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 ExamplePerfect transition between drivers

Ideal 2nd order - 12dB/Oct CrossoverCancellation problem @ x-over frequency (2kHz)

Ideal 2nd order - 12dB/Oct CrossoverImproved 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 optimizedFitting the component values to the green target AND preventing impedance below

minimum (here 3.2 ohms) is an effective solution

3-way Crossover example2nd order – 12dB/Oct with inverted phase midrange. Both acoustical and electrical phase is

well behaved

3-way Crossover example22nd 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 responses0__/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 exampleThis 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 OptimizationTotal 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 simulationIn 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 MeasurementThe 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.