Audio transformers have been a core component of pro audio equipment since the very inception of audio recording and reproduction. Despite advancements in component technology providing many smaller and more cost effective alternatives, the humble transformer remains a popular choice for equipment designers as a result of its association with big, warm or even vintage tones.
With manufacturer marketing copy peppered with phrases like “transformer saturation” and “mojo”, it can be difficult for producers and engineers to sort the fact from the fiction when it comes to these mysterious devices, so let’s dive deep into the world of audio transformers, what they do, don’t do and how you can use them to your advantage in your music!
Transformer Construction
A transformer is constructed by winding two (or sometimes more) lengths of wire around a magnetic core. In more affordable transformers this core might be made from steel, where more expensive transfers use nickel alloys for their core material. The core material affects the way the transformer saturates at low frequencies.
The coils of wire around the core are referred to as ‘windings’, and where there are two of them we call them the primary winding and the secondary winding. Each winding is made up of a number of ‘turns’ which describes the number of times the wire makes a full lap around the magnetic core. ‘Turns ratio’ describes the relationship between the number of turns on the primary and the number of turns on the secondary.
For example, a transformer with 1000 turns on the primary winding and 2000 turns on the secondary has a turns ratio of 1:2. Transformers like this, whose secondaries have a greater number of turns than their primaries, are called step-up transformers. Inversely where the number of turns on the secondary is fewer than the primary, the transformer is a step-down transformer.
Transformer Operation
When a voltage such as an audio signal is applied to the primary winding, an alternating magnetic field is generated around it and this is induced into the transformer core. This magnetic field is in turn picked up by the secondary winding and a sympathetic signal is generated at its output.
This phenomenon is known as electromagnetism and is the same principle that underlies the operation of dynamic and ribbon microphones as well as analogue tape recorders. That might be a discussion for another blog post!
Where the transformer's turns ratio is 1:1 the signal found at the secondary winding will be more or less the same as that applied to the primary, but where the windings are configured as a step-up, or step-down, the signal will be scaled in relation to the turns ratio.
Example: If we were to apply a 1V signal to the primary of a 1:2 step up transformer, we would measure around 2V at the secondary.
Not only is voltage scaled by the turns ratio, but so is impedance. If the concept of impedance is a little foggy for you, now might be a great time to brush up here. This is really useful in applications where level and impedance conversions are required, such as a direct (DI) box.
Transformer Balancing
Transformers are also often used to create a balanced signal from an unbalanced one, or vice versa. When you see a piece of equipment being described as “transformer balanced” this is what we’re talking about.
Once upon a time, transformers were the only way inputs and outputs were balanced, so vintage pro audio gear with balanced I/O will contain at least a couple in their signal path. Many mixing consoles of the 1960’s and 70’s had a transformer at every single input and output, something many credit for their great sound.
With the advent of small and cost-effective operational amplifiers (opamps) during the 70’s and 80’s, many manufacturers began creating balanced input and output stages using them, instead of the larger and more costly transformer. This made designing more compact and budget-friendly equipment possible, but some would argue made the equipment of the era less pleasing to the ear.
Personally I think it's a bit reductionist to blame the sound of an entire era of recording equipment entirely on a shift away from the use of transformers, when so many other aspects of electronic design and taste changed at the same time.
Transformer Applications
Now we know that transformers convert voltages (signal levels), convert impedance and create balanced inputs and outputs, it’s easy to see what a useful, elegant and multitasking component they really are. Let’s look at a couple of common transformer based devices and analyse how they work!
DI Boxes
The purpose of a DI box is to interface a high impedance instrument source, such as an electric guitar or bass with the input of a microphone preamplifier. In doing so the DI performs three functions.
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The level of the instrument must be attenuated so as to not overload the input of the preamp.
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The high impedance source impedance of the instrument needs to be significantly lowered to avoid high frequency loss.
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The instrument signal needs to be converted from unbalanced to balanced to avoid unwanted noise over long cable runs.
It just so happens that a single transformer can perform all three of these functions! We use a specially designed, high-ratio step-down transformer with its primary connected to the instrument and the secondary connected to a preamp.
