Fixed L-Pads

[Zilch]

Try both ways on your final setup. I'd use one leg of an L-pad to vary the Q and ascertain the optimum resistance value. I'm guessing the broad notch will give a good result, as the twin peaks are symmetrical. If not, consider dual narrow notches, though that's gonna be tough to get right, them being so close.

Again, my results are on HL91, not your horn. Try some polar measurements, too. See what your HF beamwidth is, maybe 10°, 20° and 30° off axis....

[dmtp]

I had a break at work and was working on the calculations for the LCR, but I have a question: How do I calculate the value for R? I realize that the easy thing to do is use the 16 ohm side of an L-pad, but to calculate L [L=(Q*R)/(2*pi*F0)] and then C=1/(4*pi^*F0^*L), I first need a value for R.
In a previous post EarlK said Zilch had a neat formula for calculating the value of R that would give 'x' dB attenuation when parelleled with a known impedance (which I am assuming would be the measured impedance of (LE85 || 20 ohm) at the resonant freq. Could you share this formula? Thanks!

[Zilch]

Clearly, we've departed substantively from the original N200B, isolating and assuming control of its HF boost bypass loop, and altering its frequency range of influence. In retrospect, now observing that the response of LE85 on HL91 is basically flat out to 12 kHz, what was going on in the original? Zilch wants to know, 'cause it makes no sense.

Answer: Not the compensatory VHF boost via midband attenuation we're practicing with it, rather, rising "forward" MF and HF to 10 kHz, and rolloff at 12 kHz. The single control balances an essentially constant and very familiar "Quintessential Rock and Roll" response contour with the low frequency driver (top).

EDIT: Compare to L200's LX-16, bottom, particularly 7 - 12 kHz....

[Zilch]

Could you share this formula? Thanks!

The formula is for calculating dB attenuation from L-pad resistances and driver impedance, and is linked earlier in this thread.

In this case, however, the resistance establishes the "Q" or gain of the filter, i.e., the depth of the notch. Nominally, it's set at the impedance, but adjusted up or down to accomplish the desired effect. That's why I suggest choosing it empirically, and one leg of an L-pad is a convenient variable resistance to use for making that determination....

[Earl K]

Zilch,

- Okay, I have a couple of hours before my next fitup .

Answer: Not the compensatory VHF boost via midband attenuation we're practicing with it, rather, rising "forward" MF and HF to 10 kHz, and rolloff at 12 kHz. The single control balances an essentially constant and very familiar "Quintessential Rock and Roll" response contour with the low frequency driver:

- Was that FR plot obtained with the 2308 lense on or off ?

-The 2308/L91 lense, does tend to flatten out these rising response curves .

[Earl K]

An Observation ;
- We're attempting to build an LCR series notch filter ( strapped across the load ) that has an electrical "Q" which matches ( as a recipricol ) the realworld "Q" of the observed "bump" found in the horns FR response .
- IME, even with all the theory in place, this a process where one employs "best guess" / then measures / then adjusts LC values / and then remeasures / repeating until the desired results are obtained .

In this case, however, the resistance establishes the "Q" or gain of the filter, i.e., the depth of the notch. Nominally, it's set at the impedance, but adjusted up or down to accomplish the desired effect. That's why I suggest choosing it empirically, and one leg of an L-pad is a convenient variable resistance to use for making that determination....

- Zilches suggestion has much merit . It applies a variable "R" and allows one to see ( & measure ) the results immediately ( using any adhoc LCR notch filter ).

- I had a break at work and was working on the calculations for the LCR, but I have a question: How do I calculate the value for R? I realize that the easy thing to do is use the 16 ohm side of an L-pad, but to calculate L [L=(Q*R)/(2*pi*F0)] and then C=1/(4*pi^*F0^*L), I first need a value for R.
- In a previous post EarlK said Zilch had a neat formula for calculating the value of R that would give 'x' dB attenuation when paralleled with a known impedance (which I am assuming would be the measured impedance of (LE85 || 20 ohm) at the resonant freq.
- Could you share this formula? Thanks!

