treatment
 
This article will help to understand and select audio transformers. Following sections will discuss the specifications, design and applications of an audio transformer.
AUDIO TRANSFORMERS- FUNCTIONS
 
Like any other transformer, audio transformer comprises of a primary winding, secondary winding and the core and the primary function of an audio transformer is to couple the energy from the primary windings to the secondary without introducing any distortion. Audio transformers are designed for audio range of frequencies, i.e., for frequencies ranging from 20 Hz to 20 KHz and are used primarily in audio circuits.
 
Representational Image Showing Insides of a Typical Audio Transformer
Fig. 1: Representational Image Showing Insides of a Typical Audio Transformer
 
The important functions of an audio transformer are:
1.      Impedance Matching
The impedance of the output stages of the amplifier circuits is often different from the impedance of the audio speakers. For maximum power transfer, impedance of the output amplifier should be same as that of impedance of the audio speakers. For lossless power transfer, impedance matching is done with the help of reactive elements, i.e., inductors and capacitors.
 
Since transformers can change the impedance (source sees the load with higher input impedance, while sink sees the source with lower output impedance), they are used for matching the impedance of the amplifiers and that of speakers.
 
The relationship between the impedances (primary and secondary) and turns ratio is as given below
 
Turns Ratio Proving Relation Between Impendance and Turns in an Audio Transformer
Fig. 2: Turns Ratio Proving Relation Between Impendance and Turns in an Audio Transformer
 
Where,       
            Zp = Primary Impedance
            Zs = Secondary Impedance
            Np = No of turns in a primary
            Ns= no of turns in a secondary
 
Thus, if output amplifier stage has an input impedance of 1800 W and loudspeaker impedance is 8W, a transformer is needed having turns ratio of
 
Example Showing Derivation of Turns Ratio for an Audio Transformer with Input Impendance = 1800W and Loudspeaker Impendance = 8W
Fig. 3: Example Showing Derivation of Turns Ratio for an Audio Transformer with Input Impendance = 1800W and Loudspeaker Impendance = 8W
 
With the transformer in between the amplifier and the speaker, amplifier will see an output impedance of 1800W, though actual load impedance is still the same, i.e., 8W. Similarly, loudspeaker will see an input impedance of 8 W though actual impedance is 1800W.
 
In a similar fashion (as explained above), audio transformers are used to change the impedance between the microphones and input amplifiers so that signal power is not reduced.
 
2.      DC Blocking
Typically the output amplifier’s AC signal is very low and is superimposed on the high voltage DC signal. It is often imperative to remove the DC signal from the composite signal before it enters the loudspeakers.
 
Since conventional transformers do not pass DC, audio transformers are used to prevent any DC signal from the amplifier entering the loudspeaker affecting the audio output.
 

WINDING ARRANGEMENTS

WINDING ARRANGEMENTS
 
Depending upon the function an audio transformer is used for, the windings of an audio transformer have different arrangements as given below:

1.      Phase splitting

Centre tapped Secondary arrangement is used to provide a selection of different turns ratios and for phase splitting. Phase splitting provides two anti-phase signals as it is important in some of the applications
 
Image Showing Phase Splitting Winding Arrangement in an Audio Transformer
Fig. 4: Image Showing Phase Splitting Winding Arrangement in an Audio Transformer
 
2.      Isolation
The arrangement used for provided isolation and/or impedance conversion is shown    below.
 
Image Showing Winding Arrangement used for Isolation and Impendance Conversion in an Audio Transformer
Fig. 5: Image Showing Winding Arrangement used for Isolation and Impendance Conversion in an Audio Transformer
 
The dots (used in the schematics) near the windings indicate the relative polarity of the signals on different windings.
 
3.      Multiple Loudspeakers in parallel
Public address systems use multiple loudspeakers connected to a single amplifier.  This is achieved with the help of “Constant Voltage” transformersreferred to as Line transformers. Typical values are 25 V, 70 V, 100 V and 140 V.
 
“Line” word doesn’t refer to grid electricity supply. Voltage levels, e.g. 100V, refer to the voltage obtained at the secondary of the transformer after stepping up of the input voltage. Voltage is stepped up to high voltage so that the current flowing in the long cables between the amplifier and the loud speakers is low. If high current flows in the cables, it will lead to high signal attenuation in the cables.  Again, at each loudspeaker, step-down transformers are used to perform dual tasks of increasing current by lowering the voltage and impedance matching.  Secondary windings have multiple tappings to suit for wide range of loudspeakers.
 
Winding Arrangement used in Audio Transformers for Parallel Multiple Speakers
Fig. 6: Winding Arrangement used in Audio Transformers for Parallel Multiple Speakers
 

UNDERSTANDING  SPECIFICATIONS

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 lower frequency limit is controlled by the primary inductance. If falls off 3 dB when the primary inductive reactance equals primary impedance. It is given by following formulas.

Image Showing Fall of Frequency by 3dB when Inductive Reactance in Primary Impedance

Fig. 7: Image Showing Fall of Frequency by 3dB when Inductive Reactance in Primary Impedance    

 
  (For -3dB fall off)

Image Showing Fall of Frequency by 1dB when Inductive Reactance in Primary Impedance

Fig. 8: Image Showing Fall of Frequency by 1dB when Inductive Reactance in Primary Impedance

  
(For -1dB fall off) 
 
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.
 

