General question coz I think I might be stupid...

SkinTrade

Hi all

I've been checking out all sorts of online calculators, reading up on all sorts of forums and white papers, etc.

I've got a small 2-way setup with a DRPA. I'm running a Behringer EP2500 in mono bridged mode on the subs. The specs have been posted here before, but to refresh memory: 2400W into 4ohm with 1kHz sine wave. So probably more like 2000W into 4ohm considering a full audio spectrum (20-20k).

In order to match the system with the tops I need to drop the x-over gain for the subs by 6dB. I know now, having read a few posts, that I should be changing the amp attenuator rather than the x-over gain (is that correct??). But that's all together a different question.

Now, I need that amp for somethin' else so I want to get a different amp for this application. Clearly I don't need such a powerful amp. The system never clips any amps at any time. A rock band play through this system in a smallish venue (indoors). They play softly (i.e. the drummer isn't angry at the kit and the guitarist wants to hear other members of the band too...). But the PA is pretty loud and loud enough for the venue. By my calculations, I come to the conclusion that if I drop the input level to the amp by 3dB the amp probably only outputs 1200W, and if I drop it by another 3dB then it outputs 600W. This would obviously be on the music peaks, since nothing ever clips in this system.

So if I replace this EP2500 with a 600W amp and run the sub x-over gain at 0dB I would get the same level of bass, right? Obviously I would do a proper gain structure again to ensure no clipping and need to also adjust the DRPA's limiters for the subs.

Am I talking rubbish? Am I confused? Please HELP!

Regards
Laure le Roux

Dra

What subs and power ratings (cont, program, peak)?
What is their \"listed\" -3db point.
How many on the amp?
What tops and amps do you have?

SkinTrade

Hi

I didn't provide that information because I didn't think it was relevant to the question I asked (which is really a case of mathematics, not so?) Anyhow, I'll be kinda busy at work for the next few hours so I'll post all that additional info tomorrow.

SkinTrade

What subs and power ratings (cont, program, peak)?

Custom built front load cabs (similar to Yamaha ClubV Series SW115V) with Beyma 15G450N speakers. The specs for these are:
Impedance 8 ohms
RMS Power Rating 600 Watts
Program Power 1200 Watts
Peak Power not listed, assuming 2400 Watts
Frequency Response 30Hz - 1.5kHz
Sensitivity (1W/1m) 98dB
Voice Coil Diameter 4.5\" / 114mm

What is their \"listed\" -3db point.
I can't find this information; unless I don;t know what you are referring to. Perhaps if you look at these 2 links you might find what you need:
http://profesional.beyma.com/ENGLISH/producto.php
http://www.usspeaker.com/beyma%20index-1.htm
The 15G450N is listed in the G50 series low frequency speakers and has since been upgraded with higher specs than my model.

How many on the amp?

I have a pair of subs, 8ohm each, so 4ohm load in mono bridged mode

What tops and amps do you have?

Tops are JBL TR125 specs listed here before, but to recap:
Frequency Range: 55 Hz - 16 kHz (-10 dB from rated sensitivity)
Power Capacity 225 Watts (Continuous Pink Noise EIA RS426A, 8 hrs)
Peak Pink Noise: 900 Watts
Nominal Impedance: 8 Ohms
Sensitivity: 99 dB (1W, 1m)
Nominal Dispersion: 90° Horizontal x 45° Vertical
LF Driver: 1 x 380 mm (15 in)
HF Driver: Pure Titanium Diaphragm Compression Driver. liquid cooled
Horn: Optimized Apertureâ„¢
Dimensions (HxWxD): 750 x 460 x 390 mm (29 1/2 x 18 x 15 1/4 in)
Weight: 27.2 kg (60 lb)

I drive these with a Roland SRA-800 amp in stereo mode:
360W per ch in 8ohm (20-20k)

Currently crossing at 125 on the DRPA and running the low gain on the x-over at -7dB
Like I said, I don't need the volume at that gig that the PA is capabale of in terms of the subs. The tops are probably underpowered and so are the subs at this point. However, the system is idling along and not working hard at all. What I need to do is use the EP2500 for a diferent application and replace it with another amp. So my question is: to get the same kinda bass power I have right now can I replace the EP2500 with a 600W amp? Or alternatively, since I don't need stereo, I can remove the Roland amp and use the EP2500 ch A for tops (4 ohms 650W 20-20k) and ch B for subs (4 ohms 650W 20-20k). Perhaps I can also drop the x-over pint to 100Hz and raise the HPF on the subs to 50Hz?

I am trying to understand the science behind all of this. In other words, when I drop the input level to the amp by 7dB, what kinda wattage ouput can I expect on the musical peaks given the fact the, having set up the system gain correctly and also the limiters, I never see the limiters running at all. The PA is really not working hard.