- IME, "R" can be calculated / and the result will be an "R" that is still just a guesstimate ( for a realworld LCR, working within a realworld circuit ).

- The difference between a "wild" guess ( at "R" ) and a half-assed accurate guess ( of "R" ) relies on knowing the real "working impedance" for that portion of network / ( where the LCR is going to be applied ).
- So far , only conjecture places that number at 7.5 ohms .

FORMULA ?
- FWIW: Zilches formula involves using antiLogs . So;

Divide 4 by 20 ( with 4 db, looking to be the amount of attenuation needed to flatten the 7800 hz bump / with no HF bypass circuit in place skewing the picture ). Take the antiLog of this answer . It's 1.5848931922 . Divide this into 7.5 ohms. This answer is 4.732180084 . This is the "new" frequency dependant load impedance that must be obtained to drop 4 db of level in that part of the circuit . How do we drop the ( 7.5 ? ) load resistance to 4.73210084 ohms ? Answer ; Parallel 12.82285398 of resistance ( within the LCR ) across the guess at 7.5 load . ( The DCR of any inductor chosen for use, becomes a portion of this R" ) .
- Conclusion ? 10 to 14 ohms of conjugate resistance should level out a 4 db high bump ( if the working impedance is really 7.5 ohms ) .

- You can measure the 'Q" of the real-world "bump" as I outlined earlier in this thread ( hopefully one has taken an accurate enough picture of it ) . If the 3 db down skirts ( of a 7800 ) bump are in the neighbourhood of 3000 to 3200 hz wide ( point to point ) then the bump will have a "Q" of around 2.5 ( 7800 ÷ 3120 ) . 10 times 2.5 is 25 ( & 14 times 2.5 is 35 ) . This ( 25 to 35 , "Q" ) becomes the range of the theoretical "Q" which the LCR must be designed to offer ( into a 1 ohm load )

(A) A Workable Approach to getting a real LCR filter ;

- I like Zilches suggestion of just dialing in "R" to find what out what value of "R" actually attenuates the bump by the desired amount of db . So ;

(i) I suggest building a series LCR notch filter ( strapped in parallel across the load ) that's close to resonating/notching within the area of interest . To my eyes , that's 7800 hz ( if that double-headed bump was expanded into one with an "imaginary peak" ) .

(ii)
- Since this is really just a "test" LCR to help us derive "R" , a person can use any LC combination ( within reason ) as long as they result in the necessary Fo point ( resonating frequency ) .
- The necessary working formulas were posted earlier in this thread ( that will allow one to chose a coil size first / & then derive the correct cap size ) .


- A .56 mH coil resonating with a .75uF cap has a Fo frequency of 7766 hz .
- I chose these values below based on some "intuition". ( intuition, meaning ; 25 ÷ 2 ÷ Pi ÷ 7800 giving 0.000510112 Henries / or .51 mH . Using 35 ( as the target Q ) would have resulted in a .71 mH coil . ( One can actually buy a .56mH coil . )



