Fixed L-Pads

[Zilch]

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 ) .

Indeed.

Notwithstanding wonky L-Pads, the adjustable wants to "see" 16 Ohms. With the 20-Ohm conjugate, an 8-Ohm series resistor accomplishes that. I don't understand why JBL used 20 Ohms between and 8-Ohm L-pad and and 8-Ohm driver in N3134/5.

In any case, adding that in stabilizes the L-pad (somewhat), and the new "input" impedance measures 6.3511 Ohms at 800 Hz (below).

For the record, I ran impedance curves on four LE85s on H91 horns. The one I have been using for these determinations is the red one (middle). It's not an "oddball."

For convenient comparision, I've reposted the WT2 impedance measurement on it immediately below the CLIO determination (bottom).

[I'd call them, uhmmm, "similar." ]

[dmtp]

Here are some FR plots (hope the pdf format works, not sure how to save graphs as jpgs)
Legends:
LE14A XO is close miked in box
LE85 XO is at same voltage as LE14A using standard L200B XO with fixed 2.5/5 L-pad in circuit, 16 ohm L-Pad set to 12 o'clock, and boost variable resistor (1/2 of 16 ohm L-pad) set to 5 ohm
1 ohm is same with variable high pass set to 1 ohm
1 uF is with coil and resistor by-passed
1.5 uF is just a 1.5 cap as high boost

[Earl K]

Hi Mark

- Yes, I can "read" this pdf . Those graphs actually do make some sense, given the scenarios you just tried.
- Though, now I'm wondering somewhat about the overall health of the diaphragms in your le85s / mind you / I have already expressed my reservations about believing an SPL meter from RS ( over 10K ) .

- I'll get back to you later today with an alternative schematic to try out.


- ( I assume these measurements were taken with your 6.5K "trap filter" , "out of circuit ". Correct me if I'm wrong )

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[dmtp]

Yes, this was without the trap.
I have impedance measurements for the LE85/horn combo, but I'm not sure what else would be helpful to measurefor impedance.
I have no way of knowing about the RS SPL meter (it does have A and C settings, but I forgot what they meant), on the other hand , my ears give out much above 14k anyway so perhaps it is moot. (Must have been those Moody Blues concerts in my youth!)

[Earl K]

I have no way of knowing about the RS SPL meter (it does have A and C settings, but I forgot what they meant),

Oy Vay !

- Those "A & C" filter settings have different amounts of LF & HF rolloff ( for reasons of obtaining specific types of noise measurements / for the enforcement of civic bylaws ) .

- Put it on the "C" setting . That gives the broadest frequency spectrum.
- BTW; the "official" C weighting is already 5 db down at 10K ( according to my books ).
- I have know idea what the RS SPL meter is actually "weighted" to.
- This is why "designing" networks with these things as a test mic is a really bogus idea .

on the other hand , my ears give out much above 14k anyway so perhaps it is moot. (Must have been those Moody Blues concerts in my youth!)

- My hearing also plummets at around 14K / mostly age related . I only "sense" pressure beyond that point .

- I'll get back to you later today with an alternative schematic to try out.

- Here's the schematic for you to build from what I hope is mostly existing parts .
- Maybe Zilch would like to run this through his Spice program. ( I really don't mind the help )

Some Points to ponder ;

(i) I've included my best guess for the values of the "Series" LCR strapped in parallel . Use your variable pot. to dial in the notch. A .25mH coil can be cludged together from your present .1mH and .15mH coils ( by connecting the two of them in series ). Don't stack them ! the mutual inductance from stacking will give a coil value that is smaller than .25 mH.
- If the "Q" of this filter is way off / you'll see it in new FR measurements. Let me know and I'll help you redesign for a different "Q" .
- This LCR is tuned to 6,366 hz ( Oh by the way ) / mostly because it should be able to use your current parts .

(ii) The attenuation obtained from the fixed Lpad has been increased to over 8 db.
- The 3.8 ohm resistor value can be easily obtained by wiring 2, 7.5 ohm resistors in parallel ( 3.75 is close enough ). This is likely a good idea anyway / to double the power handling of those buildout resistors .

(iii) The 8 ohm resistor that is immediately after terminal 2 of the variable Lpad is a isolation/safety resistor.
- Feel free to replace this value with something between 4 and 8 ohms ( whatever is on hand ) .
- The use of this resistor is mandatory for this newer circuit arrangement .

(iv) The 2 uF "bypass" cap is really just a guess now. Do some empirical studies by substituting in values from 1.5 uF up to 3 uF.
- The larger this cap gets / the more ( excess himid ) you're going to need to cut ( with the notch filter ).

(v)Oh yeh, for the time being, I've left out the "buried" coil under the fixed Lpad that would be needed to create a frequency dependant Lpad .

Let me know how this all goes .

EDIT ; ( some hours later )
- Mark don't build the circuit in this post . Build the last variation below in post # 54 . Zilch published this, this afternoon .
- ( Keep the fixed Lpad the way it presently is, with original values ) .
- Add my series LCR / strapped in parallel from this post ( only if you need to still tame the 6500 hz bump )
- Keep your 16 ohm variable Lpad in place and just add the 8 ohm resistor directly to terminal #2 of the pad .

