On 25 Feb 2007 01:09:01 -0800, wrote:

On Feb 20, 1:20 pm, John Fields wrote:
On 20 Feb 2007 03:42:58 -0800, wrote:

On Feb 20, 1:22 am, Jim Davis wrote:
I'm shopping for a Sony HT-DDW700 5.1 Home Theater system ,
with 6 speaker outputs : 100 watt 6 ohm .

With 6 ohm speaker outputs ,
is bad driving 8 or 4 ohm speakers ?

Thanks , Jim D

You only get max power transfer if you match the speaker impedances
exactly.

---
Not true. the amplifier looks like voltage source with close to zero
source impedance.

If it's rated to drive a 6 ohm load to 100 watts, then that means it
can supply:

P 100W
I = sqrt --- = sqrt ------ ~ 4 amperes
R 6R

through the load, and place:

E = sqrt (PR) = sqrt (100W * 6R) ~ 24.5V

across it

If you use a lower speaker impedance than the source you can overload
the amplifier and damage it.

---
Since the amplifier's source impedance is near zero ohms, that's not
like likely to happen.

Hoewever, damage can occur if there is no overcurrent shutdown and
you try to pump more than 4A into the load.
---

If you use a higher impedance speaker than source you will not damage
anything but you will get some reduction in maximum volume obtainable.

---
That's true

--
JF

In my days of playing with audio, the output impedance was important.
It was usually marked on the back of the equipment, ie 4ohms, 8ohms
etc.

---
You're probably referring to audio amplifiers with output
transformers, where the transformer was used to match the
plate-to-plate impedance of the final to the speaker impedance.

Here's a good link:

http://www.vac-amps.com/Technical_Monograph_90-9.htm


I was not aware that domestic audio equipment could have "almost zero"
impedance.
Can you explain further please. What kind of output device is this?

---
Usually a couple of transistors operating AB1 or AB2, which are
lowish-voltage high-current devices which can drive low impedances
like speakers directly. what makes their output look like an almost
zero impedance is feedback which causes the output voltage to remain
constant regardless of the load impedance. That is, if we had an
amplifier with an output voltage of 10 volts RMS, open circuit, and
we then placed an 8 ohm resistor across the source, the voltage
would remain at 10VRMS.



--
JF