Amps Flashcards
DEF:
amps
electronic device for increasing amplitude of electrical signals, mainly used for sound reproduction
how does tube/valve amp work?
cathode (-) is heated and generates electrodes which are filtered by the triode (grid) and are pulled toward to plate
the grid is given charge and depending on + or - will either repel - electrons and stop them passing through or increse the number of + electrons, amplifying the sound
why is it important to bias the tube?
operating at the extremes will cause the output to be non-linear and distorted
DEF:
transistor (amp)
component capable of varying its resistance
how does a transistor work (amp)
“valve” placed across the collector and emitter
small control voltage placed at transistor’s input allows larger output current
Where is DC bias voltage applied?
base (valve) of the transistor
grid (triode) of valve/tube
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saturation (amp)
occurs when output DC voltage isn’t sufficient to amplify the input signal
harmonics from tube clipping vs transistor clipping
transistor - odd-order harmonics
tube - even-order harmonics (more musical)
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cutoff
when there is not enough electrons being generated by the cathode to be pulled by the anode/plate
what is an op amp?
operational amplifier with high gain and high bandwidth, used for a variety of audio and video applications
howis the op-amp’s gain reduced so it’s at a level you can work with
uses a negative feedback loop where a portion of it’s output is fed, through a resistor, back into it’s input out of phase, reducing the op amps output
varying value of resistor gives different output levels controlling the op amp’s gain
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equalizers (amp)
a frequency discriminating amplifier using capacitor/resistor networks in the op amp’s negative feedback loop allowing selected frequencies to be cut or boosted
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summing amplifiers
used to provide isolations between various inputs/outputs in a signal chain
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distribution amplifiers
amp that does not provide gain but increases current signals allowing it to be delivered to multiple loads
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VCA / DCA
voltage/digitally controlled amplifier
output voltage decreases as the control voltage increases
DCA - digitally controlled DC control voltage
Power amplifier classes
A
B
AB
C
D
T
EF
G
H
Class A power Amplifiers
output draws a constant high current from supply regardless of audio passing through or not
current used to bias transistors
inneficient due to heat generation from constant bias current, but transistors have a constant steady temperature
very high sound quality
low current class A used widely in audio circuits
Class B power Amplifiers
“pull-push” amps
no current passes through output unless audio is passing
driving signal is what biases transistors
very efficient and great for battery powered equp but at low signal levels they operate near cutoff point making sound quality distorted/degraded
pairs of transistors are used to handle separate halves (+/-) of waveform, as signal hits zero crossing amp gets crossover distortion
used when you don’t need high sound quality
Class AB amps
relatively low bias current flows at all times (Class A amp)
as input signal is increased transistors are biased w/ higher current to deliver more power to speakers but since transistors are constantly biased crossover distortion is avoided
majority of high quality amplifiers are AB
Class C amps
used for driving narrow band frequencies into resonant load
used for radio to drive signal to aerial
Class D amps
uses PWM
transistors are driven by ultrasonic wave w/ mark to space ratio varied by audio signal
power dissipation in transistors is minimal
run cool and highly efficient
low pass filtering required to remove square wave component of signal
Class T amps
from the Tripath Company
ultrasonic fequency is continually varied in accordance with input amplitude
very high efficiency
Class G amps
incorporates several different voltage rails that are used progressively as signal voltage increases
very efficient (usually use low rails only) and can be smaller than Class AB amps
Class H amps
variation on Class G but supply voltage rails track input signal continuously maintaining just enough headroom to accommodate the amps output requirements
Characteristics of MOSFET amp
low distortion
good thermal tracking
simple output design
greater tolerance of adverse speaker loads without complex protection circuitry
MOSFET
Metal Oxide Field Effect Transistor widetly used since 80’s in output stages of power amps
power amp sensitivity
how much voltage input is required to produce the amp’s max rated output
must be taken into account when considering what equip will drive the amp, must not be able to deliver more voltage than the sensitiity rating or clipping will occur
what is ‘bridged operation’
where both channels are fed same signal (usually L) but one channel’s polarity is reversed (usually R)
both channels process same signal and load is connected to draw power from both channels
will be refelcted in specs
mono will be double amt of W than stereo
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power bandwidth (power amp specs)
freq response bandwidth within which an amp can sustain a specified output
must be considered when matching amps to loades (ie. subs, HF horns)
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frequency response (power amp spec)
a measuer of the limits within which an amp can reproduce frequencies at a low power output
usually measured w/ amp delivering 1W into 8Ω load
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slew rate
a measure of amp’s ability to accurate react to high level transients
defined as Vus (volts per microsecond)
Slew Rate rule of thumb
Amps of 100 W shoul dhave slew rate of 10 Vus
amps of 200 watts should have slew rate of 30 Vus (min.)
higher power amps require higher slew rates as wattage swings are more severe
Excessively high slew rates should be treated w/ caution as they can be associated with too high a bandwidth which can allow amps to pass RF signals
distortion (power amp specs)
should be 0.1 THD across entire audible range
crosstak (amp specs)
should be at least -70 dB between channels in mid frequencies
- 50 dB at freq. extremes
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signal to noise ratio (amps)
how much hiss the amp has when it’s cranked up all the way
100 dB is common for moder amps
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damping factor (amps)
numerical value indicating how well an amp can control a speaker
the ratio between the amp’s output impedance and speaker’s impedance
damping factor is more useful when frequency is given
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phase response
measurement of how well the high and low freq extremes keep in step with mids
15 degree phase lag at 20Hz and 20Khz is common
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direct coupled (amps)
output power transistors are directly connected to the speakers, except maybe low valued resistor and inductor in-between
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transformer coupled amp
transformer buffers the output and the amp uses split voltage rails of opposite polarity to have no DC output present at output terminals
+/- 40m V is acceptable max
15mV > is common
speakers connected in series
current flows serially from one speaker to next
impedance of series is equal to impedance of each speaker added together
speakers wired in parallel (impedance)
impedance is calculated like this:
pro/cons of speakers wired in series
pro:
if individual loads are low, series increases impedance
con:
if one speaker goes, series stops working
damping effect is greatly reduced
interaction between speakers is higher = more distortion
pro/con speakers wired in parallel
pro:
one speaker fail, the rest still work
con:
limited number of speakers can be connected before impedance drops below amp’s operating load
