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Author Topic: Power Amp Losing volume?  (Read 3010 times)

Thomas Le

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Power Amp Losing volume?
« on: September 17, 2022, 04:38:42 PM »

Story time:

Got an amp rack that has 4 QSC CX that are 22 years old. Just got a notification from one of the pastors on friday that the "volume was not as loud as it used to be". My thinking was that one of the main amps finally gave up and bit the magic dust. Upon inspection, all 4 are turning on still working, but after some testing and going back and forth from the amp rack room, I'm guessing one of the channels is giving a lower output even though nothing has changed in DSP land and the attenuators are at full tilt. Fortunately the way the system was spec'd,  the installer only used one amp, one channel per side, so there's two amps for FOH speakers, and I was able to switch to channel 2 on the amp. Obviously given the age, I am "working my way up the chain" on trying to get them replaced with CDi Drivecore equivalents.

The stupid question: based on this experience, can a power amplifier lose it's output/volume over time?
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John Roberts {JR}

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Re: Power Amp Losing volume?
« Reply #1 on: September 17, 2022, 04:58:10 PM »

Story time:

Got an amp rack that has 4 QSC CX that are 22 years old. Just got a notification from one of the pastors on friday that the "volume was not as loud as it used to be". My thinking was that one of the main amps finally gave up and bit the magic dust. Upon inspection, all 4 are turning on still working, but after some testing and going back and forth from the amp rack room, I'm guessing one of the channels is giving a lower output even though nothing has changed in DSP land and the attenuators are at full tilt. Fortunately the way the system was spec'd,  the installer only used one amp, one channel per side, so there's two amps for FOH speakers, and I was able to switch to channel 2 on the amp. Obviously given the age, I am "working my way up the chain" on trying to get them replaced with CDi Drivecore equivalents.

The stupid question: based on this experience, can a power amplifier lose it's output/volume over time?
Generally no with modern solid state amps and 22 YO is modern solid state...   Are they getting unusually hot? Are speaker loads in good shape?

JR

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Thomas Le

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Re: Power Amp Losing volume?
« Reply #2 on: September 17, 2022, 05:07:53 PM »

Generally no with modern solid state amps and 22 YO is modern solid state...   Are they getting unusually hot? Are speaker loads in good shape?

JR



I'll admit the amp room temperature varies wildly because it does not have good Air Conditioning flow but it does not get hot. Speakers themselves are ok once I switched over to the other channel, nothing has changed on the speaker side, all original drivers, nothing blown.
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John Roberts {JR}

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Re: Power Amp Losing volume?
« Reply #3 on: September 17, 2022, 05:15:55 PM »

I'll admit the amp room temperature varies wildly because it does not have good Air Conditioning flow but it does not get hot. Speakers themselves are ok once I switched over to the other channel, nothing has changed on the speaker side, all original drivers, nothing blown.
The amps could be tired... Loose power transistor screws could overheat leading to a soft shut down. Air flow blocked by 22 years of dust could compromise cooling.

Shorted speaker wires could cause problems, but generally failure is not graceful, more severe.

I am not familiar with innards of the CX but some amps have clip limiting that would fold back amp level/gain in case of current limiting or thermal overload.

JR
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Bob Taylor

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Re: Power Amp Losing volume?
« Reply #4 on: September 17, 2022, 05:27:58 PM »

If the Amp is down just a bit, like 6 dB, are all the inputs on the Balanced interface making connections on both Pos and Neg? And this means at the processor also. Wiggle stuff, re insert, even open up the amp and Re Seat Ribbon connectors? Flip the input/mono switches a few times to break the 22 years of corrosion.

Not played with this amp, but at first read i say it is a corroded jack or........

Bob T
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Ike Zimbel

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Re: Power Amp Losing volume?
« Reply #5 on: September 17, 2022, 06:13:17 PM »

Story time:

Got an amp rack that has 4 QSC CX that are 22 years old. Just got a notification from one of the pastors on friday that the "volume was not as loud as it used to be". My thinking was that one of the main amps finally gave up and bit the magic dust. Upon inspection, all 4 are turning on still working, but after some testing and going back and forth from the amp rack room, I'm guessing one of the channels is giving a lower output even though nothing has changed in DSP land and the attenuators are at full tilt. Fortunately the way the system was spec'd,  the installer only used one amp, one channel per side, so there's two amps for FOH speakers, and I was able to switch to channel 2 on the amp. Obviously given the age, I am "working my way up the chain" on trying to get them replaced with CDi Drivecore equivalents.

