1. Microphone and Preamp Design
Tufts University – ES 65 September 14, 2017

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

3. Impedance 10,000Ω Input 4-16Ω Output 1500Ω Input 100Ω Output
Power Amp Line Level Input
Speaker Level Output
Preamp/Console/Interface Mic Level Input Line Level Output
Microphone
10,000Ω Input
4-16Ω Output
1500Ω Input
100Ω Output
50-200Ω Output
Bridging Setup = 10:1 Load Impedance ≥ 10(Source Impedance)
Bridging Setup = 10 x
-to help avoid loss in level
-to help avoid cables impedance lending a significant impact
Older equipment doesn’t play along = 600 ohm matched thing
What does that sound like?
-what’s the difference between impedance and resistance? -impedance changes depending on frequency
-so frequency response will change
-and output level from the source will be affected
-this means we’ll have to turn the preamp up more, and that equals NOISE

4. Preamp/Console/Interface Mic Level Input
Instrument Impedance
Preamp/Console/Interface Mic Level Input
Line Level Output
Instrument
DI Box
100MΩ Input
100Ω Output
1500Ω Input
100Ω Output
~15,000Ω Output
DIs can also:
Convert unbalanced to balanced
Reduce level or ‘Pad’ input signal
Lift ground to eliminate loops & hum
Bridging Setup = 10 x
-to help avoid loss in level
-to help avoid cables impedance lending a significant impact
Older equipment doesn’t play along
What does that sound like?

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

6. Polar Patterns
-We are familiar with the "basic" polar patterns, but every microphone is slightly different
-How are these measured?
-Static sine-waves at 5-12 different frequencies, Point source loudspeaker, Anechoic chamber

7. 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
Image source:

8. 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?”
Image source:

9. Transducers & Electronics
Dynamic
Ribbon
Condensor
Vacuum Tube

10. Dynamic

11. Ribbon

12. Condensor

13. Diaphragm construction
Diaphragm Thickness of Common Microphones
Ribbons Microns
Condensors 2-6 Microns
Tympanic Membrane Microns
Dynamics >200 Microns
Source: Product Specifications,
Would you put your ear up next to a kick drum or guitar amp for four hours? Maybe don’t put your condensors through that either!

14. 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
DIRECT COLUMNS OF AIR ARE THE ENEMY OF MICROPHONES
Pop filters and windscreens diffuse the air before it can damage the diaphragm or create low-end crap in your signal

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

16. Impedance 10,000Ω Input 4-16Ω Output 1500Ω Input 100Ω Output
Power Amp Line Level Input
Speaker Level Output
Preamp/Console/Interface Mic Level Input Line Level Output
Microphone
10,000Ω Input
4-16Ω Output
1500Ω Input
100Ω Output
50-200Ω Output
Bridging Setup = 10:1 Load Impedance ≥ 10(Source Impedance)
Bridging Setup = 10 x
-to help avoid loss in level
-to help avoid cables impedance lending a significant impact
Older equipment doesn’t play along = 600 ohm matched thing
What does that sound like?
-what’s the difference between impedance and resistance? -impedance changes depending on frequency
-so frequency response will change
-and output level from the source will be affected
-this means we’ll have to turn the preamp up more, and that equals NOISE

17. Frequency Response Beta 58A Frequency response
Source:
Beta 58A Frequency response
-dotted lines represent different distances
-proximity effect compensation is built in!

18. 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?
-How is this measured?
-Swept sine wave, point source loudspeaker, anechoic chamber
-Distance is important
-Sometimes you will see measurements that represent on-axis and off-axis response (FIND SUPER-CARDIOID EXAMPLE), but often, manufacturers presume there is relative symmetry around the "on-axis" point. This is generally true for end address, but not necessarily for side address microphones!!

19. Output Sensitivity Normal Sensitivity Ranges by Use
Close-in, Handheld 2-8 mV/Pa
Normal Studio Use 7-20 mV/Pa
Distant Pickup mV/Pa
Source: Earle, Pg. 110
Output Sensitivity of Common Microphones
MD mV/Pa
AT mV/Pa
DPA/B & K mV/Pa
Source: Product Specifications
The higher the output, the less we have to boost the signal with a mic pre, so the less noise we might have.
Important to consider when choosing mics for distant pickup vs. close pickup

20. 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?
-How is this measured?
-1 khz tone, measured at 1 Pascal of pressure, from a point-source loudspeaker in an anechoic chamber, with an impedance that should be specified!! (generally, consoles and mic pres should have an impedance of >1k ohms - so it's standard to measure at 1k ohms). The "variable-impeadance" craze currently sweeping the audio industry (cloud, vipre, etc.) Generally, all modern microphones are designed to be "low-impeadance" to be complemented by a "high-impeadance" preamp input, but some of the older dynamic and ribbon microphones were actually high-impeadance, meaning they will not match well with modern mic pres. This should just be standardized and then we'd only need these impedance matching devices for vintage microphones!!

21. 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 & Dynamics Not usually specified
Source: Product Specifications
The higher the noise, the more of it we hear when boosting the signal with a mic pre. Do you want to boost signal, or noise?

22. 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…
-Equivalent Self-noise
-Important because if you are trying to pickup a quiet source, and you are using a lot of preamp gain, you don't want to boost the microphones inherent noise, you want to boost the signal picked up by the microphone!!
-However, most preamps will present noise before the microphone does
-We don't need to look at measurements, because they're simple decibel ratings and there are some general rules:
-Tubes will have the most noise (SHOW)
-Condensors will have some noise
-Dynamics and ribbons will have the least
-But these have lower output sensitivities so the preamp would need to be boosted more!!

23. “Max” SPL Measurement: This only measures the electronics!
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
-Equivalent Self-noise
-Important because if you are trying to pickup a quiet source, and you are using a lot of preamp gain, you don't want to boost the microphones inherent noise, you want to boost the signal picked up by the microphone!!
-However, most preamps will present noise before the microphone does
-We don't need to look at measurements, because they're simple decibel ratings and there are some general rules:
-Tubes will have the most noise (SHOW)
-Condensors will have some noise
-Dynamics and ribbons will have the least
-But these have lower output sensitivities so the preamp would need to be boosted more!!

24. Subjective Comparisons
Recordinghacks.com
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
Image source:

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
Local companies working in transducer and analog systems design
Fishman
Earthworks
THAT, Analog Devices
Soundwave Research / Crowely & Tripp