Regards
Laure le Roux

Dra

The -3db point is a function of the cabinet output where the spl has dropped below the base line as is goes to silence. Here we are talking about the low end. It makes no sense to sense the cab or amp trying to make sound that it can't make. Since you are dropping power to the subs changing the x-over point is a good idea.

If you use the EP2500 in stereo for tops and subs you'll give 325w to each JBL and 325 to each sub. The JBL's want 500 - 650w of amp power (headroom included) each. The subs want 1800 - 3000w of the same. Now, do you have to give them that? Obviously not. If you set you gain structure and your limiters and don't the limiters don't engage to reach desired volume, then you'll be fine. That is assuming that you are fine with running a staight mono signal.

After setting the gain structure as noted in the welcome post, play a cd and turn the \"too loud\" speakers down with the amp. Adjusting the gains on the x-over can be done, but this affect the x-over point (underlap / overlap) and requires additional testing, so the amp is best.

DRA

Gadget

So, are you trying to understand the science only here, or real world application? One further question, at least for me. What is the end goal here? Is it a learning experience, or are you asking if using a lower wattage amp is better? Because if it is the latter, the answer is a really big negative! Unless your goal is to somehow defeat the laws of physics, more power always wins. Now if your argument is that a lower wattage amp draws less edison.. then you would be wrong. because an amp, even a power hungry boat anchor like the EP2500 draws the same Idle current as the smaller amp would. The illuminating fact here is perhaps best postulated using an analogy. If we have a situation where 60 watts of average light energy is needed , and we use a 120 watt bulb, run at the level of a 60 watt bulb, (were talking incandescent bulbs here) the light bulb will have the capability of giving twice the light energy if needed, but the big benefit is longevity. The 120 watt bulb will last 3-5 times longer than the bulb running full out, 100 % of the time. Now we can also expect that the amp with the greater output will also last longer, AND won't be susceptible to power supply sag in the event the power is called for. In a conventional amp with a conventional power supply that uses a large bank of STORAGE capacitors it will draw less current from the wall in a say 50% demand situation than a balls to the wall amp will, because the reserve is still there and the amp doesn't have to use it and the wall current to keep the power supply from exhaustion... Amp manufacturers suggest (Crown International in this case) 2-4 times the program wattage of the speaker. That means in a correctly designed enclosure for that driver that 4800 watts of power could be instantaneously requested before Xmax (linear excursion limit) of the cone.

Now if it's a weight issue, get a more efficient amp but STILL keep a high power to load ratio, the same principles apply. More power equals less heat, less wear and tear on the amp, it will operate in a much more linear portion of the power curve, and you stand MUCH less chance of damage to the speaker from overheated voice coils. Since you have a 30 lb turkey in hand, leaving it lay for a 9 lb chicken in the bush doesn't look like such a good idea to me.

As I always say, too much power is NEVER enough! If you ain't got it you still want it!

Gadget

P.S. I have been systematically replacing all my old \"Boat anchor\" amps with lighter, more power efficient designs as money permits, especially in the portable applications. Instead of having a rack with 4 different wattage amps I tend to have all amps the same, that way I can turn down a more powerful amp, and still have a backup for the most powerful one in the event of a failure during a show.
:roll:

Dra

Sorry Gadget, but that's bad advice. Holding a 30lb turkey in your hand will only get your eyes poked out or, at the very least, some nasty scratches from all the violent flapping.

Other than that, good incite (insight?). I too have replaced my 40+ lb Yorkville amps (after one died) with QSC PLX3102's increase of power from 475w to 600w @ 8 ohms and 50% of the weight.

DRA

Gadget

Hey Dra.. THATS what the old boat anchors are for! You drop it on those turkeys FIRST Then they are MUCH more docile, repeated droppage can even render them suitable as pets :shock:, or dinner, and are much more humane than some other tactics... :? and did you EVER try and wring a turkey, or chickens neck :shock: There's where your eyes are in serious danger...

The incite as you know comes from years of dropping heavy amps on my feet :shock: and watching expensive speaker cones fly out on to the dance floor in flames :roll: as the the smoke alarms scream and the dancers flee in abject terror! There was that venue that the electrical conduit started arc welding itself when I turned on the amp rack :shock: P.S. using the conduit as ground AND neutral :roll: is definately a code violation :evil: ...

If you insist on pointing out the flaws in my logic these people will not want my advice much longer ...

G Spot

Dra

Sorry. I'd hate to see DBX cut you pay by 50%.
And besides, everyone knows that you use conduit for the HOT leg.

DRA

SkinTrade

Hmmm, all this chicken/turkey talk ... I might have to stop at Col. Sander's take-outs on the way home...

