Audio Transformers

37. Types of Audio Transformers

The design of audio transformers is different in many respects from the design of power transformers. The principal reason for this difference is that audio transformers must maintain a constant ratio of input to output voltage over a band of frequencies, rather than a single frequency. The problems inherent in this requirement become more severe as the width of the passband increases.

(The passband is the band over which the input to output voltage remains within prescribed limits of deviation from some arbitrary reference frequency. The reference frequency will be discussed later.) Broadband design becomes more complex with the growth of the impedance levels of the transformer windings, expanded power requirements, and with the increase of d-c currents flowing in the windings. For any given type of audio transformer, the comparative design difficulty depends upon the number of octaves to be covered in operation.

INPUT TRANSFORMERS. An input transformer couples low-level input signals to the grid of the first amplifier tube or, in a multi stage system, to the input element of the first transistor amplifier.

Because the power level is low, to avoid induction of stray voltages in its windings, an input transformer is generally shielded. Spurious voltages would, of course, be heavily amplified under these conditions and would almost certainly appear as a distortion in the output. Generally, the principal function of an input transformer is to provide the maximum possible voltage gain while still remaining within the limits set by the system's bandwidth requirements.

INTERSTAGE TRANSFORMERS. An interstage transformer also is not required to deliver power. Its primary function is to couple the plate of one voltage amplifier to the grid of another voltage amplifier operated in class A. Since grid current never flows under class-A conditions, the transformer merely serves as a voltage step-up device. Interstage transformers are not impedance-matching devices.

They must supply the maximum voltage gain possible, without reducing the frequency response of the system below the value specified by the designer.

DRIVER TRANSFORMERS. The adjective driver is applied only to those transformers that couple the plate (or plates) of an amplifier stage to the grids of a following class-AB2 or class-B stage. Since the grids of such output amplifier stages go positive during a portion of the cycle of the input signal, the transformer must be capable of supplying the necessary power. When the grids are in the negative region, there is no grid current; hence, the load resistance on the transformer secondary is quite high. As grid current begins to flow, the load resistance drops to a value determined by the magnitude of the current. This changing load reflects back to the driver tube and tends to produce serious distortion. Although this effect may be diminished by making the transformer a step down type, when this is done, a significant secondary effect appears.

A large step-down ratio places a limitation on the power that can be delivered to the driven grids. Thus, in all driver transformers, a compromise must be made between permissible distortion and required power.

OUTPUT TRANSFORMERS. Output transformers are generally employed as impedance changers. They change the impedance level of the output signal to that of the load. In addition, the transformer must provide d-c isolation from the load. Because output transformers always supply power, they suffer the same design problems as interstage types (often to a greater degree). Further more, invariably the primary winding of an output transformer carries the d-c component of the plate current of a power amplifier tube. This further complicates the design problem. We are not primarily concerned with amplifier circuits; but, to observe the differences between the various transformer types, it is helpful to study the typical circuit diagrams given in Fig. 23.

Fig. 23. A. Audio input transformer coupling a microphone to a voltage amplifier grid. B. Interstage transformer between the plate of one voltage amplifier and the grid of the next. C. Driver transformer coupling a single power amplifier to clan AB2 or 8 grids. D. Output transformer between single-ended plate and low-impedance loudspeaker.

38. Equivalent Circuits

The role played by the audio transformer in an amplifier circuit may be represented in the form of equivalent circuits for low, midband, and high frequencies. (See Fig. 24.) The limits of these frequency ranges are somewhat fluid. But it is generally accepted that the low audio frequencies range from 0 to about 400 Hz and the midband frequencies range from 400 to 4000 Hz. The high frequency end stretches from 4000 Hz to inaudible sound.

At low frequencies, the inductive reactance of the primary for any given transformer is relatively small, when compared to its reactance at the higher frequencies. This reactance behaves as though it were a shunt across the load and, effectively, reduces the voltage available for the load. The magnitude of this effect depends upon the transformer design. To prevent poor low-frequency response, the transformer primary must have a sufficiently large number of turns to maintain a comparatively high reactance, even when the frequency is low. This, in turn, demands that, to prevent saturation, there be a high "iron" content in the core design.

