Core Concepts: Microphone Amplifiers
Carlos Lellis

   By Guest Contributor   Categories: Audio EquipmentRecording

The main role of the microphone preamplifier is to raise microphone level signals to line level, although some units may offer extra features such as equalisation and compression. Microphone preamps differ in sound and while some devices may be broadly described as ‘transparent,’ others alter the nature of audio material noticeably. In addition to that, equipment deemed as ‘transparent’ will commonly respond to quiet and loud sounds in a non-linear fashion, e.g. with increasing harmonic distortion.

A preamp’s alteration of timbre must not necessarily be seen as a negative side effect of amplification, as experienced engineers frequently attempt to ‘shape’ microphone signals before routing them to the multitrack recorder, e.g. via the use of equalisation. With that in mind, it appears as a sensible procedure for recordists to audition a few different preamplification units and to do so at ‘clean,’  i.e. conservative input signals, and at higher, ‘overdriven’ levels before committing to a signal chain.

Harmonic Distortion

Preamplifiers, as all other signal processors, add harmonic content to their input signal. The level and nature of the added harmonics or distortion is an important factor influencing the efficiency and the perceived quality of devices. It is not uncommon for engineers to associate pleasing or ‘musical’ lower-order even harmonics with valve-based equipment, while linking ‘harsher’ odd harmonic content to solid-state technology. It is true that older valve-based devices have a tendency to generate ‘rich’ even harmonic content, although it is important to note that this is largely due to the topology of such equipment (class ‘A’). ‘Near-clipping’ or ‘upper headroom’ checks are a simple way to investigate the suitability of harmonic and dynamic distortion to the audio material and as a way to assess the impact of noise generated at extremes of operation.

Microphone Preamplifier Noise Floor

The terminating of the input of a preamplifier, i.e. the connecting or ‘shorting’ of pins 2 and 3 at the input stage, is a quick way to determine the noise floor of a given device. Technicians should ideally use a male XLR connector with a 200-ohm resistor wired in series between the two pins (2 and 3) for this type of testing, as this should present the preamp with the equivalent output impedance of an ‘average’ dynamic microphone.

Variable Impedance

Some preamplifiers offer user modifiable control over input impedance. Such control may be continuous or discrete (few values) and it may have a significant impact over sound quality in circumstances when high impedance microphones, e.g. some ribbon models, are connected to the preamplifier via long cable runs. In such cases, a preamp high impedance setting may be recommended, as it may make signals seem ‘bigger’ or less ‘thin’ (more low-end content). NB the altering of input impedance should have very little impact on the signal of condenser microphones.

Suggested Microphone Preamplifier Evaluation Procedure

The process of evaluating the suitability of microphone preamplification may appear arbitrary to recordists with less experience, although it is possible for a simple methodical approach to take the place of ‘blind’ trial and error.

The following is a suggested vocal microphone preamp selection routine that may allow for a quick comparison between different devices:

1)       Set up a given microphone preamplifier and DAW to record vocal signals through a well-known large diaphragm condenser microphone.

2)      Set the input gain so that very quiet vocal sounds are amplified to reach or approach the standard operating level of the preamp.

3)      Ask a vocalist or an assistant to say (or sing) percussive or transient rich words at very quiet levels, e.g. ‘test’, ‘tick’, ‘tock’, ‘click’, ‘clock’, ‘pop’, ‘pup’, etc.

4)      Monitor the device’s output signal ensuring that all transients and high-frequency content are preserved and that the noise floor is not too high.

5)      Check that a sense of intimacy, i.e. the feeling that a person speaking/whispering into the listener’s ear, is being captured.

6)      Set the input gain of the preamp so very loud singing is amplified to a level approaching the device’s clipping point.

7)      Ask a vocalist or an assistant to sing sustained vowel-based material at very loud levels, e.g. ah, eh, oh, etc.

8)      Check the added harmonic distortion for suitability.

This test can be particularly effective for the determining of whether a preamplifier is suitable for the recording of main vocals and it may also help establish the need for dynamic range processing, i.e. compression.

The first part of this procedure (steps 1 to 5) can be used for the comparison of preamps to be employed for the detailed recording of acoustic instruments. The second part (steps 6 to 8) should be effective for the comparison of preamps to be employed for the recording of loud, ‘energetic’ sources, e.g. electric guitar, snare drums, etc.

The following contains a few examples of common microphone preamplifiers used in music production.

Excerpt from Music Production: Recording by Carlos Lellis ©2013 Taylor & Francis Group. All Rights Reserved.

About the Author

Carlos Lellis Ferreira holds an SAE/Middlesex University MA in Creative Media and a diploma Summa Cum Laude from Berklee College of Music. He has worked as an engineer and producer with artists such as Rosabella Gregory, Joby Talbot and Grammy Award winners Sir John Tavener and Thomas Dybdahl amongst many others. He is currently the Audio Production programme leader at SAE Institute, UK.

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