Fixed L-Pads

[Earl K]

Hi Mark

This is a bit discouraging.

( Zilch )Are you using an 8 ohm LE85?

- Mark, the stock ( original ) diaphragm in the le85 was only available in one impedance ( no matter what the foilcals say on the back of the le85 drivers ). It's closer to 11 or 12 ohms .
- We "regulars" know this because the topic comes up pretty regularly / / I guess it's not that surprising that you're unaware of this .
- JBL ( at some point ) changed ( how they "interpreted" the drivers' "impedance curve" ) and then they changed the "listed" impedance .

In addition to the 8 ohm R4, I notice you have L3=0.08 mH vs my 0.15. C6 is the same @ 1 uF. I also do not have R5 (20 ohm).

- I'd recommend just building your hipass section according to the schematics that Zilch has published.
- The response curves he has published look a lot better than what you've achieved on your own.
- He has done extensive testing of his circuit and has "shown" the type of response one can expect to get ( if you follow directions ).
- What more motivation do you need to accept the results of his hard work ?

Or do R4/R5 form another L-pad that is further attenuating the LE85? And how does the HF bypass feeding into the 'knee' of the L-pad effect that? Hmmm, with R5 parellel LE85, the HF bypass would be 'seeing' about 8 ohms anyway, wouldn't it?

- There's nothing wrong with wanting to understand what you're building .

- One can argue that R4/R5 form another "Lpad".
- Alternatively, one can argue that R4 coupled with the preceeding variable Lpad, now form a "T" pad followed by a single "load resistor".

- Either perspective is correct. The important point to remember is that all these resistive elements represent attenuation and offer some circuit isolation .

I'm sure I have 20 & 8 ohm reisitors. I'll see if I have a 0.15 mH or another 0.08 mH coil to try out your circuit.

- Good ! Use the 0.08 mH coil. That'll at least make me feel that I wasn't wasting my time when I posted the formulas for "resonant LCRs" ( much earlier within this thread ).

( I was following the 16 ohm version posted elsewhere - not sure I still have the thread reference) I also want to try the other LE85 as all my testing to date has been done with the same unit.

- I believe that you've needlessly confused yourself into thinking that there should be 16 & 8 ohm versions for this crossover.
- So far ( from my perspective ) there's just a single usable version for the le85. That happens to be Zilches' version ( though I feel the lowpass may still need some tweaking / we'll have to see ) .

[dmtp]

OK, I couldn't find a 0.08mH coil, but I found a 0.10. Everything else matches Zilch's XO within reason (7.5 v 8 ohm, etc.)
Here are my results (note: this is just the LE85 on my tractrix horn out of the cabinet which is getting cherry veneer right now. Full XO with 7.5 ohm R for LE14A):
1. Here is the best results after tweaking both the l-pad and the hf boost. If you take 88 as the average, it is +3/-5 out to 20k.

2. Interestingly, here is the graph out to 50k. Note sure why the big rise above 20k - I can't hear that anyway

3. Finally, here is the effect of the hf boost at min (blue), mid (magenta), and max (green). Not very much effect.

[Zilch]

Hello, Mark.

The LE85s I used are all original and red wax sealed. As Earl states, they all have the same "16 Ohm" diaphragm, no matter what it says on the foilcal.

Your results look excellent, now, except for the rolloff above 12 kHz. I'm suspecting it's the horn if the HF boost is indeed ineffective there.

Do you have a standard horn you can try? My initial work with your crossover in this thread was with H91.

I get a similar 5 dB rolloff followed by a peak just above 20 kHz on 2344A horn:

http://audioheritage.org/vbulletin/s...977#post127977

[Earl K]

Hello, Mark.

The LE85s I used are all original and red wax sealed. As Earl states, they all have the same "16 Ohm" diaphragm, no matter what it says on the foilcal.

I'm suspecting it's the horn if the HF boost is indeed ineffective there.