Fig. 24. A. Simplified equivalent circuit of transformer-coupled amplifier at low audio frequencies. B. Same for mid-band frequencies. C. Same for high frequencies.

Furthermore, the size wire used in winding the transformer must be large enough to keep RP small; otherwise, the voltage drop across the resistive component of the transformer primary impedance may become large. A winding's ohmic resistance losses result in loss of signal transfer to the secondary coil. Thus, in order to meet these requirements, a high-grade transformer must be large. (Large is used in the comparative sense.) In the middle frequency range, the primary reactance is high.

Hence, it does not appear as a shunting element in the equivalent circuit. Therefore, transformer response here is usually better than at the low-frequency end.

Fig. 25. Approximate response curve of medium-grade audio transformer showing drop-off at high and low frequencies, and the effect of the series-resonant circuit formed by L8 and C8 •

Two new factors appear at high frequencies that influence frequency response. The leakage inductance becomes significant, be· haves as a series Losser element, and reduces signal amplitude to the output device. The effective distributed capacitance appears in shunt with the output since, at these frequencies, the reactance of the primary and secondary distributed capacitances becomes quite small-small enough to enable the capacitance to bypass some of the signal current away from the output device. At some point in the high-frequency range in most transformers, L. and c. reach series resonance. The result of the resonant action may, in some cases, cause a sudden rise of amplification at the resonant frequency; but, in a well-designed transformer, the magnitude of the pip just counteracts the natural drop-off that tends to occur due to C,. In any case, high-frequency response falls off rapidly at frequencies that are substantially higher than the resonant frequency of L, and C,. Figure 25 illustrates an approximate curve, showing the frequency response over the entire audio spectrum of a typical medium-grade transformer. Note especially the low- and high frequency drop-off, and the pip due to resonance.

39. Impedance Ratio of Audio Transformers

When power is transferred from one amplifier stage to another via an audio transformer, the latter becomes an impedance-matching device, as well as a coupling device. Perhaps the most familiar example of this dual function is that of a loudspeaker with a voice-coil impedance of 4 ohms, coupled to the plate circuit of an audio power amplifier through an output transformer. When manufacturers give audio ratings on power tubes, they specify the plate load impedance (or plate-to-plate load impedance for push pull amplifiers) into which the tubes must operate to deliver the rated audio power output with the rated distortion. For example, the rated load impedance for a 6V6GT output tube with 250 volts on its plate is 5000 ohms. For this impedance, the power output is 4.5 watts and the total harmonic distortion is 8%. For this application, a transformer having a primary impedance of 5000 ohms and a secondary impedance of 4 ohms (to match the voice coil) is required. The relationship between primary and secondary impedance, and the turns ratio of the transformer is:

N = __/Zp/Zs (38)

…where N is the primary to secondary turns ratio, Zp is the impedance of the primary, and Z, is the impedance of the secondary winding. For the output transformer used as an example, the turns ration would then be:

N = 25.2: 1

To construct the transformer, the primary is wound to give it an impedance of 5000 ohms at 1000 Hz and the secondary turns then adjusted to provide the ratio given in the example.

Let us further consider the problem that arises when a class-C r-f amplifier in a transmitter is to be plate modulated by a pair of audio power tubes in push-pull.

Example 6. The modulated r-f amplifier operates with 1250 volts on its plate.

When loaded by the antenna, the plate current is 250 ma. Stating the turns ratio required of the transformer, select the tubes and the modulation transformer so that 100% modulation can be obtained.

Solution. First determine the power input to the r-f amplifier.

P = E I = 1250 X 0.25 =312 watts

For 100% modulation, the audio power must be 50% of the r-f power input.

