which power amp/what the heck is class H?

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bigugly

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Jun 27, 2004
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Location
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My brother needs to upgrade the amps in his small PA rig. (currently 15 year old Peavey CS800s) So, I've been doing some surfing at the major brands websites. I'm looking to get only two amps, hopefully in the under $1K range. The PA is an Allen & Heath 16x2 into a DBX DriveRack. It's a bi-amped system with the subs in mono. My mid/hi cabs are rated @ 750W/8 ohms and each sub is @ 800W/8ohms. I was looking to get 2 of the same amps, with 600-800W stereo @ 8 ohms and more than 1600W/4 ohms in bridged mode. I have found a few that fit that bill but I would like to get some suggestions from the Group here.

Oh, I almost forgot, what in the hell is class H?
 
Class H was first developed by Hitachi decades ago and is a technique using multi voltage PS rails to reduce dissipation and improve efficiency of power amplifiers. I recall a white paper about it was published at the time.

I lump class G and class H together as similar variants on the same thing (multi rail analog amps).

There are lots of inexpensive relative high power amps available today for sound reinforcement.

A popular example is the QSC RMX2450 which is over 1200W into 8 ohms bridged. The CS800 is a small amp by today's standards. Most major amp manufacturer's have a value amp series similar to the QSC RMX with similar power points. Check out the normal suspects ; Crown, Peavey, Crest, QSC, and sundry newer players. Since these are now made in China I wouldn't be surprised to see house brands available from Best Buy some day.

JR


 
> 600-800W stereo @ 8 ohms and more than 1600W/4 ohms in bridged mode

Does not add-up.

Assuming zero sag (and no trickery with multiple supplies), bridge gives 4X power and 0.5 impedance gives 2X power. So 800W one-amp 8 ohms is 800*4*2= 6,400W bridged in 4 ohms.

> 15 year old Peavey CS800s

Peavey is still a good bet for roadhouse work. Of course an-amp-is-an-amp but also you-get-what-you-pay-for (at the same max power, a $2000 amp may be more abuse-proof than a $499 amp). Just taking that badge for reference, today's line-up is:

Peavey - CS 4000 Professional Power Amplifier
MSRP $1,299.99
Our Price $999.99
2000 watts (rms) per channel at 2 ohms
1350 watts (rms) per channel at 4 ohms
800 watts per channel at 8 ohms
4000 watts at 4 ohms bridged

It has the max-price you want, it has the 8 ohm power you want, but simple math implies significantly more bridged-4 power than you asked.

Crown's High-Brand XTi 4000 is the same price, hair lower power specs; gotta pay 10% extra for the badge and you get 10% more frills. As John says, Crown has a "low price" brand too. I think that could be Crown XLS 802D Our Price $599.99.

I think what you might consider: leave the two subs separate. Drive the inputs together but run each speaker on its own channel. That puts you right at your 800W in 8 ohms each, with less chance of complete failure and loss of thud.

Then you don't really need a 2-ohm rated amp for your 8 ohm speakers, which could save a buck and a pound. Such as PV-3800 PV-2600 or that Crown XLS802D. Cerwin Vega CV-1800 is a bit less power at even  lower price- you could do -both- amps for $900.

Class H is an engineering trick turned to marketing "feature". A Class B amp is fairly efficient at FULL power but less efficient at part power. Audio is 99% part-power. Dissipation can be quite high even at moderate levels. Class H is a small-and-large amp together, to improve part-power dissipation. This could mean less fan and sink, though non-H amps are still popular and competitive. Obviously it risks a nasty sound at the hand-over from small to large; obviously if it sounded like crap it would have vanished long ago. You simply do not care.

Get the power, get the over-engineering you want to afford, get the reputation you are happy with, consider a decent warranty because it implies careful production. You do NOT care what is inside the box: flux-condensers, vacuum tubes, or tiny elves turning giant reostats, as long at it works.
 
PRR said:
>

Class H is an engineering trick turned to marketing "feature". A Class B amp is fairly efficient at FULL power but less efficient at part power. Audio is 99% part-power. Dissipation can be quite high even at moderate levels. Class H is a small-and-large amp together, to improve part-power dissipation. This could mean less fan and sink, though non-H amps are still popular and competitive. Obviously it risks a nasty sound at the hand-over from small to large; obviously if it sounded like crap it would have vanished long ago. You simply do not care.

Just a minor amplification.. In addition to less fan and heat sink, the more efficient class G/H power stage approach allows designers to get away with a lot less copper and iron in the power transformer too. Anyone who has ever lifted a CS1200 will appreciate how in combination, this technology provides a huge size, weight, and cost savings for higher powered amps. I doubt you could even buy anything bigger than the old CS800 nowadays that is still class A/B.

I avoided specific recommendations about matching amp size to speakers as it's a little confusing. To get good peak performance you often need 2x the continuous power, but you can blow up speakers with half their rated power if you drive into heavy clipping all night, so use caution. I advise using the clip limiters built into most SR amps because that can help mitigate somewhat against operator error. 

JR
 
"Output power , 2 ohms: 9000 watt
"Power Input : 115-230V ac - 16-24A Powercon"

This seemed odd to me. So did this: 

"The SMPS has a power rating of 5000 watts + and the core/transformer has a max. power rating of 6000 watts +."

115VAC at 16A is 1,840 Watts; 230V 24A is 5,520 watts. The 9,000W rating implies 163%-489% efficiency from the wall. And as the amp can easily make 120V 60Hz, you could run one amp wall-cord from the speaker-jack of another and hit 2,300% efficiency!

But they actually claim:

"85% total conversion efficiency from ac input to speaker output, best in the business."

Ah, here's the tricky-bit.....

"Power ratings are continous pink noise (20 Hz-20 kHz) into rated impedance!"

Shades of peak-to-peak-to-peak ratings!

However, no thermodynamic law is broken or even bent. Peak/RMS ratio of pink noise is ~~10dB, so they actually claim 900W long-term heat with 9,000W peaks.

And it isn't "cheating" for most audio uses. Speech/music tends to be >=15dB, even radio-processed (handy track of XM's "60s on Six!" programming shows 15dB Pk/RMS over 8 minutes of processed gushy/gooshy pop-music). So even if in live-concert you push 5dB clipping, it's no worse than pink-noise at clipping. Continuous Sine was never about speech/music reproduction.
 
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