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Author Topic: adavantage?  (Read 18056 times)

Nathan Short

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Re: adavantage?
« Reply #50 on: February 26, 2010, 11:13:43 pm »

Thanks for the cool info Jr,  Now just gotta wait for my scope to arrive, and then I should have some much better info.

I really wish some other people would try this, or QSC itself, and Just say what they find.  But I don't really expect them to say if they find a flaw...  

Gotta wait for my new scope.
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John Roberts {JR}

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Re: adavantage?
« Reply #51 on: February 27, 2010, 09:28:52 am »

Nathan Short wrote on Fri, 26 February 2010 22:13


I really wish some other people would try this, or QSC itself, and Just say what they find.  But I don't really expect them to say if they find a flaw...  

Gotta wait for my new scope.

Why would people not talk about a problem..? You sound a little paranoid. Don't assume everybody is trying to cheat or lie to you. Amplifier specifications are generally more accurate than customer measurements.

I have no horse in this race. I used to compete with QSC, but in my judgment and experience, when they say an amp will do XYZ, it does. That and I have known Bob Lee some 30 years, and there isn't a drop of BS in him.  

In my judgment if a QSC amp had a 3 dB gain mismatch between channels, or a 3dB output difference, they wouldn't make it through final QA at the factory.

The clip(?) lights are probably not measured for precise accuracy.

We are all curious to learn what really is going on with your amp(s).

JR

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Nathan Short

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Re: adavantage?
« Reply #52 on: March 01, 2010, 05:41:40 pm »

I am not paranoid.  But If I was a manufacturer had known there was an issue, a nice PM would have shut me up right quick.

The second issue, is this, why cant QSC go out to the warehouse, open up a couple and give this a try?  Two things might happen,  "hey wow, we made this happen too!"  or "damn, we could not do this , no matter how hard we tried, and we blew up the power supply trying"  

Not one word from other users, or the manufacturer. So unless, my scope could come more quickly...   I could go around with my oscillator and bucket to any number of 10 bars in the city, and see if it was just something I never noticed, or it is just these two new ones.

And a public forum is probably not the best place for this problem to be hashed out,  but again,  no one contacted me as this happened/unfolded.  And I can return these amps any time.
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Mac Kerr

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How deep to dig
« Reply #53 on: March 01, 2010, 05:48:39 pm »

Put down the shovel, the hole is deep enough.

Mac
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Nathan Short

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Re: How deep to dig
« Reply #54 on: March 01, 2010, 06:02:26 pm »

But Mac This Rabbit Hole is still here.
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Chris Van Duker

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Re: adavantage?
« Reply #55 on: March 01, 2010, 06:32:44 pm »

Nathan,
In my experience, QSC has always been exceptionally transparent and proactive when there have been problems with their products. Two examples:

1) The original version of the PLX series had a problem with a ribbon cable which could fail after a certain period of time. QSC continued to repair that problem with these amps for free, even after the warranty had expired.

2) They had problems with the PFC power supply in the Powerlight 6.0 and 9.0 which affected (as I understand it) a fairly small minority of the units in the field. When they couldn't come up with an adequate fix for the problem, they acknowledged it and stopped selling them. I never dealt with this issue personally, but my understanding is that those who experienced the problem were well taken care of.

If I could air one complaint about QSC, it would be this: I'd really like it if they sold a 6-8KW, lightweight, mid-market amp.
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John Roberts {JR}

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Re: adavantage?
« Reply #56 on: March 01, 2010, 07:23:19 pm »

Nathan Short wrote on Mon, 01 March 2010 16:41

I am not paranoid.  But If I was a manufacturer had known there was an issue, a nice PM would have shut me up right quick.


Would it work if I PM'd you.  Laughing

Seriously nobody wants to shut you up.. People would like to help you clear up "your" problem.

Since a large number of customers have used those models without the problems you report, there is justification to assume this may be a personal problem.  
Quote:


The second issue, is this, why cant QSC go out to the warehouse, open up a couple and give this a try?  Two things might happen,  "hey wow, we made this happen too!"  or "damn, we could not do this , no matter how hard we tried, and we blew up the power supply trying"  


You haven't been adequately precise or lucid in describing the exact problem.

I have tried a number of times to coax a clear description of what is going on from you without success so far. At the moment we are waiting for you to get some test equipment while I have suggested some simple MacGuyver tests you could do for free. A speaker across the two hots will surely identify a 3 dB gain mismatch. Likewise a Y cable into both inputs could help identify gain matching or even OL LED threshold discrepancy between channels.  

Quote:


Not one word from other users, or the manufacturer. So unless, my scope could come more quickly...   I could go around with my oscillator and bucket to any number of 10 bars in the city, and see if it was just something I never noticed, or it is just these two new ones.