Do you have a standard horn you can try? My initial work with your crossover in this thread was with H91.

I get a similar 5 dB rolloff followed by a peak just above 20 kHz on 2344A horn:

Yes, Double-Checking results on a few different horns is always a good idea ( as a "reality-check" ) but ; I believe the "HF boost circuit" isn't functioning properly. Let me explain.

- As a laff ; ( multiply 159.155 by 7.5 and divide by .1 ) The answer ( in hz ) is 11,936.63 . That's the "lowpass" F3 of this .1mH inductor "working" into a 7.5 ohm circuit .
- ie; Everything in the HF boost circuit ( after @ 12K ) is "attenuated" in a 6 db per octave rolloff ( with a .1mH coil in a 7.5 ohm circuit ) .

- Now ; ( using the same formula divide 1193.6625 by .08 ) , The answer ( in hz ) is 14,920.78 . Now the "lowpass" F3 of that coil ( in the "HF boost circuit" ) has moved up to about 14,921.00 hz . That's about were I would place it , if not a little higher / such as 16K .



PS : Mark, it would be helpful if you published your most recent HiPass schematic ( which you just used for these recent TrueRTA measurements ) .

[dmtp]

I am totally imbarrasssed, but in the interest of science I will expose my stupidity.
First, the circuit I am using is Zilch's except for the value of L3 in the hi-pass circuit (I didn't have a 0.08 on hand).
http://audioheritage.org/vbulletin/s...6&postcount=76
Now the studip part. I just realized that I had the ground wire from the horn connected to the POSITIVE post of the XO instead of the NEGATIVE one! Everything else connected as in the schematic above. What is REALLY WIERD, is that connecting this the RIGHT way, results in a less flat response curve!

The aqua is the 'crossed' version and the blue is the 'correct' one! I don't understand this at all!

Now, here is what I am getting with the correct wiring and varying the value of L3. I used what I had on hand to expirement - not to doubt Zilch's hard work, just to see what happens. Anyway, 0.15 was the coil I had originally, 0.10 was sitting around, 0.06 is 0.15 in parellel with 0.10 and 0.05 is two 0.10 in parellel. I also tried a straight wire instead of L3 as EarlK suggest way back when. interestingly, with just the 0.1uF cap, LP2 has no measurable effect at all.

[Zilch]

C6 is 1.0 uF, not 0.1 uF no?

If you actually have 0.1 uF in there, the HF bypass loop isn't doing zip....

*****

With the driver connected to (+), it would be powered by the drop across the network. Difficult to contemplate, but you can see there's no highpass, 'cause the driver's playing -20 dB down at 250 Hz.

Correctly connected, it looks like you've got too much compensation dialed in, is all. Get the HF boost bypass working properly, then re-adjust....

[dmtp]

C6 is 1.0 uF, not 0.1 uF no?

Yes, my typo. Here are the results of my HF boost with L3=0.06 which is as close as I can come until I get a new order in. As you can see, just about 3 dB variation is all I get. Zilch got ~ 10dB http://www.audioheritage.org/vbullet...t=12420&page=7

Any ideas? I don't quite see how the horn would effect freq this high (1" throat measurements taken @ 3' on axis), but I do have an H91 I can try for comparison.

[dmtp]

OK, well the H91 does have better response above 10k

Here is the Tractrix taken at the exact same settings

Here is full range comparison

While it is lacking over 10k, I like the tractrix's better response in the 2k-4k region
Any ideas on flattening out the tractrix from 5k-15k? Would I need two notch filters to avoid making the 9k+ notch worse?

[Zilch]

Let me take some measurements with H91 on the crossover this weekend.

I would expect that H91, being an exponential horn, would track the voltage drive on axis.

I'm saying, let's first verify that everything is working as expected with H91, which we both have, then deal with your Tractrix, as required.

What do we know about Tractrix, generally? Exponential? Constant directivity? Over what frequency ranges?

