How did you pick your power amp?

[Jim Carr]

I am wondering how to select a power amp for a Goliath II (500 watts)
and a Son of Bertha (350 watts), both 8 ohm. I don't think a
Mono (bridged) power amp is the right thing.

My lovely old swr 350
just ain't doin' it!


How does one match cabs with different power ratings to a dedicated power amp? Wondering....

Thanks!

 

[Jim Carr]

I guess I am just asking:
What was YOUR process for deciding on your Power Amp?

How many watts and why?

 

[DanielleMuscato]

While wattage is important, there are two things to consider: 1) headroom and 2) damping factor. The more watts, the cleaner the power, especially the low end, and especially at high volume.

Higher damping factor, in my opinion, is the more important of the two. A high damping factor means cleaner, clearer, and more solid bass reproduction.

A good rule of thumb is to have 50% more wattage than the sum of your cabs RMS - so, a 500 watt cab and should have at least a 750 watt amp (or channel of an amp), etc. The more the better, though, as far as headroom is concerned. You really can't hurt a cab with too MUCH power. What damages speaker cones is attempting to reproduce distorted signals. If the amp is underpowered (causing distortion), you will damage the cones very easily. Check out the peak on your cabs... it's usually 4 times the RMS or so.

I use a Crown I-Tech 4000, one channel running each of my cabs (see signature). It's 2000 watts per side and has a damping factor of about 5300 at 30 hz. Wow! Very clean, powerful, and solid low end. It's also 28 pounds and highly recommended.

Something else to consider is the cooling system. Convection cooling, which doesn't use fans, draws more consistent amperage and therefore has less fluctuation in tone, which is especially important if you're recording and micing cabs (rather than running direct). Crown K2 power amps (1200 watts/side) are great for this, or, you can just put your power amp outside the iso booth and run the speaker cables in with the cabs and the mics.

 

[DanielleMuscato]

Also, you're right, don't bridge an amp unless you're in a pinch. They are not as efficient bridged, and really don't sound as good. Really the best thing to do, if you're running two cabs at once like that, is to run a crossover and have the frequencies best suited for each particular cab running ONLY to that cab. For example, I have my lows going to my Ampeg Classic 410HLF (I keep the horn turned down to -inf), and the highs going to my Phil Jones 8T. The PJB cab has aluminum drivers that are specifically tailored to reproduce high-end bass frequencies with extreme clarity, but are not as suited to lows (PJB makes a different cab for that - any of the cabs with the "B" (for Bottom), as opposed to "T" (top) model name). The 410 is better suited to lows. Anyway, use a different amp (or channel) for each cab. The cool thing about the I-Tech amps is that, aside from being extremely lightweight, they have built-in digital signal processing (a crossover), so you can separate the frequencies without adding another unit to your signal chain.

 

[Jim Carr]

Wow! Great advice.

But--I guess I had not considered using cross-over style filtering
for my 2 cabs.

Since, my 4x10 goliath II is a 500 watt cab (rated by swr)
and the 1x15 Son of Bertha (SOB) is a 350 watt cab (rated by swr),
I have been more concerned about finding which amp is a good
value that also can support 500 watts per channel at 8 ohms.
I plan to run the SOB at a lower power amp gain level.

Maybe you are right. Perhaps I should consider freq. division.
But which cab is really the bottom?

How many watts per channel RMS 8 ohms should I get?

thanks!

 

[DanielleMuscato]

A good way to think about it is - more than you think you'll need You can never have too much headroom.

I'd put the 1x15 on the bottom if you're using a crossover. It depends on if you want a tighter bass response vs. "thump." A 15 pumping out double-digit frequencies (if you use a crossover - say, 30-100 Hz) will give you real dance-floor bump and leave the 10s open for high end clarity. But try it both ways.

 

[DanielleMuscato]

By the way, for what you're saying, I'd get at least 800 watts/side, but really, you almost can't overdo it. Look for an amp with a high damping factor foremost, and then get the most powerful (wattage-wise) model of that amp within your budget.

 

[Fretless5verfan]

By the way, for what you're saying, I'd get at least 800 watts/side, but really, you almost can't overdo it. Look for an amp with a high damping factor foremost, and then get the most powerful (wattage-wise) model of that amp within your budget.

Agweed.

 

[Akami]

I picked largely by comparing all the lightweight amps I was able to find until I found the AI Focus which delivers what I was looking for, both in terms of sound, output level and portability.

I'm soon planning on checking into the possibility of changing to a Thunderfunk, Epifani or Bergie IP, but again it would be the combination of the above equation that would get me to either change amps or not.

I really do like the Focus.

 

[DanielleMuscato]

Jim, I don't know what your budget is, but I was just looking at your profile and based on your other equipment (nice gear, by the way!) I don't think an I-Tech would be out of line for you. They are 28 pounds, 2000 watts per channel, have on board DSP (including a fully programmable crossover), and a damping factor of 5300 at 30 Hz. They have Neutrik Speakon and 5-way binding post outputs (that accept banana, bare wire, or spade plugs), and can also run digitally. They're pricey but the damping factor, wattage, and light weight is worth it IMO.

