Audio Amplifier Problems

[PDI Dave]

Please Help Me! I have a few amplifiers that i am having problems with
and i need to determine if the amplifiers are bad or if the speaker
circuits are bad. each has upwards of 100 speakers on them (70.7 volt
system) and they keep shutting down (internal circuit breaker
tripping). they only seem to do it when all/most speakers are hooked
up, but i can't yet figure out why. I've checked the AC current with a
Fluke DMM wired in series, but it only shows what i calc to be around
70 watts (.7 amps to .98 amps) but i should be able to pull close to
120 watts. everyone i talk to tells me this is a bad way to test this
system, but nobody can tell me how to. any help will be greatly
appreciated.

120 Watt Amps
70 Volt System
Lots of Speakers...

 

[Phil Allison]

** Groper Alert !

Please Help Me! I have a few amplifiers that i am having problems with
and i need to determine if the amplifiers are bad or if the speaker
circuits are bad. each has upwards of 100 speakers on them (70.7 volt
system) and they keep shutting down (internal circuit breaker
tripping). they only seem to do it when all/most speakers are hooked
up, but i can't yet figure out why. I've checked the AC current with a
Fluke DMM wired in series, but it only shows what i calc to be around
70 watts (.7 amps to .98 amps) but i should be able to pull close to
120 watts. everyone i talk to tells me this is a bad way to test this
system, but nobody can tell me how to. any help will be greatly
appreciated.

120 Watt Amps
70 Volt System
Lots of Speakers...

** Have any idea how to calculate the load presented by all the speakers
together ?

Was your amperage test done with a sine wave signal or speech / music ?

The minimum load impedance for a 120 watt 70 volt amp is 41 ohms.

If there are similar 100 speakers, each must be have 4100 ohms load
impedance.

Also - beware the existence of low audio frequencies, a big NO-NO with
70volt systems.





........ Phil

 

[Charles Schuler]

Please Help Me! I have a few amplifiers that i am having problems with
and i need to determine if the amplifiers are bad or if the speaker
circuits are bad. each has upwards of 100 speakers on them (70.7 volt
system) and they keep shutting down (internal circuit breaker
tripping). they only seem to do it when all/most speakers are hooked
up, but i can't yet figure out why. I've checked the AC current with a
Fluke DMM wired in series, but it only shows what i calc to be around
70 watts (.7 amps to .98 amps) but i should be able to pull close to
120 watts. everyone i talk to tells me this is a bad way to test this
system, but nobody can tell me how to. any help will be greatly
appreciated.

Sounds like an overload is causing the amps to shut down. Are all of the
speakers working before it shuts down? Any chance there is a short on the
secondary side of one of the circuits?

70.7 volt systems only produce that voltage at maximum output power. How
are calculating power? Are you measuring voltage and current? Also, what
is the test signal (waveform and frequency). The meter might not be
adequate.

 

[Michael A. Terrell]

Sounds like an overload is causing the amps to shut down. Are all of the
speakers working before it shuts down? Any chance there is a short on the
secondary side of one of the circuits?

70.7 volt systems only produce that voltage at maximum output power. How
are calculating power? Are you measuring voltage and current? Also, what
is the test signal (waveform and frequency). The meter might not be
adequate.

All you have to do is add up the watts from each transformer. it
should be close to the rated output of the amplifier. The transformers
have multiple taps to select the watts at each speaker. This requires a
visual inspection of every speaker and transformer. Write down the
numbers and add them up. If the system doesn't work, test the amp into a
good dummy load, or check the wiring for a short to the building's
ground system. The whole idea of constant voltage sound systems is to
make it dirt simple to install and maintain. If the OP has 100
speakers, the taps should be set to 1 watt per speaker. He mentions
"Amplifiers", implying more than one amp. He doesn't say if it is
broken into zones, or the amplifiers are in parallel. If they are in
parallel, you have to match the gain of each amp, and make sure that all
of them are in phase.


--
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]

Sounds like an overload is causing the amps to shut down. Are all of the

speakers working before it shuts down? Any chance there is a short on the
secondary side of one of the circuits?

