Mic Preamps
Preamp specifications
 Impedance
o What load is presented to the microphone?
o Should be at least 10x the impedance of the microphone
 Mismatched impedance =
drop in level and some effect to frequency response
 Noise (Equivalent Input Noise)
o How much noise does the preamp introduce on its own?
o Should be less than -110 dB
 Maximum Gain
o How much amplification can the preamp provide?
o Should be at least 60 dB
 Might need to engage the pad to acheive this
 Maximum Input Level
o How much input can the preamp receive from the source before
distorting?
o Should be around +4 dBu (professional line level)
 Might need to engage the pad to achieve this
 Distortion
o THD+N (Total Harmonic Distortion plus Noise)
o How much distortion does the preamp introduce at MIL?
Biasing for high-gain/low-noise
 Preamps are usually the noisiest part of your signal chain because they are
amplifying a very small signal by a significant amount
o And they are amplifying after a series of resistors, which are subject to
thermal noise (unless your recording in absolute zero)
 For lower noise, we apply DC bias to the circuit
o Adjust resistors on the bias circuit, listen to output, tweak ‘til the noise is
lowest
 Transistor tolerances and durability. Temperature is a thing!
Preamp Specs & Schematics
 Yamaha O1V96
 http://www.gyraf.dk/schematics/schematics.html
 http://www.jensen-transformers.com/schematics/
 Benchmark/Burdick/Cohen History of the Active Mic Preamp
Mixers
Take many of inputs, combine and route to collected outputs
Components of a common mixer:
 Inputs
o Line (no preamp) or Mic (preamp, with gain control and sometimes pad,
phantom power, and phase reverse)
o Sometimes switchable between line and mic (like on the Mbox 2 Pro)
 But if not, beware of mismatched impedance and the respective
drop in level and change in frequency response!
 HPF (high pass filter)
o Cuts low frequencies to compensate for rumble, proximity effect and
general gunk that can clog up your mixes
 Insert
o Analog output and input so you can add an external processor
(compressor, equalizer, gate, etc.) to the signal chain
 3 or 4 band equalizer
o Semi-parimetric (sometimes they have sweepable mids)
 Aux sends
o PRE-FADER: for monitoring, recording and broadcast feeds
o POST-FADER: for effects processors like reverb and delay
 Channel Assignment Switches
o Tells the output of the channel strip where to go. To the Left/Right
masters? To a bus or submix? Maybe both!
 Pan Pots
o Depending on your channel assignment, send the signal from left to right,
and/or odd to even
 Fader
o How much of the channel output should go to the assigned destination?
o 0 = Unity Gain: the point at which the fader is not amplifying or
attenuating, and the point at which it will sound the best
 Busses/Submasters
o Take all your drums and send them to a submaster or bus so that you can
control them all at once BEFORE sending them to the master output
o Sometimes these also have insert points!
 Master Fader
o Controls the output to the world
o 0 = Unity Gain: the point at which the fader is not amplifying or
attenuating, and the point at which it will sound the best
 Aux masters
o Controls the output to the world FROM THE AUX SENDS
 Aux returns
o These are very simple line level inputs for bringing back signals from
effects processors (like reverb and delay) that you might have sent signal
to from the Aux Sends
How are digital mixers different?
 Input & Gain control, then all digital!
 Set conservative levels so you don’t clip the A/D convertor
How is the Pro Tools mixer different?
 Input & Gain control is on the Mbox, or on the Digital Mixer in the case of the
rolling rack
 You build a virtual mixer with all the inputs/inserts/busses/auxes/masters you
need
 0 on the fader doesn’t really mean much, except that you generally want to avoid
going past it to prevent overloading the busses with many tracks
How are large format consoles different?
 Big analog & digital consoles operate on the same principles as their smaller
counterparts
 Inline consoles have two faders for every channel strip:
o One that controls the individual track recording levels to tape (or
computer)
o One that controls the master output just like smaller consoles
Next Steps: build your own mic pre?
ICs for Mic Pres
 TI/Burr Brown PGA-2505
o Digital control
 TI/Burr Brown INA-103
 Analog Devices SSM-2017
 (power supplies are hard  )
DIY Kits
 http://www.diyrecordingequipment.com/
 https://seventhcircleaudio.com/
 http://www.jlmaudio.com/shop/mic-pre-amp-kits.html
 http://www.soundskulptor.com/uk/index.html
Careers at:
 Fishman
 Earthworks
 API
 THAT
 Analog Devices
 Crowley & Tripp
Referneces:
http://www.mil-media.com/preamps.html
https://sites.google.com/a/vt.edu/amp_lab/projects/high-gain-low-noise-microphonepreamplifier
http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf
http://www.ti.com.cn/cn/lit/ds/sbos003/sbos003.pdf
http://www.ti.com/lit/ds/symlink/pga2505.pdf
http://newscenter.ti.com/index.php?s=32851&item=126578
http://www.soundonsound.com/sos/mar99/articles/20tips.htm
http://www.rane.com/note148.html
http://www.allaboutcircuits.com/worksheets/class-a-bjt-amplifiers/
http://electronicdesign.com/analog/understanding-amplifier-operating-classes