Gadget, I have read your numerous replies to other posts and the theme is always more power, more power, more power...

I agree. Absolutely. And the reason why I stuck the EP2500 on the subs for that gig was to have some power in reserve. I have a limited budget. I am not in the PA rental business. I am a musician playing various gigs. As a hobby. Over weekends. But 8-5 I am an IT engineer. So although I'd love to run my PA on a 1U 1lb 100000000W amp, I can't afford it.

At this stage of my life I need to make a plan because I need an amp for another application and perhaps want to use either amp that's used for the gig in question. Either the SRA-800 or the EP2500. All I need for the other gig is around 400W per side in 8ohm. Either amp will suffice. So perhaps I can use the SRA-800 for this other gig, and use the EP2500 in stereo, tops on ch A and subs on ch B, for this existing gig mentioned above. In the process then also reduce carrying weight by X minus SRA-800 weight. Perhaps then even a 3U smaller rack! Easier to move and better for space in the car. I digress...

What I was trying to understand was what sort of wattage amp would you experienced experts recommend considering that in the current setup, I have done the gain structure, had to reduce the sub x-over gain by 7dB to balance the system, and configured the limiters so the amps don't clip when the desk is running hot. Having said all of that, I still run that gig at a lowish volume. The PA is hardly working. The limiter on the tops kick in every now and then when the vocalist belts it out a bit, but the subs are just idling and I don't recall seeing the sub limiter ever being called on.

I appreciate your analogy with the light bulb and perhaps I should rather leave the amps the way they are right now for this gig, considering it never runs hot and probably will last forever (right?!?!? )

But, budget is the ruler of the house so I need to make a plan. I could also go and buy another amp for the new gig, but if I can use the SRA-800 for the new gig and run the Behringer on tops and bass for the current gig then I don't have to spend money! Great!

So the goal here is: How do I calculate the peak wattage being put out by my amp? Let's say that an amp can produce it's maximum rated output into a certain load when just below the clip point. As I understand it, that is how most manufacturers rate their products. Correct me if I'm wrong. So an amp rated at 800W in 4ohm per side should put out close to 800W when driven to just below the clip point, i.e. the clip light just about flashes on and off. That is, off course, on the musical peaks when playing music through it. And I know an amp should never clip. But let's leave limiters out of the equation for now. So ss not to confuse me too much. So now you turn the attenuator down by 3 dB. Unless I am totally confused already, a 3dB drop in level equals half the wattage. So if the input signal remains constant, the amp should now output 400W to the speakers, right? On the signal peaks. So the averge level is probably 6dB below that which is 100W. For pink noise with a 6dB creast factor.

In my scenario, I've decreased the signal being fed to the amp by 7dB. So is the amp still capable of producing 2400W into 4ohm bridged? I think so, if the mixer level is pushed sufficiantly hot. But if I set up the gain structure correctly, then the mixer can't be pushed that hard before it will start clipping itself.

Let's look at it this way: in the audio chain, I have a desk that is running hot (just below clip) on the signal peaks. Followed by a x-over that is configured for 0dB gain on the x-over output. Let's assume the x-over input is set at unity gain. As I understand it, if your desk spec says it's mains out runs at +4dBu, then you should set the back of the DRPA to +4dBu and then the DRPA input should be at unity input gain. Right? The x-over is followed by an amp with attenuators set so that the amp just about starts to clip on the peaks. One can then probably assume that the speakers are drawing the amp's maximum rated output for that load on the signal peaks. What the speakers put out to the audience is relative to their efficiency, sensitivity, etc., and is not related to this question.

So now I go to the x-over gain and decrease the level by 3dB. How do I calculate the wattage pushed to the speakers? Is it the same rule? Signal level drop by 3dB then wattage is halved? Then if I go and drop the signal at the x-over gain by another 3dB, so it now reads -6dB, is the wattage pushed to the speakers now a 1/4 of the maximum?

If this is true, then surely the same holds for levels on the mixing console. If you are peaking at, for example +4 on your desk's meters with the amp just about clipping on the peaks, then that is where your maximum wattage output is, right? Then if I decrease the master fader on the desk so that the meter now reads +1 on the signal peaks, I guess that the amp is putting out half the maximum wattage.

I also know that an amp can produce more than it's rated output, but with a higher percentage of distortion in the signal. So when amp manufacturs state a certain maximum rated out wattage with 0.1% THD, would this be when the clip lights almost want to illuminate? I guess it varies from one manufacturer to the other, but what is the \"standard\"?

Sorry about the long post. I have a reall world need and also want to understand the logic and maths. It will surely help me to make informed decisions when blowing my cash...

