Transcript evening class - Tufts University

Microphone and Preamp
Design
Tufts University – ES 65
September 15, 2016
Evening 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
Microphone Specifications
• Variables in impedance and measurement
• How specifications are measured
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Frequency Response
Self Noise
Sensitivity
‘Max’ SPL
THD
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
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
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
• Local companies working in transducer and analog
systems design
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Fishman
Earthworks
THAT, Analog Devices
Soundwave Research / Crowely & Tripp