microphone and preamp design tufts university – me 93 september 17, 2015
TRANSCRIPT
Microphone and PreampDesignTufts University – ME 93September 17, 2015
Outline• Signal level and impedance • Classifications and specifications of microphones• Microphone construction and variables• Classifications and specifications of preamps• Preamp construction and variables• Next steps
Signal Level
Mic Level5-50 mV
Line Level
50 mV – 2V
Speaker Level50-100V
Preamp Poweramp
Impedance
50-200ΩOutput
1500ΩInput
4-16ΩOutput100Ω
Output
10,000ΩInput
Microphone
Preamp/Console/InterfaceMic Level Input
Line Level Output
Power AmpLine Level InputSpeaker Level
Output
Bridging Setup = 10:1Load Impedance ≥ 10(Source Impedance)
Instrument Impedance
~15,000ΩOutput
1500ΩInput
100ΩOutput
Instrument
Preamp/Console/InterfaceMic Level Input
Line Level Output
100MΩInput
100ΩOutput
DI Box
DIs can also:• Convert unbalanced to balanced• Reduce level or ‘Pad’ input signal• Lift ground to eliminate loops & hum
Microphones• Directionality or Polar Pattern• Transducer mechanism & electronics• Diaphragm size• Housing• Windscreen/Headbasket• Electronics• Intended Purpose
Polar Patterns
Cardioid Construction
Sound from the rear is allowed in through the labyrinth but slightly delayed to make it out-of-phase when the sound arrives at the front of the diaphragm
Cardioid Proximity Effect• 6 dB/octave rise because of its construction• Diaphragm is damped to compensate and create
a flat frequency response at normal distance• As microphone gets closer to the source, the
inverse square law means more low frequency information overcomes this dampening
• See course pack“Why does proximity effect occur?”
Transducers & Electronics• Dynamic• Ribbon• Condensor• Vacuum Tube
Dynamic
Ribbon
Condensor
Microphone Specifications• Variables in impedance and measurement• How specifications are measured• Frequency Response• Self Noise• Sensitivity• ‘Max’ SPL• THD
Frequency Response
Source: www.shurenotes.com
Frequency ResponseMeasurement:• Swept Sine Wave• Point Source Loudspeaker• Anechoic Chamber
Frequency Response changes based on impedance• Testing load not standardized!• What load was your microphone tested with?
Output Sensitivity
Source: Earle, Pg. 110
Normal Sensitivity Ranges by UseClose-in, Handheld 2-8 mV/PaNormal Studio Use 7-20 mV/PaDistant Pickup 10-50 mV/Pa
Output Sensitivity of Common MicrophonesMD421 2 mV/PaAT-4060 19.8 mV/PaDPA/B & K 4006 32 mV/Pa
Source: Product Specifications
Output SensitivityMeasurement:• 1 kHz tone at 1 Pascal of pressure• Point-source loudspeaker• Anechoic chamber
Output level changes based on impedance• Testing load not standardized!• What load was your microphone tested with?
Equivalent Self NoiseSelf-Noise of Common MicrophonesAT-4060 19 dB/A weightedDPA/B & K 4006 15 dB/A weightedShure KSM-141 14 dB/A weightedTLM-103 7 dB/A weightedRibbons & Dynamics Not usually specified
Source: Product Specifications
Equivalent Self NoiseMeasurement:• Plug-in microphone, measure the noise in dBA
When you connect to a preamp, do you want to boost signal or noise?• preamps are usually noisier than mics anyway…
“Max” SPLMeasurement:• Specialty speaker-like device that can generate 130-
140 dBSPL without distorting• When the microphone electronics start to distort,
that’s your max SPL
This only measures the electronics!• You still don’t know when your diaphragm will get
damaged
Diaphragm constructionDiaphragm Thickness of Common MicrophonesRibbons 1-3 MicronsCondensors 2-6 MicronsTympanic Membrane 30-120 MicronsDynamics >200 Microns Source: Product Specifications,
http://audilab.bme.mcgill.ca/AudiLab/ear.html
Air & microphonesSound = AC Air = DCCommon sources of direct columns of air
Kick DrumSpeaker Cones at High SPLPlosivesMoving microphonesWind (Natural & HVAC)Escaping Air Column Instruments
Subjective Comparisons• Recordinghacks.com• 3D Audio Inc.• bradfordswanson.com/mic
Preamp types• Discrete Semiconductor• Vacuum Tube• Integrated Circuit• Hybrid• Transformer vs. Active Balancing
Preamp Biasing• Preamps are often the noisiest part of a signal
chain• DC Bias helps achieve high gain & low noise• Adjust resistors while listening to the output• Tweak values until the noise is lowest• Be aware of component tolerances and durability
Preamp input section
Low-noise preamp circuit
Work with what you have• We have more than the Beatles had• Performance, placement, and gain staging are
more important than the gear• Use your ears, but be cautious of hype
Next Steps• The hardest part is the power supply…• ICs• Preamp Kits
• Jobs in transducer and analog systems design• Fishman• Earthworks• THAT, Analog Devices• Crowely & Tripp