Page 3 of 12 FirstFirst 12345 ... LastLast
Results 31 to 45 of 172

Thread: Fixed L-Pads

  1. #31
    Member dmtp's Avatar
    Join Date
    Jun 2006
    Location
    Hancock, NH
    Posts
    95
    (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 ( ??? ) .
    If my calculations are right, a 3rd order Butterworth @ 1000Hz =
    C1 =16.84 reaonabley close to 16.5
    L1 = 0.75 reaonably close to 0.8
    C2 = 50.52 about 2x JBL's 24!
    Now what does THAT do???

    Taken separately, (based on 1st order Buterworth)
    16.5uF ~ 1500
    0.8 mH ~ 1250
    24 uF ~ 1050
    but I really don't know what any of this means
    MarkT

  2. #32
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    Quote Originally Posted by Robh3606
    Why would you use an LCR trap in this case with this driver and horn combo???
    Hi, Rob!

    It was in the precedent thread on this project, a "broad peak at 7 kHz."

    http://audioheritage.org/vbulletin/s...ad.php?t=12365

    The neato horn is shown there, too....

    N200B voltage drive curves are here:

    http://audioheritage.org/vbulletin/s...3157#post43157

  3. #33
    Member dmtp's Avatar
    Join Date
    Jun 2006
    Location
    Hancock, NH
    Posts
    95
    - That means learning how to make "AC impedance measurements" for that specific part of the circuit . I call that , measuring the "Loop Impedance" .
    - How to "Take AC impedance Measurements" will be covered in your "Loudspeaker CookBook " ( somewhere ) . You should plot out the impedance curve onto graph paper .
    OK, I know how to do impedance measurements and set up a little switch box to do it. And I even graduated from graph paper to computer plots. Using a spreadsheet, I used 10^x to get a logarithmic frequency scale in 1/6 octave steps (1000, 1100, 1300.....); measure the impedance at each step and then plot a graph of x=freq, y=ohms which looks like it were plotted on standard semi-log paper (which BTW I cannot find ANYWHERE to buy!)
    I measured the impedance of the LE85/horn and found a peak of 50Ohms @ 400, down to 16 ohms @ 200 & 560, fairly flat for a bit then sloping down from 16 @ 2200 to a value of 2.7 @ 20k. (Hmmm, is this falling impedance with rising frequency the NEGATIVE Le in Zilch's data? Normally the Le of a woofer causes impedance to rise with increasing freq.???)
    Anyway, I'm not sure what parts of the circuit to include in my measurements for calculating the 'loop impedance'. Do I start at the amplifier end of the circuit and include the 18dB XO? or do I skip over that and just measure the L-pad and bypass?
    MarkT

  4. #34
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    Quote Originally Posted by Zilch
    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.
    Ahhhh, Most excellent !!!

    Thank-you Zilch !


    **** Finally, this forum has good graphical evidence of the levelling effects ( to impedance ) that the fixed pads give to those older, wonky variable types. ****

    Many Thank-Yous !


  5. #35
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    Quote Originally Posted by MarkT
    OK, I know how to do impedance measurements and set up a little switch box to do it. And I even graduated from graph paper to computer plots. Using a spreadsheet, I used 10^x to get a logarithmic frequency scale in 1/6 octave steps (1000, 1100, 1300.....); measure the impedance at each step and then plot a graph of x=freq, y=ohms which looks like it were plotted on standard semi-log paper (which BTW I cannot find ANYWHERE to buy!)
    - Good Stuff Mark !

    - Okay, since you're going to plot it within a computer program / try to do so as both a linear plot and a logarithmic plot .

    - The reason ? In a linear plot, the in between values are much easier to approximate . Here's an example of a fairly "tough" to read log plot . "Tough" in the sense of approximating values that fall in between the recognizable hash-marks . Approximating the 3 db down "skirt" values from this plot / is a "bitch" . This plot is from "Project May". Even my estimation of the "peaks" of the "bumps" don't correlate very well with the official numbers stated .
    - A linear version of this plot would be easier to read / and therefore identify the peaks ( through sight ) .