As you suggest, the compensation contour may need to be tailored accordingly, but notch filters are not the only, and perhaps not the optimum, approach for that.

Again, as earlier, I raise the question of intent. Since we're now concentrated upon VHF response, the desire would be to use this horn two-way, i.e., with no tweeter. Is that correct?

[dmtp]

What do we know about Tractrix, generally? Exponential? Constant directivity? Over what frequency ranges?

I am certainly not the best one to explain a tractix horn, but I will give it a try. A tractrix horn is neither exponential nor constant directivity. It would be closest to a catenoid. Tractrix is a unique geometry such that the wave front is always hemispherical. At any point along the curve, the length of the tangent of the curve to the axis is a constant. For any given throat diameter and frequency, the horn has a fixed length and mouth becasue at the mouth the curve becomes tangential to the surface and cannot grow further. Some folks (Bruce Edgar in particular) think that the unique characteristics give it a "better" sound, especially in the midrange.
In terms of frequency response, I did some experimentation with cone drivers and tractrix horns and found that a tractix horn is effective (i.e. has a higher SPL output than the driver alone) for about 4-5 octaves at which point the reponse falls to what it was for the driver without the horn. I have never seen anything published on the frequency range of a tractrix horn.

Again, as earlier, I raise the question of intent. Since we're now concentrated upon VHF response, the desire would be to use this horn two-way, i.e., with no tweeter. Is that correct?

Yes, this is to be a two way system.
Finally, I did a few more measurements to be sure my results were not skewed by mike placement, etc. The graph below is the average of 7 readings taken with the mike varying from at the mouth to 5' away and with the horn, sitting on the (carpet) floor facing up and sitting on a chair edge facing horizontal.

This was done with the high boost totally disconnected, LP1 @ mid. Perhaps the peak at 7-8k needs taming, but it is only a few dB, and the 'notch' @ 9k now looks less impressive as well. If I could just pump in some boost over 14k, it would be darn nice.

[Zilch]

If I could just pump in some boost over 14k, it would be darn nice.

Easy, and you know how to do that from this thread.

The C in the HF boost loop establishes where it kicks in, and the L sets where it rolls off at the top. Do your calcs, put in the desired values, and hook that boost back up.

Looks like you need ~10 dB of "assistance" up there. I believe the circuit will provide that. Set LP1 to balance with your LF, then dial in as much boost as is required with LP2 to bring the VHF up to match the MF.

Yes, it looks like you'll need a notch at 7.5 kHz. You know how to do that, too, now....

[Robh3606]

Some folks (Bruce Edgar in particular) think that the unique characteristics give it a "better" sound, especially in the midrange.

FYI Bruce uses a single cap on his 350Hz Tractrix horn driven by a 2441. That gets him good out to 10K of so. Just seems like you guys are going to an awful lot of work. Have you tried a simpler aproach with these???

Rob

[Zilch]

Have you tried a simpler aproach with these???

Naw, gotta BUILD stuff!!

***Zilch konks Robh3606 with a 28.5 uF ClarityCap***

[CONVERGENCE]

Something wrong here . Are you deaf ? Nothing beyond 10Khz. This horn should go up to 30K at 100 DB.
Get your money back from JBL.

.................................................. ....................................

[dmtp]

Easy, and you know how to do that from this thread.

OK, I'm doing SOMETHING wrong here. Based upon formulae earlier in this thread
http://www.audioheritage.org/vbullet...52&postcount=4
I derived the following:
Lowest freq for boost = 12000 (my choice)
Z=7.5 (12ohm LE85 parellel with 20 ohm)
therefore C=1.0uF
Peak boost = 16000 (my choice)
therefore L= 0.10mH
desired bandwidth = 8000 (my choice) (boost 12000-20000)
so R = 5
Here is what happened:

The boost is spread from about 3k or 4k to 20k (or beyond)
WHAT did I do wrong????