1. Preamplifier and mixers, Power Amplifiers, and Speakers
Tufts University – ES 65 October 5, 2017

2. Preamp Specifications
Impedance > 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?
Impedance is measured with an ohm meter between the positive and negative leads of the input
EIN is measured
Maximum Gain is measured by determining the range between the least amount of amplification the preamp can provide (probably 0dB or neutral) and the highest amount of amplification it can provide
Maximum Input Level is determined by increasing the level of the source until the front end of the preamp begins to distort.
Total Harmonic Distortion
With a 1 kHz signal at the stated maximum input level, the sine wave is then notch filtered out. How much distortion and noise is left after that? The percentage difference in output with and without the filter equals THD+N.

3. Preamp Specifications

4. Preamp input section

5. Low-noise preamp circuit

6. 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
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10. What is a speaker?

11. What is a speaker?
A transducer converting electrical energy to acoustic energy just like an inverted dynamic mic

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

13. Signal Level - Voltage

14. What about Watts? P = E * I E I = R Power (P) Watts Voltage (E) Volts
Current (I) Amps
Ohms are a unit of Impedance (R)
P = E * I E R
I =
A microphone with a RMS voltage of: Volts
and an impedance of: Ohms
draws a current of: Amps
exerts an average power around: Watts

15. Signal Level - Watts

16. The Passive Speaker Cabinet
1 or more speakers
Crossover
Most be paired with an external amplifier
Port
Damping
Grille
The Cabinet

17. Passive Crossovers
Amplifier

18. Active Crossovers
Amplifier
Amplifier
Amplifier

19. The Active Speaker Cabinet
1 or more speakers
Active crossover
Has it’s own amplifier(s)
Port
Damping
Grille
The Cabinet

20. Speaker Specifications
Frequency Response
Power rating
dBSPL = 1 watt of 1 meter away
Impedance
Directionality and dispersion
Weight/Form Factor

21. Amplifier Specifications
Frequency Response
Power Rating
Dependent on impedance of the load
Bridging
Slew Rate
THD
Stereo 8 Ohms,
both channels driven……….240 W/ch
Stereo 4 Ohms,
both channels driven……….400 W/ch
Mono 8 Ohms,
Bridged………………..……800 W/ch

22. Impedance & Power 1 Znet =
Multiple loudspeakers will generally be connected in parallel
How do we calculate the impedance? 1
Znet = 1 Z1 1 Z2 1 Zn + +

23. Impedance & Power Only link speakers with the same impedance
divide Z by # of speakers or just keep dividing by 2 (conservative)

24. Parameters Passive Speakers Driver Attenuators Amplifiers
Input Sensitivity
Bridging
Filters

25. Best Practices Tweeters don’t like clipping
Woofers don’t like DC/over-excursion
Amplifiers should have more power capacity than the speakers
Turn the amplifiers up so you don’t clip

26. Next Steps Speaker manufacturers close by EAW Bose Sonos Genelec
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