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Dec 28 2017

Audio Transformers Classification and Types

Warm hints: The word in this article is about 4600 and reading time is about 28 minutes.


Audio transformer, which also called a low frequency transformer, is a transformer that works in the audio range. The working frequency range is generally from 10 to 20000Hz while the impedance is often used to transform the voltage or to change the load. Audio transformer is a component in the radio, radio, television, automatic control as a voltage amplifier, power output and other components of the circuit.The frequency response of the audio transformer is uniform in the working frequency band, and the core of the audio transformer is superimposed by high magnetic material. The primary and secondary windings are tightly coupled, so that the flux across the original winding almost all has phase chain with the secondary winding, and the coupling coefficient is close to 1. This article shows what is audio transformers,how to make a audio transformer,its types and classifications in a great detail.


Article Core

Audio Transformers Classification and Types


Working frequency range


English name

Audio Transformer




A transformer working in the audio range






I.Audio Transformers Introduction

Audio Transformer Impedance Ratio

II.Audio Transformer Classification

V.Audio Transformer Example No1


VI.Types of Audio Transformers

IV.Impedance Matching




I.Audio Transformers Introduction

The lowest frequency of the passband is determined by the inductance of the original winding, and the highest frequency is determined by the leakage inductance of the transformer. To ensure that the transformer has sufficient passband, the original winding inductance is large, and the leakage inductance is small.The hysteresis loss of the core and the saturation of the magnetic circuit will cause the signal distortion. Proper load distribution and increased load current can reduce the influence of hysteresis loss. Increasing the core section and leaving the air gap can make the magnetic circuit not saturated, which can reduce the distortion of the signal. What is it?

audio transformer

Well,the audio transformer, which also called a low frequency transformer, is a transformer that works in the audio range. The working frequency range is generally from 10 to 20000Hz while the impedance is often used to transform the voltage or to change the load. Audio transformer is a component in the radio, radio, television, automatic control as a voltage amplifier, power output and other components of the circuit.The frequency response of the audio transformer is uniform in the working frequency band, and the core of the audio transformer is superimposed by high magnetic material. The primary and secondary windings are tightly coupled, so that the flux across the original winding almost all has phase chain with the secondary winding, and the coupling coefficient is close to 1.

II.Audio Transformer Classification

The audio transformer can be divided into 3 types according to its position in the electronic circuit.

  1. The output transformer :the output transformer between the output circuit and the load.

  2. The input transformer: connected between the signal source and the input of the amplifier

  3. Interstage transformer:connected between the upper level output circuit and the next input circuit.


In use, we should pay attention to the matching of the front and back impedance to avoid the signal distortion caused by the reflection. In order to maximize the load of the load, the impedance of the load must be consistent with the impedance required by the power amplifier stage through the impedance of the output transformer, neither too large nor too small. When the audio signal is amplified by an audio transformer, the voltage can be larger, but the power is reduced because of the loss of the transformer. In this way, it can not drive large loads. Therefore, the audio transformer can not replace the electronic audio signal amplifier.





To wound a better performance of the audio transformer must try to reduce the leakage inductance of the transformer and the number of turns of the primary coil takes bigger, resulting in low frequency characteristic better, while also reducing the capacitance between the lines and enhance the high frequency, but the number of coil winding and leakage inductance and capacitance between lines is three a unified contradiction, the more the number of ring leakage is bigger distributed capacitance is larger, so the audio transformer winding in the choice of materials is very important especially in the core, we should try to use high silicon steel sheet magnetic flux density larger to core shell type structure is adopted in the structure, purpose in several times under the Limited (to reduce stray) on as far as possible to increase the inductance and reduce leakage. In the low end, because the current flowing in the coil inductance is less, more easy to make core saturation caused by low frequency characteristics, in order to avoid the core magnetic saturation phenomenon, in the two iron and air gap spacers, of course this is to increase the leakage inductance of the cost.

the winding about audio transformer

In a word, it is necessary to make a reasonable choice of the selection of the core, the adjustment of the air gap and the number of the design circle. I think we can only rely on experience. At the end of said winding coil structure that, because the level is used after the tube to tube push-pull circuit, in order to prevent the two pipe load imbalance caused by iron core DC magnetization, the load on the pipe winding inductance not only to be consistent, and the DC resistance should be consistent, in addition to the distributed capacitance of the line between the less around the law, adopt hierarchical method around the edges, as shown in figure B-2 is the profile of the audio output transformer winding. The winding structure can make the total reactance of the upper and lower output tubes keep the same. From the point of view of reducing the distribution capacitance between lines, the more the layer is divided, the smaller the better, so that the frequency response characteristics of the output signal can be improved.

