How to pick mic preamps - Audio Media International

How to pick mic preamps

How to pick mic preamps
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Boomerang Sounds’ Martin Pedder explains how mic preamps give a quality to your recording.

It’s all about the iron – or maybe not,” said the owner of Eve Studios when asked what makes a good mic pre. With his studio full of the strangest vintage gear I pushed him further to discover which he likes best. “My BBC valve passive preamplifiers…” he said and went into an explanation of how they worked. By “the iron”, he is of course referring to the transformers.

I was taught that the perfect amplifier would be a piece of wire with gain. What goes in comes out exactly the same, only louder. This clearly isn’t what studio engineers actually want in a preamp. They want something that changes the sound and will cheerfully spend thousands on a box that actually adds distortion, noise, and peculiar phase shifts.

Many engineers consider the Rupert Neve-designed Amek 9098 their dream front end. The console at Gracieland Studios has 48 built-in. While doing some maintenance there recently I had to enquire why they had an Avalon and a whole rack of AML modules sitting on top of the racks. The house producer/engineer laughed: “I know. It’s silly really. But they do sound nice.”

My customers, both rich and poor, are all doing it: Jam Factory spent £1,600 on an eight-channel Avid PRE but records almost everything with a Neve 1073DPA.

80 Hertz Studios with its Neve console feels the need for an AEA and a few Chandler Germanium pres. A composer in his budget home studio uses a Joemeek threeQ when his audio interface has a perfectly good mic pre.

Console and audio interface designers put their heart into designing the best preamp they can and everyone seems to bypass it with something external. So what is going on? Let’s get a bit technical and look at what a mic pre is trying to do.

Gain

A microphone’s output ranges from below a 1mV whisper to a trumpet at over 1V. Our primary requirement is to amplify all signals to a standard level for recording, processing, and mixing. Most mic pres can add up to 60dB meaning the output can be 1,000 times louder than the input.

Pad

Designing an input for such a large range is tricky so many incorporate passive pads. Really loud signals, which would overload the input stage or transformer, will be padded down before entering a variable gain stage.

Noise

The molecules in metal generate a noise signal. So any mic and its lead will have thermal noise before you start to amplify it. Trying to approach this theoretical minimum noise requires a skilled designer.

Input impedance

Our next requirement is to choose the input impedance. A high impedance (like a guitar input) gives maximum signal but is prone to interference because a mic lead also acts like an aerial. A low impedance solves this but needs more gain (and therefore noise) to boost the reduced signal. Decades of designers have produced a compromise of 200-ohm mics feeding 1,200-ohm inputs that works pretty well.

Changing the input impedance can dramatically change the sound of the mic. It accounts for much of the variation between preamp designs. Most older ribbon mics have an output impedance around 50 ohms which is a poor match for many mic pres. Products from Focusrite, Millennia, and others have an input impedance switch. The Avalon AD2022 has a choice of five.

Phase

It is scary how many of my customers don’t know how to use a phase switch. While a drummer hits the snare, try listening to just the overhead mics. Slowly fade up the snare mic and you should expect it to continually get louder. If it dips down in level first and goes all thin sounding but then starts to increase, the snare mic needs phase inverting. Likewise, a mic in front of a bass cabinet may not be in phase with the DI output that you are blending it with.

Phantom power

Condenser mics need power for their preamplifier and some need a high voltage to polarise the capsule. Most mics will happily run from as little as 12V but some require the full industry standard 48V.

Filters

Mics can pick up very low frequency sounds like air movement and traffic rumble. This can colour the sound of a preamp and can alter the way a compressor works further down the line. The LF signal may not even be heard on small monitors. Record some silence and check the meters, then filter out any very low frequency rumble that you can’t hear.

Transformer or electronic balancing?

Transformers are big, heavy, and expensive. You wouldn’t want them in a portable recorder. They distort easily, can be susceptible to hum, and don’t pass the low bass very well. However, they are excellent at rejecting radio interference and can sound wonderful. The Portico and 9098 pre use a fascinating design that Rupert Neve calls the TLA or Transformer Like Amplifier. It mimics the nice-sounding characteristics of a transformer but has the wide frequency response, low noise, and distortion of an electronic stage. It is then coupled with a real transformer using his ‘tertiary feedback’ system.

Solid state or valve?

It is nice to have the choice and good valve designs are not noisy. When over-driven they generate even harmonics that can give a warmth to a recording that many engineers love. There are some great budget valve pres from ART, PreSonus, Samson, and SM Pro that usually have an instrument input too. Playing with the way the valve is biased can dramatically change the sound. ART makes the Tube MP Studio V3 with ‘Variable Valve Voicing’. You can just dial up 16 different valve sounds.

Modules

I am customising a lovely Classic API VP28 module for a customer who wants a separate mic, line, and DI input. It would appear that every manufacturer who designed a preamp in the 1970s is re-releasing it as a 500 series module. Take PMI, one of my favourite suppliers. It owns Joemeek, Studio Projects, Tonelux, and Trident and has mic pre modules now for all of them.

So why spend good money buying a mic pre? Because “they sound nice”. Which one? “It’s all about the iron – or maybe not.” Perhaps it’s just the designer’s fairy dust.


Expert witness


Martin Pedder is the owner of Boomerang Sounds, a specialist pro-audio retailer and studio installer based in Bury, just north of Manchester. Founded in 1980, Boomerang Sounds has installed studios for Lisa Stansfield, The Charlatans, Johnny Marr, The Stone Roses, and numerous commercial studios and radio stations. Boomerang Sounds, Britannia Mills, Cobden Street, Bury, Lancs, BL9 6AW. +44 (0)161 761 5050. sales@boomerangsounds.co.uk.

www.boomerangsounds.co.uk

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