QSC PLX 1804 STEREO POWER AMPLIFIER

MSRP: $1,049   STREET PRICE: APPROX $700

 

 

Benchmark DAC’s. Technics turntables. Most vacuum tubes. Alesis Masterlink. Realtraps. Behringer PowerCurve. What do all these products have in common? They all are “Pro Audio” products originally designed for sound reinforcement or commercial recording studio applications – not for home stereo and audiophiles. Yet they, and many other such “pro” products, have found their way into the home systems of many an audiophile. Unbeknownst to many, there is a whole world of pro speakers, amps, recorders, cables – you name it – sitting in their local guitar shops just waiting for the enterprising audiophile to discover them.


Needless to say, with few exceptions, such products are not usually covered in most home audio publications and most audiophiles would not be caught dead in a guitar shop. But we would. So we think it’s a good idea to try some of these products – many apparent bargains – for our readers.
The QSC 1804 stereo power amplifier is one such apparent bargain; 800 watts per channel in a relatively small, light chassis for street prices as low as $700. “Street price” is something to which we as audiophiles are unaccustomed but is the name of the game in the pro universe. The list price of the amp is $1,049, so you see heavy discounts are de rigueur.


CLASS WHAT?


The first thing you need to know about the QSC is that it is a switching amp. Many call this “digital” but strictly speaking, it is not. We have reviewed many Class D amps and we even did a blind shootout of 11 of ‘em, so we won’t retrace an explanation of how Class D works here. One fallciy we will mention is that Class “D” stands for “digital”. Nope. It was just the next letter of the alphabet after A,B, and C. You have heard of amplifiers that are Class D, but this amp is claimed to be Class H. It is QSC’s version of Class D. I bet you did not know that there are also Class E, F, G, H. Me thinks it’s mostly marketing. According to QSC, here are the differences between the different three output schemes:

What is the difference between Class AB, Class G, and Class H technology?


Class AB refers to the amount of idle current flowing in the outputs at zero output. Amplifiers have positive and negative output transistors which handle their respective halves of the output signal. They must "hand off" the output current to each other as the signal passes through zero. A "Class A" output stage begins to transfer current well above its "cutoff point", resulting in much current overlap. This eliminates any chance of "crossover distortion" but generates tremendous waste heat at idle, limiting the possible power of the amp. A "Class B" output stage attempts to make the transfer at exactly zero current, which is impossible to maintain perfectly and leads to "zero crossing distortion" (more commonly called "crossover distortion", a buzzy form of distortion most audible at very low levels). Class AB is the practical compromise--just enough idle current to ensure a smooth transfer between the positive and negative output transistors, without a wastefully high idle current.


POWER SUPPLY DESIGN


The other major source of waste heat, even in a class AB design, occurs at moderately high output powers. The output transistors drive the speakers by coupling a precise amount of audio voltage from the amplifier's "power supply", which is a steady reservoir of fixed voltage. Most of the time, the output transistors are called on to only deliver a fraction of the power supply voltage to the load, and the unused fraction is consumed as heat in the output devices. We can reduce the losses by providing two or more"tiers" of DC voltage, with "steering circuits" which draw from the lowest possible voltage supply. This way the waste heat in the outputs is reduced. A "Class G" design does this by using two different sets of output transistors, one coupled to the lower voltage and one to the full voltage. The signal transfers from the low to high voltage set as required. A "Class H" design uses additional circuitry to connect a single set of outputs to lower or higher voltage as required. Both approaches are capable of good results; the Class H can be designed for somewhat lower costs, especially in amps with more than two power supply "tiers".

If I read that right, QSC uses additional circuitry to lower the cost. I can see you purists cringing in the shadows.

There is quite a bit of “additional circuitry” throughout the 1804 including much circuit protection and a “clip limiter”. Though there is no technical description of this limiter, it’s probably the same limiting technology used in recording studios to limit (crush might be a better word) dynamics. On the radio and TV, the louder a song sounds the better. Compressors and limiters are used to “even out” the dynamic range and raising the overall level of the recording. Of course, that’s very anti-real and therefore anti-audiophile. We spend a lot of money to achieve realistic dynamics, both micro and macro, because that is a major factor in realistic reproduction. Such limiting and heavy duty circuit protection is very desirable in a live concert PA system. Exploding speakers would be a real drag at a Stones concert or even your local Holiday Inn lounge. In our world, “additional circuitry” usually connotes additional distortion and less transparency. On this model, that clipping limiter is not defeatable, so it’s always in the circuit. In other more expensive QSC’s it can be switched off.

In past reviews, we have also talked extensively about the relationship between amps and speakers in terms of ohms, impedance, gain and other matching ingredients. Throughout the 1804 manual and even tattooed on the rear panel, we are warned that the amp is not suitable for loads below 4 ohms. In the audiophile empire, there are many speakers that fall well below 4 ohms. Even if the “nominal load” is 4 ohms or above. As we have revealed before, speaker specs when it comes to impedance numbers are not very accurate. Owners of this amp would have to be very careful about speaker selection and owners of electrostats should stay away completely.

 


BUT HOW DOES IT SOUND?

In a word, LOUD. And that does not refer to the musical output, it refers to the frickin’ fan! My microwave is quieter than this. My computer is quieter. Maybe even the dishwasher. Again, in a concert situation where the amps are backstage or well away from listeners where the volume level is probably louder than a 747 taking off, this is not a problem. In a home, BIG problem. We can safely say that if this amp is to be located in the same room as the stereo system, it is absolutely to be avoided. The fan is that loud. Of course, you could disable the fan if you’re handy, but that would void the warranty and you could risk frying the amp. We think that’s a bad idea.

 

OTHER THAN THE FAN HOW DOES IT SOUND?


In a word, dry. There is a semi-dry quality with most if not all switching amps, but we’re not talking a touch of dry like a South African Chardonnay, we’re talking Sahara Desert arid. If you have even a bit of fondness for tube or tube-like sound, do not walk, run away now. Pairing it with a tube preamp doesn’t help much.
The soundstage is truncated compared to any stereo power amp I’ve heard and even any integrated that would qualify for even entry level high end or yes, even midfi. My Denon AV receiver used only for watching movies in surround is superior. Details are abundant but there is no air around instruments or voices and there is a grayish cast to everything. The sound reminds me of a porkchop that was accidentally left in the freezer for two years.

As we have stipulated in the past, a Stereomojo Specific Recommendation can sometimes be “NOT Recommended”. The policy of most other audio publications is to quash such reviews, but ours is not. There is more to say about the QSC 1804, but the evaluation to this point precludes us from recommending it for serious home stereo use. Perhaps that guy that has recent Klipsh towers from Best Buy on which he only plays Death Metal at insane levels would see some merit here. He probably couldn’t hear the fan with the music off anyway.

Yes, there are a lot of watts for the buck, but if cheap were everything, we’d all be driving Yugo’s - if there was one still on the road. There are undoubtly other Pro Audio products out there that may work well in the home and we will keep searching for them for our readers. Unfortunately, this is not one of them. If you know, or think you know of one, please write us. We listen.

 

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