Understanding The Channel Strip Part 1: Preamps

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  • June 30, 2017
Universal audio preamp

“What do all those knobs do?” is the timeless question, sometimes asked with tongue in cheek, other times with genuine intrigue When first approaching the mixing console – overwhelmingly complex as some may appear – there is a definitive logic to how it operates.

The desk is divided into singular vertical units: channel strips. Signals go through them, then are sent to a master fader. Understanding the strip is key to understanding a recording or mixing signal path.
 

Channel Strip

Not just the top knob on the channel strip, the selection of a preamp is imperative to producing a recording with integrity and tonal individuality.

Within each strip are components that amplify the signal, colour it to taste, send it to alternative paths and locations within a stereo field and are finally mixed in with other signals on other channel strips. These processes can happen in the analog world on real mixing consoles, or much more commonplace these days, on a computer screen.

Whichever the format, the basic principle – the sending of sound from one place to the next, through a complex matrix of inputs and outputs, is the same. In this piece, we’ll take a look at the first link in the chain: the preamp.

The Basics

The preamp’s role at the beginning of the signal path (with the exception of the microphone) is unglamorous, yet essential: microphones typically put out a very low amount of voltage and therefore needs to be amplified to work with other sound sculpting components such as equalisation and compression. The main control of the preamp is the input knob, which controls the amount of gain that is added to the signal.

Other common additions to the preamp section of a console, plugin, or standalone module include pads – for attenuating particularly strong signals; a phantom power switch – sending the 48 volts needed to power condenser microphones; a phase reversal switch – flipping the phase of a signal 180 degrees (useful in checking phase relationships when recording more than one source) and a high pass filter; for rolling off problematic low frequency information.

Universal audio preamp

Flavours

The aforementioned functionality doesn’t tell the whole story of these oft-mythologised pieces of gear. Talk to any engineer and they’re bound to have their favourites. Yes, their presence in the signal path from microphone to our ears is obligatory, yet they add so much more than mere volts. Depending on their design and how it’s being used, the humble preamp can impart a distinct personality to a sound.

In early designs, preamps that were powered by vacuum tubes were predominant. The fact they are still highly desirable says a lot about the way that tubes convey sound. They produce subtle harmonics and distort in a way that gentle and natural and as such, are prized for their musicality (an example of which, right here). On the (ever so slight) downside, preamps with valves are generally expensive, require a deal of maintenance and are sometimes prone to unwanted noise.

Solid state preamps became popular for their consistency and lifespan, yet don’t offer up the warmth of the tube-driven models. Solid state preamps became all but essential for large format consoles however, for reasons of expense and practicality.

Yet within these boundaries of efficacy, designs improved – more and more multi-tracking capability was needed as the recording industry grew and consoles from the likes of Rupert Neve and Solid State Logic achieved near mythical status; discrete input and output stages such as those that feature on the Phoenix DRS-8 give the user a varied palette of tonal colours to choose from. All manufactured on the back of preamps that relied on transistors rather than tubes.

preampsThe Digital Age

The central hub of the recording studio nowadays is the digital audio workstation, meaning that a converter is needed to transform the signal from the analog to digital realms. Conveniently, there are many audio interfaces that incorporate preamps into the unit – no more expensive standalone preamps required! Well, it’s not a completely positive development. Internal preamps on interfaces are famously dull and do little to imbue the signal with character of any description.

The folks at Universal Audio might be onto something however, with their Unison technology. Having created their own incarnations of legendary analog preamps (from the likes of Neve, API, SSL, Manley and of course, UA themselves), Unison matches the impedance behaviour and gain staging of these classic models. If you believe this video, the results are almost identical.

Humble? Utilitarian? Yes, the preamp is fit to wear these badges. But like any other chain, the signal chain is only as strong as its weakest link. The assiduous selection of a preamp is imperative to producing a recording with integrity and tonal individuality. Not just the top knob on the channel strip, the preamp needs to be considered as much for its musical contribution as it does for adding volume.

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Dan Shaw is recording and mixing engineer at Enmore Audio. He also plays bass in Wells and Circle.

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