70.7 volt systems only produce that voltage at maximum output power. How
are calculating power? Are you measuring voltage and current? Also, what
is the test signal (waveform and frequency). The meter might not be
adequate.

yeah, i does sound like an overload, except that it is inconsistent.
too inconsistent to be able to say for sure. all speakers seem to be
working fine, though with the quantities involved, i can't be sure. by
short on the secondary side, you mean on the secondary of the
transformers for the speakers? each speaker has its own transformer and
each is tapped at 1/2 watt.

yes, i'm measuring voltage and current. the test signal is a speech &
tone. the tone seems to be around 1Khz, though i don't know the
waveform. the meter has a min-max function which i've been using to
capture peak levels. the actual voltage on the circuit is more like 50.
what kind of meter do i need? or will a meter at all even be
adequate... do i need an O-scope?
good dummy load, or check the wiring for a short to the building's
ground system. The whole idea of constant voltage sound systems is to
make it dirt simple to install and maintain. If the OP has 100
speakers, the taps should be set to 1 watt per speaker. He mentions
"Amplifiers", implying more than one amp...

what is a good dummy load? there are actually more than 100 speakers,
i'm just trying to get an idea for how to fix this and was therfore
giving an idea for 1 amp and its approximate load, i'm also learning a
lot as i go. the reality of it is more like >1000 speakers divided
across 10 amplifiers that are all separate from each other except for a
shared pre-amp input. though not all amps are causing a problem,
currently only 2 are, so the rest of it i should be able to ignore. i
think the main problem circuit is around 175 1/2 watt speakers. that
means by my calcs, i should have about 87.5 watts of speakers and
should then be well within the amps capacity. any more help is greatly
appreciated.

 

[Michael A. Terrell]

yeah, i does sound like an overload, except that it is inconsistent.
too inconsistent to be able to say for sure. all speakers seem to be
working fine, though with the quantities involved, i can't be sure. by
short on the secondary side, you mean on the secondary of the
transformers for the speakers? each speaker has its own transformer and
each is tapped at 1/2 watt.

yes, i'm measuring voltage and current. the test signal is a speech &
tone. the tone seems to be around 1Khz, though i don't know the
waveform. the meter has a min-max function which i've been using to
capture peak levels. the actual voltage on the circuit is more like 50.
what kind of meter do i need? or will a meter at all even be
adequate... do i need an O-scope?

good dummy load, or check the wiring for a short to the building's
ground system. The whole idea of constant voltage sound systems is to
make it dirt simple to install and maintain. If the OP has 100
speakers, the taps should be set to 1 watt per speaker. He mentions
"Amplifiers", implying more than one amp...

what is a good dummy load? there are actually more than 100 speakers,
i'm just trying to get an idea for how to fix this and was therfore
giving an idea for 1 amp and its approximate load, i'm also learning a
lot as i go. the reality of it is more like >1000 speakers divided
across 10 amplifiers that are all separate from each other except for a
shared pre-amp input. though not all amps are causing a problem,
currently only 2 are, so the rest of it i should be able to ignore. i
think the main problem circuit is around 175 1/2 watt speakers. that
means by my calcs, i should have about 87.5 watts of speakers and
should then be well within the amp's capacity. any more help is greatly
appreciated.

Are the amplifiers in the same room? Can you swap a set of 70 volt
speaker leads between a working area and one with problems? If the
problem moves, the amp needs repaired. If not, it is in the speakers,
wiring or transformers for that area. I have found wiring cable tied to
the steel trusses that had worn spots, and was arching to ground when it
was above a certain level. In others, the line was shorted to ground,
and overloading the amp. The volume is low, so someone cranks up the
level, till the amp starts to clip.


How old are the amplifiers? You may have some failing filter
capacitors, allowing the amp top oscillate at an ultrasonic frequency.

As far as test equipment, a low distortion audio sine wave generator
and a distortion analyzer will show problems before you can hear them.

 

[Charles Schuler]

yeah, i does sound like an overload, except that it is inconsistent.
too inconsistent to be able to say for sure. all speakers seem to be
working fine, though with the quantities involved, i can't be sure. by
short on the secondary side, you mean on the secondary of the
transformers for the speakers?
Yes.

each speaker has its own transformer and
each is tapped at 1/2 watt.

That's good.

yes, i'm measuring voltage and current. the test signal is a speech

Speech is not the correct test signal for measuring power but is OK as a
basic check.
A 400 Hz sine wave is typical for testing audio systems.

tone. the tone seems to be around 1Khz, though i don't know the
waveform. the meter has a min-max function which i've been using to
capture peak levels. the actual voltage on the circuit is more like 50.
what kind of meter do i need? or will a meter at all even be
adequate... do i need an O-scope?