Thus, the audio power required is: Pa= 312/2 = 156 watts

Generally, the power capabilities of the tubes selected for such a job are made approximately 25% greater than the minimum requirements, to provide a suitable margin for possible losses.

Increased by 25%, 156 watts become 195 watts. Therefore, we would select a pair of tubes each of which provide 100 watts of audio in the push-pull system.

Reference to the tube tables in any handbook discloses several types that can be used as modulators. Let us select a pair of 242C transmitting tubes. They are rated at 1250 volts plate and have a power output of 200 watts (for a pair in class B) , into a plate-to plate load impedance of 7600 ohms.

With the power output rating, properly chosen, the turns ratio of the modulation transformer must now be calculated. For the r-f transmitting tube, the impedance that must be matched is the effective impedance of the plate circuit in normal operation. This is known as the modulating impedance:

(39)

For the r-f tube of our example, the modulating impedance is:

Zin =

The transformer must match a primary input impedance of 7600 ohms to an output impedance of 5000 ohms. The turns ratio, from Equation (38), is:

N = 1.282:1

40. Parallel-feed Method of Coupling

Fig, 26. Parallel plate feed of transformer-coupled amplifiers. Power supply must be able to compensate for voltage drop in coupling resistor.

In certain applications-especially where the audio transformer is not designed to carry significant direct currents in its primary winding-it is possible to realize the advantages of transformer coupling and at the same time avoid the problems arising from d-c current saturation. As illustrated in Fig. 26, the plate current of the first amplifier in a two-stage cascade may be fed through a plate coupling resistor. A capacitor blocks the de from the primary winding. This arrangement is satisfactory only when there is a voltage reserve in the power supply large enough to compensate for the drop that occurs in the coupling resistor. In addition, the impedance of the primary winding must be higher than ordinary transformer coupling for the same tubes requires, since the coup ling resistor shunts the primary winding and reduces the effective plate circuit impedance.

41. High-Fidelity Transformers

Opinions differ on the range of uniform response that constitutes the characteristic of a high-fidelity input or output transformer. A consensus of transformer manufacturers specifies a range of 30 Hz to 15,000 Hz. One well-known manufacturer di vides the audio frequency spectrum into these categories.

Communications Range--200 Hz to 3500 Hz. These transformers are specifically designed for receiving and transmitting equipment, such as amateur, police, railroad, and aircraft types. The frequency response for input, output, driver., and modulation transformers is within + 1 db over the stated voice range. A typically priced 5-watt output transformer in this group, $4.00.

Public Address Range--50 Hz to 10,000 Hz. These transformers are designed for typical public address applications. The frequency response is within + 0.5 db over the entire range. A typically priced 5-watt unit in this group, $11.00.

Full-frequency Range--30 Hz to 15,000 Hz. Frequency response ± 1 db over the full range given. Cost for a typical 5-watt unit is $17.00.

With respect to the full-frequency range high-fidelity transformers, the manufacturer guarantees an exceptionally low percentage of distortion over the entire range both at low and high frequencies. Also included in all these units is a hum-bucking coil and core construction that provides maximum neutralization of stray magnetic fields.

Full-frequency transformers differ in design from less costly transformers in many respects. The quantity and quality of core material is generally superior, in that its hysteresis loop has a substantially smaller area. These cores do not saturate easily. The distributed capacitance of the windings is generally held to the lowest possible figure and the leakage inductance is minute.

The power output rating of an output transformer is dependent upon two factors. The maximum permissible current (which is determined by the temperature rise of the transformer windings) and the maximum permissible voltage (which is limited by the flux density in the core for the unsaturated condition). Since the magnetization curve of any core material is nonlinear, when the flux density is high, the inductance of the winding will vary. This variation occurs within each individual cycle and gives rise to distortion. Therefore, the maximum voltage that can be applied to the transformer is a function of the maximum permissible distortion. With low-grade output transformers, distortion appearing at frequencies well within the transformer's "flat" range is not an uncommon occurrence. Therefore, if distortion is to be minimized, the flux density in the core must be kept at a low level. Thus, high-fidelity components generally contain more iron than their less expensive counterparts. This is particularly true if good response is desired at very low audio frequencies.

