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Old 12th July 2004, 05:01 AM   #1
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Default Crown XLS Problem

Hi all. I've got a question about a bit of a problem I am having with my Crown XLS 402 amplifier. I am using this amp to power a 15" Adire Tempest of mine, and for over 6 months have used the amp to power the Tempest in a stereo 8Ohm configuration (each 8ohm coil gets a seperate output from the Crown) in a ported box. Recently however I became tired of the 4cuft box and decided to go w/a small sealed with a linkwitz transform. In order to compensate for the higher power requirements needed when using a linkwitz transform, I decided to try the Tempest in a 4Ohm (putting the voice coils in parallel) and bridging the Crown amp w/this 4Ohm load. I changed the input XLR connectors as per the amps instructions (invert the second channel) and all seemed well upon initial testing. However, I have found that if I crank the volume on my preamp, the sub begins to make what sounds like a loud cracking noise. My intial thoughts were the speaker was damaged somehow, but upon further testing I don't believe that to be the case. I was able to drive the sub very hard using 90Hz and 35Hz tones with other equipment and did not get any crackling. So the preamp/amp was left. I dragged out the scope and checked the preamp during while the crackling was taking place and the preamp looked good, so it must be the amp. I put the scope in parallel with the speaker, moved the volume on the preamp up, and watched the signal. When the crackling took place I could view for a split second large spikes. So I used the store function of my HP scope (1MB memory per channel yay!) and kept storing until I finally got a spike stored. Upon examining I found that this noise wasn't just one spike but three. There were two "digital" looking spikes (meaning half of a square wave) maybe a few (5-10) microseconds apart and with a width of maybe 5 of less microseconds each w/a value of about +60volts on the crest of a 30v p-p wave and on the trough of the same wave there was another spike of sorts, but not digital looking, more exponential looking in nature and in the negative direction w/a value of -60 volts. This spike's width was a bit wider than the "digital" spikes.

I was a bit baffled by this. I have watched the output of the amp on the scope and have seen 40V rms (not p-p) for very brief periods (which is where I would expect to see problems if any should exist), but this crackling at least appears at a much lower voltage as well. It may be a bit of a stretch but does anyone have ideas about what this could be or what could be causing it, or other tests to try or things to watch for? I even ran the amp in non-bridged mode with the 4ohm load on one channel and the same results happened. Also of note is the amp has a clip indicator and the light was not going off, so its not a clipping issue. I have gotten advice from others that it might be the current limiting circuitry kicking in but the stored wave didn't make much sense to me if that were to be the case.

If anyone wants more info or other tests to try let me know.

Thanks guys!
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Old 12th July 2004, 06:20 AM   #2
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Hi Impoliticus,

My first reaction when you said that you hear a "crack" sound with a tempest in a small sealed box is that you're bottoming the speaker out. I don't have any experience with a tempest (yet)- but- what's the excursion like when this happens? What I came up with to explain the spikes was the idea that it was reaching an excursion limit and hard stopping, causing the "crack" and some back-EMF.

Very unlikely, now that I think of it. The smaller box size ought to provide a stiffer compliance and limit excursion, don't you think? I wonder if you could recreate these spikes with a dummy load.

Another question - do these cracks only happen when you are touching the pre-amp? I have seen volume pots go bad and crack horribly when you change the volume. I know you say that the preamp is good.. just another gut reaction to ask this question.

Hopefully one of those suggestions or questions were useful to you or someone more knowledgeable than I.


-Joe



PS- I feel the need to ask a question about this amp: Is it very susceptible to ground loop hum? My fraternity has this particular amp, and it has a low pitched hum. The problem was "solved" by some genius by wedging a pencil under the top panel!! If you pull the pencil out, the hum starts again. If you push the panel in, the hum goes away. What gives? I was going to try to fix the problem for them; but unless it's something easy I don't think I'll go to the trouble.

just seems wrong to stick a pencil into an amp.
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Old 12th July 2004, 08:29 AM   #3
djk is offline djk
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Driving a 4 ohm load in mono will activate the foldback current limiting.

On some amplifiers this will make a loud 'crack' that almost sounds like a gunshot.

On a 'scope you will see a narrow spike that goes in the opposite direction of the source signal, all the way to the opposite rail, and then back again. Rail-to-rail, full speed ahead.

The current limiting activates and the output drive is removed. Of course the driver now becomes a flyback transformer with the voltage trying to go to infinity to keep the current flowing, so the voltage goes to the other rail.

The current has now dropped to a reasonable value so the current limiting de-activates, and the whole cycle starts over again.