Yes, as I posted , 70.7 volts is realized only at maximum output ... so at
50 vols, the system is operating below its capability (assuming all is
normal).

what is a good dummy load? there are actually more than 100 speakers,
i'm just trying to get an idea for how to fix this and was therfore
giving an idea for 1 amp and its approximate load, i'm also learning a
lot as i go. the reality of it is more like >1000 speakers divided
across 10 amplifiers that are all separate from each other except for a
shared pre-amp input. though not all amps are causing a problem,
currently only 2 are, so the rest of it i should be able to ignore. i
think the main problem circuit is around 175 1/2 watt speakers. that
means by my calcs, i should have about 87.5 watts of speakers and
should then be well within the amps capacity. any more help is greatly
appreciated.

Is there an 8 ohm or 4 ohm output? If so, you can use a resistor of that
resistance, rated at 100 W (or somewhat less for a brief test). I use an
oscilloscope connected across the dummy load and a triangle wave test signal
at 400 Hz.

 

[PDI Dave]

Are the amplifiers in the same room? Can you swap a set of 70 volt
speaker leads between a working area and one with problems? If the
problem moves, the amp needs repaired. If not, it is in the speakers,
wiring or transformers for that area. I have found wiring cable tied to
the steel trusses that had worn spots, and was arching to ground when it
was above a certain level. In others, the line was shorted to ground,
and overloading the amp. The volume is low, so someone cranks up the
level, till the amp starts to clip.


How old are the amplifiers? You may have some failing filter
capacitors, allowing the amp top oscillate at an ultrasonic frequency.

As far as test equipment, a low distortion audio sine wave generator
and a distortion analyzer will show problems before you can hear them.

Thanks for all the input. all the amps are in 2 racks right next to
each other. i have tried moving leads between amps to check for amp vs.
circuit problems, but its too inconsistent. I switched one and let it
run for maybe an hour or more and the problem seemed to be fixed
(different amp doesn't trip = original is weak) but then a day or two
later it started again on the new amp. about a week before that a
different amp had been tripping and after checking speakers i found two
that were not functioning. i replaced them and the problem stopped...
for a few days, then started again. it's also tricky because these
particular model of amps supposedly had a recall around the time that
they were manufactured (2004?) but they have since discontinued
production. so i need to verify for myself with absolute certainty that
the amps are good or bad. also they've been sitting in a construction
site with lots of dust, humidity, and uncontrolled temperatures for
presumably the last two years.

 

[PDI Dave]

Thanks for your help. well, i dont know if there are any shorts on the
transformer secondaries, and unless there is a way to check from the
main circuit, the labor involved to pull down each one to check would
be ridiculous. as far as test signals i think i have a 1Khz Sine wave
signal on my laptop that i can try to load. is there someplace i might
find the test tone of which you speak? also, where would i find a 100
watt resistor? i think the biggest iv'e ever seen is 5 watts. and i may
be showing some more ignorance here, but if i load/play the test tone
to the dummy resistor, then the scope shows a waveform & freq. that
match the test tone, does that mean the amps are ok? if so, then i just
need a huge resistor, test tone and scope also, do you know if there
is a way to find out the min/max frequencies that an amp could push to
such a circuit. i think i read that impedance is dependant on
frequency, and that if the impedance were to rise or fall too much,
either too much current or voltage would be needed and could cause
problems. is that to far-fetched?

 

[Charles Schuler]

Thanks for your help. well, i dont know if there are any shorts on the
transformer secondaries, and unless there is a way to check from the
main circuit, the labor involved to pull down each one to check would
be ridiculous.

I know that, and that is why I asked if there was normal output at all
locations before the amp shut down. A dead speaker would be a big clue. I
installed a 70.7 volt system in a church and it later turned into a
nightmare. Carpet installers shorted secondary circuits and ... you do not
need to know the rest. S**T happens!


as far as test signals i think i have a 1Khz Sine wave

signal on my laptop that i can try to load. is there someplace i might
find the test tone of which you speak?

You can indeed use a laptop because there are free-ware signal generators
out there ... Google is your friend.


also, where would i find a 100

watt resistor?

