SO HERE’S ANOTHER USE OF THE 2ND ORDER ALLPASS FILTER (not a recommendation by the way)   Leave a comment

Well at the end of April down here in the Netherlands we have a party all over our small country called the Queens day which also had a minor addition this year . The queen gave way for our new king so the party got a bit bigger .

I got hired at a party to do the system tech job and also i had to mix all the performing bands .

During the system alignment i ran in to a “small” problem . Turned out there was a difference between the left and right Top of the set-up .

So here is the transfer trace of the left Mid high cabinet .Top lft high driver correct

And here is the transfer trace of the right Mid high cabinet .Top rght high driver pol rev

On the right Mid high cabinet there are some strong hints that something is wrong . Just before the 4K mark you see a very steep angle on the phase trace and a rather large dip in the frequency response that is not there on the left mid high cabinet .

So what is the “problem” here : The high driver in the right Mid-high has been wrongly connected so it’s a polarity reversal (which i did not ask for) . Also i did not have the tools to open it up and reverse the connection on the high driver nor did i have time for opening it up .

First both drivers no processing yet to clearly see the difference between the both mid high’s .Compare top lft vs rght

The dark blue trace shows no gap in the frequency response and also no drop in the phase trace . From 4K up to 16K a 180° of phase difference is clearly visible between both traces .

So because time was a big issue on this job i had to change the phase response of the left cabinet by applying a 2nd order allpass .

Top lft with ap2 to match the right side 

So the red-ish trace is my new left and now both trace are a some kind of a match with the help of the 2nd order allpass filter somewhere in the 5K zone with a rather high bandwidth . I normally would prefer ro open the cabinet up and fix the problem or get another 1 in but no time and on this day it’s impossible to rent in another 1 from anyone because nobody can be reached .

It’s a “solution” not a pretty one but you can go on . The only thing left is a rather large dip in the frequency response on the right which wont go away via eq because the mid and high driver are out of phase in the x-over zone . Shit happens (all the time) so i had to live with that .

I’ll be posting more problems like this if i run in to them in the future .  


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