(mad laughing in backround)

The most expensive Crown amps used a multiple RC ladder with an integrator (ODEP), but this is their lower line of product.

http://wardsweb.org/audio/docs/MK%20...schematic2.jpg

R132, 133 and C118, 120 form an RC time constant to delay activation on transients.

You could add something simple like this SAE and it would help.
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Old 12th July 2004, 04:41 PM   #4
MikeW is offline MikeW  United States
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Diving a 4 ohm load in that mode each channel is only seeing half of the load, 2 ohms. It is better to go back to 8 ohms or run the amp in parrallel mode into the 4 ohms.
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Old 13th July 2004, 07:33 AM   #5
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In response to Joe, no the problem is not evident when I only touch the preamp. Also, the amp hasn't seemed to be very susceptible to ground loop hum. I've only experienced some noise AC due to poorly shielded RCA to XLR cables. I have however found that the amp oscillates if I hookup a scope to the outputs when I put the ground for the probe to the negative output terminal.


In response to djk, thanks for the suggestions. Although, the spikes I observed were in the same direction as the source signal, all three of the spikes. I may try something like you suggested there and see if that helps, an RC circuit is so simple to make/implement it would be a crime not to give it a try.


In response to Mike, the amp is stated as running fine under 2ohm stereo loads. Take that for what its worth. Crown also advertises the amp in a 4 ohm bridged mode as well, making me think it should be able to handle such a configuration. Additionally, the amp cannot be run in parallel mode. It is not spec'd for such and the manual makes no mention of running the amp in this mode, so I dare not try it. Other Crown amps specifically mention paralleling ability but my model does not so I would assume iy isn't a good idea to try/do it on this model.


Thanks to all of you for the comments. If anyone else has some other ideas keep 'em coming. Once I find out more info I'll post it.
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Old 13th July 2004, 10:06 AM   #6
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"the amp is stated as running fine under 2ohm stereo loads"

My XLS 602 manual warns that the amplifier might overload when operating into 2 ohm loads. It is not recommended.

It's also possible that your speakers are actually presenting a much lower impedance at some frequencies.
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Old 14th July 2004, 07:32 AM   #7
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Angry XLS

I noticed the cracking sound from an XLS when driving speakers whose impeadance dips dramatically at various low frequncies, looking at the impeadance VS freq graph I noticed that at some freqs the impeadance was barely 1/2 an ohm, it was at these points the amp would really crack! Temporary solution was to stop using it on LF content.
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Old 14th July 2004, 06:11 PM   #8
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Frank, thanks for alerting me to that fact. The manual has a list of configurations smattered across it and 2Ohm stereo and 4Ohm bridged are plastered all over the place. I must have glanced over the 2Ohm part, but I looked through my manual and saw the same thing.

Again, Mike and Frank, its interesting both of you suggested that the impedence may drop dramatically at certain frequencys and this might be whats causing or contributing to it. This could also be the reason why on a single channel 4Ohm config the amp does the same thing but to a bit lesser extent. Maybe a zobel of sorts would help. The only problem is I purchased this amp for the sole amplification of LF, so I don't have another use for it. I'll find some time and check out the impedance vs freq of the driver and see what I can find/do. Thanks guys.
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Old 26th December 2004, 07:22 PM   #9
hakank is offline hakank  Sweden
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Hi Impoliticus,

I have just experienced the same problems as you have. I have a XLS 402 driving a 4 ohm JBL SUB1500 subwoofer in bridged mode. This has been working fine for some month, but a couple of days ago I heard exactly the same cracking sound as you did. I have also expected that the woofer was damaged but it wasn't.
Do you still have the same problem? If not, how did you fix it?

Thanks, Håkan
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Old 30th December 2004, 05:52 AM   #10
Enzo is offline Enzo  United States
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WHy not call customer service at Crown? They usually have helpful tech types there to answer questions. My former business partner left to go down there and do exactly that. They are right next to you there in Indiana. Send them an email or pick up the phone. They can tell you if you are indeed operating within spec and they may have other insights.

Joe - that pencil thing doesn't sound like a ground loop. You don't generally cure loops by pressing on the panel. SOunds like the lid was magnetically coupling to the transformer or was vibrating to the fan. SOmetimes large free areas of sheet metal can vibrate. The automobile industry has cured that for ages by sticking rubber strips on the back sides of panels. Make sure the hardware is all real tight, and if the top still buzzes, either adhere a strip of rubber underneath, or replace the pencil with rubber bumpers. Those little rubber "feet" with glue on the back can be stuck to the panel or lid in a spot that will dampen any buzzing, by adding support. The foot will bear against something to keep down the vibration. Stick the foot on where it will sit on the transformer or some other interior surface.
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