Audio Amplifier Problems

The amps 70 volt circuit hooks up directly to the amps. there are terminals on the back for 70volt, 25 volt & i think 4 & 8 ohm. the actual amp is a rauland-borg FAX-120.

If you're going to attach it to the 70V output and want it to absorb 120W

41 ohms is about right.

--

Bye.
   Jasen

41 ohms isn't a standard value, so he'll end up with a pile of different resistances and wattages. The stadard values of hig wattage resistors aren't E96, or even E48. 50 ohms is close enough, but 47 or 39 MIGHT be availiable.

Tell you what Jasen, why don't you tell him, and everyone else where to buy a 120 Watt 41 ohm resistor without having it custom made, at an outrageous price?

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Lol, so just to be clear, if i use a 100 watt resistor, then that is the load that will be placed on the amp, so 50 would be 50 and 300 would trip the amp. can i hook up my laptop's line out to an amplifiers input with no ill effect? i'm thinking that would be easier for test tones. Thanks to everyone for all of this info.

Merry Christmas

I think you should have some one help you with this. You have your theory about resistor wattage's a little twisted.

The wattage of a Resistor is the amount of power it can dissipate with out damage. It does not generate any kind of power for any connected device. It just simply absorbs it if you want to put it in that context.

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"I\'m never wrong, once i thought i was, but was mistaken"
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Dave, 70.7 volt systems are (70.7 * 70.7= 4998.49) This is rounded up to 5000 to make the calculations simple. So, 70.7 volt systems are

5000 ohms per watt. A 100 watt load would be 5000/100 = 50 ohms. Your half watt speakers would be 5000/.5, or 10,000 ohms each, and in parallel with all the other speakers connect to that amp. Rather than do a lot of math to calculate the load, you work with the actual wattage and just add up the numbers to get the total load in watts.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

no, it's the most power the resistor is designed to handle continuously,

The amount of power the resistor will receive from the source depends on the resistance of the resistor and the voltage of the supply

the power is voltage squared divided by resistance. for very large resistors it may be less expensive/more convenient to use heating elements of the aprpriate size...

Bye. Jasen

lol. no i don't think that resistors generate power. i just don't get why i would use a 100 watt resistor to test a 120 watt amp IF the wattage that the resistor can handle is less than the total applied to it, shouldn't the resistor overheat and fail? IF it works the other way around and the wattage that the resistor dissipates causes the amp to push that same wattage, then 100 watts makes sense because that would push the amp to 80-someodd percentage of its capacity. but to say it differently, if the power that the amp pushes because of the dummy load is based on resistance, then shouldn't my wattage be higher than what the amp could ever generate? so i would need a >120watt 50ohm resistor to verify that the amp "Aint Broke"

"PDI Dave"

** Always take time and a *considerable overload* to damage a resistor.

Blowing cool air on a resistor doubles or triples its power rating.

Placing it in cold water increases it tenfold.

YOU are so far out of you depth Dave - you may drown anytime.

FFS - get a pro to check out the PA system.

...... Phil

Ok, so then if i put a 10k 1/2 watt resistor across the 70 volt output, that would equal 1 speaker's load on the amp, and if a scope shows that my test tone waveform stays true & in frequency, then that would prove that that amp is capable of driving one speaker? obviously a one speaker test is not what i need, but is that the gist? and i need a resistor with a wattage greater than the amp's rated wattage so no resistors are lost? or it doesn't matter because you've calc'ed the proper resistance to pull 100 watts, so no more than that will be drawn...

A combination of resistors is OK. Ten 80 ohm resistors in parallel, for example. Eight 1 ohm resistors in series ... whatever.

Also, if you don't want to buy a dummy load, you could use two 100-W light bulbs (in parallel) to load the 70.7 volt output.

"Charles Schuler"

** NOT AGAIN !!!!!!!!!!!!!!!!!!

Arrrrrrrrggggggghhhhhhhhh !!!!!!

Don't any of you jerkoffs know the resistance of a lamp filament is NOT CONSTANT !!

A 100W, 120 volt lamp has a cold resistance of about 10 ohms - so two in parallel = 5 ohms !!!

You cannot expect to use that as a test load in lieu of a 41 ohm resistor.

...... Phil

Lol, well guys, just thought I'd share the info. i have found out that we are just going to replace both racks of amps and hope for the best... however, i'd still like to find out a little more about these systems. do you all know of any recommended reading for the intricacies of 70.7 volt systems? also, does a single test tone fed into a dummy load actually drive the amp at full capacity or would it be more like multiple tones @ multiple frequencies. i ask because i wired my laptop into the amp rack and did some testing. i noticed that a single test tone didn't result in any problems and everything worked fine, but if i started sending multiple tones to the amps, then things started to go south. 4 tones ranging from 200hz to 1Khz really started to cause the same types of problems that are actually happening with the message that is being used. why is this? can a speaker produce more than 1 tone at any given instance in time?

"PDI Dave"

** Piss the hell off Dave - you are an utter idiot.

...... Phil

Not off the top of my head, but you might try a google search - I just did a quick google of stuff with "10.7 volt" in it, and came up with lots of material. AFAIUI, at 100% gain, a sine wave output has an amplitude of

70.7VRMS. And it's a constant-voltage output, so each speaker decides how much power it wants, by the turns ratio of its own transformer.

Well, technically, "at any given instan[t] in time", there can't be any such thing as a tone, but only one specific voltage value. But when you look at a graph of volts vs. time (x = time, think oscilloscope), then a single tone is, surprise, a sine wave! :-)

Now, when you add in another tone, and look at it on a scope, you can't see both tones, because there's only one trace - what you see is the sum of their instantaneous values.

Now, this _could_ lead to an overload, and from what you say it sounds like that's what's happening, but at this point I'm at the edge of my knowledge - maybe it's something like driving the one tone at a certain level, and then adding in the other tone without reducing the first one - the amp has a limit as to what it can output, so for the times when the peaks in the two tone signals add to more than 70.7V (or maybe 100V, which is the peak of 70.7RMS), which sends the amp into overload. Try turning down the gain on tone 1 before you start introducing tone 2, and so on, and see if you can get your hands on a scope, so you can see what the waveforms are doing.

Good Luck! Rich

A single tone would do it if the rms amplitude of the tone is 70.7V at the output. a combination of tones (or as is often used pink noise) that comes out with an RMS of 70.7V would do the same.

The power of the multiple tones is combined, if the tones are worth 60W each the four of them together would be asking for 240 watts.

I may be that you just need to reduce the volume of the input to the amps a little, something could have caused it to increase triggering these problems, or there may be a part in the amp that's aging and needs replacing.

Bye. Jasen

Yo PDI Dave... you seem to be able to use the laptop as a signal generator... how about a sweep from hi to lo freq at about quarter power? This tests my theory that elcheapo 70v transformers stop acting like transformers about 200hz and look more like shorts, causing the amp to poop out and sound like dog breath. Does that describe the symptoms you are experienceing? Fix is to put a hi pass filter at the amp input, then get some better transformers (maybe there are only a couple of bad apples in the bunch)

yeah, that kind of sums up what is happening. except the system never sounds bad, it just shuts down via internal circuit breakers on the amplifiers. so whenever it decides to go, it just stops. I was able to get some of them to fail during some sweeps. i don't know how i'd determine quarter power though. the laptop has only volume controls, so setting at around 25% seems inspecific. is it possible to have a bad speaker(s)/transformer(s) that could cause the amps to fail while still functioning and making noise? I'm trying to get a hold of a scope that will work for this type of problem as well as the data aspects of my job, but until then, I've just got my meter. (and a busted up audio system :( )