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Page 1

Audio Transformers
by

Bill Whitlock

Jensen Transformers, Inc.

9304 Deering Avenue

Chatsworth, CA 91311

This work first published by Focal Press in 2001 as

Chapter 11

Handbook for Sound Engineers, Third Edition

Glen Ballou, Editor

Copyright © 2001, 2006 Bill Whitlock
All rights reserved

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Bill Whitlock Audio Transformers Page 1
Handbook for Sound Engineers, 3 Editionrd



1 Audio Transformer Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1 Basic Principles and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.1 Magnetic Fields and Induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.2 Windings and Turns Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.3 Excitation Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Realities of Practical Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.1 Core Materials and Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.2 Winding Resistances and Auto-Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.3 Leakage Inductance and Winding Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.4 Winding Capacitances and Faraday Shields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2.5 Magnetic Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 General Application Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.3.1 Maximum Signal Level, Distortion, and Source Impedance . . . . . . . . . . . . . . . . . . . . . . 9

1.3.2 Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.3 Insertion Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.3.4 Sources with Zero Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3.5 Bi-Directional Reflection of Impedances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3.6 Transformer Noise Figure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.3.7 Basic Classification by Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2 Audio Transformers for Specific Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1 Equipment-Level Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.1 Microphone Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.2 Line Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.3 Moving Coil Phono Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1.4 Line Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1.5 Inter-Stage and Power Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.6 Microphone Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2 System-Level Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.1 Microphone Isolation or “Splitter” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.2 Microphone Impedance Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.3 Line to Microphone Input or “Direct Box” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.4 Line Isolation or “Hum Eliminators” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2.5 Speaker Distribution or “Constant Voltage” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.6 Telephone Isolation or “Repeat Coil” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2.7 Telephone Directional Coupling or “Hybrid” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.8 Moving Coil Phono Step-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3 Measurements and Data Sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1 Testing and Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1.1 Transmission Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1.2 Balance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1.3 Resistances, Capacitances, and Other Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2 Data Sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2.1 Data to Impress or to Inform? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2.2 Comprehensive Data Sheet Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4 Installation and M aintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.1 A Few Installation Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2 De-Magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

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Bill Whitlock Audio Transformers Page 14
Handbook for Sound Engineers, 3 Editionrd

1.3.7 Basic Classification by Application

Many aspects of transformer performance, such as level-handling, distortion, and bandwidth, depend critically on the impedance

of the driving source and, in some cases, the resistance and capacitance of the load. These impedances play such an important role

that they essentially classify audio transformers into two basic types. Most simply stated, output transformers are used when load

impedances are low, as in line drivers, while input transformers are used when load impedances are high, as in line receivers. The

conflicting technical requirements for output and input types make their design and physical construction very different. Of course,

some audio transformer applications need features of both input and output transformers and are not so easily classified.

Output transformers must have very low leakage inductance in order to maintain high-frequency bandwidth with capacitive loads.

Because of this, they rarely use Faraday shields and are often multi-filar wound. For low insertion loss, they use relatively few

turns of large wire to decrease winding resistances. Since they use fewer turns and operate at relatively high signal levels, output

transformers seldom use magnetic shielding. 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, frequently have a Faraday

shield, and are usually enclosed in at least one magnetic shield.

2 Audio Transformers for Specific Applications

Broadly speaking, audio transformers are used because they have two very useful properties. First, they can benefit circuit

performance by transforming circuit impedances, to optimize amplifier noise performance for example. Second, because there is

no direct electrical connection between its primary and secondary windings, a transformer provides electrical or galvanic isolation

between two circuits. As discussed in Chapter 37, isolation in signal circuits is a powerful technique to prevent or cure noise

problems caused by normal ground voltage differences in audio systems. To be truly useful, a transformer should take full

advantage of one or both of these properties but not compromise audio performance in terms of bandwidth, distortion, or noise.

