|
Part number: 7
|
Haos Redro
|
Last update: 14/06/2009
|
|
|
|
Second order allpass filter module
|
|
|
|
|
Resources:
| 30 |
Overall schematic |
FIG | PDF |
| 31 |
PCB layout |
FIG | PDF (copper) | PDF (copper+component) |
|
|
|
|
Description:
A second order allpass filter module for delay correction of active crossovers. This is frequently required to compensate for the phase shift between drivers of a multiway loudspeaker caused by the distance between the acoustic centres of the drivers.
Circuit description:
At DC, C1 and C2 are open-circuit and both stages operate as non-inverting buffers. At very high frequencies C1 and C2 are near short-circuit and both stages operate as inverting buffers. The phase sift between the input and output occurs within a frequency range determined by the values of the RC networks R5-C1 and R6-C2. This phase shift is caused by a time delay introduced by the circuit. As a general guide, the time constant of each stage should be significantly less than the period of the crossover frequency to reduce frequency dependence of the delay time close the crossover point. The actual required delay is not entirely dependent on the driver offset alone and experimentation is often required to find the optimal delay time.
|
Table 1: Component function
| R1-R4 |
Feedback potential dividers. Normally these should all be the same value. |
| R5-C1,R6-C2 |
These form two first-order lowpass filters. |
| C3,C4 |
Reduces the gain of IC1A and IC1B at very high frequencies. Required for stability with some types of operational amplifier. |
| C5,C6 |
Supply rail decoupling capacitors. |
|
Table 2: Design formulae
| Delay time (for multiway loudspeakers): |
Td = (d/c)-1 |
Where d represents the distance between the acoustic centres of the drivers and c represents the speed of sound in air, typically 344ms-1 |
| Delay of each stage: |
Td = 2RC |
The total delay is the sum of the delay introduced by each of the two stages, where RC represents R5 and C1 or R6 and C2. |
| Initial component selection: |
RC < (20*Fc)-1 |
This is to ensure that the delay time is largely frequency-independent near the crossover point. |
|
PCB layout:
The PCB is 42mm long and 27.5mm tall and uses a single-sided layout. All three connectors are aligned along the bottom edge of the board to allow the module to be fitted vertically to a motherboard.
Images:
Completed module |
|
|
|
|
|
|
Copyright © 2009 Taiyo Rawle. This document is released under the terms of the TAPR Open Hardware License version 1.0 or at your option any later version as published by TAPR: http://www.tapr.org/ohl.html
|
