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Older XLS602 - One side lacks hi end

XLS 602 Hi End Muffled C-Audio GB602

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#1 bruinsfan37

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Posted 03 April 2014 - 07:10 PM

I have an older XLS602, the three space model, that I had bough refurbished back in the early 2000's. The amp has always had an issue where audio sounds muffled in the mids and lacks hi end on one side. Back then I thought that it was my PA speakers (cheap 15" Pyle Pro full range speakers) but once I replaced those with JBL and others, I heard the same thing. I've tested the amp in several different scenarios ruling out every other piece of gear. I've tried to compensate with a graphic but I does't quite do the job to make the audio sound exactly the same.

I'm wondering if something was done when the amp was refurbished and there is a capacitor or resistor that is different on one side vs the other and thats causing the timbre change. Visually inspecting the inside doesn't show anything obvious.

The funny thing is when opening the amp, the circuit board is actually branded C-Audio P1071 and GB202, GB402 and GB602 are also stamped on the inside.

Can anyone shed some light on why the two sides sound different?

#2 dakos

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Posted 04 April 2014 - 06:17 AM

Hello,
I think first we should verify that the problem is indeed in the amp and not in any other part of your rig. You should make a clear note which side is the faulty one, then switch the speakers. If the problem persists on the same side then you should switch the XLR inputs to the amp. Only then when the problem continues to be on the same side I would say the amp is the faulty part.
Avi

#3 joust

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Posted 04 April 2014 - 08:21 AM

Hello Bruins fan! This is a Canadiens fan! Bruins are doing well this year! Good luck to the Bruins and hope to see them against our team in the semis! :)

I seem to remember that the input circuitry power was provided by a few zener diodes, located directly on the main board. Verify for proper voltages. Should be in the region of +/-15V.
Also, there is an output filter circuit, located close to the output connector. I've seen some fried components (resistors, caps) on this small board. Could cause unexpected issues.


Alain

#4 bruinsfan37

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Posted 04 April 2014 - 08:50 AM

Thanks for the reply! I should have laid out the tests I've done that lead me to believe the amp is the issue. I made the signal chain as simple as it could be to rule out as many variables as possible right off the bat. The test scenario was:

A Sennheiser e845 into various channels on a PreSonus Studiolive 16.4.2 that had been zeroed out (no eq, fx, bussing, panning centered - just mic pre to fader to master and out the main L and R) -- XLR cables into the back of the 602 channels 1 and 2 -- banana to 1/4" speaker cables to a pair of Harbinger HX121M floor monitors.

When rolling up the input on the 602 one side at a time, channel 1 sounded clear and normal. Channel 2 wasn't as crisp and sounded muffled in the mids when turned up to the same level as channel 1. - Then I swapped the input XLR's but the trouble remained on channel 2. Switched the banana cables and the muffled side changed to the other speaker.

I tried this same test on a second pair of HX121's and in a pair of JBL JRX125's and got the same results. Then later with a stand-alone mic pre (to rule out the PreSonus) and different mic cables right into the amp - same issue.

This amp has sounded this way from day one (it was bought refurbished ), but at the time I was brand new to pro audio and never thought the issue could be the amp and not the speakers I was using. Haven't really used it in the years since then, but now I'm curious to pick the brains of the folks on this forum to see if they might know whats going on.

Thanks again!

#5 joust

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Posted 07 April 2014 - 10:09 AM

Hello Bruins fan,

You didn't confirm that you've openned up the amp and had a view inside.
Can you verify the state of the output filter circuit I asked about? (You should have one good side!)
Do you see any burnt components? Can you measure these components?
Do both sides appear to be of similar loudness or is the muffled side kind of weak compared to the good side?
Did you (can you?) 'scope' the output signals?
What happens to the signal when you have no (removed) load?
What happens to the signal when loads are re-applied?

Note : There really isn't anything particular about the amp; Usually, with sound issues similar to yours, the input or output circuit circuit would be at fault, not the power (psu or o/p devices) circuits.

Alain

#6 bruinsfan37

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Posted 07 April 2014 - 02:23 PM

Hi Alain,

I'll take another look inside after work this evening, but the last time I opened up 602 a couple days ago, I couldn't find anything visually that hinted any issues... no popped or bulging caps, no burnt resistors or anything else that looked obvious. Everything looked very neat except for the gobs of adhesive on the quads of 4700u capacitors. I'm afraid my means of testing are limited to a soldering iron, digital multimeter and a basic understanding of electrical engineering. I've built simple circuit boards and speaker crossovers in the past, but nothing on the scale of a power amp like this, so please forgive me if my posts are on the simplistic side.

I've got a copy of the schematic for the amp, so I'll see what results I can get testing some components with the multimeter (safely that is :))

I'll be able to hook the amp up to my floor monitors again tomorrow night when I'm back at my practice space where the rest of the system lives (Oh the joys of condominium living) and get more specific answers as to level difference between the sides.

Thanks,
Will

PS - The B's have had an impressive season so far, I just hope they can carry on that success in the playoffs. Wouldn't mind a stop in Montreal this spring either... Its always exciting when the B's and Habs meet in the post season!

#7 bruinsfan37

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Posted 07 April 2014 - 09:29 PM

So this evening I was able to open up the amp once more and really get a good look at the inner workings. Since I wasn't able to unsolder any components from the main board and in and output boards, I checked around with the multimeter, comparing the readings of components from side A against their twins on side B, also making sure to observe the correct polarity. Seeing as they are two identical sides, I thought the readings should be about the same.

I checked the zener diodes powering the input board and they checked out fine. Everything looked fine visually on the output board as well, getting near identical readings from matching components on each side.

#8 dakos

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Posted 10 April 2014 - 05:30 AM

May be it's time to take out the oscilloscope and compare the channels with a 10-16KHz signal connected to the input. It sounds to me like this signal might be the thing that would bring the problem to the surface but since I'm not the expert here we'll the other bruinie tell us what he thinks...
Avi

#9 joust

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Posted 10 April 2014 - 09:26 AM

Avi, I am unfortunately not a Bruins' fan! I'm a Habs fan! And more to the point, a hockey fan; All games are good!

And, as Avi comments, this is 'oscilloscope time'!
Signal input : sine (better if you have square waves!) @ 1 kHz; 1.2VAC (+4dB)
Compare input signals with the signals at the level controls and then test output signal with and without loads:
- Without loads output signal should be strong;
- With loads, signal should be same but perhaps 20% lower than without loads;
It is possible that you may have a faulty coupling cap in the input section, on the small input board.
I still think that the output filter board could have been damaged; Check and compare the signal directly at the output, but before the output coil and again at the terminals.

Hope this helps!

Alain

#10 dakos

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Posted 10 April 2014 - 01:26 PM

Getting a square wave is not a problem, just download a free signal generator with square wave function (many available) for your iPhone or Android device, run it through your console to amplify the signal to 1.2V (using the oscilloscope to verify the voltage) and connect to the amp.

Let us know how you progress :-)
Avi

#11 bruinsfan37

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Posted 14 April 2014 - 11:48 AM

Thanks for the replies! If only I had a scope to be able to see whats going on with signal passing through the amp. Without one, I think I'll have to take it to a repair shop and have them take a look and fix it if possible. The closest authorized Crown shop is about 1.5 hours away in Lawrence, MA (I'm in RI.) Can anyone recommend any other places in New England?