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Bill Whitlock Audio Transformers Page 15
Handbook for Sound Engineers, 3 Editionrd

Figure 29 - Low-Noise Unity-Gain Balanced Line Input Stage

Figure 28 - Microphone Pre-Amplifier with 40 dB Overall Gain

2.1 Equipment-Level Applications

2.1.1 Microphone Input

A microphone input transformer is driven by the

nominal 150 Ù (or 200 Ù in Europe) 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. As discussed in chapter

21, this optimum impedance may range from 500 Ù

to over 15 kÙ , depending on the amplifier. For this

reason, microphone input transformers are made with

turns ratios ranging from 1:2 to 1:10 or higher. The

circuit of Figure 28 uses a 1:5 turns ratio transformer, making the microphone appear as a 3.7 kÙ driving source to the IC

amplifier, which optimizes its noise. The input impedance of the transformer is about 1.5 kÙ . It is important that this impedance

remain reasonably flat with frequency to avoid altering microphone response by loading it excessively at frequency extremes.

In all balanced signal connections, common-mode noise can exist due to ground voltage differences or magnetic or electrostatic

fields acting on the inter-connecting cable. It is called common-mode noise because it appears equally on the two signal lines, at

least in theory. Perhaps the most important function of a balanced input is to reject (not respond to) this common-mode noise. A

figure comparing the ratio of its differential or normal signal response to its common-mode response is called common-mode

rejection ratio or CMRR. An input transformer must have two attributes to achieve high CMRR. 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. This is usually accomplished by precision winding of the primary to evenly distribute

capacitances and the incorporation of a Faraday shield between primary and secondary. Because the common-mode input

impedances of a transformer consist only of capacitances of about 50 pF, transformer CMRR is maintained in real-world systems

where the source impedances of devices driving the balanced line and the capacitances of the cable itself are not matched with

great precision [3].

Because tolerable common-mode voltage is limited only by winding insulation, transformers are well suited for phantom power

applications. The standard arrangement using precision resistors is shown in Figure 28. Resistors of lesser precision may degrade

CMRR. Feeding phantom power through a center tap on the primary requires that both the number of turns and the dc resistance

on either side of the tap be precisely matched to avoid small dc offset voltages across the primary. Normal tolerances on winding

radius and wire resistance make this a less precise method than the resistor pair in most practical transformer designs. Virtually all

microphone input transformers will require loading on the secondary to control high-frequency response. For the circuit in the

figure, network R1, R2, and C1 shape the high-frequency response to a Bessel roll-off curve. Because they operate at very low

signal levels, most microphone input transformers also have magnetic shielding.

2.1.2 Line Input

A line input transformer is driven by a balanced line and, most

often, drives a ground-referenced (unbalanced) amplifier

stage. As discussed in Chapter 37, modern voltage-matched

interconnections require that line inputs have impedances of

10 kÙ or more, traditionally called “bridging.” In the circuit of

Figure 29, a 4:1 step-down transformer is used which has an

input impedance of about 40 kÙ .

High common-mode noise rejection or CMRR is achieved in

line input transformers using the same techniques as those for

microphones. Again, because its common-mode input

impedances consist of small capacitances, a good input

transformer will exhibit high CMRR even when signal sources

are real-world equipment. Electronically-balanced stages, especially simple differential amplifiers, are very susceptible to tiny

impedance imbalances in driving sources. However, they usually have impressive CMRR figures when the signal source is a

laboratory generator. The pitfalls of measurement techniques will be discussed in section 3.1.

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Bill Whitlock Audio Transformers Page 28
Handbook for Sound Engineers, 3 Editionrd

4 Installation and Maintenance

4.1 A Few Installation Tips

! Remember that there are very tiny wires inside an audio transformer. Its wire leads should never be used like a handle to pick

it up. The internal bonds are strong, but one strong tug might result in an open winding.

! Be careful with sharp tools. A gouge through the outer wrapper of an output transformer can nick or cut an internal winding.

! Use either the supplied screws or ones no longer than recommended to mount transformers in shield cans. If the screws are

too long, they’ll bore right into the windings — big problem!

! Be careful about using magnetized tools. If a screwdriver will pick up a paper clip, it shouldn’t be used to install an audio

transformer.

! Don’t drop a transformer. It can distort the fit of the laminations in output transformers and affect their low-frequency

response. Mechanical stress (as in denting) of the magnetic shield can of an input transformer will reduce its effectiveness as a

shield. For the same reason, don’t over-tighten the clamp on transformers mounted with them.

! Twisting helps avoid hum pickup from ambient ac magnetic fields. This is especially true for mic level lines in splitters, for

example. Separately twist the leads from each winding — twisting the leads from all windings together can reduce noise

rejection or CMRR.

