Circuit Impedance ( not 16 ohms ) Part "A"
Quote:
Originally Posted by MarkT
,,,, snip ,, Any idea (or speculation) on WHY JBL would do that? It would have been just as easy to create fixed L-pad that maintained 16 ohms.
- My speculation ( for one reason ) is that by changing impedances within separate portions of the same network, JBLs' crossover designers, can economize on the overall price of parts.
- Ie; Smaller Coil sizes can be used when "passives" are working into lower impedances ( though cap sizes do need to go up ). All in all , the price of copper ( by the pound ) likely overshadows the extra cost of the necessary larger size ( Mylar or Electrolytic type ) caps .
- There may also be a "damping" preference . I know that a low value parallel resistor does offer more circuit damping ( at least to my ears ) when used with a compression driver .
:)
Circuit Impedance ( not 16 ohms ) Part "B"
Quote:
Originally Posted by MarkT
Yes, I had just (independently!) figured that out - that explains why none of the values made any sense when calculated against 16 ohms! Any idea (or speculation) on WHY JBL ,,,,, snip, snip
(A) Okay, now that you realize that the actual load impedance ( that the crossover portion of the N200b ) "sees" is actually in the neighbourhood of 6.3 ohms / calculate ( through your text-book formulas ) what the F3 point is for the following ;
(i) 16.5 uF cap = gives an F3 of ??? hz into a 6.3 ohm load
(ii) .8 mH coil = gives an F3 of ??? hz into a 6.3 ohm load
(iii) 24 uF cap = gives an F3 of ??? hz into a 6.3 ohm load
(B) Then calculate what the textbook values are ( caps & coils ) of a 3-pole Butterworth network ( HP portion only ), crossing at 800 hz / or 900 hz or 1000 hz ( ??? ) .
(C) Once you've finished point (B) , do the same exercise of calcs. for F3 ( as in point "A" ) for the new values of "textbook-derived" passives .
;)
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Y'all are ruthless taskmasters!
Quote:
Originally Posted by Earl K
Unfortunately ( for the sake of clarity ) these impedance graphs were made without the "fixed" 4.5 db Lpad in place.
Hadta scrounge up the correct R's this morning.
Z800 = 6.2417 Ohms, measured.
The impedance the actual filter faces is quite stable with this topology. Compare to that of just the driver and horn in #18, above, top.
Le is calculated at 1 kHz, and the impedance is dropping at that frequency, so it's negative.
Le at all sampled frequencies is given in the WT2 full sweep data file.
[Seems no matter what, Fs remains the same.... :p ]
"LPad #2" wired as a variable potentiometer
Hi Zilch
Quote:
Originally Posted by you
You mean F3 of the bypass loop, or F3 of the basic highpass? I need to know where to look.
[Easily remedied with these fancy spring breadboarding thingies, if so.... ]
- I'm referring to the position of "LP-2" in the bypass loop .
- I'm not saying, that it's in the wrong postion . I am saying that at that position, with it offering variable resistance ( in parallel with the midrange loop below it ), in a small way it also changes the working impedance that the 1uF cap ( and .08mH coil ) "sees" . No biggy :p
:)