Mouser, Digi-Key, Allied and others ... Google is your friend.

i think the biggest iv'e ever seen is 5 watts. and i may

be showing some more ignorance here, but if i load/play the test tone
to the dummy resistor, then the scope shows a waveform & freq. that
match the test tone, does that mean the amps are ok? if so, then i just
need a huge resistor, test tone and scope also, do you know if there
is a way to find out the min/max frequencies that an amp could push to
such a circuit. i think i read that impedance is dependant on
frequency, and that if the impedance were to rise or fall too much,
either too much current or voltage would be needed and could cause
problems. is that to far-fetched?

Impedance is certainly not constant in these systems. But, that is not the
issue here. I suggested a triangular test waveform because, with an
oscilloscope across the output, it is easy to spot clipping and crossover
distortion.

I don't want to insult you, but you might want to consider some outside help
with this. Not via the Internet. An experienced audio technician to go to
the site with you.

 

[Michael A. Terrell]

I know that, and that is why I asked if there was normal output at all
locations before the amp shut down. A dead speaker would be a big clue. I
installed a 70.7 volt system in a church and it later turned into a
nightmare. Carpet installers shorted secondary circuits and ... you do not
need to know the rest. S**T happens!

Standard practice is one transformer per speaker, and mounted to the
speaker frame or cabinet. This prevents the secondary circuit from
being exposed.

as far as test signals i think i have a 1Khz Sine wave

You can indeed use a laptop because there are free-ware signal generators
out there ... Google is your friend.

also, where would i find a 100

Mouser, Digi-Key, Allied and others ... Google is your friend.

i think the biggest iv'e ever seen is 5 watts. and i may

Impedance is certainly not constant in these systems. But, that is not the
issue here. I suggested a triangular test waveform because, with an
oscilloscope across the output, it is easy to spot clipping and crossover
distortion.

I don't want to insult you, but you might want to consider some outside help
with this. Not via the Internet. An experienced audio technician to go to
the site with you.

I started servicing these types of systems in the mid '60s. We didn't
have anything fancy to troubleshoot with. Most problems were found to
be where someone had tampered with the wiring so try to raise or lower
the volume. I have seen messes that were so bad that they had to be
ripped out, and start over.

Personally, I would have split a system like this into zones of 10
speakers with a home run back to the equipment room for each zone. A
"66" series telephone stake block will handle 25 zones. You remove the
bridging clips to isolate a zone. Then if you have problems, you can
easily isolate it. A "Test tac" made for telephone work and a speaker
with a matching transformer (with a volume control) can be plugged into
any circuit to listen to the amp.

There are impedance bridges available to read the impedance @ 1 KHz,
but I used the test speaker and a small FM receiver and 5 watt amp with
both 25 and 70 volt outputs to feed unknown wiring. It wasn't uncommon
for some idiot to find a wiring cabinet and rip all the wiring apart,
and rip all the tags off, as well.

 

[jasen]

yeah, i does sound like an overload, except that it is inconsistent.
too inconsistent to be able to say for sure. all speakers seem to be
working fine, though with the quantities involved, i can't be sure. by
short on the secondary side, you mean on the secondary of the
transformers for the speakers? each speaker has its own transformer and
each is tapped at 1/2 watt.

what is a good dummy load?

for a 120W load on a 70.7 volt system:

Ideally a 150W 41 ohm resistor, you don't need to buy one, you may be able
to find one, or something close enough, you want something that doesn't vary
in resistance as it heat up nichrome wire is good - it's wat heating
elements are made from.

about 345W worth of 120V heating element, or 1382W worth of 240V
heating element.

there are actually more than 100 speakers,
i'm just trying to get an idea for how to fix this and was therfore
giving an idea for 1 amp and its approximate load, i'm also learning a
lot as i go. the reality of it is more like >1000 speakers divided
across 10 amplifiers that are all separate from each other except for a
shared pre-amp input. though not all amps are causing a problem,
currently only 2 are,

can you swap them round? see if the problem moves with the amp or if it's
tied to the position.

Is it possible that a splice in the speaker cable is getting wet?

Bye.
Jasen

 

[BobG]

Sounds like an overload is causing the amps to shut down. Are all of the===========================================
The Don Davis book says a good way to find a rogue speaker that has
bypassed the transformer is hook a filament transformer to the 70 volt
line... the speaker that is receiving 6vac at 60hz will speak loudly
for a while then smoke. I suppose a variac would also work.

 

[Michael A. Terrell]

The Don Davis book says a good way to find a rogue speaker that has
bypassed the transformer is hook a filament transformer to the 70 volt
line... the speaker that is receiving 6vac at 60hz will speak loudly
for a while then smoke. I suppose a variac would also work.

