Microphone and Preamp Design Tufts University – ME 93 September 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 Level 5-50 mV Line Level 50 mV – 2V Speaker Level V Preamp Power amp

Impedance Ω Output 1500Ω Input 4-16Ω Output 100Ω Output 10,000Ω Input Microphone Preamp/Console/Interface Mic Level Input Line Level Output Power Amp Line Level Input Speaker Level Output Bridging Setup = 10:1 Load Impedance ≥ 10(Source Impedance)

Instrument Impedance ~15,000Ω Output 1500Ω Input 100Ω Output Instrument Preamp/Console/Interface Mic Level Input Line Level Output 100MΩ Input 100Ω Output 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:

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 Source: Earle, Pg. 110 Normal Sensitivity Ranges by Use Close-in, Handheld 2-8 mV/Pa Normal Studio Use7-20 mV/Pa Distant Pickup10-50 mV/Pa Output Sensitivity of Common Microphones MD4212 mV/Pa AT mV/Pa DPA/B & K 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 dB/A weighted DPA/B & K dB/A weighted Shure KSM dB/A weighted TLM dB/A weighted Ribbons & DynamicsNot 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 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 Ribbons1-3 Microns Condensors2-6 Microns Tympanic Membrane Microns Dynamics>200 Microns Source: Product Specifications,

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 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