1. VX Limiter
The Virtual Xover pseudo-peak limiter (RMS
calibrated) is at the core of the Linea Research VX Suite. This section will guide you
through the usage and functionality of the
VX Limiter, which can be operated in two
distinct modes: VX Mode on or VX Mode off.
VX Mode Off: Conventional Limiter When
VX Mode is off, the VX Limiter behaves
similarly to a conventional limiter, but with
enhanced functionality. It operates in a
multiband form, meaning different frequency ranges are independently limited. Each
band has optimized attack and release
characteristics, which results in a more natural sound and reduces the effect known as
“pumping.” In this mode, the only controls
you need to adjust are:
•Threshold: This sets the limiter’s activation
point, calibrated to the RMS dB value.
•Overshoot: This controls the maximum
output level above the threshold that the
limiter will allow, typically set between 6 to
8 dB. Avoid setting this value too low (less
than 4 dB), as it can result in a harsher
sound. This mode is suitable for most applications, especially when passive crossover
networks are not involved.
VX Mode On: Virtual Crossover When VX
Mode is on, you gain additional control with
a virtual crossover. This introduces a low-latency, linear-phase crossover, creating two
independent limiters per output. Each limiter
can have its own Threshold and time-constant characteristics. This feature is more
than just a frequency-conscious limiter. The
output spectrum changes based on the level of limiting applied to each band, mimicking the behaviour of a bi-amped system with
individual limiters for each driver. This can
provide better protection for specific components, such as a tweeter, by allowing
for a lower threshold and faster attack/release times for higher frequencies. When VX
Mode is engaged, the following parameters
become available:
•Threshold: Set this to the desired RMS dB
value.
•Overshoot: Set this between 6 and 8 dB
for optimal results.
•Split Frequency: This is the crossover
frequency, typically matching the passive
crossover network.
•Threshold Hi: This sets the limiting threshold for higher frequencies, relative to the
main threshold. For example, if the main
threshold is set to 50V, and the Threshold
Hi is set to -6 dB, the high frequencies will
be limited to 25V (6 dB lower).
•Overshoot Hi: Similar to the primary Overshoot control, but for high frequencies. A
value between 6 and 8 dB is typical. By using these controls, you can fine-tune the VX
Limiter to suit a wide range of applications,
providing precise and targeted audio limiting with minimal impact on overall sound
quality.
2. Thermal Limiter
Thermal limiting is crucial for protecting the
most vulnerable part of a loudspeaker—the
voice coil—from overheating. When a loudspeaker is in use, much of the electrical energy applied to the driver converts into heat
within the voice coil. The close proximity of
the voice coil to the magnet assembly allows heat to transfer between them, but this
heat transfer is not perfect. As a result, the
temperature of the voice coil can rise faster
than the magnet assembly.
Estimating Voice Coil Temperature
While it is possible to estimate the temperature of the voice coil based on the input signal, doing so accurately requires knowledge
of many detailed driver parameters, which is
often impractical. Instead, the Thermal Limiter provides a powerful solution using just
three key parameters:
1. Threshold level
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2. Attack time
3. Release time
This approach models the voice coil’s temperature and adjusts the output signal to
ensure the RMS power stays within safe
limits, applying attack and release characteristics to simulate the thermal behavior of
the driver’s voice coil and magnet assembly.
Adjustable Parameters for Thermal Limiting
1. Threshold - This parameter sets the
continuous RMS voltage that the driver
can safely withstand, calibrated at the amplifier’s output. You can disable the Thermal
Limiter by setting the Threshold to its maximum “Off” value.
2. Attack Time - This defines how quickly
the driver heats up, measured in seconds.
Though this specification is rarely published, it can be calculated based on the thermal Time Constant of the magnet assembly, which is roughly proportional to its
mass and volume. For smaller high-frequency drivers (HF drivers), an Attack time of
around 50 ms is typical, while larger cone
drivers may require values up to 5 seconds.
Using faster attack times offers more protection but at the expense of lower average
output levels.
3. Excursion Limiter
The Excursion Limiter is designed to protect your loudspeaker driver from excessive
movement (excursion) of the cone and voice
coil, which can result in mechanical damage. This excessive excursion is primarily a
concern at low frequencies, where the driver
is most vulnerable.
To properly configure the Excursion Limiter,
you need to understand the driver’s excursion vs. Frequency curves at various voltage
levels. Choose the curve where the slope
steeply intersects the specified X-max value
for the driver. Note the peak voltage and
frequency at this point, as they will be used
to configure the Excursion Limiter.
The Excursion Limiter typically requires only
two parameters for setup:
Threshold - This is the peak voltage at the
point where the excursion reaches the driver’s X-max. You can calculate this by multiplying the RMS voltage by 1.414 (the square
root of 2). The threshold is calibrated at the
amplifier’s output. To disable the Excursion
Limiter, set the Threshold to its maximum
“Off” value.
Frequency - This is the frequency corresponding to the peak voltage noted above,
where the driver reaches its X-max.
Minimum Frequency (Optional) - Some applications allow the use of a Minimum Frequency (Min) parameter. This sets a frequency below which the limiter’s action levels off. By default, this is set to 5 Hz and
typically does not need to be adjusted unless the application specifically requires it.