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AUDIO OUT AUDIO OUT L R By Jake Rothman Transformers in audio – Part 1 although he only used it to demonstrate kicks on a compass. True transformer action based on mutual induction was not revealed until around 1836. I’m still amazed when I see a toroidal transformer in a modern Hi-Fi amplifier, as illustrated in Fig.2. It must be one of the oldest, and leastchanged electronic components still in production nearly two centuries after it was invented. Toroidal transformers are regarded as the best choice for audio power supplies by audiophiles because of their low emitted noise. Commercial switch-mode power supplies are (often) much cheaper and more compact, but they have short lives and add highfrequency noise on top of mains frequency noise. It’s important to note that when we say ‘audio transformer’ there are two basic kinds – ‘power transformers’, which are found in power supplies, and ‘signal transformers’ which range from tiny input transformers to large high-power amplifier output transformers. We will cover both types. Fig.1. A reconstruction of Faraday’s original transformer. Two coils on an iron ring. The cloth covered wire is from under the floorboards of my 1901-built Rectory. The clergy were at the forefront of technology back then. T he transformer has always had a vital role in audio electronics – right from the start of audio technology with Victorian telephone systems up to the latest studio digital-to-analogue converters. The transformer can trace its lineage directly from Faraday’s 1831 law of electromagnetic induction, which states that voltage is proportional to the rate of change of magnetic flux (V = dΦ/dt where Φ is the magnetic flux). Later, Faraday made the first original transformer with two coils of cotton-covered wire wound on an iron ring (similar to the one in Fig.1), There is no alternative Modern audio (low) frequency transformers are still made from copper, iron and nickel; and their cost generally follows commodity price ups and downs. They are also big and heavy. Accountants regard them as an anachronism to be eliminated, and favour silicon devices, which benefit from mass production and are much more resilient in terms of raw materials pricing. Audio engineers, however, appreciate the total electrical isolation transformers provide between one circuit and another. This eliminates the risk of interference, hum loops and domino destruction effects. I’ve been to many a design meeting arguing to include them in a product, often to be met with ‘can’t you do it with op amps, opto-isolators or simulate the transfer function in software?’ The answer is always, ‘no not really’. By transferring electrical energy and signals via a magnetic circuit, there is no direct wired connection, which is why Britain’s Health and Safety Directorate does not recognise a sliver of reverse-biased silicon as offering viable fail-safe isolation. Also, impedance matching – varying the relative output voltage and current of a circuit – is best accomplished with a transformer, thereby optimising power transfer and noise minimisation. Linear thinking Fig.2. Faraday’s transformer still lives on in modern amplifiers. Top-right, a 28-0-28V 100VA toroid in a Cyrus 2 amplifier made by Holden and Fisher. 60 Unlike opto-isolators, transformers are relatively linear. Their small inherent non-linearity generates the ‘transformer sound’, a distortion characteristic of magnetic systems beloved by producers of electronic and pop music. With the rise of digital recording, the distortion from magnetic tape is now no more, so today this is provided by transformers. Software plug-ins can simulate it to a degree, but these don’t seem to have caught on. Practical Electronics | July | 2022 n Long life Transformers are one of the longest-lived electronic components. After all, there is not much to go wrong apart from a wire breaking or shorting and this can usually be fixed with a bit of rewinding. There is one exception, and that is the transformer with an internal non-resettable thermal fuse embedded in the windings. These have a short fatigue life and cause more transformers to go to landfill than anything else. The good thing is that being made out of metal, transformers are truly recyclable. I’m always surprised how much cash I get from scrap merchants from the pile of failed devices I build up every few years. It is quite common to see old amplifiers where every part has been