DESIGN OF AUDIO TRANSFORMERS 

DESIGN OF AUDIO TRANSFORMERS
 
Most important elements in the design of an audio transformer are the selection of core type, size and material, turn ratio, windings and casing.

 

a)      Core Size & Material

Choice of core size and its material depends upon frequency response, power levels, and harmonic distortion. Audio transformers should preferably use both high-permeability cores and the largest number of coil turns to create high primary inductance.

The most commonly used audio transformer core materials are M6 steel (a steel alloy containing 6% silicon) and 49% nickel or 84% nickel (alloys containing 49% or 84% nickel plus iron and molybdenum). Nickel alloys are substantially more expensive than steel but being high permeability materials require lesser number of turns.
Size of the core is determined based on the power requirements. Standard lamination tables needs to be referred. Once core is chosen, turns ratio to provide required inductance for a given size and magnetic material is obtained using the lamination catalogs.
 
b)     Turns Ratio

Voltage induced into the secondary winding depends on the turns ratio of the transformer. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.

If the turns ratio and the input voltage are known, the output voltage can be determined as follows
 
Formula For Determining Output Voltage when Turns Ratio and Input Voltage are Known
Fig. 9: Formula For Determining Output Voltage when Turns Ratio and Input Voltage are Known
 
Where, E1 & E2 are primary & secondary voltages, N1 & N2 are the number of turns in primary & secondary winding.
 
c)      Windings

Windings, primary as well as secondary, are the coils of conducting wires as a coil of conductors create a higher magnetic flux compared to the flux created by a single conductor.

 
The material used for the windings is application specific. Insulated solid copper wire is used for small power and signal transformers.
 
d)     Hum Reduction

When signal levels are low, it is often essential to keep external magnetic fields to the minimum. For this purpose, transformers are enclosed in a case of high permeability materials.

 

APPLICATIONS

APPLICATIONS

 
Audio output transformers are used when load impedances are low, as in line drivers, while audio input transformers are used when load impedances are high, as in line receivers.
Output transformers have very low leakage inductance in order to maintain high-frequency bandwidth with capacitive loads. For low insertion loss, they use relatively few turns of large wire to decrease winding resistances. They use fewer turns and operate at relatively high signal levels.
On the other hand, input transformers directly drive the usually high-resistance, low capacitance input of amplifier circuitry. Many input transformers operate at relatively low signal levels, often have a Faraday shield, and are enclosed in magnetic shields.
 
1.      Microphone Input

A microphone input transformer is driven by the nominal 150 W source impedance of professional microphones. One of its most important functions is to transform this impedance to a generally higher one more suited to optimum noise performance. The optimum impedance may range from 500 W to over 15 kW, depending on the amplifier. For this reason, microphone input transformers are made withturns ratios ranging from 1:2 to 1:10 or higher.

 

High CMRR is desirable form an input transformer. To achieve this, they must have two attributes. First, the capacitances of its two inputs (to ground) must be very well matched and as low as possible. Second, it must have minimal capacitance between its primary and secondary windings.

 
2.      Line Input

Line input transformers are generally driven by balanced line and drives an unbalanced line. They also transform the impedance as microphone input transformers and have high CMRRs.

 

3.      Moving Coil Phono Input

Moving coil phonographs are very low impedance devices (~3W). Due to this, it is very difficult to achieve good noise performance in an amplifier. Hence, the transformer is used in step-up configuration so that amplifier sees an impedance of 600 W in order to achieve good noise performance.

 

4.      Line Output

Typically, audio transformer is driven by an amplifier and loaded by several thousand pF cable capacitance and high input impedance of line receiver.Therefore, a line output transformer should have low output impedance which remains low at high frequencies. This requires both low resistance windings and very low leakage inductance.

 

5.      Interstage and Power Output

Interstage coupling transformers used to be popular in vacuum tube based designs. They used 1:1 to 1:3 turns ratio and classical push pull power amplifier in the output stage.

 
6.      Microphone output

Ribbon microphones& Dynamic microphones use step up transformers whereas condenser microphones use step-down transformers. Ribbon elements have impedance of the order of 1 W and hence need a step up transformer with turns ratio of 1:12 or more to transform its impedance to somewhere around 150 W. Similarly, step-up is required for dynamic microphones having impedances of the order of 10-30W. On the other hand, condenser microphones have high impedance and use step down transformer so that amplifier sees smaller input impedances.

 
7.      Speaker Distribution

When number of low impedance speakers are located at a distance from the  power amplifiers, the speakers are connected with the help of technique used in grid electricity supply, i.e., use of step-up transformers at transmission end and step-down transformer at receiving end.

 
8.      Telephone directional coupling or “Hybrid”

Telephone “hybrid” circuits use bridge nulling principles to separate signals which may be transmitted and received simultaneously on a 2-wire line. This nulling depends critically on well-controlled impedances in all branches of the circuits. This nulling is what suppresses the transmit signal (your ownvoice) in the receiver of your phone while allowing you to hear the receivesignal (the other party).

 

Telephone Directional Coupling in an Audio Transformer

Fig. 10: Telephone Directional Coupling in an Audio Transformer

Comments

"In a similar fashion (as explained above), audio transformers are used to change the impedance between the microphones and input amplifiers so that signal power is not reduced."

What RUBBISH   !!!!!

 

"Thus, if output amplifier stage has an input impedance of 1800 W"

What RUBBISH  !!!!