Microphone Pre-Amps...

 

A mic pre is a preamplifier for the weak signals that come from microphones.  The preamplifier boosts the signal to a standard line level, which all recorders can capture.  How to hook up a preamp?  Simple.  Every stand alone preamp has a line level output, to which you connect to the line level input of your mixer or audio interface.  

Let there be no doubt that with the Mic pre you pay for sound quality.  That is what the whole game is about!  But the preamp alone is not wholly responsible for quality, it is just one component.  Your a\d converters, the cleanliness of your signal paths, the acoustic properties of your recording room, your choice of microphone and choice of monitors (so you can hear the fine differences) are all critical to quality. Those who want "the best" sound quality might spend $4,000 on preamps, $3500 on converters, $1500 on an audio interface and $5,000 on mics and $3,000 on room treatment.  That's $17,000 just to get into the pro ballpark, still far from "the best", and we haven't included the cost of monitors, DAW, software or cables.  So my first bit of advice is to scale your choices to your overall budget.

It takes about $500 to get a dual preamp (and about $300 for a single) that is definitely and noticeably better than the "average" preamps on audio interfaces and mixers. But will it give you the "Pro Sound".  The rock bottom for that is probably $1000.  You may realize several times--you are Not Ready for this plunge.  OK, we understand. Just work on your audio skills.  Huh?  Again, position the mic better, use a pop filter, set a better useable level, position the monitors better, improve the vocalist's techniques, fix the room with whatever you have to fix it with, learn how to mix a vocal so it stays above the band, learn compression, applying eq, reverb, sends, returns.  In a word, experiment.   This approach will give you much more sound quality than replacing one mediocre preamp with another.  In a way, you guys that can't afford a great preamp now are blessed. 

Applying gain to the microphone signal is the main job that the preamp does.  Ideally, the preamp amplifies the signal and does not add any additional noise or hum of its own.  The tell-tale sign of a poor preamp is that you hear lots of internal hiss as you raise the gain all the way up to 60db. You turn up a hi quality preamp the same amount and the hiss is very faint, you can barely hear it.  Everything is loud and crystal clear. That is what we want.

A problem with many mixer preamps is that hum can creep into the recording bus from other equipment connected to the mixer.  That is why the best preamps are not on mixers.  A good, stand alone preamp will amplify all sorts of sounds coming in from the mic which may be noisy, such as air conditioning vents, your refrigerator in the kitchen, traffic noise a block away, and breathing in the room but won't add any more noise problems.  While the poor preamp often buries these low level sounds in hiss (and just lowers the quality of everything going through it),  a good preamp exposes environmental problems and allows you to fix some of them. You might find yourself moving the mic away from the vents, closing the kitchen door, telling the vocalist to move their head slightly to the side, turning off the guitarist's buzzing amp, throwing a blanket over the computer.   See, you can use the quietness of a great preamp to diagnose and fix problems before hitting the record button.  This is one way a great preamp can improve your recordings.   Its like you suddenly turned off that buzzing in your brain and can now hear things as they are. 

Preamps come in different formats.  Some have simply one single mono preamp in the box.  People who record only 1 track at a time don't always realize that even with just ONE mono preamp of quality you can sound tremendously better.  There are also units with dual preamps, which are effective for recording with 2 mics in stereo.  There are also units with four and even eight preamps in one box.  How many you need, of course, depends on what you are recording.  Vocals are usually recorded in mono, while acoustic guitars are sometimes recorded in stereo.  Recording a drum kit requires at least 3 preamps and some use a lot more.  Recording a whole band at once might require one for each member's vocal, one for their amps and several for drums.  It does make a lot of sense to have one great preamp for vocals and a good stereo preamp.  If you are on a strict budget, consider the 2 channel preamp first, as you can use just one side of it in mono for your vocal and still have stereo capability.  

Digital outputs

By no means should you consider that because a preamp that has a/d converters it is sonically superior. The classic "wire plus gain" preamp design is analog in, analog out with no digital signal path.  More and more, though, as time marches on, a\d converters are being added to mic preamps.  The true benefit of digital outputs on the preamp is that you can bypass the a\d going into the audio interface.  Your digital signal, after it leaves the preamp, stays digital all the way to the storage medium.  Can you tell the difference?  Depends.  If your soundcard has inferior converters, they are best to be bypassed whenever possible.  Of course, if your monitors do not let you hear fine details, you'll probably not hear much difference.     

