Crossover For 123A-1 and LE5-2

[GordonW]

Harvey, did you ever get a chance to tinker around with any of the S-70 Alpha or Alpha III systems? The idea of a 123A in 6 cubic feet with a passive radiator has sounded quite interesting to me.

In fact, I've assembled the drivers (123A, LE5-2, LE20) to BUILD a reproduction S-70... as soon as I get around to building some cabinets. The only change from original, is that I'm using PR300s rather than PR12s... I've found that "looser" suspended passive radiators (foam surround) seem to sound better than stiffly suspended ones (fabric surround)... the bass is usually smoother and less 'boomy' even with the same f3 point and same effective mass on the radiator diaprhagm.

I'm probably going to try the C533, the original L100, and the later L100 crossover designs in it, to see how they compare, both listening and testing with LMS/LEAP. I will also be designing a crossover, from scratch, in LMS/LEAP, to see what these drivers can do with "modern" thought in crossover design factored in.

Regards,
Gordon.

[4313B]

Thanks Harvey!

Very cool Gordon!

[4313B]

Here's a few more networks

[AlexanderS]

Hi, all,

my 5 cents. I am trying now to cross 123a1 with another silk dome tweeter, and measurements (Bruel & Kier high accuracy meter) show 123 is peaky at 5 kHz. It rolls of smoothly up to 2 kHz, and then makes a peak at 5 kHz.

This makes me a doubt how JBL used it in full range.

I can place the curve.

Anybody met the same?

Alex

[GordonW]

If you would, please do post the curve. My take, is that by 5KHz, the response of the 123A is probably down enough, that the peak doesn't go above the "0 dB" line... ie, even at the top of the peak, it's BELOW the passband level of the driver. In those days, this was probably thought to be good enough... it wouldn't really upset the overall FR curve, though it might wreak a bit of havoc with polar response...

My take, would be to "assist" the rolloff at 2KHz, with first order crossover and a Zobel, and maybe additionally a notch filter at 5Khz. Given that, you should be able to use a GOOD, EFFICIENT silk dome tweeter successfully. Might I suggest the SEAS 27TFFC? It's one of the only silk domes that simultaneously has a) enough low-end excursion- ie, it can handle low frequencies (between 1500 and 2500 Hz) without distorting or sounding strained, and b) enough EFFICIENCY, to actually KEEP UP with a 123A, which is effectively about 94 dB/2.83V. I'd be inclined to put a 2KHz second-order highpass filter with a Q of about .577 on the 27TFFC, and just tweak both the highpass and the inductor value on the woofer, to get the best possible blend...

Regards,
Gordon.

[AlexanderS]

That's the curve, JBL123-A1 is in free air, so there is normal roll off at low end.

Done with my 2a3 SE amp (it has flat responce, and about 2.5 Ohm output), the measurement is in 1/3 octave pink noise, with Bruel meter.

Alex

[Wallacefl]

I have an old black pair of 123a woofers from an early L-88 and have looked at using the S70 Alpha III crossover design but would like to alter the midrange high pass from 7000hz down to 3500hz. Any recommendations would be appreciated. My goal is to avoid the up sloping peak in the LE5-2 midrange.

[Wallacefl]

I have an old black pair of 123a woofers from an early L-88 and have looked at using the S70 Alpha III crossover design but would like to alter the midrange high pass from 7000hz down to 3500hz. Any recommendations would be appreciated. My goal is to avoid the up sloping peak in the LE5-2 midrange.

No thoughts?