The stupid question: based on this experience, can a power amplifier lose it's output/volume over time?
I know that the earlier QSC's can. One of the features of the QSC design was split level power supply rails. I don't remember the exact voltages, but say +/-35v and +/- 70v.
The idea was that for lower levels the amps would run on the lower voltage rails, switching over to the higher voltage rails on peaks, louder passages etc. When I managed a soundco back in the 90's we had probably around 300+ QSC amps, 3500's, Mx-1500/1500a, EX2500, EX4000 in heavily used inventory. Almost the only failure we ever encountered, and that very rarely, was the circuitry that would switch between the two sets of power supply rails would stop working. So, it IS possible for an amp of that design to only run on the lower supply rails, and that would be on only one channel because that circuitry was on a per channel basis. And, the symptom was the amp worked fine, but wouldn't produce full volume. I can't swear to this, but I believe it was a FET transistor that did the switching, and is what would fail.
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Thomas Le

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Re: Power Amp Losing volume?
« Reply #6 on: September 17, 2022, 06:17:37 PM »

If the Amp is down just a bit, like 6 dB, are all the inputs on the Balanced interface making connections on both Pos and Neg? And this means at the processor also. Wiggle stuff, re insert, even open up the amp and Re Seat Ribbon connectors? Flip the input/mono switches a few times to break the 22 years of corrosion.

Not played with this amp, but at first read i say it is a corroded jack or........

Bob T

yeah I checked the phoenix connections, all snug. When I changed the amp channel, the perceived loudness of the system was "back to normal", so the input cable is ok.
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Steve-White

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Re: Power Amp Losing volume?
« Reply #7 on: September 17, 2022, 06:51:07 PM »

yeah I checked the phoenix connections, all snug. When I changed the amp channel, the perceived loudness of the system was "back to normal", so the input cable is ok.

Any and all connections could be suspect.  Even inside the amps.  Sometimes just a simple remove and re-seat can cause problems to disappear.  This applies to both XLR connectors and connections within the amps.  Heat and cool cycles in the amps can cause issues after years of use.

I would wonder if the line feeds are contiguous from the FOH to the amps?  i.e. no connectors along the path?
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Paul G. OBrien

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Re: Power Amp Losing volume?
« Reply #8 on: September 18, 2022, 05:53:25 PM »

When I changed the amp channel, the perceived loudness of the system was "back to normal", so the input cable is ok.
Simply disconnecting and reconnecting the wire may have "fixed" the problem (if it is corrosion), a test for this would be to now reconnect the wire to the original channel.. if the signal level is back to where it should be then you have confirmation.
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John Roberts {JR}

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Re: Power Amp Losing volume?
« Reply #9 on: September 19, 2022, 11:20:19 AM »

I know that the earlier QSC's can. One of the features of the QSC design was split level power supply rails. I don't remember the exact voltages, but say +/-35v and +/- 70v.
The idea was that for lower levels the amps would run on the lower voltage rails, switching over to the higher voltage rails on peaks, louder passages etc. When I managed a soundco back in the 90's we had probably around 300+ QSC amps, 3500's, Mx-1500/1500a, EX2500, EX4000 in heavily used inventory. Almost the only failure we ever encountered, and that very rarely, was the circuitry that would switch between the two sets of power supply rails would stop working. So, it IS possible for an amp of that design to only run on the lower supply rails, and that would be on only one channel because that circuitry was on a per channel basis. And, the symptom was the amp worked fine, but wouldn't produce full volume. I can't swear to this, but I believe it was a FET transistor that did the switching, and is what would fail.
If the upper rail of class g/h amp is not working the gain will be the same, but it will clip sooner. This may be audible in full tilt use, but not obvious for normal listening levels. 
If the Amp is down just a bit, like 6 dB, are all the inputs on the Balanced interface making connections on both Pos and Neg? And this means at the processor also. Wiggle stuff, re insert, even open up the amp and Re Seat Ribbon connectors? Flip the input/mono switches a few times to break the 22 years of corrosion.

