6 ohm amp driving 4 ohm speaker ?

[Jim Davis]

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

 

[[email protected]]

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.
If you use a lower speaker impedance than the source you can overload
the amplifier and damage it.
If you use a higher impedance speaker than source you will not damage
anything but you will get some reduction in maximum volume obtainable.
This advice is brought to you by Sirkituk -- please visit my site at
www.geocities.com/sirkituk

 

[Jack]

Since the source impedance is near zero - That's exactly what will happen.
The 4 ohm speakers will draw 6 amps from the amplifier and overload it by
50% which it may or may not survive.

 

[jasen]

Since the source impedance is near zero - That's exactly what will happen.
The 4 ohm speakers will draw 6 amps from the amplifier and overload it by
50% which it may or may not survive.

only if you turn the volume up too far.

 

[[email protected]]

---
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


---
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.

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.
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?

Sirkituk

 

[PeterD]

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?

Impedance is 'directional'. That is the impedance that an amplifier
(or whatever...) drives does not necessarly equal the back impedance
of the amplifer.

Say a signal goes to teh speaker. For whatever reasons, some of that
signal is reflected back towards the amplifier. The ideal goal is that
the amplifier appear as a very low impedance to that signal, to help
prevent its being reflected a second time back to teh speakers...

Same basic concept as batteries--an ideal battery has an internal
impedance (resistance, since it is DC) of zero or almost zero. Were
the battery's resistance be any significant value at all, then its
current supplyling capabilities would be rather limited.

 

[[email protected]]

---
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


---
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.

Thank you thats a big help.
Ed

 

[[email protected]]

Impedance is 'directional'. That is the impedance that an amplifier
(or whatever...) drives does not necessarly equal the back impedance
of the amplifer.

Say a signal goes to teh speaker. For whatever reasons, some of that
signal is reflected back towards the amplifier. The ideal goal is that
the amplifier appear as a very low impedance to that signal, to help
prevent its being reflected a second time back to teh speakers...

Same basic concept as batteries--an ideal battery has an internal
impedance (resistance, since it is DC) of zero or almost zero. Were
the battery's resistance be any significant value at all, then its
current supplyling capabilities would be rather limited.

Thanks that's a big help
Ed