replaced – except for the transformers and the metalwork. I always reuse old audio transformers and toroids. Primary and secondary characteristics Unlike most semiconductors, the transformer is a simple device and its design and application follow simple repeatable, scalable maths. Like most Victorian technology, its physics is classical, rather than semiconductors’ quantum nature. It’s simply the electrical version of a gear box; voltage can be thought of as speed, and current as torque. When you change gear in a car, an optimum ‘impedance match’ between the engine speed and road speed is being achieved. Just as with gear ratios, transformers have voltage ratios, determined by the relative number of turns on the two windings. To put it simply, a transformer with twice as many turns on the output winding as the input winding will double the output voltage. This is called the ‘turns ratio’, a vital transformer parameter, and in this case, it would be expressed as 1:2. However, there is no such thing as free energy, so to preserve the energy balance between input and output, the output current is proportionally reduced. Overall, the energy transfer efficiency of well-made small power transformers is high, typically over 90%. (For very large grid transformers, operating with megawatts of power, the efficiency can approach 99.75%, making them just about the most efficient of all machines.) The input winding of a transformer is called the ‘primary’ and the output winding the ‘secondary’. These terms are merely designations, a transformer is inherently bidirectional and can be used both ways – but do watch out for voltage ratings. If it increases the voltage, it is called a ‘step-up’ transformer and for a decrease, ‘step down’. Practical Electronics | July | 2022 Calculating the turns ratio 2 V pk- pk (the ratio of primary turns (NP) 1 V pk- pk to secondary turns (NS)) is sim1 :2 ple. For example, a transformer S ignal with 1000 turns on the primary generator ( L ow- im pedanc e and 100 turns on the secondary output) has a 10 to 1 ratio, expressed as ‘10:1’. However, since it is just a T ransform er Osc illosc ope under test ratio, a transformer with 50 turns on the primary and five on the secondary is also a 10:1 device. Fig.3. Measuring the turns ratio by determining the The absolute number of turns voltage ratio using a ‘scope and signal generator. depends on the application. High-voltage, low-current (high-impedchecked with an oscilloscope and sigance) applications have thousands of nal generator, as shown in Fig.3. You turns of thin wire. High-current, low-voltcan also check the current ratio, which age (low-impedance) devices have a few is inversely proportional to the turns tens of turns of thick wire. ratio, but measuring currents is often Not surprisingly, the turns ratio (n) less convenient than voltages. equals the voltage ratio – but do note There is another vital ratio, which is it is the reciprocal of the current ratio: less easy to understand and analyse – the impedance ratio. The impedance ratio is n = NP/NS = IS/IP the turns ratio squared – remember to square both sides, even if you are just squaring ‘1’. Essentially, it represents From the above, you can see that since: the apparent change in impedance looking from one circuit to another when NP/NS = IS/IP linked by a transformer. In other words the output and input impedances can be We can rearrange rhe expression so that: modified to whatever is needed. This is important in audio because N PI P = N SI S it allows us to match the output of an amplifier to a load (eg, a speaker), or Confirming that in an (ideal) transthe output of a transducer (eg, a miformer, power into the primary equals crophone) to the input of an amplifier. power out of the secondary. But why is this useful and what do we mean by matching? Impedance ratio and When it comes to connecting an amimpedance matching plifier to a speaker, transformers were The turns ratio is easy to specify and originally used with valve amplifiers. understand, and also to test by lookThese typically produced high voltages ing at the voltage ratio. This can be Fig.4. The first audio transformers were originally used in telephones. They survived in landlines until quite recently – here is one from a British Telecom phone from 1981. It was used for impedance matching and partial sidetone cancellation (sidetone is one’s own voice in the earpiece). 