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