What common electronic device can be used to raise or lower AC voltages, block DC voltage, remove noise, and is completely passive? Get to know the transformer, a gadget as useful as it is old school!
Basic Electricity 101 says that a current passing through a conductor creates a magnetic field around the conductor. Some may recall a simple kid’s experiment using a battery, steel nail, and piece of wire to make an electromagnet. Conversely, when a conductor passes through a magnetic field, a current is induced in the conductor.
A simple transformer uses two loops of insulated wire wound on a common metal core. Without a physical connection between the wire loops DC voltage cannot flow. But when AC current is applied to one loop (known as the primary winding) it causes a magnetic field that is rapidly growing and collapsing around the wire. The magnetic field passes through the core and induces a similar current in the secondary winding. This is sometimes referred to as magnetic coupling.
Power Applications
Transformers are used everywhere in the electrical power grid, within buildings, and within equipment to change voltage levels. This is probably their most commonly known application, even to the general public, as they are often visible as large metal boxes behind buildings, or cylindrical cans mounted on power poles.
Power transmission lines operate at extremely high voltage in order to reduce losses caused by wire resistance over long distances. By varying the number of turns in the primary and secondary windings (the turns ratio) a transformer can “step down” the transmission voltage to something more practical for various needs. (See Fig. 1)
For power applications this is usually done in several stages at different points in the distribution system; in switchyards, on power poles, in underground vaults, or inside buildings. In commercial buildings transformers are used to convert 3-phase high-voltage supplied by the power company into various lower voltages used by infrastructure and equipment.
Similarly, converting voltage to use equipment internationally is easily done by transformers stepping-up or down between the 120V and 240V service in different countries. Although this has largely been eliminated by the widespread use of switch-mode power supplies that accept a range of input voltages. Transformers, as a rule, don’t care about the power line frequency, and cannot change it.
Within AV equipment, internal circuits generally run on fairly low DC voltages. Linear power supplies, whether internal or external, generally always use a transformer to convert the incoming line voltage to something that can feed the rest of the power supply (and then convert to DC). Switch-mode power supplies instead have a transformer at the output. For more see Practical Power at www.svconline.com/author/eric-wenocur.
Balanced Audio
Remember that “balanced” audio is the practice of sending two identical signals, of opposite polarity, between equipment. The differential input of the receiver inverts one side so that those signals add positively, while any noise picked up on both wires is canceled out. Before the advent of electronically balanced audio circuits using transistors or op-amps, the phone companies (such as Bell Telephone in the US) realized that differential signals would result in better audio quality and used transformers to create balanced line pairs. For more see Analog Audio Interfacing at www.svconline.com/author/eric-wenocur.
For a transformer to function there needs to be a voltage difference between the two legs of the primary winding. When that’s the case a similar signal will appear on the secondary and is, by definition, a differential pair because it comes from each end of the winding. But the transformer actually doesn’t care if the primary connection is a balanced pair, or if one side of the winding is connected to the local signal ground, as long as there is a voltage difference. So transformers can convert unbalanced to balanced, and vice-versa, and are often used for that purpose.
To reiterate, whatever is connected to the primary, whether a differential balanced pair or unbalanced signal and ground, is conveyed to the secondary. What comes out of the secondary is essentially a “fresh” signal and requires no connection to the ground reference of the previous circuit.
The implications of this feature are quite significant. If hum is being caused by voltage flowing on ground conductors, that circuit can be broken by inserting a transformer in the audio path. The transformer will pass the audio, with no other connection between the two devices or systems—a function known as galvanic isolation. Many, if not all, commercially available audio “hum eliminators” are simply transformers. (See Fig. 2)
Products that passively send audio on CAT5 or 6 cable also use transformers for the reasons above. Whatever signal goes in, whether balanced or unbalanced, is sent along on the CAT cable as a balanced line (the twisted pairs of CAT cable are ideal for this). The receiving end transformer produces a clean signal, isolated from the originating system.