4.2 De-Magnetization

Some subtle problems are created when transformer cores and/or their shield cans become magnetized. Generally, cores become

magnetized by having dc flow in a winding, even for a fraction of a second. It can leave the core weakly magnetized. Steel cores,

because of their wider hysteresis loops, are generally the most prone to such magnetization. The only way to know if the core has

some permanent magnetization is to perform distortion measurements. A transformer with an un-magnetized core will exhibit

nearly pure third harmonic distortion, with virtually no even order harmonic distortion while magnetized ones will show

significant even order distortion, possibly with 2nd harmonic even exceeding 3rd. A test signal at a level about 30 or 40 dB below

rated maximum operating level at 20 or 30 Hz is typically the most revealing because it maximizes the contribution of hysteresis

distortion.

Microphone input transformers used with phantom power are exposed to this possibility whenever a microphone is connected or

disconnected from a powered input. However, distortion tests before and after exposure to the worst-case 7 mA current pulses

have shown that the effects are indeed subtle. Third harmonic distortion, which normally dominates transformer distortions, is

unaffected. Second harmonic, which normally is near the measurement threshold, is typically increased by about 20 dB but is still

some 15 dB lower than the third harmonic. Is it audible? Some say yes. But even this distortion disappears into the noise floor

above a few hundred Hz. In any case, it can be prevented by connecting and disconnecting microphones only when phantom

power is off. However, such magnetized transformers can be de-magnetized.

Demagnetizing of low level transformers can generally be done with any audio generator having a continuously variable output It

may take a booster of some sort to get enough level for output transformers (be sure there’s no dc offset at its output!). The idea is

to drive the transformer deeply into saturation (5% THD or more) and slowly bring the level down to zero. Saturation will, of

course, be easiest at a very low frequency. How much level it takes will depend on the transformer. If you’re lucky, the level

required may not be hazardous to the surrounding electronics and the de-magnetizing can be accomplished without disconnecting

the transformer. Start with the generator set to 20 Hz and its minimum output level, connect it to the transformer, then slowly (over

a period of a few seconds) increase the level into saturation — maintain it for a few seconds — then slowly turn it back down to

minimum. For the vast majority of transformers, this process will leave them in a demagnetized state.

Shield cans are usually magnetized by having a brief encounter with a strongly magnetized tool. Sometimes, transformers are

unknowingly mounted on a magnetized chassis. When the shield can of an input transformer becomes magnetized, the result is

microphonic behavior of the transformer. Even though quality input transformers are "potted" with a semi-rigid epoxy compound

to prevent breakage of very fine wires, tiny movements between core and can activate what is essentially a variable reluctance

microphone. In this case, a good strong tape head de-magnetizer can be used to de-magnetize the can. At the end of the production

line, most transformers are routinely demagnetized with a very strong de-magnetizer just prior to shipment. Although I haven't

tried it, I would expect that something like a degausser for 2" video tape (remember that!) would also de-magnetize even a large

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Bill Whitlock Audio Transformers Page 29
Handbook for Sound Engineers, 3 Editionrd

steel-core output transformer.

References

[1] Magnetic Shield Corporation, Frequently Asked Questions, www.magnetic-shield.com/faq.html.

[2] Sowter, G.A.V., Soft Magnetic Materials for Audio Transformers: History, Production, and Applications, Journal of the

Audio Engineering Society, October 1987, www.sowter.co.uk/pdf/GAVS.pdf.

[3] Whitlock, Bill, Balanced Lines in Audio: Fact, Fiction, and Transformers, Journal of the Audio Engineering Society, June

1995, pp 454-464.

[4] Smith, F. Langford, Radiotron Designer’s Handbook, Wireless Press, Sydney, 4 Edition, 1953, p 208.th

[5] Woolf, Lawrence, RMS Watt, or Not?, Electronics World, December 1998, pp 1043-1045.

[6] Smith, F. Langford, op. cit., p 227.

Co-Netic® is a registered trademark of Magnetic Shield Corp.

HyMu® is a registered trademark of Carpenter Technology Corp.

Mumetal® is a registered trademark of Telcon Metals, Ltd.

Permalloy® is a registered trademark of B & D Industrial & Mining Services, Inc.

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