Love the site and all that you do!!!
I have extensive experience with this series of QSC amps. The "Power Light 2" family. The family includes CX (basic install) DA (cinema) PLX 2 (premium music store/musician) and Power Light 2 (pro touring) I ran my sound company using the CX, which are available used for nearly nothing. New, the list price is several thousand per amp.
The amp you tested is the lowest power dual rail in the series, and actually my least favorite. Low voltage rail is 50 something volts and high if I recall is around 100. My two favorite amps from the series are the second from the bottom (CX502/PLX1602/DCA1622) 83v single rail, and the top of the series (CX1102/PLX3402/DCA3422) 83v bottom rail, 135v top rail. The CX502 uses the same transformer as the CX1102 but the high rail driver circuitry is missing.
The amp you tested would have delivered better 4 ohm performance (along with being more gentle to the wall power) if you populated the two missing 200v caps, which are available from QSC as factory parts. Also, the pro touring versions use 1000uf caps in the second stage of the power supply, where all the other amps in the series use 470uf caps. The 1000uf caps are also available from QSC as factory parts. Used the way I did,at its limit, driving double 18" ported subs down to 25hz hour after hour after hour... the extra cap storage makes a huge difference in both bass sound quality and how the amp draws power from the wall or distro. (We always used a proper distro!) Despite what the ad copy says... when driving subs at the limit, these amps want LOTS of AC current.
The CX502/PLX1602/DCA1622 with all six 200v caps installed and the secondary caps upgraded to the 1000uf "touring" version is a remarkable amp with lots of power and good current delivery. (The larger CX1102 version basically doubles power everywhere which is totally not necessary in a domestic environment unless your wall outlet can deliver tons of current and you want to brag about 700 - 1200 watts per channel in your hifi...
The basic topology is 115khz switching power supply feeding a class a/b output stage of one or two voltage rails. There are two electrolytic caps in the signal path - one before the volume control and one before the amp driver stage. I always removed the first (pre volume) cap as its there simply to prevent any dc from making the volume control sound noisy. Immediate subjective improvement in sound quality from removing that first cap. I have experimented (domestic use only!) with removing the second cap, swapping preamp opamp, (lm4562 sounds noticably better than the stock 33078!) film cap bypass on power supply caps, etc. and the amp does respond well to those tweaks. If I was trying to drive Maggies or similar the QSC CX properly modded would be my second choice go-to amp. (I currently use deeply modded Ashley FTX Series 3 amps. Fully discreet, all mosfet, thru-hole boards, etc.)
Used domestically, (properly modded) this series of QSC amps will lean toward the brighter side of neutral, with good stereo width, some stereo image depth, a playful "growl" to the bass, and an insistent sense of tons of power looking for a party.
Used domestically, and unmodded, the amps will sound very "matter of fact" with not much imaging depth (thanks to two electrolytic caps and 33078 opamp in the input section, plus a 5532 opamp if you engage the limiter.) The prodigious power will be obvious but if you're looking for holographic imaging, beauty of tone, etc. then these amps are not the droids you seek.
Regarding fan noise, this series of amps benefits from re-tightening the heat sink screws after a few years. As a classic a/b output stage with about 1 amp of idle current, the heatsinks need to be working correctly to keep the fans at their slowest setting in a domestic environment. At its slowest speed, the fan should be absolutely quiet. Maybe the amp you tested has a bum fan? (I've never had a fan issue with dozens and dozens of those amps working year after year in the worst possible environments imaginable.)
I've attached the schematic (PLX 2402 version) of the amp that you tested.