Not played with this amp, but at first read i say it is a corroded jack or........

Bob T
+1 missing one leg of differential source could look like -6dB loss.

JR
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Thomas Le

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Re: Power Amp Losing volume?
« Reply #10 on: September 19, 2022, 04:17:11 PM »

Simply disconnecting and reconnecting the wire may have "fixed" the problem (if it is corrosion), a test for this would be to now reconnect the wire to the original channel.. if the signal level is back to where it should be then you have confirmation.

I'll see if I can check sometime this saturday.

If the upper rail of class g/h amp is not working the gain will be the same, but it will clip sooner. This may be audible in full tilt use, but not obvious for normal listening levels.  +1 missing one leg of differential source could look like -6dB loss.

JR

I did check for any loose wiring, they were tightly screwed in.
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Craig Hauber

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Re: Power Amp Losing volume?
« Reply #11 on: October 28, 2022, 01:16:42 PM »

I'll see if I can check sometime this saturday.

I did check for any loose wiring, they were tightly screwed in.
I use a lot of CX series in my portable rigs and the attenuator pots get dirty (or something)  When I now experience an imbalance the first thing I do is to exercise the pot a bit and it usually clears up the problem.
It will pass signal but at a significantly reduced level.

Just my personal experience with repurposed CX series amps that lived their previous lives in 24-7-365 always-on installs
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Brian Jojade

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Re: Power Amp Losing volume?
« Reply #12 on: November 29, 2022, 04:13:03 PM »

Story time:

Fortunately the way the system was spec'd,  the installer only used one amp, one channel per side, so there's two amps for FOH speakers, and I was able to switch to channel 2 on the amp.

So you're saying that 2 of the amp channels simply aren't being used?  Or are the amps in BRIDGE mode?

If the installer put in the amps and only used half of the amp, that's a pretty silly waste of money for the design, but handy for you now.  If switching to channel 2 on the amp fixed it, then who cares about fixing the first side?

Or, you can move everything over to the 2nd amp and pull out the problem unit to put on the bench for testing and repair.
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Thomas Le

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Re: Power Amp Losing volume?
« Reply #13 on: November 30, 2022, 12:17:10 PM »

So you're saying that 2 of the amp channels simply aren't being used?  Or are the amps in BRIDGE mode?

If the installer put in the amps and only used half of the amp, that's a pretty silly waste of money for the design, but handy for you now.  If switching to channel 2 on the amp fixed it, then who cares about fixing the first side?

Or, you can move everything over to the 2nd amp and pull out the problem unit to put on the bench for testing and repair.

Nope its not in bridge mode, yeah its a weird spec, buying two amps but only using one channel. This was before my time so I don't know. Yeah I was meaning to get around to noodling with it but I didn't have time haha and "if it ain't broke" mentality so I just left it there...

Eventually I did a proposal to upgrade to all Crown CDi Drivecore (the one with the screen) so that it can be network manageable. We'll see how that goes...
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doug johnson2

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Re: Power Amp Losing volume?
« Reply #14 on: November 30, 2022, 04:51:12 PM »

I don't have a lot of time with the CX amps but, have delt with the MX series.  I have had issues similar to this that were the result of problems volume pots.  Cleaning and exercising them may or may not temporarily fix the issue but, if it went away, it would come back in short order.  IIRC, it was a known issue and QSC offered a replacement kit long after the MX series were discontinued.
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Thomas Le

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Re: Power Amp Losing volume?
« Reply #15 on: December 05, 2022, 08:51:34 AM »

In the end, the CX amps are going to be removed and I'm going to order the CDi Drivecore equivalents.
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Ivan Beaver

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Re: Power Amp Losing volume?
« Reply #16 on: December 05, 2022, 01:19:48 PM »


Are SURE the amplifier does not have the same output, and not that someones hearing is not what they remember.

Maybe the mix/blend was different years ago, maybe the operators are not running it as loud (that is assuming you are running the system at max output).

What makes you think the amplifier has "lost power"?  Besides someone saying it is not as loud as before.
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Re: Power Amp Losing volume?
« Reply #16 on: December 05, 2022, 01:19:48 PM »


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