Fixed L-Pads

[Zilch]

As built:

[dmtp]

I am happy to report that I have today oprdered the Beheringer ECM8000 with a preamp to use with an old (P3) computer I have laying around. Haven't decided between TrueRTA and ETF. It is not so much the cost ($100 v. 150) but the steepness of the learning curve that worries me.
Anyway, while waiting for my new toys, I started playing with the SPL plots making guesstimate adjustments for the RS meter, but focusing on the XO point instead of the high end. (I used the formula 20log((10^A/20)+(10^B/20)) for combined response.) It appears that I will have quite a notch around 600. I remember one thread where the builder "fixed" this by adding a 16 ohm resistor in series with the woofer, but noone thought that was a good idea. Looking at the separate curves, it is obvious that the woofer is rolling off too soon. This appears to be due to the 24uF cap. Keeping the coil @ 2.25, I found three choices for XO freq/cap:

1. Butterworth, freq = 625, cap = 28 (closest to what we have)
2. LR, freq = 900, cap = 14
3. Bessel, freq = 775, cap = 18

On the assumption that the hi-pass is a third order Butterworth (somewhat 'tweaked' by the second cap value as already discussed), is there a compelling reason to use a Butterworth alignment (second order) for the woofer, or will L-R or Bessel work as well? I am much more familiar with L-R XOs and it looks like a 900 XO would be a lot better. I think I'll try different cap values and see what I get.
Attached image:
Orange = LE14A with XO as in Zilch's post above
Purple = LE85 with 6300-6db, 10k+10dB and smoothed
Turquoise = same dropped 2 dB
Red - combined

[Zilch]

I've always played it with 2235H. I recall deciding in the end that the 600 Hz thing wasn't real, upon measuring from further back. That's all documented in the "Keeper" thread.

Adding resistance in series with the woofer is a BAD idea. The amp loses control. It's best practice to use a minimum DCR inductor for the filter, even, to keep the damping as low as possible. It just makes no sense to go and add in more.

*****

Bear in mind that it's the acoustic rolloff that ultimately matters, i.e., the driver(s) behavior in combination with the filter. I can run that for you here; plenty of LE14s around. Tell me again what is the box size and tuning you're using with it so I can get close to what you have.

Like you, I worried and fretted about all kinds of stuff relating to that crossover design. In the end, it all just came together and worked with 2235H:

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

My analysis was that the driver rolloff combined with the second order filter to make more like a third order rolloff. Note also that there's a bump in the LF lowpass; it's not a textbook alignment. The crossing is at -6 dB. I betcha Earl can advise as to how to tweak that as well, if necessary, once we get the measurements for him.

I'm not understanding your scale on the y-axis there in #77, above. Still 3 dB/div?

[dmtp]

I'm not understanding your scale on the y-axis there in #77, above....

Yeah, I missed that. Somehow it lost the second digit. Major dividsions are 3 dB, minor 1 dB. Numbers should be: 70, 73, 76, 79, 82, 85, 88, 91, 94, 97, 100, 103
My box is ~3.9cf. minus bracing and horn ~ 3.6 net. Two 3" PVC vents x 6" = 34 Hz by theory, impedance measurements suggest 36.

[Zilch]

It's "Plug-'n'-Play" with LE14H-3 (below). Both controls set to "Normal," i.e., middle.

I'll set up Citation 7.4 (3.5 cuft.) with LE14A tomorrow, probably.

[Earl K]

Hi Mark

I am happy to report that I have today oprdered the Beheringer ECM8000 with a preamp to use with an old (P3) computer I have laying around.

- That's good news .
- Don't forget about the soundcard requirements for any of these software packages.
- Does your computer have an adequate card ? ( Sound Card requirements are posted at software companies websites ). If you don't see your card listed / email them and ask if it's supported / ie; usable with their software .

Haven't decided between TrueRTA and ETF. It is not so much the cost ($100 v. 150) but the steepness of the learning curve that worries me.

- Ask Jack Bouska about the learning curve for ETF5.
- Also ask him what power of computer does he figure a person needs to adequately handle the processing overhead of this software . A P3 might not be adequate .
- You may not need a MLS based system for your casual use / only you can decide that. I'm biased towards "TrueAudio" as a company / so I am not impartial . I'd likely buy the 1/6 octave version ( 70.00 ) for TrueRTA to start with / and then upgrade later . Each version is upgradable with a simple entry code / ie ; the "lessor" versions are full-blown TrueRTA packages that are just partially disabled / meaning an upgrade is a simple email "call & response" away .

[dmtp]

I'll set up Citation 7.4 (3.5 cuft.) with LE14A tomorrow, probably.