IV.Impedance Matching

One of the main applications for audio frequency transformers is in impedance matching. Audio transformers are ideal for balancing amplifiers and loads together that have different input/output impedances in order to achieve maximum power transfer. For example, a typical loudspeaker impedance ranges from 4 to 16 ohms whereas the impedance of a transistor amplifiers output stage can be several hundred ohms. A classic example of this is the LT700 Audio Transformer which can be used in the output stage of an amplifier to drive a loudspeaker.

We know that for a transformer, the ratio between the number of coil turns on the primary winding (NP) to the number of coil turns on the secondary winding (NS) is called the “turns ratio”. Since the same amount of voltage is induced within each single coil turn of both windings, the primary to secondary voltage ratio (VP/VS) will therefore be the same value as the turns ratio.

Impedance matching audio transformers always give their impedance ratio value from one winding to another by the square of the their turns ratio. That is, their impedance ratio is equal to its turns ratio squared and also its primary to secondary voltage ratio squared as shown.

Let’s see the video about how audio transformers work for impedance matching:


Audio Transformer Impedance Ratio

Where ZP is the primary winding impedance, ZS is the secondary winding impedance, (NP/NS) is the transformers turns ratio, and (VP/VS) is the transformers voltage ratio.

So for instance, an impedance matching audio transformer that has a turns ratio (or voltage ratio) of say 2:1, will have an impedance ratio of 4:1.

Audio Transformer Impedance Ratio

V.Audio Transformer Example No1

An audio transformer with a turns ratio of 15:1 is to be used to match the output of a power amplifier to a loudspeaker. If the output impedance of the amplifier is 120Ω’s, calculate the nominal impedance of the loudspeaker required for maximum power transfer.

audio transformer impedance ratio

Then the power amplifier can efficiently drive an 8-ohm speaker.

How important is the impact of audio transformers on sound quality?

In many of the audio equipment, audio transformer is a very important factor, for the voltage and current plays an important role in the regulation, once these factors change, will affect the sound quality etc. series, so the audio transformer for sound quality will have a great impact.

The quality of audio transformer directly affects the stability of the equipment. The unstable audio transformer in the work will have a great impact on the sound quality of the device, of which the noise and electromagnetic interference are the main way of expression.

VI.Types of Audio Transformers

Generally,audio transformers are divided into three types: isolation transformers,input transformers and output transformers.

  • A. Isolation transformers

In the electronic tube audio amplifier circuit, sometimes due to the special requirements of amplifier circuit and the need to the front stage and the circuit level off, so that no voltage current before and after the class between the direct connection, but the need for audio signals and can not be interrupted, then the audio isolation transformer can play its due role (of course a good quality of the capacitor can also play the same role).

The work of the isolation transformer is accomplished by electromagnetic and magnetoelectric conversion. It has two windings and a pair of iron cores.

isolation transformer

The electric signal of the front - stage amplifier of the electron tube acts on the iron core by the primary coil of the isolating transformer. With the difference between the frequency and amplitude of the pre - stage audio signal, the magnetic field that changes in the core will be produced in the iron core due to the effect of the primary coil. The secondary coil picked up the corresponding electrical signal through the change of the magnetic field in the same core. Its initial voltage is 1:1. In other words,a qualified isolating transformer can also detect 1V’s undistorted audio and electrical signals if its primary audio signal is added to 1V and its load is appropriate on its secondary.

Well,there are also some isolated transformers with a certain voltage and impedance transformation.Their ratio of pressure to pressure is not equal to 1. But the isolated transformer is not an isolated transformer in the pure sense, but has the nature of some input and output transformers.

  • B.Input transformers


input transformer

In the electronic tube amplifying circuit, the input transformer usually has three functions:

  • a.Input coupling and function.

It can be made from the microphone cartridge, the former circuit etc., weak signals, by connecting the input transformer, the electromagnetic, isolation, magnetoelectric conversion, the electrical signal transmission to the next level of electron tube amplifier.

  • b.Voltage amplification.

Because the input transformer is made up of two independent coils of primary and secondary parts, the voltage of the output terminal can be adjusted according to the requirement as long as the ratio of the primary coil to the secondary coil is changed. In normal circumstances, the change of this voltage is proportional to the change in the coil number. For example, the primary winding of the input transformer is 500 circles, and the secondary winding is 1000 circles. At this time, the audio voltage of the primary transformer plus Liv for the input transformer can detect the 2V audio voltage at the secondary level. In an old-fashioned phonograph cartridge using a moving coil in the transformer is generally used as a 1:10 channel, so as to enhance the audio voltage of about 0 ImV to the standard ImV, the input to the equalizer circuit and amplifying balanced cartridge.

