Fixed L-Pads

[Zilch]

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....

[Earl K]

The boost is spread from about 3k or 4k to 20k (or beyond)

- That ( curve ) looks quite normal when the Variable Lpad ( MF ) that controls the midrange content ( in the horn circuit ) is dialed down too low / and the HF "bypass circuit" is allowed to run too hot .

WHAT did I do wrong????

- I'll suggest that there is a significant conceptual problem that needs to be addressed first.
- Also, for the moment, consider not using in this HF bypass circuit, the series LCR (" for boost" ) .
- Just use a plain cap ( no coil ) until you understand how a simplified "HF Bypass Circuit" works and how you can manipulate it .

(i) The overall "HF Boost Circuit" ;
- It's a bit of a misnomer because generally , passive circuits do most of their work through attenuating signas .
- A "HF Boost Circuit" is really a "HF Bypass Circuit".
- It needs to be designed to work "hand in glove" with the MF attenuation circuit .
- ie; Dialin too much attenuation into the midband and ( & by extension ) a wide open HF bypass circuit will appear to offer way too much HF content ( or HF/MF content of the wrong type ).

- The typical HF bypass circuit ( with no LCR , just a C and an R ) allows one to send the bypassed High Frequencies directly to the horn driver at a predetermined F3 point ( say 20K hz which is chosen through the value of the capacitor within that bypass circuit ).
- Everything one hears ( through this HF bypass circuit ) / is
below the chosen F3 point,. It's down in the caps rolloff area ( the "stopband" ) and this HF content is being attenuated at a rate of 6 db per octave .
- It's worth repeating that typically ( in JBL "bypass" networks ), one is usually listening to HF content that is below the F3 point of the cap ( Altec is different ). A lot of the audible HF content is down in the capacitors' "stopband" ( which is below the caps F3 point ).

(ii) An Example:
- Since "F3" is already 3 db down ( chosen as 20K hz for this example ), another octave lower ( 10K hz ), the HF bypass cap has "allowed in" frequencies ( in the 10K area ) which are 6 db lower ( than 20K ). This gives total HF attenuation ( again at 10K ) of 9 db.
- Now, if one has dialed in 15 db of midband attenuation ( like you've probably done in the variable Tpad ) the HF bypass circuit will continue to addin MF to content at the constant attenuation rate of 6 db per octave . So one will get this unwanted "MidRange content , 15db down (in the 5 to 10K region ) more or less because that is where the "Tpad" kicks in it's contribution of frequency content . ( Believe it or not , I've tried to simplify this and may still need to edit a lot of this ).

- The above example outline the general HF bypass concept . It still doesn't touch on all the wrinkles that come into designing a realworld, "working" circuit .

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

- As far as I can recall, you still haven't run ( & graphed & published ) an AC impedance study ( measured at the horndriver, with a fully built network in place ). I did see the woofers' impedance curve ( with Zobel ). Therefore, one doesn't really know what the circuits' "working impedance" is at that particular spot ( just after the "T" pad ). We still don't know what AC impedance the HF bypass circuit actually "sees" . So 7.5 ohms is still just conjecture.
-Therefore , choosing new values for the passives in your HF bypass circuit is still a crap shoot .

(iii) "R"
- The "R" ( as is typically used in a HF bypass circuit ) is there to limit the drive voltage of the circuit. It's usually accomplished through a 0 to 30 ohm pot. ( & is usually located at the start of the HF bypass circuit ). This resistance, controls the voltage flow through this part of the circuit.
- Since you referenced my first post in this thread that only just touched on basic LCR theory, I'll assume you've tried to work "R" into your LCR resonance formula . I wouldn't do that in this case. Think of the "R" in a HF bypass circuit as mostly controlling the voltage flow through the circuit .
- OTOH, "R" is a very critical component of the LCR formulae when one is deriving the needed component values for an effective series notch filter ( strapped in parallel across the load ) .

What remedies do I suggest ?

- Build one HiPass circuit "exactly" as Zilch has outlined . But be aware, this may not be your final circuit . This will be a worthwhile exercise to see if you can duplicate Zilches circuit / as well as his published FR measurements . You gotta "walk before you can run" .

