Making resonance sound beautiful
In electronic engineering, every design comes with drawbacks. When possible, clever engineers find workarounds to minimise potential issues or even eliminate them. Once in a great while, however, a solution of this type turns out to have hidden benefits, as legendary designer Rupert Neve learned. While designing what would become the iconic Focurite ISA preamp range, he tapped into the potential of a circuit called a Zobel network.
To understand Zobel networks, it's important to understand an element or two about impedance. We talk about it all the time – low-impedance mic cable runs, guitar amp cabinet impedance, high-impedance instrument inputs, and more – but what exactly is it? Impedance is what it sounds like – the amount that electricity is impeded from flowing through a circuit.
At first, we might think, “Impedance? Oh, you mean resistance!" It's true that resistance is a component of impedance, but it's not the only one. Resistors are joined in their effects by capacitors and by inductors (transformers and other wire coils). Resistance, capacitance, and inductance come together to create impedance, and their sizes relative to one another will not create a steady value, but one that changes as the frequency of the electrical signal changes.
The important part of this effect is that starting at a certain frequency and going up, resonance will be created. The signal at that frequency bounces back and forth up and down the signal line, building in strength as more current is fed into it. If you've ever heard a howl of feedback from a microphone too close to a speaker, you've heard resonance.
In real-world circuits, these resonances can be broad, but eventually their power increases exponentially, and the effect ranges from problematic to nasty to downright destructive. For example, when a power amp is connected to a speaker, the speaker's impedance can cause a resonance that can eventually destroy the amp. To counter this, we add a Zobel network to the circuit.
A Zobel network is a carefully designed network of resistors that can be tuned to cancel out the effects of capacitance and inductance. What had been a complicated function becomes, in theory, a pure resistance – with no resonances. Zobel networks are tuned to a particular frequency, a range of frequencies, or are designed to be tunable as needed; they have been in use in broadcast telecommunications for nearly a hundred years, and — for obvious reasons — they're often found in power amps.
Even though most of the audio literature about Zobel networks has to do with power amps, resonances can occur anywhere, and Zobel networks can be found combating them in many places. One such place is Focusrite's ISA preamp design.
These preamps have an input transformer that acts as a passive amplifier: the two coils are wound in a 1:5 ratio, boosting voltage and reducing current. Left to itself, the transformer and its associated resistive and capacitive components will reflect signals back down the line to the microphone. As explained, this function is very complex and nonlinear, and in this particular case, we start to hear the effect of the resonance at about 2 kHz. It continues to climb and shoots up quite dramatically at higher frequencies.
However, when we install a Zobel network on the secondary coil of the transformer, the signal that bounces back through the primary can now be controlled… and, as Rupert Neve discovered, tuned so that the formerly unpleasant resonance becomes a presence peak, turning an irritating artifact into something euphonious.
Mr. Neve, when developing the ISA at AIR Studios, tuned the preamp's Zobel network by ear to get what sounded best to him, using mics available at the time. These mics' impedances tended to be in the range of 100 to 200 ohms, and that means the preamp's sonic character will change if the mic's impedance is outside this range. Older mics will have measurably higher impedance, so the resonance will be damped down even more, leading to what we hear as a “warmer" tonality. Conversely, modern mics with markedly lower impedance will have a slightly higher resonance that we perceive as “air".
This sort of interaction between a microphone and an analogue preamp like the ISA is part of the magic of miking, and the Zobel network is a big part of making that happen.
Words: Mike Metlay, with many thanks to Rob Jenkins for illuminating technical insights.