Regards
Laure le Roux

Dra

Isn't it...
+6db = double the volume (-6db = 1/2 the volume)
2x volume = 10x watts (1/2 volume = 1/10th watts)
+6db = 10x watts (-6db = 1/10th watts)

???

DRA

SkinTrade

Dra, thanks for adding to my confusion. :?

What about this then?
http://www.crownaudio.com/apps_htm/desi ... -power.htm

If I enter 2400 in the Power A box and 1200 in the power B box and click my mouse in the dB box (which you have to do otherwise the damn script doesn't seem to work properly!!!), then I click on calculate and it gives me -3.
So the difference in dB between 2400W and 1200W is -3dB. Huh?

But is this calculator really relevant to what I'm doing? Or planning to do? I want to calculate the difference in wattage output when dropping the input signal by 7dB.

Now the EP2500 specs only refer to a 1k sine wave into a 4ohm load when operating in mono bridge, so if one can use the ratio between the published values for 1k sine wave and 20-20k when operating in stereo mode (750W vs 650W) then you can probably assume that the EP2500 will put out 2080W (20-20k) in mono bridge mode.

So if my gain in the DRPA x-over is at 0dB, driving the amp to clipping level should output 2080W in mono bridge mode.

If I enter 2080 in the Power A box, enter -7 in the dB box and clear the data in the Power B box and then click on calculate, the answer I get is 415W.

So, according to this calculator, my amp will output only 415W if the signal changes by -7dB. Currently I am running the PA with this -7dB x-over setting, but the amp is nowhere near clipping. So if I run both subs of 1 side of the EP2500 I will theoretically have 650W into 4ohm. But that is near clip level. So if I then drop the attenuators by 3dB according to the gain setup procedure, I will only output 325.8dB.

I am going round and round in circles. I need the formula for working out what happens when you drop the input signal level by xdB and what other factors impact this formula.

Someone out there please HELP!

Dra

Then maybe it's....
+10db = double the volume (-10db = 1/2 the volume)
2x volume = 10x watts (1/2 volume = 1/10th watts)
+10db = 10x watts (-10db = 1/10th watts)

If you really want some \"scientific\" answers go to Pro Sound Web. Those guys lay the smack down. Sometimes they'll go so far as to factor in humidity and wind speed to your equation.

Good luck.

DRA

kpippen

Dra wrote:

Those guys lay the smack down.

Heroin?...

Kev

Dra

In the sound production community? Come on! :roll:

DRA


PS - We're possibly getting our first major snow in about 8-10 years (1 - 2 inches). Not a loaf of bread or gallon of milk to be found.

kpippen

Dra wrote:

We're possibly getting our first major snow in about 8-10 years (1 - 2 inches).

Cocaine?...

Yeah,...and it's about time ya'll learned how to drive in it...

Kev

Dra

Driving in snow is easy. Go slow, keep moving and stay off the brakes. Unfortunately, not many know that here in Dixie.

I bet Gadget could teach us a thing or two.

DRA

kpippen

Gadget keeps a team hooked year round...



Kev

Gadget

GAWD...we have 2 feet of snow.. It's colder than a lead spike in the Klondike, (it was 54 degrees BELOW -0- !!!!!!!!!!!!!!!!!!!! yesterday morning)... Its a balmy 15 below zero right now with a \"real feel index\" of -41 :shock: so QUIT WHINING!
G
Wanna come up and do some fishing! Maybe go snowmobiling... How about a nice hike in the woods!

SkinTrade

Hi

Just some more feedback on this issue. The guys at ProSoundWeb seem to agree that a -3dB drop in level anywhere in the input chain to the amp will result in half the wattage output. So if you are currently outputting 600W and you drop the input signal by 3dB then you will output 300W. IF EVERYTHING ELSE REMAINS CONSTANT. They also said that is the theory, and real world might be slightly different.

So, I finally had the courage to test this live. I changed the dip switches on the EP2500 to run it in stereo mode. I then changed the wiring of the outputs to the subs so that I was running both subs off of ch A. I configured the limiter output level back to 0dB for the subs (as opposed to -7dB, when the amp was in bridged mono mode).

Volume seemed to be pretty close to what we previously had. I reconfigured the limiter also just to ensure there would be no clipping on the EP2500. Obviously all these level changes affected the threshold where the limiter kicks in.

I think the bottom line is this: the theory is correct, or at least, reasonably clsoe to a real world scenario. What I achieved here is that I now know I can leave the Roland SRA800 at home or use it elsewhere and only bring an EP2500 to this gig. I scored 3U of rack space, so now I can use a smaller rack and I also scored about 20kG less weight to carry. And for that convenience I am happy to be \"underpowered\" for my speakers, but knowing that what I have will produce enough volume for the particular application it is being used for.

Regards
L