(iv)Comments ;
- It's really a mystery to me why your results are so drastically different than Zilches', especially when you state that your circuits are virtually identical to his.
- Also, considering some of the early measurement and wiring gaffs that I'm aware of ( from the last couple of months ) / I'm inclined to think the fault lies somewhere in those areas . That's why I believe it's a much better idea to see if you can simply duplicate Zilches efforts ( before tweaking this circuit for your particular horn ).
- He's published many graphs that show what happens when he turns the "drive" pot up or down ( in his HF bypass circuit ) . His circuit works. When he adjusts the ( HF ) drive-pot , only frequencies from 10K and up are adjusted.
- I also know that he has properly balanced the le14 woofer to the MFs' variable Tpad ( since he also published those studies ).
- Therefore I know all parts of his circuit, balance out well / and appear to be virtually finished .

[dmtp]

That ( curve ) looks quite normal when the Variable Lpad ( MF ) that controls the midrange content ( in the horn circuit ) is dialed down too low / and the HF "bypass circuit" is allowed to run too hot .

I didn't state, but the L-pad (MF) is set at mid position, the circuit has both the fixed 2.5/5 L-pad and the following 8 resistors. The HF bypass had the 16ohm Lp2 but I set it at a measured 5 ohms to match my 'calcualted' R.

- The typical HF bypass circuit ( with no LCR , just a C and an R ) allows one to send the bypassed High Frequencies directly to the horn driver at a predetermined F3 point ( say 20K hz which is chosen through the value of the capacitor within that bypass circuit ).

OK, if I understand this right and if we accept 7.5 as the working impedence (more later), and an F3 of 20k, that would mean a C=1.06 = 1/(2*pi*Xc*F) which is what we had. I will try putting Lp2 before C and try measuring that.

As far as I can recall, you still haven't run ( & graphed & published ) an AC impedance study ( measured at the horndriver, with a fully built network in place ).

I'm confused as to exactly what to include and where to measure? Do I take my measurements at the terminals of the LE85 with the freq generator driving through everything (except the HF bypass)? What about the Lo Pass (LE14A) part of the network? Is that attached too? And I assume I still calibrate the setup by substituting the 10 ohm R for the LE85 and set my DVM to read 10mV through the whole network?
I did the LE85 alone and it is shown below:

[Zilch]

Here's the impedance curves from earlier in this thread. The elements are out of the circuit:

http://audioheritage.org/vbulletin/s...871#post126871

See the headers for what they are.

[dmtp]

Interesting. As Bruce suggests, the tractrix seems to have far less variation than the exponential horn in terms of peaks and valleys. (of course I only started @ 500 so did not include the resonant peak - I was worried about driving the LE85 below 500 - probably needlessly. My curve is also based on descrete measurements @ 1/6 octave so might have missed some.

[Earl K]

MarkT;

- Please confirm that your le85s still have their original "16 ohm" diaphragms.
- Are the red wax seals still in place ?
- Measure the DC resistance of the coils and post the result .

I'm confused as to exactly what to include and where to measure?

- Well, not to worry, nobody ( to the best of my remembrance ) has ever done this before and posted the results to this forum .
- ie; it's new ground that's never been covered to date .
- Measure at the compression drivers' terminals .

Do I take my measurements at the terminals of the LE85 with the freq generator driving through everything (except the HF bypass)?

- Yes, the freq generator drives signal through the horn portion of the network .
- Take measurents with and without the HF bypass in place .

What about the Lo Pass (LE14A) part of the network? Is that attached too?

- At this time, it's not necessary to include the woofer portion .

And I assume I still calibrate the setup by substituting the 10 ohm R for the LE85 and set my DVM to read 10mV through the whole network?

- The calibration process is the same ( just as you've described ) .

I did the LE85 alone and it is shown below:

???? Really ????

