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Power Amp Losing volume?

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Ike Zimbel:

--- Quote from: Thomas Le 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?

--- End quote ---
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.

Thomas Le:

--- Quote from: Bob Taylor 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

--- End quote ---

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.

Steve-White:

--- Quote from: Thomas Le on September 17, 2022, 06:17:37 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.

--- End quote ---

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?

Paul G. OBrien:

--- Quote from: Thomas Le on September 17, 2022, 06:17:37 PM ---When I changed the amp channel, the perceived loudness of the system was "back to normal", so the input cable is ok.
--- End quote ---
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.

John Roberts {JR}:

--- Quote from: Ike Zimbel on September 17, 2022, 06:13:17 PM ---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.

--- End quote ---
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. 

--- Quote from: Bob Taylor 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

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

JR

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