Crossover - Ponkala/audiopoli-protone GitHub Wiki
This page is subject to change as we get the initial PCBA's from the fab.
The lowpass filter for the woofer is a combination of a LR4 lowpass and a notch filter. For the tweeter we used LR2 highpass in combination with two notch filters. The notch filters are vital to the design, as they are the ones that actually make the response smooth.
The filters are designed as described on on linkwitzlab. The website is absolutely amazing for active filter design with good explanation on the mathematics. The matlab script filtercalc uses that as the source for the equations.
Voltage regulation
The crossover also includes step down circuitry using the LM317 and LM337 linear regulators. This enables us to provide the crossover the required supply voltage from the power amplifier's auxiliary power terminals (+-25 V). The first iteration lowered the voltage down to +-15 V, but since it caused the operation amplifiers (OPA1604) to heat up significantly, we opted for +-7 V for the final version. After testing the new regulator configuration the thermals are much more manageable. The entire crossover uses around 30 mA idle current.
Why OPA1604? Because it's offered in quad-package and it provides really low noise and distortion. The quad-pack was kindof a must-have, to enable the crossover to be as low cost as possible.
The total cost of a single PCBA became 25€.
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| Crossover as it came from the manufacturer |
One thing worth noting is that the BOM accidentally had a 33k resistor instead of 33, which forced us to do some patchwork.
Frequency response
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| Crossover response. Solid lines are simulations and dashed are measurements |
Measuring the crossover transfer function confirms the simulation results. The results were within the expected range for the chosen component tolerances.
Schematics
Layout
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| Crossover layout with dimensions |