(iii) Passives values of .75uF & .56mH ( Offers a Resonance Point of 7766 hz ).
-These two passives will "theoretically" offer a ( full "notch" ) "Q" of 27.325 ( into a 1 ohm circuit ) . 27.325 ÷ (10 to 14 ohms , our range of "R", I guesstimated ) should offer a final filter "Q" that's in the range of 2.7 ( to just below 2 ).
- As Zilch suggested, wire up a portion of a Lpad that gives usable resistance ( 0 - 16 ohms from terminals 3 and 2 when using a 16 ohm variable Lpad / or / whatever range of resistance is available when using terminals 2 & 1, usually 60 ohms to a low of .5 ohms ?? ) .
- "Q" is also a short form for "Quality Factor" . One will discover the realworld "Quality Factor" of their chosen components by building with them / and then measuring the notch results .
- If the notch depth or width isn't "as expected or calculated" / then this will indicate that the values of L&C need shifting. This is realworld. The "Quality Factor" for these component types now have to be factored into the final equation . This usually means buying better quality capacitors ( they have "Q" that affects how well they resonate ), as well as using larger value coils ( larger than what text-book theory actually suggests ) .
- Using larger coil sizes will drive up the theoretical "Q" but this allows for ( balances out ) the lower ( ie; sloppy ) "Quality Factor" inherent in real world inductors .
- One can also study JBLs' network schematics to get hints of what size of coils they found worked( in their real world circuits ). Of course we don't know the "Q" of the bump they were notching / so this study is of limited value . But it is still worth the study .


Mark , I hope this isn't all too confusing

PS ;
- First do this exercise with the HF bypass circuit disconnected .
- The HF bypass circuit has the capacity to completely skew the initial results . In this projects case / your HF bypass circuit is not finished / and in reality, should be "fixed" prior to mucking about with LCR notch-filters .

[Earl K]

Mark,

About your HF bypass circuit ;

If you are convinced that you haven't miswired something and if you are convinced that the MF variable Lpad offers enough attenuation to the whole horn circuit , then ;

- Start reducing the value of C ( in the HF bypass circuit ) . And for now get rid of any drive pot and get rid of the resonating coil . ( You can attempt to add them back later in the process ) .

(i) If the HF bypass circuit ( when the MF is properly balanced to the le14 ) has bled into the final FR ) too much 5 to 10 K information or lower / then simply reduce the value of the cap by a similar amount ( in octave terms ).
- ( 5 to 10K represents one octave of information ).
- Halving the size of a cap shifts its F3 up by one octave.
- By extension , shifting the F3 up one octave will reduce all content below the F3 by a comparable amount. ie ; what was at say minus 6db will now be at minus 12 db ).

(ii)
- If the HF bypass circuit is "contaminating" 10K by adding an addition 6 db of unwanted information / then shifting the F3 point of the bypass cap upwards 1 octave will, reduce this contamination by 6 db.
- As I said earlier, everything we are hearing/seeing ( the net contribution for this HF "bypass" circuit ) comes from below the caps F3 point / therefore shifting the F3 point ( of that cap ) determines how much level ( of any one frequency ) is added into the final FR plot ( until the original MF signal swamps out the HF contribution ) .
- So everything "contributed" ( from this "bypass" circuit ) is electrically filtered at an attenuation rate of 6 db per octave .


- So try a .5 uF ( bypass cap, if you are presently using 1 uF ), then remeasure and post your results .




ps ;

Can't be right. C6 is already 1.0 uF.

Earl's gonna have to resolve this one.

Me, I'd be trying lower value C6's....

- Yeh, what he said !

- I've lost track, is C6 the same cap I'm referring to in the bypass circuit ?
( I don't keep this schematic taped to my computers' monitor )

[Zilch]

Was that FR plot obtained with the 2308 lense on or off ?

Lens on. I measured the flattening earlier, but didnt' save it, of course....

Edit: See below.

I've lost track, is C6 the same cap I'm referring to in the bypass circuit ?
( I don't keep this schematic taped to my computers' monitor )

Yup.

[[**Looks around for Earl's spycam....**]]

[dmtp]

- So everything "contributed" ( from this "bypass" circuit ) is electrically filtered at an attenuation rate of 6 db per octave .

So if I understand it correctly, the best we could hope for from this circuit is something on the order of 4dB from 12k-20k (assuming I keep increasing C6 until there is no "boost" @ 12k). If OTOH, I allow the "boost" to bleed down into the 6k-12k range, I will have a bigger "hump" to attenuate with the LCR, but I could get more like 10 dB of "boost" @ 20k (by dropping the "zero boost" point by an octave and therefore gaining 6 dB.)