If you've got the time to go bar hopping with test loads, you should be able to find the time to answer Bob's questions.

I am still unclear what this error is? As I have said a number of times, if you actually have a 3 dB gain difference between the two channels, QSC will fix it..  But that is not proved to be the case at least to me.
Quote:


And a public forum is probably not the best place for this problem to be hashed out,  but again,  no one contacted me as this happened/unfolded.  And I can return these amps any time.


No this is a fine forum, but you need to meet QSC half way and answer Bob's questions... So he can determine what you are seeing. This is not semantic games play. In technical troubleshooting, and I suspect Bob has decades of experience doing so, there is a need for following a very concise checklist of questions.

This is not a "the world" against Nathan thread, but you are testing the patience of people who aren't being paid to be nice to you, like Bob is.  Laughing  

We all want you to resolve you questions with insight and awareness, not conspiracy theories and suspicion of deviousness.

JR
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Nathan Short

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Re: adavantage?
« Reply #57 on: March 01, 2010, 08:00:25 pm »

I have pretty much figured that the problem is at the onset of clip light indication.  

Unfortunately I do not have the luxury of doing more tests this past week as bids and shop work have taken all my time and a lot of 15hr days.

At different gain pot settings, driven by different levels of input, swapping both sides of the amps input to make sure the problem is not upstream in the signal flow.  My basic conclusion is that at 30Hz and lower, and at 15kHz and higher,  the ability of channel one to grossly outperform the output of channel 2 is unmistakable and possibly dangerous to my loudspeakers.

I would suspect the clip limiter is in some way at fault.

Channel 1 is consistantly able to put out 81v as measured by a voltmeter at the load of 4ohm.  Channel 2 is able to consistantly put out in the 60v range.

This is the symptom, and I need a doctor to help diagnose the disease. Or design flaw,  I would normally not drive an amp quite this hard in an install, but the test is to show me these weaknesses and I need to be prepared for all sorts of DJ abuse scenarios.  

I will most certainly keep at it, when I have more time, and my new tools, I will gather many more notes, post a video, and grab some big old beater subs To also use as a load.

I just wish someone else would try it.

Until then I am going to enjoy the PL340's and 380's that are proving to do an excellent job.
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John Roberts {JR}

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Re: adavantage?
« Reply #58 on: March 01, 2010, 08:42:21 pm »

Nathan Short wrote on Mon, 01 March 2010 19:00

I have pretty much figured that the problem is at the onset of clip light indication.  

Unfortunately I do not have the luxury of doing more tests this past week as bids and shop work have taken all my time and a lot of 15hr days.

At different gain pot settings, driven by different levels of input, swapping both sides of the amps input to make sure the problem is not upstream in the signal flow.  My basic conclusion is that at 30Hz and lower, and at 15kHz and higher,  the ability of channel one to grossly outperform the output of channel 2 is unmistakable and possibly dangerous to my loudspeakers.

I would suspect the clip limiter is in some way at fault.

Channel 1 is consistantly able to put out 81v as measured by a voltmeter at the load of 4ohm.  Channel 2 is able to consistantly put out in the 60v range.

This is the symptom, and I need a doctor to help diagnose the disease. Or design flaw,  I would normally not drive an amp quite this hard in an install, but the test is to show me these weaknesses and I need to be prepared for all sorts of DJ abuse scenarios.  

I will most certainly keep at it, when I have more time, and my new tools, I will gather many more notes, post a video, and grab some big old beater subs To also use as a load.

I just wish someone else would try it.

Until then I am going to enjoy the PL340's and 380's that are proving to do an excellent job.


I believe it has already been suggested that 81V is a possible valid output level, so if one channel only puts out 60, That is the suspicious one. If it is limiting at 60V due to a valid external reason (like too low load impedance) or invalid reason like internal circuit fault, the OL indicator would fire early for either case.

The obvious test is, do both channels put out rated power into the identical, valid  test load, with identical input voltage?

I don't think your worry is one channel putting out too much in some faulty mode, but if anything one channel not putting out rated voltage.

If the loads and inputs are the same (and valid), and the outputs are that different, the amp is bad. Keep in mind that exact symptom could also be caused by a bad load, so that is worth confirming.  

JR
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Nathan Short

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Re: adavantage?
« Reply #59 on: March 01, 2010, 10:06:18 pm »

You know JR  that brings up a great point, there might be some odd impedance problem at both ends of the audio spectrum.  I am gonna hook up my woofer tester and run an impedance sweep on both of the bucket loads.  I am pretty sure I tried both channels on of most of the amps on both dummy loads, but as I was taking mostly averages, when the amplifiers put out within 6v +or-  I was pretty happy.  That is why the 81v>60v was the one that really got me worried.
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