In our own DI products (DI-20, DI-20 and SS-6) we use a transformer that is custom wound just for Franklin Audio, with a very high impedance primary winding. This makes it sound crystal clean and clear, even on passive guitars and basses.
Re-Amp Boxes
The role of a re-amp box (reamp, re-amplifier) is to take balanced line level signals such as those from an audio interface and convert them to high impedance, unbalanced instrument signals. It allows the user to take prerecorded material and run it out through pedals and amps for a plethora of creative applications.
For a re-amp box to do its job we need 3 things:
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Attenuation from line to instrument level (~12dB).
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Conversion from a balanced signal to an unbalanced one.
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Conversion from a low impedance output to a high(ish) one to emulate the output of an electric instrument.
Once again the transformer proves an elegant solution to the problem. A 10:1 step-down transformer can provide the necessary level attenuation, at the same time as properly converting the signal from balanced to unbalanced, removing unwanted noise in the process.
Unfortunately the transformer works against us here in terms of impedance. The secondary will present a very low impedance to the guitar effects or amps and while this is a good thing from a technical standpoint, it doesn’t accurately emulate the behaviour of guitars and basses. That’s why in our own re-amp, the RA-10, we added an impedance control after the transformer which allows the user to raise the output impedance and emulate the conditions created by a wide range of electric instruments.
Transformer Sound
A big misconception about transformers is that they add a great deal of saturation, harmonic distortion or vintage magic to audio signals. In reality, the signal level required to actually overload and saturate a good quality transformer is greater than most any device in your studio could actually generate. When engineers and producers drive preamps and compressors into distortion, it’s usually components like transistors, opamps and tubes that create these pleasing and exciting harmonics. Saturation is often incorrectly attributed to the transformers.
It’s not impossible to push a transformer into saturation but it would really only occur when:
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The transformer is such low quality that the cheap core material saturates at moderate signal levels, or,
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The signal level pushing the transformer is so significant that every other device in the studio including the digital converter is distorting as well!
A really good transformer is designed to not distort, and that is a key factor in choosing a transformer to be included in a Franklin device. We want our users to be able to hit their gear with a wide range of signal levels without having to be overly concerned with gain staging during the creative throws of making music!
Active devices that use opamps or transistors and low voltage power supplies in place of transformers might clip and sound weak or unpleasant when hit with hot signal levels, where a passive Franklin box will handle most anything you throw at it with ease.
Below is a harmonic analysis of the Franklin FA-02 transformer which is used in our DI products, vs a budget transformer from an electronics hobby store. We’re feeding them both a 100Hz signal to represent a worst case scenario because transformer saturation is worst at low frequencies.
FA-02
Generic Transformer
We can see the harmonic distortion of the FA-02 is so low it has fallen off the display, where the small generic transformer saturates heavily, with a particularly concerning low harmonic an octave below the test tone.
Closing Thoughts
I’ve personally formed my opinions about different makes and models of transformers using tests and measurement (and of course lots of listening) because I believe the ear alone is too susceptible to marketing, preconceptions and placebo effect. Producers and engineers have described to me in no uncertain terms how much they prefer X legacy brand transformer manufacturer over Y brand - but when pushed, admit they’ve never done anything close to a scientific test or a blind listening shootout.
One hill I’m willing to die on is that above a certain threshold of quality, more expensive transformers offer extremely diminishing returns. In our exhaustive research selecting transformers for our products we discovered that small and very cheap transformers uniformly performed terribly, with unpleasant distortion and unacceptably limited bandwidth. However, all of the good transformers we tested performed so similarly I had to check we were definitely looking at the correct results.
Between our own transformers and my collection of high-end name brand transformers, new and old, the deviation in frequency response and harmonic distortion was truly small enough to be considered negligible at most.
So long as the transformers in your device are not bad enough to be bad, then they’re good enough to be great, and you should lose no sleep over whether or not swapping out one brand for another might give your audio device some special magic that you’re missing out on.
Audio transformers offer an incredible all-in-one-solution to many common engineering problems, and good ones offer a solid and robust sound with incredible amounts of headroom. That’s why I love transformers and we’ll continue to use them in Franklin devices wherever we can!