- Your posted "Z" curve is quite close to what I would expect to see from a measurement that was taken with the HF bypass "shunt" in place ( over a 16 ohm Lpad ) / but feeding into a 8 ohm diaphragm ( 5.??? ohms actually ), & with the 20 ohm "load resistor" strapped across the driver . ( & Measured before the variable Plad ). Hence my first piece of direction within this post .
- Since "my scenario" doesn't match your written text, I'll have to believe that there's a problem with your impedance measuring methods.

- Look at Zilchs' impedance studies HERE
- Yours' should approximate one of his from that batch of "Z" runs. The HF ac impedance isn't greatly effected by the difference between your Tractrix horn and that of a H91. Again , " I'll have to believe that there's either a problem with your impedance measuring methods " or you aren't measuring a real le85 diaphragm . There are many impedance curves of le85s on this forum ( most done by Zilch ) and none of them decline to 3 ohms at 20K ( or even 5 ohms at 10K ) . In fact, those curves typically climb after 10K unless there's alot of resistive padding in place .

[dmtp]

Just use a plain cap ( no coil ) until you understand how a simplified "HF Bypass Circuit" works and how you can manipulate it .

It will probably be a couple of days before I can get the impedence maeaurements, but I did some more SPL measurements.
I am using the entire network as published by Zilch earlier "as built" with 7.5 ohm resistor for LE14A EXCEPT NO HI BYPASS.
This was taken with the MF L-pad @ mid, 16ohm section of Lpad2 ahead of 1uF cap. The curves are NC, min, mid, max;
This is the HF detail

Now with the MF down to 9:30 (NC, min,mid,max)

Now with MF @ max (NC, min,mid,max)

Obviouly the amount of upper mid is greatly effected by the MF control position as EarlK suggested it would be, but the HF (>10K) is still pretty minimal.

[dmtp]

- Please confirm that your le85s still have their original "16 ohm" diaphragms.
- Are the red wax seals still in place ?
- Measure the DC resistance of the coils and post the result .

Original as far as I know. red wax seals intact.
DCR = 6.9 on each

[Earl K]

Interesting. As Bruce suggests, the tractrix seems to have far less variation than the exponential horn in terms of peaks and valleys. (of course I only started @ 500 so did not include the resonant peak '''snip,,,,,, My curve is also based on descrete measurements @ 1/6 octave so might have missed some.

- I believe this "tractrix" assertion of yours is based on "wishfull thinking" as much as anything concrete. ie; I don't believe your impedance studies .

- It would really be worth your while ( and ours ) , if you would practise duplicating Zilches' AC impedance measurements. Use your le85 ( on a H91 ). Adhere to the circumstances employed by Zilch ( in post # 18 ) . ie ; match the various pad settings , & use ( or don't ) the 20 ohm load resistor .

[Earl K]

Original as far as I know. red wax seals intact.
DCR = 6.9 on each

Okay, thanks for that confirmation .

[Earl K]

Mark,

Re; the new FR studies in post # 127 .

I am using the entire network as published by Zilch earlier "as built" with 7.5 ohm resistor for LE14A EXCEPT NO HI BYPASS.

A recap of conditions ;
(i) - Built with the same values as Zilches "modified" N200 .
(ii) - Built without a HF bypass circuit in place .
(iii) - A 7.5 ohm dummy load standing in for a real woofer .

Observations;

(i) The MF variable Lpad is not working correctly.
- It should be equally attenuating all frequencies, down to the hipass crossover point of @ 850 kz ( not just the higher frequencies ) .
(ii) Since it's obviously not accomplishing equal attenuation we need to find out why .

Some Remedies ;

(i) Remove the 7.5 or 8 ohm resistor that was inserted after the variable Lpad / making it into a "T" Pad .
- Redo your measurements with that resistor out of the circuit .

(ii) Make sure you've wired the 16 ohm variable Lpad, correctly into the circuit .
- With the shaft pointed towards you, the terminal numbers are ; # 3, #2 , & then #1 .



PS ;

This was taken with the MF L-pad @ mid, 16ohm section of Lpad2 ahead of 1uF cap. The curves are NC, min,

- This statement contradicts the line above it ( which I have quoted at the beginning of my post ).
- ie; You actually have built a HF bypass circuit .
- Therefore, once again , I don't know what circuit conditions where actually employed for these pics .