  • c. Change the way of connection

The another function of input transformers is to change the way of connection. Take an example, in the input circuit of microphone amplifier or mixer, the output of the microphone is balanced (in order to improve the anti-interference capability in the transmission process), while the actual amplifying circuit is unbalanced. This change can be done easily as long as the initial winding changes of the transmission transformer are changed. 

Another example of an electronic tube push-pull output amplifier, amplifier circuit level is single ended, and to the last stage to make the electrical signal and the One divides into two., half signal phase (flip 180 degrees), as long as the secondary set input transformer stage two identical coils, and the the two coils are connected into a road for positive output (coil output, tail head ground), another way for the negative output (output head and tail coil grounding).

  • C.output transformers

output transformer

In the tube audio transformer,there are two functions about output transformers: 

One is to complete the matching of output impedance; the other is to complete the conversion of voltage to current, so that the higher voltage signal of the last stage of electron tube can be converted to the electrical signal with lower output current and larger current.





Transformers are interesting, despite their apparent simplicity. For anyone who hasn't already done so, I recommend that you read the Beginners' Guide to Transformers. There are three sections, mainly dealing with power transformers but also covering general principles. Most people don't really think deeply about transformers in any of their applications, but they are by far the most fascinating of all the passive components.Using resonance to obtain a bit of low frequency boost is not something I've seen discussed, and this is a technique that can be used if necessary. You also won't find much info about the interaction of capacitors and transformers (unwanted resonance), and the required capacitance and optional series resistance can only be determined after careful measurement and some experimentation.


The circuit you use to drive the transformer must be capable of supplying enough current to feed the load and the transformer's magnetising current. This becomes more critical at low frequencies. If you need an output level of (say) +24dBV (just under 16V RMS), you will need either a small power amp IC or a step-up transformer because most opamps can't be operated at ±25V or more. Don't expect to get more than 10mA peak from most opamps (although some can provide up to 25mA peaks), and be prepared to engineer the drive circuit carefully or you won't get the performance you expect.Using a negative impedance driver can improve performance dramatically, but it does come with serious caveats. It will be necessary to test the complete system very carefully to ensure that it can never become unstable with any frequency, amplitude or load. It's worthwhile doing a Web search on the topic if you are interested, as there is some information available. However, nothing I have seen mentions the unstable regions or gives any warning at all that bad things can happen once the drive circuit cannot handle the combination of the load and the transformer's saturation current. Most articles seem to assume that the negative impedance circuit will be directly connected to the transformer (no series capacitor), but don't offer any info on how to remove DC created by the NIC itself, nor do they warn you that a NIC can have very high gain at DC.


In general, I recommend that you select a transformer that is designed for your application, and use a low impedance (not negative impedance) source. While the use of a series capacitor is usually a good idea to prevent any DC in the windings, make sure that you test it thoroughly to ensure that resonance is well below the lowest frequency of interest. Ideally, the drive circuit will include a high pass filter to prevent any subsonic frequencies from reaching the transformer.


As a final check, I did a listening test with transformer #1 and the Figure 7 negative impedance driver. The average voltage was around 1V RMS from an FM tuner, and the error signal (across R4) showed a surprising amount of activity with most of the music that was playing at the time. While I thought I could hear a difference between 'traditional' voltage drive and the NIC, I couldn't be certain. If there was a difference it was rather subtle, but the sound did seem a little cleaner with the NIC, especially with bass-heavy material. However, it's far easier to reduce the level a little to reduce distortion to be within acceptable limits. That is a simpler method, and doesn't require messing around with negative impedance and the subtle problems it can create. Remember that it's always a good idea to include a capacitor in series with the primary, but make sure it's large enough so it doesn't create a series resonant circuit at any frequency above ~10Hz or so, and that there is no bass boost as a result. This needs to be measured so you can be sure.


Book Recommendation

  • Handbook of Transformer Design and Applications


This second edition updates what has become a standard reference on transformer design and applications, and now includes a selection of computer solutions to many transformer circuit problems. Every chapter reflects the latest technology advances - and the section on inverter transformers has been expanded to cover more fully the increasingly important subject of power supplies.

--William M. Flanagan  (Author)

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    • http://www.kaotonglu.com/comment/html/?832.html on 2018/9/12 6:15:11

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