Pro Amplifiers for use in HT

Why?

Why does live music usually have a more emotional impact than recorded music?

Why are real gun shots so much more startling than the movie versions?

Well there are a lot of reasons but one of the biggest is dynamic range. Humans are very sensitive to changes; motion catches our eye, we like lots of contrast in our video images, and we like to hear the full range of sounds from the quietest to the loudest. What's this have to do with home theater? To get the maximum sonic and emotional impact from a home theater system it must be able to reproduce the quiet parts with great fidelity and the loud parts with great fidelity. Fidelity means that the reproduced sound is true to the source, in both frequency (pitch) and intensity. Most good quality home theater speakers, subwoofers and amplifiers can reproduce the frequency response properly but many compress or "go flat" on the high intensity scenes. Not a big deal if you only watch movies with nothing but dialog and background music, but when you pop in the latest action blockbuster you want to feel the action. You don't want your system to do this:

This graph shows what happens when a signal is compressed, as the volume is increased (each trace is a louder volume setting) the low frequencies no longer increase the same as the upper frequencies, resulting in loss of dynamic range (and likely some distortion to boot).

Even if you never crank up your movies, the dynamics in movie soundtracks can bring on compression at even moderate volumes. THX knows this, this is why they specify that systems be able to reproduce 108 dBC SPL from the satellites and 115 dBC SPL from the sub channel. They know you won't normally listen at these levels but transients like crashes and explosions can easily hit these levels.

THX Certified Theater Reference (PDF)

What can be done?

To reproduce the full impact of a modern soundtrack, the whole audio chain must be up to the task. This includes the souce, pre/pro or receiver, power amplifiers and speakers. For most set ups the source and pre/pro or receiver are pretty far down the list of components likely to introduce compression. The main culprits are inadequate power and speaker limitations.

In my theater, I've used a Denon Receiver for a number of years with good results but always felt it was missing some "impact". Over the last year and a half I've gradually replaced the Denon and AR speakers with Pro Audio class gear.

Why Pro Audio, several reasons:

Performance: Designed to play loud and clear, todays pro gear has smoother frequency responses, better looks and more choices than ever before.

Price vs. Performace: I'm always looking for the most for the money, as far as cost per watt and level of impact delivered, pro gear is tough to beat.

Availability: Some features and extreme dynamics are only available on very high end HT gear, while many of these features are widely and affordably available by using pro gear.

What are the drawbacks?

Noise: Many pro amps have noisy fans

Looks: Pro gear isn't as beautiful as home theater specific gear

Design Issues: Pro gear isn't nearly as plug and play as home theater gear, with speaker directivity, input level mismatches, and mouting issues, using pro gear takes more effort to get right.

Ground Loops and Hum: Often results when interfacing pro and home gear, hard to track down and eliminate.

Is it worth it?

It is for me for sure. If you've never heard a movie like "Master and Commander"or "The Matrix" reproduced with full uncompressed dynamics at reference levels you'd be better off not hearing it If you do you'll find yourself heading down a similar path. You're sitting watching with normal dialog levels and sound effects when WHAM, a cannon ball rips into the scene, wrenching your insides, making your clothing blow in the breeze and your hair stand on end, quite a rush....

How is it done?

If you've decided to try out using pro amplifiers with your home theater, here's how I did it, you can read how others did it in this thread on AVS Forum.

I purchased two QSC RMX-850 Amplifiers, with 200 watts per channel into 8 ohms. Mounted them in the rack, used the pre outs for the center, left, right and bass shakers and connected them to the amplifier inputs using an RCA to 1/4" cable. I moved the speaker wiring to the output terminals of the amplifiers.

I've got a few concerns...

Q: Won't that much power damage my speakers?

A: It is possible, but due to the transient nature of music and sound effects, it is unlikely. If you have very low power rated speakers, or like to listen very loudly a lot of the time, you will want to make sure the speakers are rated to match the amplifier, otherwise you should be fine. Underpowered amps and compressed dynamics can lead to more problems than a strong amp that is running with low distortion.

Q: How do you turn them off and on with the remote?

A: I don't, at least not the RMX's. The SRA's have trigger inputs that can sense when the system is on and power up the amp. The RMX's would require a power strip with a trigger to turn them on automatically. For now I just flip all the switches on when I enter the theater for a movie.

Q: Those Pro Amps are a bit ugly.

A: Some more than others, a small price to pay for pure power.

Q: Are all Pro Amps equal? What are some good brands?

A: Not all are equal. Some have loud fans some quiet, some have huge beefy power supplies, others are a tad wimpy. Different protection circuits, different filters, different sizes, on and on. Some good brands are QSC (my choice), Crown (I've used these also), Crest, Ashly, Carver, Hafler. The old adage, you get what you pay for and if it sounds too good to be true it probably is both hold with amplifier purchases.

Q: You make it sound like Pro Amps provide some sort of magic watts or provide some inherent advantage over normal home theater amps, what gives?

A: Watts are still watts. There is no special advantage to a well designed pro amp over a well designed home amp. In general pro amps can be had for a lower $/watt than home amps of similar quality and power, they also tend to be very heavy duty. For me, pro amps are an integral part of an overall high impact home theater audio system.

This didn't work out as well as I'd hoped, while I had plenty of power and it was clean, I also had a very loud 60Hz ground loop hum. Good info on ground loop problems with pro gear here, from Rane.

I tried grounding cases, I tried several different cables, I tried DI boxes, and nothing worked, I got the level of hum down, but never gone. I finally had success with an Ebtech Hum Eliminator ground loop isolator. Jensen and a few others also make good models. This unit allowed me to patch from the receiver to the Ebtech, then to the amps, with the Ebtech Hum Eliminator providing transformer isolation and breaking the ground loop.

The fans in the RMX-850's are not loud, but certainly audible, too audible for my taste. I tried adding a clear plastic plate over the vents, this helped but not enough. Eventually I opened up the cases and disconnected the fans, I added a large but low speed fan to the rack behind the amplifiers to make up the cooling (see pic below). They run very cool and 98% silent.

My next purchase was a pair of QSC SRA amplifiers, these amps are one of the few pro amps that takes an unbalanced or balanced audio signal. They also have very very quiet fans (yeah!). These were pressed into duty for the main left and right, leaving an RMX-850 open for the surrounds. The other SRA was for the new LAB Subs I was building (and now enjoying).

While this arrangement worked pretty well, I finally came to the conclusion that the best possible solution was to replace the receiver with a processor that had balanced audio outputs. I did a lot of looking and research before eventually purchasing a gently used B&K Reference 50. I re-connected everything using XLR balanced interconnects and never looked back.