Meyer Sound CQ-2 - Manual (Page 6)

6
Limiting and Protection
Circuitry
TruPower Limiting ™ System
Conventional limiters assume that the resistance of a
speaker remains constant and set the limiting threshold
by measuring voltage only. This method is inaccurate
because the speaker’s resistance changes in response
to the frequency content of the source material and
thermal variations in the speaker’s voice coil and magnet.
Conventional limiters begin limiting prematurely,
which under-utilizes system headroom and deprives
the speaker of its full dynamic range.
The TruPower Limiting (TPL) system accounts for
varying speaker impedance by measuring current, in
addition to voltage, to compute the power dissipation
and voice coil temperature. TPL
• allows the speaker to deliver its highest SPL
across its entire frequency range during limiting;
• eliminates long-term power compression when
operated at high levels for extended periods;
• protects the drivers and extends the lifetime of
amplifier and driver components.
Hi Limit and Lo Limit LEDs on the user panel indicate
TPL activity for the high and low frequency drivers,
respectively. The limiters for each driver function
independently and do not affect the signal when the
LEDs are inactive. Limiting begins when the driver
temperature exceeds the maximum safe level and
ceases when the temperature returns to normal.
The CQ performs within its acoustical specifications
and operates at a normal temperature if the limit LEDs
are on for no longer than two seconds, and off for at
least one second. If the LEDs remain on for longer
than three seconds, the CQ is hard limiting with the
following negative consequences:
• Increasing the input level will not increase the
volume.
• The system distorts due to clipping and
nonlinear driver operation.
• Unequal limiting between the low and high
frequency drivers alters the frequency response.
• Driver and amplifier components are subjected
to maximum heat, which shortens their life span.
Troubleshooting with TPL
The TPL LEDs can indicate serious driver problems, if
interpreted correctly. If one CQ in a system exhibits
substantially more TPL activity than others receiving the
same audio signal, then the driver corresponding to
the excessively active LED may have a short circuit. This
is a potentially dangerous condition for the electronics;
shut the CQ down immediately.
The TPL circuit does not activate if there is no power
dissipation in the driver, regardless of the input signal
level. Therefore, if all CQs in a system receiving the same
audio signal exhibit TPL activity except one, then that
unit may have an open voice coil; disconnect it and
contact Meyer Sound for repair information.
TPL can indicate an imbalance in a configuration of
speakers by functioning like a spectrum analyzer. If the
speakers in a subwoofer, mid-bass, or mid-hi subsystem
begin to limit before reaching the required operating
level for the entire system, then that subsystem needs to
be supplemented with additional speakers.
NOTE: Although the TPL limiters exhibit smooth sonic
characteristics, we do not recommend using them for
intentional compression effects. Use an outboard
compressor/limiter to compress a mixed signal.