Provided here, is an Excel spreadsheet calculator to find the capacitor and resistor values for an Allpass or Delay filter for electronically aligning the phase of treble and mid/bass speakers.
On a flat speaker baffle the treble unit's voice coil is in front of the mid/bass unit voice coil, often between 35 and 45 mm. Around the crossover frequency there will be some cancellation due to this physical misalignment. A Delay Filter can electronically 'move' the treble speaker into alignment.
|This simulation shows how the misaligned voice coils of the treble and mid drivers creates a dead zone at a certain frequency right at the listening height.||Here the sound sources are physically aligned producing a clean wave in the critical zone.||And here the sound sources are misaligned but the phase of the top source has been altered - as with a delay filter. The result is close to the physical alignment.|
(These simulations were made with a wonderful Ripple Tank Simulator java applet written by Paul Falstad.)
The Excel spreadsheet has no macros and should save ok, if however, your security does not allow XLS files to download you can download the 'zip' file. Right click a link and select 'Save As'.
Download the All Pass Filter Calculator Excel Spreadsheet
Download the All Pass Filter Calculator ZIP file
This is a screenshot of the spreadsheet. It makes calculating values easy.
This calculator calculates for using 2 delay networks in series for each channel.
Use the spinners on the left to dial in the desired treble unit offset in millimeters and the Xover frequency.
The graph will display an ideal response also shown in the light green column. Resistor and Capacitor values can then be adjusted to achieve the best match between 'ideal' and the 'filter response'. The white and light blue areas in the blue column denote the critical frequency ranges where a good match is important. Remember to use the 'Polarity Inverted' switch if component values are getting too extreme! With a 24dB Xover the central area of the crossover frequency (within 1 octave) is critical.
The phase matched plane for speakers is described as having their voice coils aligned. This generally means aligning points just in front of the speaker magnet. However, in testing I found that the offset I required was around 4mm less than I would have calculated in this way which was helpful for keeping component values low and easier matching.
Allpass (delay) Filter Construction
Here is the circuit schematic with 2 sections, one inverting and one non-inverting.
And here are the 2 sections above on a single circuit board. This image can be downloaded and scaled using an art programme and used as a guide for drilling by sticking the image down onto a blank PCB. This is how I made my circuit boards. The grid is 2.4mm. You will need 2 of these for stereo. The big input cap is a high quality 470nF (1uF recommended) for protecting the circuit from DC. The opamps are NE5532N though any good opamp with same pinouts (i.e. TL072) will do. Where component values are shown, these can be used for RF filtering and input an output buffering.
Here is the internal power supply which uses regulated 15 or 12 volt AC source - see Rod Elliot's website for component values. He designed this wonderful supply and can provide circuit boards. The version on his website is a 15v, I made mine 12v and it seems fine. Also have a look at Rod's other audio pages - he explains why bi-amping is such a good idea and why phase alignment of speakers is important.
This is what my finished project looks like. The 'test' psu I made was so good I kept it! The case is 20cm wide. The boards are temporarily held on ceramic tiles during testing. The tiles add weight that helps keep the unit stable.
An on/off and bypass switches are included. The signal comes from the active crossover treble output and goes to the treble amplifier after filtering. When the filter is switched on the difference is quite audible as notes around the crossover frequency gain volume and clarity.
I hope to add comprehensive instructions soon as this is a really worthwhile project and within the scope of beginners (like me) so please check back later.
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