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Dud HF response on one speaker (Ls3/5a)


Steffen
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I'm posting this in the DIY forum, hoping for some real expertise :) Also, this might turn into a DIY situation...

 

I'm setting up a new listening room and took some preliminary measurements prior to room treatment, final speaker placement, etc. In these measurements I noticed a curious dip in HF response from one of the speakers, around the 5.6kHz mark (about 8dB down), and to a lesser extent at 9kHz and 13kHz, before the speaker catches up again around 15kHz. Please see the REW plots below.

 

As you can see, the room response introduces a fair amount of unevenness, but the HF dip remains when the speakers swap places in the room. It also remains when I swap speaker cables at the back of the amp, so the issue is not generated by the electronics. What I'm wondering is, does this point to a dud component on the crossover, or to a dud tweeter? I'm thinking the former, since the tweeter manages to catch up at higher frequencies.

 

I hadn't noticed this issue with these speakers before, but I haven't measured them in a while. The speakers are Rogers Ls3/5a (11 Ohm) from ca. 1992. Crossover frequency is 3kHz. Both speakers are at the exact same distance from the microphone position and equally spaced from front and side walls.

 

L-R-graph.thumb.jpg.279ad713a38f555d85e3378e55680e5c.jpg

Edited by Steffen
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It could be a faulty or out of spec component in the HF crossover.  If you are handy with a soldering iron and have access to a LCR meter it will quickly identify the out of spec or defective part.  Other issues could be a dry joint on the PCB.  

 

Cheers,

Alan R.

LS3-5A 11 ohm crossover.jpg

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Thanks guys, sounds like some surgery may be required. Which is a bummer, since these cabinets are near impossible to open. Some 20 years ago I sealed the baffles with Blu-Tack, to make the speakers pass the "box test" (Ls5/3a aficionados will recognise this). That turned out to be a mistake... (What's a good material for sealing speaker cabinets BTW?)

 

Oh, and the measurements are unfortunately perfectly repeatable. I measured many times in different configurations, hoping this was a fluke, but that 5.6kHz dip always shows up prominently.

 

Ah well, there's hoping it's a crossover component, they're much easier to replace than a T27.

 

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 it's been a while since i opened up one of these but i thought you just accessed the crossover from the front although it would be good to swap the tweeters over to save some time and rule out the driver as if you do have to go through the cross over it is very complicated

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Yes, swapping the tweeters sounds like a good idea to pinpoint the issue. And yes, that crossover is quite complex. On the other hand, this means I have to open both speakers. Then again, that may be a good idea anyway to exchange the same part in both speakers...

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Tweeter measurements should be taken at 0.5m distance - the farther you move the bigger chance is picking up a side reflection or two. Make sure that the measurement is correct before you start replacing crossover components. Although they do measure like completely different speakers. Also keep in mind that tweeters really need to be matched if you want the exact same response. Not even Scanspeak can achieve to make two absolutely identical tweeters, AND you are talking about speakers from 1992.  

Edited by Decky
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Alright, thanks everybody for your suggestions. I swapped the tweeters keeping everything else the same and took near field measurements on axis (1kHz-20kHz, windowed at 5ms left and right) before and after. The bad HF response followed the tweeter. Looks like I'll be shelling out for a new pair of T27s.

 

And yes, the distortion measurement was a good clue. On the good speaker distortions fell steadily with frequency, whereas on the bad one they shot up just before the crossover frequency.

 

I'm looking forward to my new LS3/5as :) 

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  • 5 weeks later...

By way of an update, I've replaced the tweeters with new drivers from Falcon Acoustics (thanks Jerry!), and the speakers sound like new. Tweeter response on close-range measurement and 1ms window is now identical for both speakers and much flatter, distortions are mostly gone. I can't seem to window the distortion plot, so I can spot a nearby resonance from a picture frame in there :) 

 

I also got new damping foam and gasket strips from Falcon. On the cross-overs I replaced R4 and R13, they looked rather toasty (probably related to the tweeter damage).

 

IMG_3010.jpeg.f42bbf69de36bf045a5ad1768bda966b.jpeg

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