To state it in different terms, it only makes sense, sonically, to use a digital out on a preamp if it's converters are better sounding than the ones on your audio interface.   If you bit the bullet and bought $4,000 stand alone converters you'd be a bit daft to bypass them in favor of inferior converters on the preamp.  The only exceptions are matters of convenience or if the preamp has some form of digital processing inside.  If the signal is being converted to digital inside the box, then bypassing the d\a out of the preamp and the a\d into the audio interface is a good thing.  In terms of convenience, in the multi-channel preamp, you may see an ADAT option, which can send up to 8 amplified mics at once bypassing, yep, 8 converters on the audio interface. It can be convenient to have one thin lightpipe cable instead of 8 fat XLRs connecting the preamps to the audio interface.  Or if you have an audio interface with an unused ADAT input, it makes a lot of sense to add an 8 channel preamp with ADAT output.
 

Onboard compressors, EQs, de-essers, limiters and gates

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Do you need these extra features on your preamp?  Many say no.  And  most of them are professionals.  But I urge you to examine this question for yourself and your studio environment.  Those with a professional studio room that is pristinely quiet with a large selection of mics and outboard processors truly don't need these items.  The homeys in their  converted bedrooms with only one mic an no outboard processors may benefit greatly from them. 

With 24 bit recording, there is more "room" for recording an audio signal.  Compression or limiting may not be necessary going in as it can be effectively added later by software, and many software compressors do a better and more accurate job later in the mix.  Once you compress going in you are stuck with it.  There is no "undo" after hardware compression.  However, when recording live to 16 bit media, the equation changes.  There is less headroom in a 16 bit system and if you record your level too low you will have to amplify the system noise to bring up the level during a mix.  A little compression before hitting the recorder can give you an optimally leveled track.   If you record to analog cassette tape a compressor is practically required to get a decent track.   Recording live shows or field recording also benefit from compression as you can never control for abrupt volume changes that can go through the roof.  Finally, some instruments are so peaky by nature you know you want compression to even out the signal.  Acoustic guitars and drums fall into this category, even when recording at 24 bit.

Note that some preamps allow you to add an external compressor rather than supplying you with one onboard.  In this case, on the back of the preamp there will be insert jacks.  No inserts? You can also route the line out of the preamp to the line in of a compressor and feed the compressor output to a recorder.  You can keep your signal balanced this way.  With inserts, you are forced to go unbalanced.

A gate can be an effective tool.  A gate will lower the signal automatically when it drops below a defined threshold.  If you have a great, quiet room there is less or no need for a gate.  But many of us do not have that perfect room in our home studios and have to contend with kids and birds screaming in the yard, hard drives grinding away, trucks going down the street and distant aircraft, all of which will be faithfully recorded if you have a nice mic and pre.  A well-setup gate can dramatically improve the signal to noise ratio, providing the illusion of recording in a dead silent room.  Poorly setup it can make a track sound unnatural, with the breathing of environmental noise coming in and going out.  The true solution is to improve the room, but for those of us who cannot, the gate can improve a track immensely.

There are workarounds to having a gate.  You can always "zero-out" data in your sequencer's audio editor with a silence command.  This arguably, is superior to gating because with the mouse and screen at high zoom levels you can zero out noise with surgical precision.

A de-esser is a variable eq circuit which kicks in when a threshold of a certain focused frequency is reached.  It is used to reduce sibilance, the "ess" and "shh" sounds that some vocalists cannot control.  Often in a recording situation, it is better to try a different mic than turn on a de-esser.  You might also position the vocalist slightly off axis to reduce these sounds.  It is also possible to eq them out later or apply a software de-esser.

EQ, or using an equalizer on the source, is something that can usually be done later in the mix.  Software equalizers are at a high state of the art.  if you are recording live to direct to media feeds where there is no chance for post production, the EQ can save a recording.

Hi pass filtering (also called bass rolloff) is important as many mics pick up an astounding amount of bass.  You can do this at the mic, on the preamp, or later in the mix but it is often better to attack this problem at the source, the Mic.  If the mic doesn't have a rolloff, then the pre's rolloff can be used. 
 

Summing up

So now its back to you.  What features do you need?  You back through the article and make a list of the features you want and then go compare your list with the features on the preamps available. You should be able to identify exactly what you want, and hopefully you will find a product that suits your needs. OK, I hope i have helped you identify some issues that surround the selection of the ideal mic preamp for your studio.  Indeed it is a major step towards improving your sound quality.  I hope i have demonstrated that the preamp alone is not the solution, but simply one that puts excellent sound quality within our reach.