Fig. 27. Response curve of a typical ultra-miniature en capsulated transistor transformer. Courtesy, Chicago Standard Transformer Corp., transformer type UME-15.

42. Transistor Transformers

In recent years, transistor transformers have become generally available. Usually, these units are extremely tiny, so that they may be used to advantage in miniaturized equipment such as pocket broadcast radios, portable test equipment, etc. Often, to make them impervious to moisture and changes in altitude, these transformers are encapsulated. As might be expected, the low-frequency response of such transformers is generally not as good as that found in high- quality, large transformers (see Fig. 27). But their high-frequency response is more than acceptable. The transformer whose response curve appears in Fig. 27 has a primary impedance of 1200 ohms, a secondary impedance of 3.2 ohms for a miniature loudspeaker, a primary current rating of 2 ma maximum, and an output level of 100 mw.

Fig. 28. Response curves of typical transistor transformers.

The response curves for the remainder of this line of transformers are shown in Fig. 28. The typical curves given in this figure are representative of transformers having the following important characteristics:

A. Output o-r driver transformer. Primary impedance is 10,000 ohms. Secondary impedance is 500 ohms. Power rating is 100 mw.

B. Single or push-pull output. Primary impedance is 300 ohms. Secondary impedance is 12 ohms. Power level is 500 mw.

D. Input. Primary impedance is 200,000 ohms. Secondary impedance is 1000 ohms. Power level is 25 mw.

E. Single or push-pull output. Primary impedance is 7500 ohms. Secondary impedance is 12 ohms. Power level is 500 mw.

From these few examples (this particular line carries 29 distinct types) , it is evident that an extremely wide range of primary and secondary impedances are available. Another complete line of transistor transformers contains 77 distinct types with impedance ranges as given below. The multitude of different types permits the selection of almost any combination of primary to secondary impedances for matching the many types of transistors in use. In the groups listed, the power levels range from 100 mw up to 350 mw.

A. Output. Primary impedance is 10,000 to 48 ohms. Secondary impedance is 500 to 3.2 ohms.

B. Driver. Primary impedance is 20,000 to 1500 ohms. Secondary impedance is 3000 to 200 ohms.

C. Input. Primary impedance is 500,000 to 3 ohms. Secondary impedance is 80,000 to 30 ohms.

Transistor transformers for power transistors may now be obtained for entertainment and experimental applications. They differ from the ones discussed above in that they have a much higher primary current rating and a higher power output rating.

For example, a unit rated at 6-watt power handling ability can carry 500 ma of unbalanced d-c primary current. Its primary impedance is 48 ohms and its secondary impedance is 8.2 ohms. The frequency response is ± 2 db over the range from 70 to 20,000 Hz.

43. QUIZ

1. How do audio transformers differ from power transformers in design and construction?

2. Detail the requirements for the design of input, interstage, driver, and output audio transformers.

3. Describe the behavior of audio transformers at low-, intermediate-, and high-audio frequencies, using the specific equivalent circuit in your explanation.

4. Find the primary-to-secondary turns ratio of a transformer that has a primary impedance of 7500 ohms, if it is to match a loudspeaker with a voice coil impedance of 16 ohms.

5. What is the most important advantage of parallel feed transformer coupling? What is one disadvantage?

6. Outline the frequency response requirements for a transformer designed for the communications range, the high fidelity range, and the public address range.

7. Explain why the flux density in a transformer must be kept low in order to avoid distortion.

8. Why is the frequency response in the low portion of the audio spectrum poor in transistor transformers?

9. Discuss the relative performances of the transformers whose response curves are given in Fig. 28.

10. Explain why a transformer having a hysteresis loop of small area can be expected to give better frequency response than one in which the loop has a large area.