So, do you guys just have all these drivers and cabinets sitting around somewhere? Must amount to a small warehouse (and a not so small fortune!) I thought my addiction was bad - I build 1 or 2 new systems each year - but frequently they are relatively inexpensive ones for my kids or out by the pool, etc. My current 'reference' system is a slightly modified version of Siegfried Linkwitz's Phoenix system bi-amped with a 400W ss amp for the 4 woofers and a 70W tube amp (modified Dyna 70)for the mid/hi's. I can't wait to see how the JBL's compare!

[Earl K]

Looking at the separate curves, it is obvious that the woofer is rolling off too soon. This appears to be due to the 24uF cap.

- It is most likely a combination of both the coil value & the presnt cap value .
- For a 7.5 ohm load / figure out the individual F3 points for the values of your 2 passive components .

- Keeping the coil @ 2.25, I found three choices for XO freq/cap:

1. Butterworth, freq = 625, cap = 28 (closest to what we have)
2. LR, freq = 900, cap = 14
3. Bessel, freq = 775, cap = 18

- You've lost me here with your version of "reverse-engineering " against a single known value . But really , that doesn't matter now .

- Remember these "Filter" relationships ;

(a) 2-pole Butterworth Transform ; the F3 of each individual pole ( coil & cap ) when compared to each other, create a spread that is 1.0 octaves apart / & they are not equally arranged around ( above & below ) the resulting F3 point .

(b) 2-pole Bessel Transform ; the F3 of each individual pole ( coil & cap ) when compared to each other, create a spread that is 1.5 octaves apart / & they are not equally arranged around ( above & below ) the resulting F3 point .

(c) 2-pole LR Transform ; the F3 of each individual pole ( coil & cap ) when compared, create a spread that is 2.0 octaves apart / & they are in fact equally arranged around ( above & below ) the resulting F3 point .

- Now, for a 1000hz crossover point , determine ( for each transform type ) the necessary values of the coil & cap ( on a 7.5 ohm load ) .
- Then calculate the F3 of each derived value against the 7.5 ohm load .
- Then look again at the F3s' for your existing passives' .
- These comparisons will start to illuminate the lowpass situation and possible remedies .

On the assumption that the hi-pass is a third order Butterworth (somewhat 'tweaked' by the second cap value as already discussed), is there a compelling reason to use a Butterworth alignment (second order) for the woofer, or will L-R or Bessel work as well? I am much more familiar with L-R XOs

- Forget the "dogma" ( & everything that this implies ) that is inherent in using the names of the standard transforms.

and it looks like a 900 XO would be a lot better. I think I'll try different cap values and see what I get.

- That's a good idea .
- Try a bunch of different values for the coil & cap / keepng in mind that you are trying to fit together 2 different "transforms" that were never designed to fit together.
- You're looking to achieve "the best fit" given the ( unchangable ) circumstances that the two drivers are physically separated and voice coils are way out of alignment.
- Don't be afraid to flip polarity on the horn driver when looking for better energy summing within the crossover area.

- You are now into an area of empirical experimentation . ( Good Luck ! )



ps; Why do all this you may ask ? Because real-world drivers don't output "flatlines" as FR / therefore every situation has some extra "EQ" built into the choice of filter slope .

[Zilch]

So, do you guys just have all these drivers and cabinets sitting around somewhere? Must amount to a small warehouse....

It's mostly an array of "stuff," collected over time at minimal expense. I only put real money into leading-edge components not otherwise available.

The LE14H-3s I measured above are running in L55 cabinets (2 cuft) that forum member Don C was taking to the dump. Do I care what they look like? Not one whit.

For the purposes of this "hobby," what matters is having the requisite platforms available for convenient experimentation. If there's some result that I really like, I'll build new cabinets from scratch.

There's exceptions, of course. L200, for example, is not only an excellent research platform, it's a good-looking 5 cuft. cabinet that can be developed through upgrade into quite a nice finished system.

L101, on the other hand, there's not much to do but stack a horn on top and admire the fretwork grille and marble top, which is fine, too....

http://audioheritage.org/vbulletin/s...2383#post92383

That's Citation 7.4 on the right there, with a refoamed flea-market LE14A in it. I got that (the cabinet) in a late-night exchange with forum member Ti Dome in an underground parking lot in Sunnyvale. It was raining, as I now recall. Citation 7.4 barely fit in the venerable Zilchmobile.

[Scroll up to post #414 for a closer look....]

[dmtp]

- For a 7.5 ohm load / figure out the individual F3 points for the values of your 2 passive components .

