Peter Larsen, Crossover Design by software

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ALMA Europe 2009 Paper Presentation:
Crossover Design
by
Software
Peter Larsen
The Purpose of the Crossover:
1. Protect midrange and tweeter from LF overload
2. Obtain smooth transition between drivers
3. Equalize frequency response
4. Control off-axis response
5. Obtain smooth power response
6. Minimize number of components to reduce costs
Ideal 3rd order - 18dB/Oct Crossover Example
Perfect transition between drivers
Ideal 2nd order - 12dB/Oct Crossover
Cancellation problem @ x-over frequency (2kHz)
Ideal 2nd order - 12dB/Oct Crossover
Improved response with Tweeter inverted
Tweeter inv phase
Theoretical Solution with Real Drivers
This is the ideal 2nd order -12 dB/Oct crossover again, now using real drivers in phase.
The transition is imperfect and the response is not smooth.
2nd order - 12dB/Oct Crossover optimized
Fitting the component values to the green target AND preventing impedance below
minimum (here 3.2 ohms) is an effective solution
3-way Crossover example
2nd order – 12dB/Oct with inverted phase midrange. Both acoustical and electrical phase is
well behaved
3-way Crossover example2
2nd order – 12dB/Oct with midrange in phase. The midrange and tweeter position was
moved by simulated time delay
2-way crossover optimized with off-axis responses
0__/15__/30__/45__ deg off-axis SPL. The 30deg response was optimized
with the sloping green target for controlling the power response
RMS Power in circuit
The max RMS Power per IEC 60268-1 weighting may be calculated in all resistive
components with the actual x-over. Also the calculated power in each driver is particularly
important, because it can be used to simulate the power compression
4th order - 24dB/Oct Crossover example
This woofer section has RC impedance compensation, which can be optimized for linear
impedance above Fs. However in this case all 6 components were optimized to the 4th order
Butterworth target
4th order - 24dB/Oct Crossover
Total response after individual woofer and tweeter sections were optimized to 4th order
Butterworth target. The total result can be further improved with system optimization
4th order - 24dB/Oct Crossover Optimization
Total response after system optimization___. Removing redundant components reduced the
components down to a total of 6___, saving 6 components . The difference is less than 0.5dB.
Import of responses from FEM simulation
In stead of real drivers, you may import simulated responses from Finite Element software. The total
system can then be verified before building prototypes!
Requirements for Active Crossovers:
1. Driver response to be controlled in pass band
2. Driver phase response important
3. Well behaved driver off-axis response necessary for achieving good
power response
4. Frequency response and baffle EQ still necessary
5. Midrange and tweeter must be protected from thermal and excursion
overload
6. Actual power in drivers should be considered to minimize compression
and prevent spectral frequency balance problems of system
Simulation v Measurement
The crossover simulation accuracy is very high: The difference is often from measurement
due to slightly changed microphone position and cable resistance etc.
ADVANTAGES with Software Simulation:
• SAVE Development time
• Shorten Time to Market
• Save Component and Costs
• Extremely accurate results
• Automatic guard against too low impedance
• Calculation of POWER for drivers & components
• Control of Power Response with off-axis responses
Resume
Passive crossovers do not behave as expected from
simple filter theory due to the varying driver impedances.
This is well handled in modern software, and the
crossover circuit can be optimised to obtain a given
acoustic response both on- and off-axis while considering
necessary equalisation.
Design examples are given, and the results include power
calculations of all components and delay of individual
drivers.
Active crossovers do not have problems with driver
impedance, but equalisation and power/excursion
limitations in drivers still exist, which will be briefly
explained.
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