Impedance

Dennis

So, if we know that an amplifier will put out 500W FTC per channel into 4 Ohms and 350W FTC per channel into 8 Ohms, how do we accurately calculate what that amplifier will put out into 16 Ohms.

No, this is not a quiz.

Dennis

Gadget

\"IF\" there were an amp with an 8 ohm output...then it would be 1/2 the rating or 175 watts...

The output impedance of the amp is much less than the impedance of the speaker....

The damping factor Df = Zin / Zout tells you what Zout is.

Zout = Zin/Df.

If the damping factor Df = 200 and the loudspeaker impedance is Zin = 4 ohms, the output impedance of the amplifier is Zout = 4 / 200 = 0.02 ohms.

You see, there is no \"8 ohm amplifier\" on the market with a 8 ohm output impedance.

Then there is the line loss calculations that tell us the little if any of the current is actually getting to the speaker...
right...


So in reality there is no sense in attempting to calculate the amp power available without a degree in physics, and/or the garganutbrain computer from the 'Hitchhikers guide to the galaxy\"

right?

Dennis

OK, so perhaps I asked the wrong question OR asked the question incorrectly.

The power rating specs I listed above are for a QSC MX 1500 which I am using to power one JBL 2447 16 Ohm compression driver per side. Since I have dropped the XO point to 933Hz, the driver is using a lot more amplifier power than it did at a XO point of 1.2kHz. I would like to determine if I am using the correct size amp.

This is the first weekend I have had off in a month and I am sitting in front of my computer with a bottomless cup of coffee doing usless projects.

Dennis

Dra

It is odd that some amps are (or at least state) a 200-8 / 400-4 / 800-2 format, while others are 200-8 / 350-4 / 675-2. Why is that? power supply? Output class? What?

DRA

Dennis

Dra

I came across this article while surfing the web for answers today. It may partially answer your question...or may not help at all.

http://sound.westhost.com/project56.htm

Dennis

Gadget

Well Dennis there are so many ways to look at it...I think your under powered...I think you have just enough to get yourself in trouble because you don't have any real headroom...the thing is on spec alone you look marginal, and of course what exactly ARE the specs? the Driver shows a minimum impedance of 12 ohms...but then at what FREQUENCY? Above 500 hz 100 watts and these are CONTINUOUS ratings... so dynamic/peak power 400 watts right ...and again that @ 500 hz...Above 1k that rises to 150 watts that would by 150/300/600... and your crossover is close to that... I run my B&C 750's with a Carver PM1000... that's 450 watts RMS @ 8 ohms @1029hz, and I see some material when I'm CRANKIN that is approaching peak I have actually set the limiter on the last street dance... I should have brought more speakers/power, but I was alright...just pushing it...

See, the thing is that As I said before there are so many factors...Frequency related impedance... back EMF, line resistance, amp damping factor...yada yada...They war about this all the time over at the PSW, and I guess if you wanted to, that might be the best place to ask...

G

Dennis

Yeah, I thought it was under powered...it felt underpowered but with the 16 Ohm compression drivers, I couldn't make the calculations to verify it was under powered. I think we have a few spare QSC RMX2450 kicking around the shop which are rated at 450/8 Ohms. Maybe using 8 Ohm diaphragms instead of 16 Ohm would buy me a bit as well...but the thought of actually spending money on this old JBL system does not sit well with me.

At the last festival I did, the BSS Omnidrive output meters were hitting red a lot on the high outputs but the limiters kept the amplifiers from clipping.

All I planned to do today was transfer my JBL settings on the Omnidrive over to my DR260 and use them as a starting point to re-tune the system with the DriveRack. I then got sidetracked with my underpowered amplifier and decided I had better resolve that issue before setting up my computer and a measurement mic.

Dennis

Gadget

Dennis...Why not eliminate the REAL problem on those speakers and put in a set of P Audio Neo 2\" drivers? They are smooth.. brilliant and you could get the WHOLE driver for the cost of JUST the diaphragms on those JBL's AND because they say JBL you could sell them for MORE than the cost of the P Audio upgrades... WIN WIN! P Audio is the worlds largest speaker manufacturer ( raw drivers ) AND they are more efficient since they are NEO magneted ... :idea:

http://www.loudspeakersplus.com/product ... c=joholre8

Check your PM in a bit...

G

Dennis

Question: Why not eliminate the REAL problem and leave the JBL in the shop. Take out the Meyer instead.

Answer: The JBL is a challenge and there is a sense of accomplishment making it sound even half way good.

Dennis

Gadget

:roll: ok... I see a conundrum here...IF I had a Meyer system sitting idle... \"I\" would make sure the JBLs' sat idle...bringing the \" wanna be\" guns out to replace the cannons????

I was assuming you were looking for solutions for the JBL's...My bad...
G

Dennis

Well, the \"wanna be\" guns are probably best suited to the \"wanna be\" budgets which are abundant this year. But more to the point, there are those of us who are more into the process of tuning and alignment than we are into mixing bands. Spending a few hours of company time tweeking the JBL rig with audible results the boss can hear is a plus for me. Now that you and I have come to the conclusion that the 2447's are underpowered, I will have to sub in larger amplifiers, do a new gain structure, and break out the computer and measurement mic and re-tweek the system from scratch. On company time. This is a solution I like. The P-Audio driver solution kinda sounds like cheating.

Dennis

Gadget

Ah...I understand..I just wish I had the business to be able to make those choices... might be fun to make a JBL system REALLY sound good for a change though... and let them all wonder...HOW you :shock: did it!