On Wednesday, December 31, 2014 7:09:55 AM UTC-5, wrote:
On Mon, 29 Dec 2014 23:19:22 -0500, wrote:
OK, lets get something straight. Impedence is NOT just a test or a
number. Impedence is the AC version of resistance. It is a measure of
the load placed on a circuit.
Say an amplifier outputs 20 volts and can handle a maximum of 5 amps
output. That is 100 watts PP output. If the speaker has a 4 ohm
impedence, at 20 volts it will draw 5 amps for 100 watts pp power
output
If you have an 8 ohm speaker, it will only draw 2.5 amps, which will
only ouput 50 watts.
If on the other hand you connect a 2 ohm speaker to that amplifier it
will draw 10 amps - which will overload the amplifier, and if the
amplifier is capable of pumping out the required 10 amps, it will
produce 200 watts, and fry the speaker.
Always err on the side of too much impedence rather than too little.
Connecting speakers in series, you add the two impedences together. If
you connect them in parallel you reduce the impedence.Rt= 1(1/R2)=
1(1/R2) - so 2 8 ohm speakers in series is 16 ohms, and in parallel
is 4 ohms. A 4 ohm and a 8 ohm in series is 12 ohms and in parallel =
2.66 ohms.
So the OP with a speaker with 2 6 ohm drivers would connect the 2 in
series to get 12 ohms of impedence to connect to the amplifier
designed for 6 ohm speakers and will get reduced output but will not
damage either the speakers or the amp.
This thread brings up something I've wondered for a long time. Back
around the 1950s and 60s, they used 70 volt (primary) transformers at
EACH speaker in a large building, such as an auditorium, or when
speakers sent a signal to rooms all over a school or factory. These
smallish transformers had a 70volt primary and the secondary usually had
a ground, with 4, 8, and 16 ohms, which went to usually just one
speaker. All those 70volt lines went back to the source amplifier,
which had a 70 volt output, coming from the amplifier's outpit
transformer, (and there was usually also at least an 8 ohm output as
well on the amp.
Anyhow, these amplifiers were high power vacuum tube amps, and had some
massive output transformers.
Back then, I played around with a lot of electronics, and got my hands
on several of those old tube amps, and a coffee can full of those (at
the speaker) 70v transformers. I learned at that time, that the reason
they did the system that way, was because there would be too much loss
running an 8 ohm line for hundreds of feet, as well as serious impedence
problems, if there were 10 or 20 speakers around the building (which
makes sense).
What I never did fully understand is whether the number of transformers
placed onto the amplifier's output could overload the amp, if too many
were used?????
Sure, there is no free lunch. The amp that's driving it has to be
sized to the total load and if the load exceeds that, then it's overloaded.
The concept here appears to be the same as with power distribution. Instead
of sending 120V from the plant to your house, they step up the voltage,
the step it back down. You can send the same amount of power long distances
two ways:
A - higher voltage and lower current
B - lower voltage and higher current
Choice A gives you less loss due to resistance in the lines, ability to use
smaller conductors, etc.
My other question is whether they still use those 70volt transformers on
modern semi-conductor amplifiers, and if so, how would they derive 70v
from a bank of output power transistors, since they dont have audio
output transformers anymore?
IDK, Pico says yes and provided a link, but the link appears broken.
I don't see why they can't still be using audio output transformers for
special applications like you're talking about. They are just a transformer,
still available, etc.
---
One further comment. Someone mentioned the ridiculous output ratings on
modern equipment. I was at Walmart a few years ago, and noticed a
boombox, which had two speakers about 5 inches in size, and ran on
either a wall transformer or 6 or 8 D-cell batteries. In huge letters,
it said 1000 WATTS POWER. I about laughed my pants off.
I have an old guitar amp, rated at 70W RMS, (120W Peak). It drives a
heavy 15 inch speaker, and will make pictures on the wall shake if I
turn it up. But this "toy" boombox running on flashlight batteries and
driving two 5" speakers is supposed to put out 1000W..... NOT
The entire sound system for most loud rock bands is only 500W RMS. A
huge outdoor concert that covers many acres, usually runs 5,000 Watts to
10,000 Watts.
I just wonder how they can rate those boomboxes so ridiculously high and
not get in legal trouble????
It's an interesting question. That's as extreme an example as I've ever
heard. Most of the consumer audio companies have been playing spec games
for a long time. Too many customers thing that 1000W automatically means
that the system is way better than one with 50W, that they really could use
all that power with their speakers, even if it were available, etc.