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A little cheaper, but still very good, is the CTS series. Damping factor >3000 between 30 and 100 Hz. They're not nearly as lightweight, and have the unfortunate design drawback of having the attenuators on the back panel (!), but GREAT sounding for bass.

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[Kenny Allyn]

I've been using SWR cabs with the Ampeg pre and a Crown power amp (see profile) for years with great results ... it replaced my SVT ...

 

[bgavin]

I wanted a PLX.

Knowing that, I watched eBay until one came along at a very good price. It turned out, that was the one I wanted.

I've done this 3 times. Never fails.

 

[Jim Carr]

Dave & Kenny-

Thanks for the good advice. I can see that the damping factor
on the QSC rmx series and Crown XLS series is only 200 or 300.

Hmmmnnnn. Guess I've got to look deeper and save more money-
but thats Cool.

 

[seamonkey]

If you're going with a good modem amp I wouldn't worry too much about damping factor. Search around and there's old and new papers on damping.

i.e.
THE DAMPING FACTOR DEBATE
by George L. Augspurger, JBL Inc.
Electronics World, January 1967

"CONCLUSIONS

It should be obvious at this point that the quoted damping factor of an amplifier is important only if the figure lies somewhere below 20 or so. Changing the damping factor from 2 to 20 does change the performance of the loudspeaker system (for better or for worse, depending upon the speaker). But trying to prove that a damping factor of 200 or even more is somehow better than one of 20 is pretty unconvincing because the effective difference in the practical case cited is only that between 1.25 and 1.32.

But someone is bound to insist that exhaustive tests have been made with such and such amplifier and that a very high damping factor is better than one down around 10 or 15. "The bass is just a little cleaner, just a little more natural and open" is the way the argument usually runs.

In a given situation, this may very well be true. R_s is a byproduct of negative feedback. The more such feedback that is thrown into a power amplifier circuit, the lower the generator impedance and the higher the damping factor. The point is simply that if a lot of feedback has to be used to lick the distortion in a particular circuit, fine - use it. But don't believe that the reason it sounds good because of some astronomically high damping factor."

 

[paulraphael]

I would ignore damping factor numbers (at least for powerful solid state amps) and pay a lot more attention to sound. If you can't audition the amp yourself, find a thorough review or reports from friends who have compared them.

Sometimes there's a big divide between what the the published specs lead you to expect and what you actually hear.

Also keep in mind that these pro audio amps we use for bass are really mid-fi designs. They tend to have a lot of sonic flaws. No one knows how to take true high end sound, and deliver a thousand watts of it in a package we can lift and can afford.

So as good as the best of these amps sound, they're all a different mix of compromises, and they can sound dramatically different. You'll probably find some of the biggest differences in their ability to deliver transient peaks. Some amps do an amazing job. Others, either through lack of energy storage, slow response, or overly aggressive protection schemes, sound flat and muted in compariston. You might also hear differences in coloration, low end extension, smoothness in the mids and trebble, and detail and openness.

In general, I agree with everyone who says you're not going to regret more power.

 

[Jim Carr]

Wow-

The "JBL guy's" article is interesting.
I am starting to think that I need to buy from a dealer
who will let me return the amp if it
doesn't do the job.

I want to be able to use any 1 or 2 of my SWR cabs.
One is rated at 500 watts, (goliath II), and the other 2 are
rated at 350 (son-of-bertha and son-of-goliath III).

Maybe if I run a two channel amp, I can mix the watt 500 4x10
with either the 2x10 sog or the 1x15 sob....(hee hee).
with attenuators obviously set lower for the 350 watt cab.

I've read that 1.6 x (speaker rms) is a good rule of thumb for headroom.
That means 800 watts per channel @ 8 ohms.

Any one tried the Crown XLS 802d ??

 

[Richard Lindsey]

You really can't hurt a cab with too MUCH power. What damages speaker cones is attempting to reproduce distorted signals. If the amp is underpowered (causing distortion), you will damage the cones very easily.

This has been debunked numerous times around here. Look for posts by Mark Reccord, Bob Lee, Jerrold Tiers, and billfitzmaurice.

 

[Fuzzbass]

How did I pick my power amp? Stewart World 2.1: 2100 watts bridged, roughly 750 watts/channel into 4 ohms... just 17 pounds!

My lightweight Stew sounds great. I was going to say "good enough", but that's faint praise... I get great tone from my rig!

This has been debunked numerous times around here. Look for posts by Mark Reccord, Bob Lee, Jerrold Tiers, and billfitzmaurice.

+1.

I also agree with seamonkey that damping factor isn't a concern with modern amps.

 

[Akami]

This has been debunked numerous times around here. Look for posts by Mark Reccord, Bob Lee, Jerrold Tiers, and billfitzmaurice.

The talk of underpowering blowing speakers got started by tweeters getting blown by distortion which will not adversely affect cone drivers nearly as easily.
Especially a problem with stereo speakers not designed for pro applications.

 

[DaveDeVille]

How did I pick my power amp? Stewart World 2.1: 2100 watts bridged,
roughly 750 watts/channel into 4 ohms... just 17 pounds!
What I do know is that my lightweight Stew sounds great.
I was going to say "good enough", but that's faint praise...
I get great tone from my rig!

...agreed .
or , +1 , ... whatever ...