Recently I needed to get audio between a video source and a PA system in different parts of a building. A previous person had done this by extending the HDMI using HDBaseT on CAT5, and then dis-embedding the audio at the far end. A reasonable idea, but there was more hum than audio in the final output! I knew that the power distribution was radically different in the two areas of the building, so I decided to disembed the audio at the source, and send it via a balanced audio extender, using the existing CAT5 cable. This removed the audio ground connection between the two systems and eliminated the hum (as well as rejecting any noise along the way).
Along with converting between balanced and unbalanced connections, the same transformer can also provide level-matching. For many years I used the Rane Balance Buddy product to interface consumer VHS decks and other gear in professional systems. The Balance Buddy is entirely passive and the transformers were wound to give 12dB of gain or loss to match pro and consumer levels. And even though it has appropriate XLR jacks for “input” and “output” it could actually be used in either direction. The transformers don’t care. (See Fig. 3)
Transformers fell out of favor for differential audio transport due to size and cost, compared with electronic circuits. But they are still used in many pieces of equipment, such as mic preamps, direct boxes, and signal splitters, because of their sonic characteristics and universal compatibility with just about anything.
Other Handy Functions
The fact that transformers will pass an AC voltage, but block DC, also makes them useful when dealing with “phantom” power on microphone circuits. DC that is connected to a winding centertap will be added to the AC signal on that side of the transformer and sent along to the mic. Within the mic another transformer will prevent the DC from going where it’s not wanted. Capacitors can also be used for this purpose, so designers of microphones and preamps make choices based on cost, size and the sonic qualities they desire. (See Fig. 4)
Not coincidentally, ethernet wiring is also balanced and uses transformers as connection points. In general, every RJ45 jack used for ethernet has four tiny transformers inside, or on a nearby chip, one for each twisted pair. Not only does this provide ground isolation and noise immunity, but it allows POE (power over Ethernet) to work as phantom power does. The requirement for transformer coupling in wired ethernet is a key reason that it can be run over 100m and between disparate data rooms with relative impunity.
Another clever device is the rotary transformer, used to transfer signals between two rotating components. These were used extensively in videotape machines, and other tape-recording formats, to couple the signals from the rotating head drum to the rest of the electronics. The alternative, slip-rings, is less reliable and prone to wear.
Going back to audio, transformers are used in distributed speaker systems for what’s known (slightly misleadingly) as constant voltage operation. As with high-voltage distribution for power lines, a transformer at the power amp steps-up the output level to a nominal 70 or 100V (at full power) so the signal can run over longer distances and smaller wires. Each speaker has a step-down transformer that matches the high-voltage line to its own impedance, such as 8 ohms. Raising the voltage maintains the transferred power, but lowers the current, which means less loss from cable resistance. (See Fig. 5)
Using Transformers
Transformers come in many specialized forms with varying specifications. Designs and materials can be tailored to adjust for parameters like core saturation, signal loss, power handling, frequency response, etc. Form factors also vary greatly and are usually specific to the application. It’s important to know how “good” a transformer needs to be in a given situation.
Those designed for optimum audio performance, particularly with microphone signals, can be exotic and expensive. Manufacturers such as Jensen and Lundahl are sought out when frequency response and distortion are critical. In other cases, better specs may not be worth the extra cost. In the earlier example, feeding TV sound to a PA system, I don’t know what kind of transformers are in the extenders. They aren’t expensive but are fine for the task at hand.
Using transformers is a bit different than using electronic interfaces because both terminals of a winding must be connected for any signal to flow at all. Even if the transformer has a ground or shield connection, it should not be part of the audio circuit in any way. So situations where you might leave one side of an electronically balanced output unconnected (such as directly feeding an unbalanced input) would result in no audio from a transformer output. A look inside the box, or at the schematic, would confirm the trouble. In that case the other leg of the transformer output can connect to signal ground, which will make it a functioning unbalanced output.
I should note that the cable shield being connected to the audio circuit, as would be necessary to use half of a balanced input or output, can invite ground-related noise (the classic “Pin 1 problem”). But this is how a great deal of equipment is built, and sometimes connecting this way is acceptable, if not ideal. If noise is a problem, put in a transformer!