Not if there are several ohms resistance in the cable between the amp
and the speaker. He said that there are about 100 speakers per amp.
What are the chances that it is on the end closest to the amp? If there
is no transformer, you can hear the difference, anyway. BTDT for over
30 years.


--
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]

for a 120W load on a 70.7 volt system:

Ideally a 150W 41 ohm resistor, you don't need to buy one, you may be able
to find one, or something close enough, you want something that doesn't vary
in resistance as it heat up nichrome wire is good - it's wat heating
elements are made from.

about 345W worth of 120V heating element, or 1382W worth of 240V
heating element.


can you swap them round? see if the problem moves with the amp or if it's
tied to the position.

Is it possible that a splice in the speaker cable is getting wet?


Bye.
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?

its very unlikely that anything is getting wet. all outdoor devices are
weatherproof and in weatherproof backboxes and they're somewhat
protected from rainfall by the floors above them. everything else is
indoors, mostly in electrical rooms with no water.

 

[PDI Dave]

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)

 

[PDI Dave]

I started servicing these types of systems in the mid '60s. We didn't

have anything fancy to troubleshoot with. Most problems were found to
be where someone had tampered with the wiring so try to raise or lower
the volume. I have seen messes that were so bad that they had to be
ripped out, and start over.

Personally, I would have split a system like this into zones of 10
speakers with a home run back to the equipment room for each zone. A
"66" series telephone stake block will handle 25 zones. You remove the
bridging clips to isolate a zone. Then if you have problems, you can
easily isolate it. A "Test tac" made for telephone work and a speaker
with a matching transformer (with a volume control) can be plugged into
any circuit to listen to the amp.

There are impedance bridges available to read the impedance @ 1 KHz,
but I used the test speaker and a small FM receiver and 5 watt amp with
both 25 and 70 volt outputs to feed unknown wiring. It wasn't uncommon
for some idiot to find a wiring cabinet and rip all the wiring apart,
and rip all the tags off, as well.

Thanks for the help michael. they're actually split into zones of
around 21 speakers each. each zone can be manually turned on, so i
don't even need to re-connect anything to isolate a given zone. the
problem doesn't happen on any given zone though, its more like a
variable combination of zones I'm really hoping i don't have to tear
out too much. but thanks to all of you guys, i at least have a few
directions to go in. is an impedance bridge the same thing as an
impedance meter?

 

[BobG]

100W is 70V across about 50 ohms. All the other transformers are higher
impedance and bridge off this one. 1/2W is 2V across 8 ohms. 21
speakers 2W each in a zone need 10.5W, and you could run about 10 zones
with that 100W amp, 210 speakers per amp. The voltage gain from the 70V
bus to the speaker is 70V/2V or 35, so the impedance seen on the
primary is the secondary impedance scaled by the turns ratio squared.
The turns ratio squared is 35^2 or about 1200 on the primary x 8 on
the secondary, so you each primary is about 9600 ohms, 210 of them is
about a 45ohm load on the amp and the 70V bus. Cheap 70V transformers
have poor low freq resp... system might work ok from 200hz up, and
overload at low freqs. Stick a resistor in series and watch the volts
across it as you sweep the freq below 200. Tell me if I identified the
prob. The fix is fatter transformers or a hi pass filter in front of
the amp.

 

[Michael A. Terrell]

Thanks for the help michael. they're actually split into zones of
around 21 speakers each. each zone can be manually turned on, so i
don't even need to re-connect anything to isolate a given zone. the
problem doesn't happen on any given zone though, its more like a
variable combination of zones I'm really hoping i don't have to tear
out too much. but thanks to all of you guys, i at least have a few
directions to go in. is an impedance bridge the same thing as an
impedance meter?

<http://www.mcminone.com/category.asp?catalog_name=MCMProducts&category_name=3830519&Page=1>
has several models listed, with some details on the individual pages.
Most impedance bridges are at higher frequencies, but if it will do 1
KHz in the right impedance range it may work. The meters allow you to
set the line voltage which will help if it is a problem of insulation
breaking down, Since you can zone the speakers easily, you may want to
check each group to see if there is any leakage to ground. An ohm meter
will show some problems, but a "Megger" at a hundred volts or so will
will find arc over problems.


--
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

 

[Michael A. Terrell]

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?

You want to load the 70 volt output. 50 ohms is close enough for
testing.


--
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