Here are some quick notes I took from various sources...
Inductor – pass high, block low
Capacitor – block low, pass high
Both – creates mids
Passive crossover network – no power supply needed to operate: amplified by external source first
Passive units waste power – cause more heat than sound and can’t be made loud
Passive system, like monitor speakers are usually passive because they don’t need to be loud
When passive and active systems are combined, passive is used for the tweeters (especially the very high frequency ones) while active is used for the mids/lows
Impedance (Resistance with frequency sensitivity) needs to be near 0 in the wires and the load impedance must be fixed for a perfect passive system
Back-emf also plays a role – to stop, the speakers need near 0 ohms resistivity, but that’s impossible with a passive system
Inductors saturate at high frequency levels, causing distortion. Inductors also pick up other signals very well, mixing them into your audio – cell phones cause distortion in speakers
Active crossover networks fix most of the problems – tucked away safely
- No power loss, due to the active circuit having its own power supply
- Far more efficient, allowing louder music
- Damping is control of loudspeaker’s cone
- dB/oct = slope rate
- Modern speakers use 3
rd order (18 dB/oct) and 4
th order (24 dB/Oct) are the most common
- High frequency is more efficient than low frequency
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Ground loops – creates a closed loop
- Current creates a hum when it passes through an audio ground signal
- Balanced interconnects reject ground loop the best
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- Balanced equipment usually designed wrong - cost and size
- Unbalanced systems are used for home audio – currents exist, but don’t get large enough
- Random stuff in the circuits (rails, chassis, etc.) have very low impedance (resistance that affects frequency) in the thousandths of an ohm
- Cable shielding needs to be used to avoid interference… all the way up to the metal chassis
- Balanced and unbalanced systems are incompatible – causes more hum
- Signal ground isn’t used because of how much unbalanced audio equipment exists
Connecting unbalanced + balanced
- Transformer Isolating all unbalanced connections
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- - requires internal grounding
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- Alternate – remove shielding at one end – decreases low frequency noise
- Also increases high frequency noise since shield can act as antenna (RF interference)
Summary
- Unbalanced cables should be less than ten feet/3 meters
- Balanced > transformer isolation > special cable assemblies
Digital Analog Conversion
- When converted to digital, nothing can change sonic properties
- 16-Bit is CD quality (Resolution = 2^16 = 65,536) Sampling rate is usually 44.1 kHz
- 24 bits (16,777,216) sampling rate 96 kHz is usually the highest it goes – differences extend out of human hearing
- Oversampling increases noise generated – white noise (same across all frequencies)
- Noise shaping moves the noise generated out of the audible range and into the inaudible range
- Dithering causes a tremble by switching between quantization levels drastically lowers distortion levels
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Vacuum Tubes vs Solid State (transistors)
- For amps, this difference is important
- Vacuum has little negative feedback, more musical than solid state, high tolerance against overloads, higher voltages means higher dynamic range, mostly independent of temp, capacitance varies very little with signal voltage, simpler circuit designs, Class A or AB operation (minimizes distortion), Output transformer protects speakers from tube failure, easier maintenance since tubes are easily replaced
- Bulkier (impractical for portable use), high voltage required + high power consumption, lots of waste heat, lower power efficiency, tubes are fragile, more prone to microphonics (distortion heard), shorter lifetime for tubes (1-5 years), higher cost
- Transistors have lower cost, smaller size, can be made into an integrated circuit, lower power consumption, less waste heat, low voltage,
- Higher distortion, complex circuits required for low distortion, sharp clipping (overdriven amplifier), capacitance varies with voltage, large variations between amps, stored charge effects cause signal delay, vary with temperature, less efficient cooling, Class B Totem circuits cause higher crossover distortion, less tolerant of overloads/voltage spikes, can damage speakers, capacitance coupling requires high-value electrolytic capacitors which have worse performance, pick up radio frequencies, not easily replaceable, older versions not available after 20 years
Noise Cancellation
- Two left boxes are the noise gathering -> noise cancellation (inverts phase by changing the polarity of the voltage)
- Box on the right is where the two signals are summed up
- Below that is where the music gets amplified
Edit: the pictures didn't show up, but they aren't that important the information about noise cancellation was taken from
http://www.soe.rutgers.edu/sites/default/files/gset/silence.pdf
