1. Microphone and Preamp Design Tufts University – ME 93 September 17, 2015

2. 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

3. Signal Level Mic Level 5-50 mV Line Level 50 mV – 2V Speaker Level 50-100V Preamp Power amp

4. Impedance 50-200Ω 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)

5. 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

6. Microphones Directionality or Polar Pattern Transducer mechanism & electronics Diaphragm size Housing Windscreen/Headbasket Electronics Intended Purpose

7. Polar Patterns

8. 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

9. 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?”

10. Transducers & Electronics Dynamic Ribbon Condensor Vacuum Tube

11. Dynamic

12. Ribbon

13. Condensor

14. Microphone Specifications Variables in impedance and measurement How specifications are measured Frequency Response Self Noise Sensitivity ‘Max’ SPL THD

15. Frequency Response Source: www.shurenotes.com

16. 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?

17. 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-406019.8 mV/Pa DPA/B & K 400632 mV/Pa Source: Product Specifications

18. 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?

19. Equivalent Self Noise Self-Noise of Common Microphones AT-406019 dB/A weighted DPA/B & K 400615 dB/A weighted Shure KSM-14114 dB/A weighted TLM-103 7 dB/A weighted Ribbons & DynamicsNot usually specified Source: Product Specifications

20. 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…

21. “Max” SPL Measurement: 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

22. Diaphragm construction Diaphragm Thickness of Common Microphones Ribbons1-3 Microns Condensors2-6 Microns Tympanic Membrane30-120 Microns Dynamics>200 Microns Source: Product Specifications, http://audilab.bme.mcgill.ca/AudiLab/ear.html

23. 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

24. Subjective Comparisons Recordinghacks.com 3D Audio Inc. bradfordswanson.com/mic

25. Preamp types Discrete Semiconductor Vacuum Tube Integrated Circuit Hybrid Transformer vs. Active Balancing

26. 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

27. Preamp input section

28. Low-noise preamp circuit

29. 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

30. 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