Audio Amplifier Problems

[BobG]

Yeah, i can swap them around. i have and right now it's too
inconsistent to be sure of any results. why do you suggest a 41ohm
resistor instead of a 4 or 8?

========================================
One thing you havent mentioned yet Dave is if the amps have a 70V
transformer, or are driving the 70V bus directly... probably in bridge
mode... so 70V wires need to be connected to the red terminals....
what type of amps do you have?

 

[PDI Dave]

========================================
One thing you havent mentioned yet Dave is if the amps have a 70V
transformer, or are driving the 70V bus directly... probably in bridge
mode... so 70V wires need to be connected to the red terminals....
what type of amps do you have?

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.

 

[jasen]

Yeah, i can swap them around. i have and right now it's too
inconsistent to be sure of any results. why do you suggest a 41ohm
resistor instead of a 4 or 8?

If you're going to attach it to the 70V output and want it to absorb 120W
41 ohms is about right.

 

[Michael A. Terrell]

If you're going to attach it to the 70V output and want it to absorb 120W
41 ohms is about right.

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

 

[PDI Dave]

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.

 

[PDI Dave]

Merry Christmas

 

[Jamie]

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.

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.

 

[Michael A. Terrell]

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.

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

 

[jasen]

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,

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

 

[PDI Dave]

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.

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"

 

[Phil Allison]

"PDI Dave"

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?

** 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

 

[PDI Dave]

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

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...

 

[Charles Schuler]

Thanks charles for all of your help, no insult taken. however, i pretty
much am the outside help. this audio system is just a small portion of
the rest of the system, of which i am quite familiar. so i'm the guy
that gets to figure it out, and by the time its all said and done, i
will have learned a lot, so its actually a good challenge. So just to
confirm... an 8 ohm resistor rated for 100 watts (does this actually
matter as long as its big enough? eg. a 150 watt or 300 watt would be
equally effective?) gets tied to the 8 ohm output and if waveform &
frequency stay true, then the amp is good??? after that i can go after
the circuits. also i have been told an impedance meter is invaluable,
but couldn't i just take amp/volt measurments (Z=V/A)

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.

 

[Phil Allison]

"Charles Schuler"

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.

** 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

 

[PDI Dave]

"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?

 

[Phil Allison]

"PDI Dave"



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





....... Phil

 

[Rich Grise]

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?

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.

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?

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

 

[jasen]

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.

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.

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?

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

 

[BobG]

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)

 

[PDI Dave]

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 )