# Help me understand power amp specs!

Discussion in 'Amps and Cabs [BG]' started by CaptainWally, Nov 20, 2000.

1. ### CaptainWally

Oct 21, 2000
Sandy Eggo, CA
First of all, I am ultimately seeking an amp
that can run a couple of acmes (4ohms). I'm
thinking 600W/Acme will give me some head room.
I think it would be way cool if I could get away
with a Stewart World 1.2 (1 rack space, 11 lbs.),
but I can't figure out if it's enough power.
The amp is 1200 W bridged mono at 4 ohms, but does
that means it's 600W/channel in stereo? According
to the specs (below), it shows only 350Wx2 @ 4 ohms,
but this is at 1% THD @ 1kHz whatever THAT means.
Help me understand!

Thanks,
Wal

SPECIFICATIONS
WORLD 1.2
Maximum Output at 1% THD @ 1kHz:
580W @ 2 ohms single channel
420W @ 4 ohms single channel
240W @ 8 ohms single channel
Output Power: (1kHz, 0.1%THD)
500W x 2 @ 2 ohms per channel
350W x 2 @ 4 ohms per channel
FTC Power Rating <0.1% THD, 20hz-20khz
(both channels driven): 200W x 2 @ 8 ohms per channel
Rated Power Output, Mono 20Hz-20kHz:
1200W x I @ 4 ohms bridged
700W x 1 @ 8 ohms bridged
400W x 1 @ l6 ohms bridged
Frequency Response +0dB, -5dB: 20Hz-20kHz
Bandwidth +0dB, -3dB: 15 Hz - 45kHz
Signal-to-Noise Ratio (A-wtd., 650w @ 4 ohms): >100 dB
Input Impedance: 20k ohms

2. ### throbbinnut

SPECIFICATIONS
WORLD 1.2
Maximum Output at 1% THD @ 1kHz:
580W @ 2 ohms single channel
420W @ 4 ohms single channel
240W @ 8 ohms single channel

>>>The above is the absolute max power out for a limited time on a single channel running by itself. If you were to push this indefinitely, something would fry in the amp eventually. Note the Total Harmonic Distortion, it's higher than the next set of specs. That means it's right at the point of clipping the output.

Output Power: (1kHz, 0.1%THD)
500W x 2 @ 2 ohms per channel
350W x 2 @ 4 ohms per channel

>>>This is normal use maximum rating. The amp can do this continuously with no problems with basically a clean output signal on both channels.

FTC Power Rating <0.1% THD, 20hz-20khz
(both channels driven): 200W x 2 @ 8 ohms per channel

>>>Not real sure what FTC means, just looks like a continuation of the above normal use maximums, but for 8 Ohm loads on each side, note the even lower THD due to the increased impedance and decreased power.

Rated Power Output, Mono 20Hz-20kHz:
1200W x I @ 4 ohms bridged
700W x 1 @ 8 ohms bridged
400W x 1 @ l6 ohms bridged

>>>This is the bridged rated output, continuous use, no problems, indefinitely.

Frequency Response +0dB, -5dB: 20Hz-20kHz
Bandwidth +0dB, -3dB: 15 Hz - 45kHz

>>>I have no clue why they give differing specs here. Seems like the FR should be -3dB, and the Bandwidth should be -5dB. Oh well. No matter. It's better than most HI-FI amps with these numbers.

Signal-to-Noise Ratio (A-wtd., 650w @ 4 ohms): >100 dB

>>>This means very low noise, as compared to full output power.

Input Impedance: 20k ohms

>>>This is what the output of your pre-amp cares about. 20K means you can't just plug in a standard passive bass. You'll need a preamp of some sort, and the preamp will need to be able to handle a 20K output load, which it will no problem.

I could be wrong on any of the above, but for the most part it's right.

Chris

3. ### CaptainWally

Oct 21, 2000
Sandy Eggo, CA

To put the question in a way I can grasp:

I have an Ampeg SVTIII prog @ 450W @ 4 ohms.

It's a little weak in running a Single Acme. (I
think Ampeg kind of fudged this figure...possible?)

Anyway, with the Stewart 1.2, will I get 600
"equivalent watts" to the 450 W I have now?

Cheers!
W

4. ### throbbinnut

Well, if you bridge the amp and run it into a 4 Ohm load, then you will get 1200W according to the specs. So it would be louder than the Ampeg you have. I'm sure if you put the Ampeg on a test bench and measured the output power, it would do what was advertised.

I think the cabinet must be kinda inefficient if it doesn't sound loud with the Ampeg. It's a design tradeoff, and Acme probably traded high efficiency for wider frequency response. In other words, you need more power to make it sound as loud as a bigger, more efficient cab, but the frequency range goes lower and higher than the high efficiency cab would. Compare a VW Bug to a Ferrari, handling versus gas mileage. The Bug gets better gas mileage (more efficient), but the Ferrari has better response (analogous to wider bandwidth).

If you were to run 2 4 ohm cabs, one on each channel, you would get 350Watts into each cab, according to the specs.

Chris

5. ### brianrostGold Supporting Member

Apr 26, 2000
Boston, Taxachusetts
FTC means Federal Trade Commission and in amp power specs means this:

By law wattage must be disclosed as RMS wattage into a specified load impedance, specified bandwidth, specified distortion with all channels driven (two in a stereo amp). The amp must be able to run at 1/3 rated power indefinitely without thermal shutdown and at full power for one hour with continuous sine wave or pink noise input (more severe than playing your bass through it!).

Notice that this lowers the wattage considerably! The FTC rating for 8 ophms is 200 watts, but the 1KHz rating (i.e. NOT full bandwidth) at near clipping when only running one channel (i.e. power supply doesn't have to divert any juice to the OTHER channel) is 20% higher.

The difference between rating at 1 KHz vs. full bandwidth (the usual 20-20K) is that the amp can't usually put out as much juice at 20 Hz, so the only way to goose up the wattage spec is to use the 1K rating. You sometimes see home stereo amps with FTC ratings only spec'ing reponse to 40, 60, 80 even 100 Hz because that way they can claim the amp as "100 watts" or something where if you needed response to 20 Hz it might only be rated at 60 or 70 watts. Often extending the low end response is just a matter of needing a beefier power supply.

Bridging is a mixed bag, you use both amp channels wired together to double the power but you also increase the heat. Bridged into 4 ohms is the same as running stereo into 2 ohms (easily seen from the wattage specs). Running at 2 ohms is theoretically safe if the manufacturer says so but the amp runs a lot hotter and is more prone to failure.

Most bass amps are NOT rated by FTC rules, Ampeg is just one example. Check out the "up to 1600 watt output power" SVT-IV which at FTC ratings is really 1200 watts (25% less!!!). This is why you shouldn't just take the wattage numbers in the ads at face value. A watt is a watt, but how the watt is measured makes a big difference. The Stewart is legitimately rated as a 1200 watt amp.

6. ### CaptainWally

Oct 21, 2000
Sandy Eggo, CA

Ok, so in all likelihood, even thought the
Stewart is only putting out 350 legitimate watts per
channel, is is probably just as powerful as my
non-FTC rated 450W Ampeg SVTIII-pro?

Does anyone want to weigh in and offer an opinion
of whether this is enough to run Acmes? For those
who don't know, Acme Speakers are very inefficient,
although hi-fi. :=)

Cheers.