21
Power Amplifier Stage Protection Circuits
The Halo Integrated protects itself from external fault conditions such as excessive heat, load
impedance that is too low, or a short-circuited speaker connection or wire. Under such conditions
the On-Off button will glow red. After you correct the fault, the Halo Integrated will resume
operation and the On-Off button will glow blue.
Current Overload (short circuit or load impedance below 1 ohm)
Specialized current sensors constantly monitor the current flow through the Halo Integrated's
output stage transistors. If the current draw exceeds a predetermined safe level due to a load
impedance below 1 ohm or a short circuit at the speaker terminals, the output protection relay
will open immediately to prevent damage to the output transistors or other parts. This will
cause the On-Off button to glow red. Immediately turn off the Halo Integrated and disconnect
the speakers from the Integrated’s speaker terminals. Turn the unit back on to see if the On-
Off button now glows blue, indicating the fault has been removed. Carefully check your
speaker wires and speaker connections for a short circuit and correct the problem.
Overheating Condition
If the unit overheats the On-Off button will glow red and the power amp section will shut down.
If such a condition occurs you must provide better ventilation around the amplifier or check
speakers for a fault that is causing the Halo Integrated to overheat. The over-temperature
protection can also be activated if you are trying to drive more than one pair of speakers for an
extended time and/or at extremely high listening levels. If the protection circuit has been
activated because of overheating, normal operation will resume after the unit has cooled
down.
DC Servo Protection
The Halo Integrated power amp stage is direct (DC) coupled from its input to its speaker
terminals. Since direct current (DC) burns out speakers every power amplifier must have
some way to insure that DC never reaches the + or - speaker terminals. Most amplifiers
simply use trim controls to reduce their DC offset or capacitors that block DC. Unfortunately,
trim controls can allow DC offset to creep up over time, and even the most expensive
capacitors in the audio signal path attenuate bass response and place a veil between you and
your music.
Parasound power amplifiers overcome this problem by incorporating ingenious and fast-acting
DC servo circuits that monitor the audio signal (but are not
in
the audio signal path) to keep
DC offset at the output of the Halo Integrated close to zero without using coupling and blocking
capacitors. This advanced circuitry never needs adjustment or maintenance.and the results
are startling clarity, freedom from listening fatigue, and formidable bass response.
Faulty source components and particularly vacuum tube preamps with leaky output coupling
capacitors can have a large DC offset in their outputs that exceeds the capabilities of the DC
servos. In this case the protection circuit will activate. If the On-Off button glows red try
disconnecting your source components after trying the other two steps listed above.
Contact Parasound
If you believe you have confirmed the above three fault conditions are not present and the On-
Off button still glows red it could indicate an internal problem. Please contact Parasound’s
Technical Service Department.