    Attached Images Attached Images  

  6. #36
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    Quote Originally Posted by dmtp
    Anyway, I'm not sure what parts of the circuit to include in my measurements for calculating the 'loop impedance'. Do I start at the amplifier end of the circuit and include the 18dB XO? or do I skip over that and just measure the L-pad and bypass?
    - Build the complete hipass circuit / excluding your LCR trap ( the one presently notching around 6000 hz ). Connect the network to the source ( amp ) and the load ( horn/driver combo ) . The amp can be on and idling .

    - Measure the ac-impedance of the "loop" ( connecting the test leads ) to the drivers' terminals .



  7. #37
    Member dmtp's Avatar
    Join Date
    Jun 2006
    Location
    Hancock, NH
    Posts
    95
    - A linear version of this plot would be easier to read / and therefore indentify the peaks ( through sight ) .


    The other alternative that I do, is once I identify a peak on one of the "standard" frequencies in my plot, I simply run the Freq generator up and down until I find the true peak - even if it isn't one of the freq I would normally plot. I then do the same thing to zero in on the -3dB points. I do this for tuning bass enclosures, but I guess I can do it for the horn too. Probably this weekend before I get to do more measurements.
    MarkT

  8. #38
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    Quote Originally Posted by MarkT
    Probably this weekend before I get to do more measurements.
    Okay!

    Post your FR & "Z" plots once you've made them .


  9. #39
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    Quote Originally Posted by Earl K
    Finally, this forum has good graphical evidence of the levelling effects (to impedance) that the fixed pads give to those older, wonky variable types.
    I had to go look to see if we kept the fixed attenuation in the "Keeper" crossovers.

    We did.

    [Whew.... ]

  10. #40
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    Quote Originally Posted by dmtp
    but I really don't know what any of this means
    Look at the measured voltage drives I got for N200B:

    http://audioheritage.org/vbulletin/s...4678#post94678

    Ignore the UHF, which was separated out (the bypass boost loop removed and lowpass added) to make it three-way in that iteration.

    The stock N200B HF sim with bypass HF boost in place is here:

    http://audioheritage.org/vbulletin/s...3157#post43157

  11. #41
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682

    3-Pole Bump Filter

    Quote Originally Posted by dmtp
    If my calculations are right, a 3rd order Butterworth @ 1000Hz =
    C1 =16.84 reaonabley close to 16.5
    L1 = 0.75 reaonably close to 0.8
    C2 = 50.52 about 2x JBL's 24!
    Now what does THAT do???

    Taken separately, (based on 1st order Buterworth)
    16.5uF ~ 1500
    0.8 mH ~ 1250
    24 uF ~ 1050

    but I really don't know what any of this means
    Hi Mark,

    - The reason I asked you to do those calcs. is so that you begin to understand that your 3-pole HiPass is not a standard Butterworth type filter. It's best thought of, as a HiPass "bump-filter" that provides a bit a gain before the 18db per octave, attenuation kicks in. The "bump" is achieved through resonance and occurs within the filters'' working / "knee" area.

    - Earlier I had commented that your usage of a N200B network was a wise starting point, because I'm guessing your "Tractrix" horn may need a bit of low-mid bump ( EQ ) before crossing over to the le14a. It's just a hunch and since we haven't seen any "raw" FR plots ( of the horn/driver combo ) / I don't know if the hunch is right or wrong .

    - Anyways, if you don't need/want this EQ "bump", you can get rid of it by implementing the standard values for a 1000hz, 3-pole Butterworth filter ( ie. essentially, double the value of your 24uF cap ) .

    - If you don't believe there's a builtin resonant EQ "bump" / then look at these voltage drives ( courtesy of Giskard and Zilch ) .