My lord I'm getting an education - far better than Dickerson;s book!
OK here goes:
L=2.25 = 500Hz
C=24=700

- Now, for a 1000hz crossover point , determine ( for each transform type ) the necessary values of the coil & cap ( on a 7.5 ohm load ) .
- Then calculate the F3 of each derived value against the 7.5 ohm load .
- Then look again at the F3s' for your existing passives' .

OK, 7.5, 1000Hz
LR
L=2.39 = 500Hz
C=10.6=2k
Butterworth
L=1.69=700
C=15=1400
Bessel
L=2.07=600
C=12=1800
Current
L=2.25=500
C=24=700
Obviously the cap in the current design has a lot lower F3 than any of the standard XOs. That would account for a dip between 500-1000 (or tame a peak).
But even if one was going to use a standard XO, how would you choose between the LR/Butterworth/Bessel? Don't they all accomplish the same thing? I'm learning, but still have quite a ways to go it seems!

[Earl K]

OK here goes:
L=2.25 = 500Hz
C=24=700

Actually my calcs for a load of 7.5 ohms , put the 2 F3(s) at ;

L = 2.25 mH = @ 530. hz
C = 24 uF = @ 885. hz

- The point to this is, that anytime the difference between the two is less than 1 octave wide , you've built up a "bump filter" of some sort. Through ringing/resonance they have a bit of gain in the region of the F3(s), along with an accelerated transition to the final slope. Bump filters also have compromised group delay properities ( as well as transient response ) .
- Why use one ? To get the builtin EQ ( gain ) .
- Who ( what networks ) used them ? Just about all the older 43xx series of 3 & 4 ways ( with 2307 horns ).
- Do you need one ? I don't know without seeing accurate FR studies of the le14a. Since JBLs' published FR drawings show a bit of rising response from 300 hz and up / your project may not need any "boost' in the crossover region ( though you still need to fill in that "frequency hole" / if indeed it actually exists ).

But even if one was going to use a standard XO, how would you choose between the LR/Butterworth/Bessel?

- In your case, by an educated guess .
- ie ; get us some accurate response studies of your le14a in its' cabinet & we'll help you "guess".

Don't they all accomplish the same thing? I'm learning, but still have quite a ways to go it seems!

- No, they all have different characteristics in the transitional / crossover region .
- Ultimately ( maybe a couple of octaves later ), they do achieve similar attenuation slopes .

[Zilch]

I don't know how 1 kHz got into this; it's an 800 Hz crossover.

Are you wanting to move it up to 1 kHz and I missed that part?

[Earl K]

I don't know how 1 kHz got into this; it's an 800 Hz crossover.

Who are you asking Zilch ?

- From my perspective / the actual Fc point is just a bit of a side issue / though I would prefer to mantain a 800 hz area crossover point .
- ( IMO ) A more important concern is whether a person feels the need to design with "bump-filters" to achieve a decent Fc .
- For instance; Does the le14a really need a bump filter in this region ?
- It could be that the typical Tractrix horn and Oblate Spheroid ( like the older 2307 ) need some EQ "help" in their lowest octave ( assuming typical DIY sizes such as we've seen here lately ) . Id' like to see all the raw FR studies before making sweeping generalisations .
- That's the reason I'm still talking It's a bit of a 2 way investigation ( maybe 3-way )

Are you wanting to move it up to 1 kHz and I missed that part?

- No, I'm just working towards creating some network design perspectives and protocols .
- So far I'm guessing you haven't picked up on my "hints" for designing a 3-pole / "bumped" / 800hz HP ( ie; start with a 1000hz Butterworth HP & then migrate the bottom pole upwards ) .

[Zilch]

Who are you asking Zilch ?

Uhmm, it was a "rhetorical" question....

I assumed the 1 kHz "illustrative," but began to think I might have missed something, is all.

Re: The "bump," we've already got that, both HF and LF, I believe, needed or not.

I'll get the LF response this evening, with and without the filter. It's hard to accomplish anything real during the day here.

Gotta get it up off the floor if the results are gonna be meaningful.

The ugly truth of crossover design, below, using "old school" Vector spring breadboading pins on perfboard. No soldering.

[SOUNDS good, tho.... ]

[Zilch]

1) Nearfield (~1/4") response of LE14H-3 and LE14A. L55 port is on the back; Citation 7.4 port is next to the woofer, hence extended LF response; mic is "hearing" a bit of the port output. Bigger box is part of that, as well. It's NL200B LF filter, of course.

2) Hi res version of same.

3) Zoomed to the area of interest.

4) L55 DUT.

5) Citation 7.4 DUT.

Zilch deems them "Similar."

Filter gives a 1 dB "bump" 100 - 200 Hz.

Where'd THAT come from, L200B?