Microphone preamplifiers strengthen a microphone’s signal, so that the signal might be used by other audio equipment. On their own, most microphones do not produce a strong enough signal for recording devices or mixing consoles to use without sacrificing quality. Microphone preamplifiers improve a microphone’s signal, bringing it up to line-level quality, which is the signal-strength level that most sound engineering devices require.
Microphone preamplifiers are also referred to as microphone preamps, mic preamps, preamps, and mic pres.
History of Preamps
As with all electronics, the first microphone preamps were much larger and less sophisticated than modern-day models. Preamps from the 1940s were characterized as large, temperamental tubes that would hiss and hum – sometimes ruining an entire take. Over time, preamps were upgraded. Quieter metal films replaced carbon resistors, and polypropylene capacitors replaced old, paper versions. The electrolytic filters decreased in size. Thanks to decades of improvements, the preamps available today are much smaller and produce better sound that the first vacuum-tube preamps.
Types of Transformers
There are two categories of microphone preamps: solid-state and tube. Solid-state preamps have transistors or op amps within them. (Op amps contain integrated circuits and several transistors). These are designed to be cleaner than tube models, adding less un-wanted noise but producing a more sterile sound. Tube preamps offer a more natural tone, but they can introduce noise into the sound stream. Some manufactures produce preamps that are solid-state, but contain a tube; this is an attempt to merge both types’ desirable qualities.
Both solid-state and tube mic preamps can come with or without a transformer. Transformers have two coils in them, which do not completely touch, creating some insulation. Preamplifiers with transformers can be useful in situations where there is high interference.
When selecting a preamp and considering whether to purchase one with a preamp, one fo the most important considerations is the output impedence of the mic and the input impedence of the preamp. If these match perfectly, there will be no EQ effect (equalization effect); however, any slight difference will result in an EQ effect.
How Preamps Work
The output of a typical microphone is low, often less than 100 microvolts. A preamplifier increases the sound by as much as 70 dB, up to 10 volts. At this level, audio mixers and other external equipment is able to process the sound.
The Unique Sound of Preamps
Solid-state preamplifiers are designed to eliminate unwanted noise, but even at the cost of creating a shallow sound. Tube preamps, on the other hand, produce a richer tone, which enhances the musical qualities of a recording or live performance.
Each preamplifier alters sound uniquely. Some force microphones to alter the quality of sound they produce, by loading them with low impedance. Others add their own characteristics to the sound, before sending it onto the preamplifiers that are built into an audio mixer.
Who Uses Preamps
Preamps are used by audio engineers, audio mixers and anyone else who produces recordings. People who work on large-scale projects often select tube preamplifiers with excellent transformers, as these produce a rich, clear sound. Those who are on a tighter budget, though, often opt for less expensive equipment, such solid-state preamplifiers that do not have transformers.
Tubes and transformers can greatly help a musical recording. Purchasing cheap equipment, though, usually negatively impacts the sound produced. Most audiophiles would rather use the best solid-state preamps that do not have preamps than cheap tube preamps with transformers. If one can afford to invest in high-quality tube preamps and transformers, however, it is often advisable.
Advantages and Disadvantages of Preamps
Solid-State Preamps:
- Cost less
- Do not introduce noise
- Produce a sterile sound
Tube Preamps:
- Cost more
- Can pick up noise
- Create a more robust tone
Transformers:
- Are useful in high-interference settings
- Cheap ones usually produce poor-quality sound
Preamps Available Today
There are many manufacturers of preamplifiers, which range in price.
Burl Audio’s B1D Microphone Preamp and DI is an inexpensive preamp and DI box. It comes with both an input and output transformer, as well as a capacitor-free circuit path. The gain of 70 dB is standard within the industry. This is an excellent model for musicians just starting out, who have to watch their expenses.
On the other end of the price spectrum, the GainStation8 offers superb quality in both studio settings and live performances. With 60 volts, it provides an excellent gain of 130 dB. This strength helps in live situations, where signals must often travel significant distances.
In between these two extremes, the Rupert Nerve 5032-H Single Channel Mic Preamp and 3 Band Equalizer (EQ) is a great value. This single-channel preamp and EQ provide quality to please the most picky audiophiles, while also appeasing budgets.
Glossary of Common Features
Ammount of Gain: The amount of gain is how much a preamp strengthens the sound.
Roll-Off Impedance: Roll-off impedance is the level at which there is a sudden increase in impedance.
DI: A DI box connects high-impedance, unbalanced signals to the low-impedance, balanced input of some equipment.
Mic/Line Level: Line level is the strength at which most audio equipment can process a signal. Mic level, which is much lower, is the output level of a microphone.
Class-A: Class-a preamps are constantly on, which is inefficient but produces excellent sound.
Class-AB: Class-ab preamps turn off one side of the preamp, saving energy.
Multi Channel: Multi-channel preamps can handle multiple signals at once.
Channel Strip: Some preamps are part of a channel strip, which also can include compressors, noise gates, equalization (EQ) and enhancers.