[Zilch]

I'm looking at Earl's model calculations in this post:

http://audioheritage.org/vbulletin/s...&postcount=109

which is telling me the L should be .06 mH (0.0597) for 20 kHz,

and if you're using 0.1 mH, you're shutting down the boost at 12 kHz (11.937).

I'm gonna go do the measurements I promised now....

[dmtp]

Quote:
Originally Posted by MarkT
This was taken with the MF L-pad @ mid, 16ohm section of Lpad2 ahead of 1uF cap. The curves are NC, min,

- This statement contradicts the line above it ( which I have quoted at the beginning of my post ).
- ie; You actually have built a HF bypass circuit .
- Therefore, once again , I don't know what circuit conditions where actually employed for these pics . Today 01:09 PM

Sorry for the confusion. The bottom curve is with NO HF BYPASS (i.e. NC= not connected). I then did three curves with the HF BYPASS consisting of a 16 ohm section of L-pad followed by just a 1.0 uF cap as suggested. L-pad set @ min, mid, max. So we have 1 curve with NO HF BYPASS and three with variable R preceding the cap. The same is true for the following graphs showing MF @ 9:30 and max. Four curves - lowest is NO HF BYPASS, then min, mid, max on 16 ohm
If you compare the green (bottom) curve of each of the three posted graphs, you will see that the L-Pad #1 DOES change the entire curve from 95dB, to 100dB to 105dB (eyeball average). Again, sorry for the confusion on the posts, I probably should have shown the three MF settings on one graph. I was trying to see if the position of the MF did effect the HF effect as EarlK had suggested earlier. Of course, it did. I think everything is working as it should except very little effect of HF bypass.

[Zilch]

This is LE85 on exponential HL91, which approximates your quasi-exponential Tractrix.

1) Disconnected HF boost, what's it look like with no compensation? Familiar, is what. Red

2) Boost reconnected, MF and HF at mid setting, also familiar, Orange

3) Same, but kick the HF boost to max, Green.

4) Change L from 0.08 mH to 0.06 mH. Yup, VHF rises. Blue

5) How 'bout pushing it a bit further, to 23,873 Hz with 0.05 mH. Yup, a bit better, but the returns have diminished. Violet

6) O.K., now let's work the low end. Change C from 1.0 uf to 0.5 uF. Good. Cyan

7) Can we lower it more? 0.33 uF pulls down the VHF more, nearly back to where we started with no boost, so that may be too low. Black

So, use L = 0.05 mH and C = 0.5 uF, then apply your notch filter to the result. That's about the best we're going to get with this circuit and your driver/horn combination. Try that and post your result.

There's other topoligies which might be used to "tailor" compensation above 12 kHz only if you find this simply doesn't work for you.

I'll go listen now....

Edit: Sounds fine, but quite beamy in the vertical. I gotta be smack on axis to hear the cymbals' sizzle. Full range, bottom. We accomplished exactly what Earl predicted would be achievable, i.e., 9 dB of boost at the top.

I did try one of the alternative topologies on hand here, AM6212/00. It virtually superimposes the cyan curve.

[dmtp]

Thanks, Zilch. Although I haven't gotten to trying to duplicate your impedence measurements yet (Work getting in the way again!), I am glad to see that you have essentially duplicated my results, so it looks like we are dealing with the same things.
I agree that the cyan curve looka best and it just so happens that two 0.10mH coils can make 0.05 and two 1.0uF caps can make 0.5 so I have on hand enough to duplicate that for a trial (may be a few days).

So, use L = 0.05 mH and C = 0.5 uF, then apply your notch filter to the result. That's about the best we're going to get with this circuit and your driver/horn combination. Try that and post your result.

For the notch (LCR || LE85) would you just focus on the range from ~ 5.5k-9.5k or would you go al the way to 14k? I'm worried that going to 14k will exagerate the existing notch @ 9.5
I'll measure on my horn before building LCR, but it looks like ~4 dB centered @ 7.5k with a bandwidth of 4k (which is a Q of ~0.5, right?)