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In both editions, the left and right channel opamps are rail-to-rail, capable of swinging output
voltage to within 0.5V of each supply rail. This makes maximum use of the available battery
voltage, providing improved output clipping/overload headroom.
•
The opamps in all variants have very high slew rate, wide bandwidth, low output impedance
and low distortion.
•
All resistor values are chosen to reduce DC offset, minimize stray capacitance and
inductance (which may cause opamp instability), and to maintain low noise.
•
Low value resistors at the output of the left and right channel opamps offer short circuit
protection. These resistors are wrapped within the global feedback loop so that they do not
cause the effective output impedance to be compromised.
•
Ferrite beads at the output of each opamp isolate headphone cable capacitance, and keeps
the opamps stable.
Fully direct-coupled
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There are no signal-degrading coupling capacitors at the input, output or negative feedback
loop.
This is a true DC amplifier. Care should be taken to ensure that the input source does not
have DC offset at its output.
Suitable for a wide variety of headphones
•
Can be used with low-impedance and most high-impedance headphones.
Versatile power supply/battery charging circuit
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Both 7-cell ("8.4V") and 8-cell ("9.6V") rechargeable NiMH battery types are supported. A
battery capacity of 230mAH or greater is recommended for long run times.
•
A 7812 voltage regulator allows the use of an AC-to-DC adapter ("wallwart") that outputs
anywhere between 15V DC to 24V DC (actual voltage) with a minimum current rating of
300mA for battery charging and for running the amplifier. Onboard voltage regulation allows a
low cost unregulated linear AC-to-DC adapter to be used, while protecting the opamps from
being damaged by over-voltage.
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The charging circuit, utilizing a LM317L voltage regulator wired as a constant current source,
safely charges the battery at 16mA without risk of damage. A depleted battery can be
charged back to capacity overnight.
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A zener diode protects the amplifier from damage if the battery becomes disconnected while
the AC-to-DC adapter is plugged in.
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A power-on indicator LED is on the front panel, and a battery charging LED is on the rear
panel.
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Ni-Cd batteries are not recommended due to the poorer charge capacity and memory effect.
Li-Po, Li-Ion, or any non-rechargeable battery should not be used to avoid an explosion
hazard.
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Two 470µF reservoir capacitors (for a total of 940µF) provide ample charge to feed the
current demands from the amplifier. These capacitors are
before
the power switch, so they
are always charged as long as the battery is connected. This avoids a large surge current
during power-on, preserving the integrity of the switch contacts. It also ensures that the
opamps' supply rails turn on and off instantaneously, eliminating noise.
Designed to be easy to build
•
A specific target case, DC power jack, input and output jacks, volume control, battery contacts
and other parts make this a highly integrated design.
•
All parts are through-hole except for the two SOIC-8 opamps, making the amplifier very easy
to solder and assemble.
•
All parts are board-mounted, there are no wires to connect.