Originally Posted by
merlin
Sorry Ian you are losing me totally I'm afraid - I suspect we may be a crossed purposes. I am just trying to confirm the effect the series protection cap has on the roll of in the stop band.
A capacitor in series results in a 6db per octave attenuation below a set frequency as I have always undertood it.
Yes in theory that is correct..but in the real world it may measure differently due to the variation on the impediance of the compression driver mounted on the horn.
Therefore, if a signal comes in an octave below the F3 point and that signal was originally say 100db, it will be attenuated to 91db. Is that correct? Again in theory that would be right.
Now if that signal was really originally 109db but has been attenuateed to 100db by an active filter an octave higher before the capacitor sees it, it's still going to end up at 91db isn't it?
Merlin, This is difficult to discuss and I will simulate the voltage drives later in the day for you and post the cascaded attenuation into a simple 8 ohm load. Technically the attenuation will sum negatively at any point at the capacitor -3 point and below at 6db per octave
And that would be a total attenuation of the music signal of 18db.At a particular point on the curve yes.
My goal is to use a first order slope - specifically in the octave around the crossover. There is always going to have to be a protection cap an octave lower isn't there? Yes.
So it's only a first order slope in the crossover area itself. Correct
Another reason is it sounds shootloads better than using 3rd order. It can do depending on the overall design.
I see Pass Labs recommend the lowest possible order too. In my case, the horn/compression driver starts to roll off at 6db per ocatave acoustically below 900hz, whilst the bass unit, a 1500AL has a rising response above 500hz of 6db per octave. I'm therefore trying to use a third order slope on the 1500al and a first order on the horn, resulting in matching second order slopes at the crossover point. I hope that makes sense!
I understand.
Put another way, in trying to answer my quandry, if we follow guidelines and insert a series cap 1 octave below the crossover point, does that not mean that a 1st order slope becomes a 2nd order slope below this lower point, and that a 2nd order would become a third order and 3rd a fourth?