JBL 2452H: what minimum frequency cut-off used with PT horn?

[Massilia]

(already posted in PA section)

Hello,

I use a 3-way triamp'ed system built of a 18" 2268H, a CMCD-81H (8" 2169H driver) and a 2452H 1.5" driver loaded by a 60° PT Horn (JBL progressive transition horn).

Cut-off frequencies are 350Hz and 2KHz, active, 24dB/oct slopes, with parametric EQ to make the response almost flat using a XTA digital active crossover.
-> So the 1.5" 2452H is used between 2kHz and 20KHz.
The size of the PT horn is about 10" X 5".


The natural measured frequency response of the 2452H with the PT horn is extended below 2kHz.

Problem:
At rather high sound level (at home, let's say above 100dB/1m), it becomes a little agressive in the mid frequencies. CMCD 8" exhibits very little distortion and should not be the problem. Furthermore, its unsmooth response above 2kHz is strongly filtered (24dB/oct slope + EQ attenuations)
Hearing close to the loudspeakers, it seems indeed this is the 2452 who sounds harsh.

So here are my questions :
. what is the lowest recommanded frequency cut-off for a 2452H with such of horn ?
. Is there some distorsion problems with the 2452H used above 2kHz?
. if 2kHz is too low for that 1.5" / PT association, should a JBL D2430K driver (the HF section of the JBL M2) be the solution? in the M2 the cut-off is 800Hz, but the horn area is a lot higher than the PT one.
I note M2 HF horn is not usable for my system since its dispersion is 120° width, against 60° for the PT horn (and also the 8" CMCD).

Thanks in advance,

Regards

Bruno

 

[weltersys]

Cut-off frequencies are 350Hz and 2KHz, active, 24dB/oct slopes, with parametric EQ to make the response almost flat using a XTA digital active crossover.
1)So the 1.5" 2452H is used between 2kHz and 20KHz.[/B]
The size of the PT horn is about 10" X 5".
2)The natural measured frequency response of the 2452H with the PT horn is extended below 2kHz.
3)At rather high sound level (at home, let's say above 100dB/1m), it becomes a little agressive in the mid frequencies. Hearing close to the loudspeakers, it seems indeed this is the 2452 who sounds harsh.
4)What is the lowest recommanded frequency cut-off for a 2452H with such of horn ?
5)Is there some distorsion problems with the 2452H used above 2kHz?
6)if 2kHz is too low for that 1.5" / PT association, should a JBL D2430K driver (the HF section of the JBL M2) be the solution?

Bruno,

1) The vertical height of the horn would loose vertical pattern control below 2000 Hz.
2) That would indicate crossover frequency is not a problem as far as excursion.
3) "Flat" response can sound "aggressive" or "harsh", especially with music with lots of compressed mid content. Above 100dB at one meter the distortion in the HF driver begins to rise, you may be hearing that. Any trash in the HF coil gap can make the driver sound bad, but that generally sounds worst at lower levels. It is also possible that the diaphragm has small tears in either the surround or the many embossed figures it has, which really sounds bad.
4) That would be power/excursion dependent, but at 2kHz you should not be running anywhere near those limits at only 100 dB at one meter. The specification sheet for the particular cabinet should give recommended crossover frequencies. The 4" diaphragm can be used down to 500 Hz or lower on horns that load lower, your crossover frequency should be no problem.
5) Yes, though they are fairly typical of compression drivers, double digit distortion at over a few watts in the upper operating range.
6) The D2430K should have a bit less distortion than the 2452H, but no miracles- perhaps 3 dB less 2kHz and up.

Art

 

[Massilia]

Hi Art,

Thank you very much for your detailed and expert response.

So, perhaps a problem with the compression (diaphragm corrupted). I will make some FFTs at different frequencies and SPL to check it.

Thanks again

And have a merry christmas!

Bruno

 

[weltersys]

Hi Art,

Thank you very much for your detailed and expert response.

So, perhaps a problem with the compression (diaphragm corrupted). I will make some FFTs at different frequencies and SPL to check it.

Thanks again

And have a merry christmas!

Bruno

Bruno,

Merry Christmas to you too!
A slow sine wave sweep will reveal diaphragm coil rubbing or cracks very audibly, sweep from around 100 Hz up, starting at a fraction of a volt, go up to 2.8 volts for 8 ohm, 4 volts for 16 ohm.

Art

 

[Massilia]

Thank you very much Art! I will try it,

Thanks

Bruno