    Attached Images Attached Images   

  12. #42
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682

    Lost HF

    Hi Mark,

    - Here are 2 pics of voltage drives that quite clearly indicate that the amount of HF drive-energy, available to the N200b is less ( about 5 db ) when compared to the N3135 ( 4435 ).

    - I believe it's that inline 5 ohm resistor that's keeping the N200B from providing the HF levels you are after. Once it's removed ( as well as the coil ) and you increase the size of the 1uF cap to something close to 1.7uF or 2 uF / you should obtain more HF . ( As mentioned earlier; you can resize the coil and reinstall it if you want to try to recreate a "LCR boost" )

    - The most significant point of these V-Drives (for me anyways ), is that the N3134/N3135 allows the HF ( in the HiPass section ) to return back to "0" level ( no attenuation ) .

    - Notice the "bump" in the hipass section of the N3135/N3134 .

    - These voltage drives are courtesy of Giskard ( with N200B, Lpad" data provided by Zilch ) . The second pic is mostly all N200B V-Drive .

    Attached Images Attached Images   

  13. #43
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    Mark has not yet stated whether his objective is to eliminate the variable MF L-pad or not, i.e., we don't know his purpose in desiring to incorporate more attenuation in the fixed one. Doing that will provide more "gain" to the HF bypass, as well.

    The problem I encountered with using the stock N200B crossover two-way was that I could not get enough gain without doing something like that, because just turning the adjustable L-Pad down to achieve it ALSO attenuates the HF boost as its parallel leg approaches zero resistance. See the white curve in Giskard's SIM, immediately above, which is what occurs at 8:00 setting, 7:00 being "off." That's the most compensation "gain" I could achieve with the stock circuit.

    I'm suggesting Mark's adaption of the the circuit might best increase the attenuation at the fixed pad, but retain the adjustable one to provide a higher impedance (in the upper portion of its operating range) into which the bypass loop may operate effectively; removing it will force the HF bypass to "see" the relatively low impedance of the fixed pad, presently 5 Ohms, in parallel with 20 Ohms and the driver impedance, presumably to become even lower when additional attenuation is incorporated there.

    Further, for safety's sake, I would retain some small fixed resistance in the bypass loop, perhaps 2 Ohms or so, as, under the condition of both controls being set to zero, it will otherwise see a direct short to common, limited only by the DCR of the bypass inductor. N3134/5 accomplishes this isolation (and also isolation of the MF and HF adjustments,) with a series resistance after the MF adjustable pad, another option, perhaps, in lieu of increasing attenuation at the fixed pad.

    Note also how the slope of the compensation increases with the increased gain, in particular in the area where Mark is presently using a notch filter. We don't know the FR response of his horn/driver combination, but it's looking to me as if the increased gain achieved via increased attenuation at the fixed pad may also provide more optimum compensation for his setup.

    At the time I was using these two-way, I did not want to mess with modifying my factory stock N200B crossovers to make them work with 2344(A) horns for quasi-4430. I had paid good money for them on eBay.

    When I subsequently opened them up for other reasons, I discovered they were trash (well, one of them was, anyway) and would require rebuild for any future use, anyway. Thus, the "Keepers," I built from scratch with much improved quality....

  14. #44
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    - Good Points about the potential for a short circuit !

    Quote Originally Posted by Zilch
    Mark has not yet stated whether his objective is to eliminate the variable MF L-pad or not, i.e., we don't know his purpose in desiring to incorporate more attenuation in the fixed one. Doing that will provide more "gain" to the HF bypass, as well.
    - True enough /though the "gain" is only "by comparison" .
    - I stand by my observation that the HF bypass circuit needs to directly "see" the driver. Your short circuit concerns can be still addressed .