When next I get a chance to play (probably Sat.), I will first get high detail SPL from 5k up and then try to dial in the VHF boost needed (using just the cap, no L, no R). Once that looks good, I will re-evaluate the 5k up SPL to determine the height and width of the "bump" to make up the LCR. Depending on how well I can make up these circuits with what I have on hand, I'll post results as I go.
I'll also try to get the impedence measured with the H91 and the tractrix for comparison to get a better idea what the imdepence we are dealing with REALLY is. (To be sure, I will measure LE85 alone and LE85 || 20 ohm, right? or do we need to look at the WHOLE network?)

[dmtp]

Answer: Not the compensatory VHF boost via midband attenuation we're practicing with it, rather, rising "forward" MF and HF to 10 kHz, and rolloff at 12 kHz. The single control balances an essentially constant and very familiar "Quintessential Rock and Roll" response contour with the low frequency driver (top).

Do I correctly interpret this to mean that the SPL response we are seeing is what was considered "desirable" for a "Rock & Roll" speaker? "The JBL sound" with "forward" MF? Hmmmmm

[Zilch]

Do I correctly interpret this to mean that the SPL response we are seeing is what was considered "desirable" for a "Rock & Roll" speaker? "The JBL sound" with "forward" MF? Hmmmmm

Yes, one element of "West Coast Sound." L200B apparently implemented it to somewhat of an extreme in the HF (see below). You'll need boomy bass and a couple of other things to complete the package.

From the frequency response curves, the original L200(A) was a better emulation; it's as if some marketing type liked the sound even more with the lens off, thus defining L200B. Let's not pretend any of this analysis provides insight into the actual product development process, tho.

We coveted juke box sound. Anybody have the FR curves for '60s & '70s vintage Seeburgs?

[There was a tweeter in there somewhere, I'm CERTAIN.... ]

[Robh3606]

Do I correctly interpret this to mean that the SPL response we are seeing is what was considered "desirable" for a "Rock & Roll" speaker? "The JBL sound" with "forward" MF? Hmmmmm

Well that depends. Look at the difference in the amount of boost you are seeing over the same range as the posted 4311 graph. Worst case with the pads set to 0 at about 4K you would see about +2/+3 dB. You are seeing quite a bit more in the posted graphs. That extra 2/3dB rise over that range is going to make a big difference in how they sound. The 4311 could sound forward. You don't want foward to turn into shrill.

Rob

[Zilch]

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.

Perhaps forum horn theorists can fill in the details, but I'd suspect the "rub" in attempting to use a 500 Hz Tractrix for extended VHF is severely compromised power response, as occurs with HL91 in the vertical.

That's why I'm asking dmtp to do some rudimentary polar response measurements once the boosted VHF is working. The hemispherical wavefront may behave differently up there....

[Robh3606]

as occurs with HL91 in the vertical

Hello Zilch

Actually that would be both axis. The lense is what changes the pattern otherwise it's a round horn so it would be symetrical. You should read this thread.

http://www.audioheritage.org/vbullet...ad.php?t=12967

Rob

[Zilch]

For horns with exponential, or Tractrix flare, the most rapid horn wall curvature changes occur towards the mouth of the horn, with the sides exhibiting less curvature near the throat. This means that waves will only diffract into the full angular coverage, defined by the exit angles of the mouth, for wavelengths which are longer than the axial length of the horn. For shorter wavelengths, the coverage pattern approximately conforms to the angle of the horn walls at the ½ wavelength axial position. For most of the other horn curves in fig 7, the angles range from 8°-12° at an axial distance of 3”, meaning that these horns (exponential, Tractrix, hyperbolic and spherical) will be unable to support a beam width of more than +/- 10° at 9kHz and above.

The refractive lens mitigates it in the horizontal, but at expense to the vertical:

http://audioheritage.org/vbulletin/s...9&postcount=74