61 we do the reverse – a low microphone output impedance is matched to a high amplifier input impedance. This not only optimises power transfer but more importantly optimises the SNR (signal-to-noise ratio) into an amplifier, because for a given amplifier there will be an optimum source impedance for minimum noise. An early, but longlived example of this was in telephone systems, which needed to match the low impedance of carbon microphones with the higher impedance of long telephone lines to minimise losses. A transformer was used at the other end to match the receiver/earphone impedance. This transformer also blocked DC. Fig.4 shows the transformer in a 1981 British Telecom phone. A good approximation to the impedance ratio can be calculated from the turns ratio, but for the best performance in a circuit you need to take into account the transformer’s non-ideal characteristics, which we will cover next month. That said, the lazy (and very effective) way to find the impedance ratio is to use an impedance meter, such as the Peak LCR45 shown in Fig.5. Fig.5. An impedance meter is a good way to check transformer action. Here, the output of a low-impedance (22Ω) turntable moving coil pickup is stepped up to 5kΩ with a 14:1 transformer (Vigortronix VTX-101-003. but low currents, which meant they had a high output impedance. Speakers work with high currents and low voltages, so they offer a low impedance. Clearly, this created an impedance mis-match between the output of the valve amplifier and the speaker input. In other words, in terms of power transfer the two systems are poorly matched because for maximum power transfer the two impedances should be the same. A transformer makes the output of the amplifier appear to have a lower impedance and conversely the speaker looks like a higher impedance – so, with an appropriate turns ratio (and hence impedance ratio) the two can be better matched. When it comes to matching transducers and the inputs of amplifiers, Multiple windings The number of windings on a transformer need not just be limited to two, most 820Ω 1W 75Ω +33V 470kΩ 1000pF 6.8kΩ 1.6mA 2:1:1 10kΩ DCR 5Ω 470Ω 2W 2.2kΩ 680µF 25V DCR 11Ω BC107 330µF 50V 2N3055 0.47Ω + BD140 + +43V Iq = 85mA 28mA 6.8µF 25V 10kΩ 1500µF 50V VA1040 130Ω NTC 100Ω + 47Ω 130Ω + 250kΩ 2N3055 Input 100Ω 33kΩ 270Ω 47Ω + 47µF 10V 22Ω VA1040 130Ω NTC 8-15Ω Iq and centre 0.47Ω set point 100nF 47Ω 0V 820Ω 1W Negative feedback 10Ω 0V 2.4nF Fig.6. Push-pull driver transformer circuit from the Rogers Ravensbrook, a classic late-1960s transistor audio amplifier. The circuit shown here uses a phase splitter to allow just one polarity (NPN) of driver transistor to be used. This amplifier sounded excellent with 15Ω LS3/5A loudspeakers. 62 Practical Electronics | July | 2022 Turns ratio = 0.5+0.5:2+1 (or Turns ratio = 1(CT):2+1 S Start (S) S Turns ratio 2 F Centre tap (CT) Finish (F) S Turns ratio 1 F F Primaries Secondaries Electrostatic screen Fig.7. Push-pull driver transformers – one with a centre-tapped winding and one with split windings for output stages without a final output transformer, such as the Ravensbrook. provide two antiphase outputs (often called ‘phase splitting’). This was vital in early amplifiers when only one polarity of valves or early transistors was readily available for push-pull power amplifier outputs. The phase splitter transformer meant you could build a push-pull amplifier with just NPN transistors, as shown in Fig.6. (Note the phase splitter is not an output transformer for impedance matching with speakers.) Fig.7 shows a couple of driver transformers suitable for phase splitting. types have four: two primaries and two secondaries which offer several useful combinations from one transformer. For example, with power transformers you can put the primaries in series for 240V in, or in parallel for a 120V supply. And for the secondaries, you can join the two together at one end for a centre-tapped supply, or in series for double the voltage or in parallel for double the current. For signal transformers in audio circuits, multiple secondary tappings can Fig.8. Phase is indicated by dots or winding start (S) and finish (F). Dot and S are the positive going half of the cycle. Going dotty about phase The phase of an AC voltage on a winding is indicated on a transformer with a dot, which indicates a positive-going cycle, as shown in Fig.8. The start and finish of a winding can also be used to indicate phasing with the initials ‘S’ and ‘F’. 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