Microphone and Preamp
Design
Tufts University – ME 93
September 17, 2015
Outline
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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
Level
5-50 mV
Preamp
Line
Level
50 mV – 2V
Power
amp
Speaker
Level
50-100V
Impedance
Microphone
Preamp/Console/Interface
Mic Level Input
Line Level Output
1500Ω
Input
50-200Ω
Output
100Ω
Output
Power Amp
Line Level Input
Speaker Level
Output
10,000Ω
Input
Bridging Setup = 10:1
Load Impedance ≥ 10(Source Impedance)
4-16Ω
Output
Instrument Impedance
Instrument
DI Box
100MΩ
Input
~15,000Ω
Output
Preamp/Console/Interface
Mic Level Input
Line Level Output
100Ω 1500Ω
Output Input
100Ω
Output
DIs can also:
• Convert unbalanced to balanced
• Reduce level or ‘Pad’ input signal
• Lift ground to eliminate loops & hum
Microphones
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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 outof-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
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Dynamic
Ribbon
Condensor
Vacuum Tube
Dynamic
Ribbon
Condensor
Microphone Specifications
• Variables in impedance and measurement
• How specifications are measured
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Frequency Response
Self Noise
Sensitivity
‘Max’ SPL
THD
Frequency Response
Source: www.shurenotes.com
Frequency Response
Measurement:
• 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
Normal Sensitivity Ranges by Use
Close-in, Handheld
2-8 mV/Pa
Normal Studio Use
7-20 mV/Pa
Distant Pickup
10-50 mV/Pa
Source: Earle, Pg. 110
Output Sensitivity of Common Microphones
MD421
2 mV/Pa
AT-4060
19.8 mV/Pa
DPA/B & K 4006
32 mV/Pa
Source: Product Specifications
Output Sensitivity
Measurement:
• 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 Noise
Self-Noise of Common Microphones
AT-4060
19 dB/A weighted
DPA/B & K 4006
15 dB/A weighted
Shure KSM-141
14 dB/A weighted
TLM-103
7 dB/A weighted
Ribbons & Dynamics
Not usually specified
Source: Product Specifications
Equivalent Self Noise
Measurement:
• 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” SPL
Measurement:
• Specialty speaker-like device that can generate 130140 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 construction
Diaphragm Thickness of Common Microphones
Ribbons
1-3 Microns
Condensors
2-6 Microns
Tympanic Membrane
30-120 Microns
Dynamics
>200 Microns
Source: Product Specifications,
http://audilab.bme.mcgill.ca/AudiLab/ear.html
Air & microphones
Sound = AC
Air = DC
Common sources of direct columns of air
Kick Drum
Speaker Cones at High SPL
Plosives
Moving microphones
Wind (Natural & HVAC)
Escaping Air Column Instruments
Subjective Comparisons
• Recordinghacks.com
• 3D Audio Inc.
• bradfordswanson.com/mic
Preamp types
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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
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Fishman
Earthworks
THAT, Analog Devices
Crowely & Tripp