September 14Th - Microphone Design Considerations; Mic Preamps

September 14th - Microphone design considerations; Mic Preamps Note: Pg.1 was to be presented in the afternoon class, pgs 2+3 in the evening, but they have been combined because we ran out of time this afternoon

Level & Impedance Mic level ~5-50 mV, 50-200 Ohm (low) impedance Instrument Level ~50mV - 1 V 15,000 Ohm (high) impedance Line level ~50mV - 2 V 100-1500 Ohm impedance Speaker level ~50-100 V 4-16 Ohm impedance More on levels

How do we match all these disparate levels? Mics/Instruments go into preamps, which boost the signal to line level then Line level devices go into power amps then Power amps go into speakers then Boom goes the dynamite

How do we match all these disparate impedances? Bridging Setup = input impedance should always be at least 10x source impedance -to help avoid loss in level -to help avoid cables impedance lending a significant impact -e.g.: mic preamp inputs are usually around 1500 ohm to match the 50-200 ohm output of microphones

Some vintage equipment doesn’t play along. Everything used to be matched to 600 ohms (except power amplifier outputs and speakers)

What does that sound like when you attach vintage equipment to modern devices? -impedance changes depending on frequency, so frequency response will change -output level from the source will be affected -this means we’ll have to turn the preamp up more, and that means MORE NOISE Polar Patterns  Measurement & Variables  Proximity Effect http://www.soundonsound.com/sos/sep00/articles/direction.htm

Different types of designs and construction methods  Dynamic diaphragms, housings, schematics and intended uses  Ribbon diaphragms, housings, schematics and intended uses & hazards  Condenser diaphragms, housings, schematics and intended uses

Additional considerations  Diaphragm o Small o Large o Single-element o Dual-element  Directionality o Proximity Effect  Housing  Windscreen/Headbasket o Pop filters o Reflexion filters o Anything near a microphone will color the sound of a microphone . Remember the music stand!  Electronics o Transformer vs. transformerless  What is it’s intended purpose?  Durability

Microphone Specifications  Variables in impedance and measurement  Frequency response  Self noise  Sensitivity  ‘Max’ SPL

Comparing microphones quantitatively and qualitatively  Recordinghacks.com  bradfordswanson.com/mic  3Daudioinc.com Mic Preamps Preamp specifications  Impedance o What load is presented to the microphone? o Should be at least 10x the impedance of the microphone . Mismatched impedance = drop in level and some effect to frequency response  Noise (Equivalent Input Noise) o How much noise does the preamp introduce on its own? o Should be less than -110 dB  Maximum Gain o How much amplification can the preamp provide? o Should be at least 60 dB . Might need to engage the pad to acheive this  Maximum Input Level o How much input can the preamp receive from the source before distorting? o Should be around +4 dBu (professional line level) . Might need to engage the pad to achieve this  Distortion o THD+N (Total Harmonic Distortion plus Noise) o How much distortion does the preamp introduce at MIL?

Biasing for high-gain/low-noise  Preamps are usually the noisiest part of your signal chain because they are amplifying a very small signal by a significant amount o And they are amplifying after a series of resistors, which are subject to thermal noise (unless your recording in absolute zero)  For lower noise, we apply DC bias to the circuit o Adjust resistors on the bias circuit, listen to output, tweak ‘til the noise is lowest  Transistor tolerances and durability. Temperature is a thing!  More information

Preamp Specs & Schematics  Yamaha O1V96  http://www.gyraf.dk/schematics/schematics.html  http://www.jensen-transformers.com/schematics/  Benchmark/Burdick/Cohen History of the Active Mic Preamp