Preamps and Analog MixersTufts University – ME 93October 1, 2015

Outline• Review signal level and impedance • Preamp construction and variables• Mixer construction and variables• Classifications and specifications of preamps &

mixers• Next steps

Signal Level

Mic Level5-50 mV

Line Level

50 mV – 2V

Speaker Level50-100V

Preamp Poweramp

Impedance

50-200ΩOutput

1500ΩInput

4-16ΩOutput

100ΩOutput

10,000ΩInput

Microphone

Preamp/Mixer/InterfaceMic Level Input

Line Level Output

Power AmpLine Level InputSpeaker Level

Output

Bridging Setup = 10:1Load Impedance ≥ 10(Source Impedance)

Preamp SpecificationsImpedance > 1000 Ohms • What load is presented to the microphone?

Noise (EIN) < -110 dB (relative to +4 dBu)• How much noise does the preamp introduce?

Maximum Gain > 60 dB • How much amplification can the preamp provide?

Maximum Input Level > +4 dBu (Vrms)• How much input level can the preamp receive from the

microphone before distorting?Distortion (THD+N) Below 0.05%• How much distortion does the preamp introduce at MIL?

Preamp Specifications

Preamp input section

Low-noise preamp circuit

Preamp Biasing• Preamps are often the noisiest part of a signal

chain• Thermal noise• Be aware of component tolerances and durability

• DC Bias helps achieve low noise• Adjust resistors while listening to the output• Tweak values of R1 & R2 until the noise is low

8x4x2 Mixer signal flow

INPUTs

1

2

3

4

BUSSES

MASTERS

L

R

Channel Assignment

s

Pan Pots(L/Odd)(R/Even)

Faders

OutputAssignment

s

Pan Pots(L/Odd)(R/Even)

Faders

CR1604 Channel Strip

Digital EQ, inserts, routing, mixing and logic

01V96 Channel Strip

01V96 Channel Strip

Pro Tools Channel Strip

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