[Zilch]

Adj = -6 dB
Notch = 10 Ohms

[Earl K]

Zilch

Notwithstanding wonky L-Pads, the adjustable wants to "see" 16 Ohms. With the 20-Ohm conjugate, an 8-Ohm series resistor accomplishes that.

- That's the "R" value that I included in the modified N200b .

I don't understand why JBL used 20 Ohms between and 8-Ohm L-pad and and 8-Ohm driver in N3134/5.

- Actually, the 3134(s)' 7.5 ohm conjugate when paralled with the AC load of the 2425h/2344 horn/driver will average down the AC impedance spikes to around 5.3 ohms ( looking at the big 18 ohm spike seen at around 1100 hz ). - So logically, a resistor value of around 3 to 5 ohms would have made more sense ( if the goal was strictly to provide some resistive isolation and impedance matching for the 8 ohm variable Lpad ).
- But, I think those were just secondary goals with the primary goal being to create the proper conditions for a variable "bump filter" to work from.

- Giskards' Voltage Drives that I have reposted above, clearly show the variable bump filter in some "wild" resonant action .

[Earl K]

Adj = -6 dB
Notch = 10 Ohms

Thanks !

[Zilch]

Well, here's more to consider, using the original bypass:

I like killing it above 20 kHz.

No joke, actually. I suspect part of what makes two-way "boosted" compression driver HF sound nasty is subharmonics of overdriven UHF above what we can hear.

I've thought that since I first saw 311xA and N200b voltage drives.

That's why I'm showin' it out to 40 kHz.

[Zilch's theory #117.... ]

[Earl K]

Thanks Zilch.

- It's amazing how effective that LCR boost actually is . Mark, I'd keep it in based on what Zilch just posted.

- I'm pretty sure that ( MarkT ) does need a notch filter of sorts .

- I'm concerned that the circuit ( both variations ) now look like there's too much overall attenuation .
- Is that why you went back to the original values for the fixed pad ?

- I figure the drivers' sensitivities are really only a 12 -14db spread ( when considering the different impedance of the drivers and size of that horn ) .
- What do you think ?

[Zilch]

I think those were just secondary goals with the primary goal being to create the proper conditions for a variable "bump filter" to work from.

Also, N3134/5 has no fixed padding. The 20 Ohms is generating the requisite attenuation, it would appear....

[Zilch]

- I'm concerned that the circuit ( both variations ) now look like there's too much overall attenuation .
- Is that why you went back to the original fixed pad configuration ?

Yes. I'm showing ~15 dB differential with -6 dB on the adjustable, so at "zero," it'll just be ~9 dB. We're in the range.

While Mark's original question here was how to get the additional attenuation he believes he needs, putting in that 8 Ohms gives ~3 dB more, so I kept the original fixed L-Pad values.

He can easily adjust it there, if required, now that we know the impedance at that point is 6.35 Ohms.

I'm not certain he's going to need a notch, since the midband attenuation slope is steeper, but that's easily added as well.

I'm going to try and resolve the question of how good a mic the RS meter is, also. If it's good, then his driver isn't generating VHF, or his horn is killing it.

LE85 plays WAY better up top than Mark measures....

[Ian Mackenzie]

Also, N3134/5 has no fixed padding. The 20 Ohms is generating the requisite attenuation, it would appear....

Zilch, R5 in the schematic is used to trim the level of the driver overall, the values in both 3134 /35 are different, 10R in the 3135 to increase the level to match the 2234's, only a couple of db.

[Zilch]

Zilch, R5 in the schematic is used to trim the level of the driver overall, the values in both 3134 /35 are different, 10R in the 3135 to increase the level to match the 2234's, only a couple of db.

Thanks, Ian. I see that now.

These references are to the N3134/5 schematics:

R4 sets the compensation differential, and R5 then establishes its overall balance with the woofer(s).

[It's NOT merely a gratuitous conjugate.... ]

[Earl K]

I'm going to try and resolve the question of how good a mic the RS meter is, also. If it's good, then his driver isn't generating VHF, or his horn is killing it.

- That'll be very useful .

LE85 plays WAY better up top than Mark measures....

- Yes, I remember ( from the Q&D 4430 & the Widget studies ) . I wonder if Mark has seen all the relevant info on the le85

I'm not certain he's going to need a notch, since the midband attenuation slope is steeper, but that's easily added as well.

- Yes, he'll find that out soon enough . If Mark does needs it, this thread ( on padding ??? ) now contains, virtually all the formulae I know for manipulating LCR filters ( apart from determining what value of "R" should be used ) .

- I'll admit, I was surprised the 2 uF cap extended its "energy adding effects" down so low / well down into the 2K region of the midband ( I guess it was due to that the extra padding that was put inplace ). All along I've been saying the Notch Filter was going to need to deal with some extra energy up in the "presence" frequencies / but I wasn't expecting this much extra energy down lower / a properly designed LCR wouldn't reach that low ( assuming the Q was correctly calculated ) .