    Quote Originally Posted by Zilch
    The problem I encountered with using the stock N200B crossover two-way was that I could not get enough gain without doing something like that, because just turning the adjustable L-Pad down to achieve it ALSO attenuates the HF boost as its parallel leg approaches zero resistance. See the white curve in Giskard's SIM, immediately above.
    - Yes, I understand this point. The variable Pad ends up attenuating the complete curve in one fell swoop.
    - One fix to that is to add resistance directly after the variable Lpad / effectively turning it into a "T" pad . I've done that with a few "bench" projects of this sort . The 3134/5 also uses this approach / remove its' 20 ohm ( inline ) resistor and those networks now have the same "problem" .



    Quote Originally Posted by Zilch
    I'm suggesting Mark's adaption of the the circuit might best increase the attenuation at the fixed pad, but retain the adjustable one to provide a higher impedance (in the upper portion of its operating range) into which the bypass loop may operate effectively; removing it will force the HF bypass to "see" the relatively low impedance of the fixed pad, presently 5 Ohms, in parallel with 20 Ohms and the driver impedance, presumably to become even lower when additional attenuation is incorporated there.
    - Right, the fixed Lpad needs a 15 ohm load (to the driver side of it ) for the HF bypass to actually work ( and for the "Fixed Lpad" to rpovide the correct 6.3 ohm load the preceeding passive components. Therefore the variable Lpad needs to remain ( or at least an inline resistor of similar value ).

    - My concern is if the attenuation of the fixed Lpad is increased too much ( and subsequently ) the variable Lpad is opened up too far, as in "wide open" / the reflected impedance the fixed Lpad "sees" will drop down from 15 ohms towards 7.5 ohms ( 20 ohms plus 12 / the conjugate with the drivers' "Z" ) . This downard change in load impedance will bugger everthing that precedes it within the circuit. This can of course be fixed with a simple resistor of appropriate value.


    Quote Originally Posted by Zilch
    Further, for safety's sake, I would retain some small fixed resistance in the bypass loop, perhaps 2 Ohms or so, as, under the condition of both controls being set to zero, it will otherwise see a direct short to common, limited only by the DCR of the bypass inductor.
    - We are on the same page here / except I would add the "safety" resistor directly after the 16 ohm variable Lpad ( creating a "Tpad" ). Altec does this as well / BTW / in the N1201a. ( The 20 ohm conjugate may need to be changed to a different value ). Tinkering in this area still means having to maintain a 15 ohm load ( for the preceding fixed Lpad ) .

    Note also how the slope of the compensation increases with the increased gain, in particular in the area where Mark is presently using a notch filter. We don't know the FR response of his horn/driver combination, but it's looking to me as if the increased gain achieved via increased attenuation at the fixed pad may also provide more optimum compensation for his setup.
    That's not gain / it's midband attenuation !

    At the time I was using these two-way, I did not want to mess with modifying my factory stock N200B crossovers to make them work with 2344(A) horns for quasi-4430. I had paid good money for them on eBay.

    When I subsequently opened them up for other reasons, I discovered they were trash (well, one of them was, anyway) and would require rebuild for any future use, anyway. Thus, the "Keepers," I built from scratch with much improved quality....
    I also noted, that at the time, you were still staring at that goofy 4LED thingy you called an "RTA" . Like Giskard said " You've come a long way " .

    <>

  15. #45
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    Quote Originally Posted by Earl K
    I also noted, that at the time, you were still staring at that goofy 4LED thingy you called an "RTA" . Like Giskard said " You've come a long way " .
    Heh, heh.

    Well, it served to immediately convince me that I needed adequate instrumentation if I was going to accomplish anything worthwhile in this.

    Thank you again, Mr. Widget, for loaning me my first RTA!!!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. L-Pads, 8 or 16?
    By dmtp in forum Lansing Product Technical Help
    Replies: 4
    Last Post: 08-29-2006, 11:16 AM
  2. Any recommendations where to get my h/k Citation 22 fixed??
    By Tweak48 in forum General Audio Discussion
    Replies: 0
    Last Post: 01-16-2005, 12:06 PM
  3. 16-ohm L pads
    By jim henderson in forum Lansing Product Technical Help
    Replies: 1
    Last Post: 11-06-2003, 10:10 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •