Chapter 2. From Analog Sound to Digital Audio
2.5. Microphone Preamplifiers
When the sound has been captured by the microphone (we should say “read by the microphone), it comes out in the form of electrical current tension values. Those values are usually very small since the air pressure moving the membrane is not very strong: on average, 0.1 mV for a ribbon microphone and 10 mV for a dynamic microphone; guitar pickup levels are higher at 150-200 mV (see reference  for details).
Audio equipment is designed to work with much higher voltages (around 1V, the line level we previously defined), so this electrical current must be amplified. That is the role of preamplifiers, or preamps or pres (pronounced “preez”): help the signal gain power. How these devices amplify the signal is important since we know that any electronic component put in the electrical flow will alter the original signal. For a detailed discussion, read this document.
A preamp consists in a chain of electronic components. First, the input, usually a (balanced) male XLR input since that is what microphones usually use as outputs. The next set of components can be put under the banner of the input section and is optional, depending on the preamp model; some have these components (and more), and some have none. The first component in the input is usually a phantom power source for condenser microphones. The next component might be a polarity switch which would allow you to flip the polarity of the incoming signal. Then, a pad switch might allow you to make the source quieter if it is too loud by inserting a set of coupled resistances in series in the circuit. Finally, a resistance inserted in parallel with the circuit might be available to change the impedance of the circuit; the switch itself might be labeled “Low Z”, for example.
Once these formalities have been executed, the real deal starts with the main gain stage and the actual increase in tension for which the preamp is built. This increase is achieved by inserting in the circuit a series of electronic components such as transistors, vacuum tubes, operational amplifiers (op-amps) and/or a combination of all the above, each raising the gain by some fixed amount called gain stage.
These components can change the way the signal is shaped and add frequencies that the original signal did not have: that is where the secret sauce of mic pres is made and a jealously kept secret. If those effects are not desired (a so-called clean signal is preferred), a copy of the original signal is inserted after the gain stage and mixed with the processed signal to try and get rid of the unwanted effect(s); this works because the gain stage flips the polarity of the signal (that is how transistors work, see this article for more details). Finally, the output buffer takes the amplified signal and feeds it to the next device in the signal chain. Other components such as a trim switch might help you adapt the pre’s output to the next device input sensitivity.
One such line of components, built to amplify an analog signal, is called a channel. You will hear engineers talk about “4 channels of input”: this means that they can record four analog sources and amplifies those signals simultaneously.
Preamps can be classified in multiple fashions, but the most useful one is the one which categorizes them by what type of amplifiers are used for the main gain stage: solid-state or tube. Solid-state devices are devices which have current flow only through solid matter, by opposition to tube devices which have current go through vacuum tubes. Solid-state amplifiers include all types of transistors, bipolar junction transistors, field-effect transistors, and op-amps; these transistors can be put on integrated circuits (IC) or separately integrated in the signal flow (the famous discrete design which minimizes interferences between signals in the device itself). There are preamps which gain stage the signal through a solid-state amplifier before sending it through a tube; technically speaking, they are a solid-state preamp.
On the topic of tube vs. solid-state amplifiers and which ones sound the best, we are touching upon religion, much like in the analog vs. digital debate. The only sensible thing which can be said is that people favor the tube / analog sound because those designs produce mostly even harmonics which are musical to our ears (the famous warmth that people talk about). Tube amps are usually both noisier and less accurate than solid-state amps, but we have learned to like the “old” sound better and that is what we now expect. I strongly believe that if we were to present both amplified sounds next to the source to a Martian audiophile, our Martian friend would prefer the solid-state / digital sound for its better accuracy.
In the table below, classic preamps are listed. They are chosen because they were made from respected and well-known manufacturers. The features column contains the number of channels (“c”), the type of electronics inside (“SS” for solid-state and “T” for tube) and the design (see below). The last column gives an indication on how most people perceive the sound coming out of the preamp – do not quote me on that!
|A Designs||Pacifica||2c, SS, A||$2100||$1050||Thick|
|API||3124+||4c, SS, AB||$2800||$700|
|Avalon||AD2022||2c, SS, A||$3000||$1500|
|BAE Audio||1073MP||1c, SS, A||$1000||$1000||Neve 1073 clone|
|FMR Audio||RNP 8380||2c, SS, A||$450||$225||Clean|
|Focusrite||ISA One||1c, SS||$500||$500||“Colored”|
|George Massenburg Labs||GML 8304||4c, SS, A||$3600||$900||Clean|
|Golden Age||Pre73 MK II||1c, SS, A||$350||$350||Neve 1073 clone|
|Grace Design||m101||1c, SS||$765||$765||Clean|
|Great River||MP-2NV||2c, SS, A||$2275||$1138||Neve 1073 clone|
|Neve||1073||1c, SS, A||—||—||Big, punchy|
|Telefunken||V72||1c, T, A||$1300||$1300||Thick|
|True Systems||P-Solo||1c, SS||$600||$600||“Colored”|
|Universal Audio||LA-610 MKII||1c, T||$1600||$1600||Warm|
Another classification considers the design of the internals of the preamp. There are three main designs: Class A, Class B and Class AB. Class A designs signal the fact that both the positive and negative voltages are simultaneously amplified. This means that the output signal’s shape is very close to the original (no distortion), but also that the device will run very hot since it is functioning all the time; this means more risk of failure. Class B solves this problem by having two distinct amplifiers, one to amplify the positive and one the negative voltages; the disadvantage is that at the crossover between both sides, the signal may be distorted since the amplifying devices take some small time to start working; thus, they are perceived to be less accurate than their Class A counterparts. The third type, Class AB, tries to take the best of both worlds by having the following amplifier starting up before the crossover is reached so that when it does, the distortion is minimized. As you can see, Class A does not mean that it is the best class, only that it is one design amongst others.
Manufacturers have been very creative in bundling different devices together in a single cased product, appearing as a single device to the unsuspecting consumer. Such bundles include the audio interface, which consists in one or more mic preamps, along with AD and DA converters. Another bundle is the channel strip, putting together mic preamps and outboard gear (see here) such as EQ (EQ stands for equalizer) and compressors.
To close this short summary of how preamps work, I have always wondered what the difference was between, on one hand, turning the knob on the preamp clockwise, and on the other, turning the knob on my speakers clockwise. To me, both knob turning exercises achieved the same goal: making the music louder. As it turns out, the difference is significant: the knob on the preamp amplifies the signal before processing: it gives the processors down the chain more “meat” to work with. The knob on the speaker only amplifies the signal after processing.
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