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Surround sound speakers & amp impedance mismatch

B

Bob Engelhardt

Jan 1, 1970
0
My step son & family have bought a second home and are furnishing it
with 2nd hand stuff. My neighbor donated a Sony receiver/surround sound
without speakers. I found speakers at our dump's "swap shop". Trouble
is the amp uses 3 ohm speakers and the ones that I found are 8 ohm.

The speakers will need 1.4 times the voltage to output the same power,
but levels are non-linear between the dial & the output, so I'm not sure
if I should expect the amp to have enough power (voltage). In other
words, if a level of "5" would be normal for 3 ohm speakers, would a
level of "10" be enough to drive 8 ohm ones?

Fidelity is not a big concern - this is for a TV system. But if
distortion is going to be major, I could match impedance by adding a 5
ohm resistor in parallel with each speaker. But then the power demand
on the amp would be that much greater.

I could use some advice.

Thanks,
Bob
 
My step son & family have bought a second home and are furnishing it
with 2nd hand stuff. My neighbor donated a Sony receiver/surround sound
without speakers. I found speakers at our dump's "swap shop". Trouble
is the amp uses 3 ohm speakers and the ones that I found are 8 ohm.

Most likely, "3 ohm" refers to the minimum allowed speaker impedance,
going any lower could permanently damage the amplifier. Thus any 4-8
ohm speaker would be suitable for this amplifier.
The speakers will need 1.4 times the voltage to output the same power,

A surround sound amplifier might be specified as 5x100 W, while the
power supply might be able to deliver perhaps 150 W constantly :).
The 100 W output power might have been measured with one channel
driven at a time and even then only for a short time (and then the
number is multiplied by 5).

In practice, the power supply voltage will sag, when multiple
channels are driven at the same time, thus reducing the output
available from each channel.

By using 8 ohm speakers instead of 4 ohm speakers, the speaker will
draw less current and the power supply voltage will not sag as much,
delivering a slightly higher output voltage to the speaker, before
distortion starts.

With a stabilized power supply, the audio output level would drop by 3
dB when going from 4 to 8 ohm (assuming same speaker efficiency),
however, with a badly regulated power supply, the loss in SPL might be
just 1-2 dB.
 
B

Bob Engelhardt

Jan 1, 1970
0
John said:
How loud do you need it to be? I'd just try it. The power and
impedance ratings on amps and speakers are almost meaningless, and
speaker efficiencies are all over the place anyhow. And you do want
this to be cheap.

Not loud. You got the "cheap" part right <G>. Thanks, Bob
 
B

Bob Engelhardt

Jan 1, 1970
0
Dave said:
You're assuming that the 3-ohm and 8-ohm speakers would have the same
acoustic efficiency. That's not necessarily true. Speaker
efficiencies vary all over the place, depending on the driver design,
size of enclosure, and type of enclosure.

That's something that I didn't even think about, but it makes sense.
...
In other words, ... you won't have to
turn the volume control up very far at all to compensate for this...
Good!
No reason at all to do that.

For one thing... the difference in speaker impedances won't cause
"distortion"... just decreased loudness at any given volume control
setting (perhaps). All you need to do to compensate, is turn the
volume control up a bit.

Good, again. The concerns that I had were enough power & distortion.
'Looks like we're OK on both counts.
For a second thing... you do not need to "match" impedances, in an
audio amplification situation like this... the amp-to-speaker
relationship is *not* one of matched impedances (either identical or
conjugate). It's not like matching impedances on a transmission line.
The actual output impedance of the amplifier is quite low (probably
less than an ohm). Transistor audio power amplifiers are usually
perfectly happy driving impedances that are "too high". They aren't
happy driving impedances that are "too low".

Thanks. This is the kind of "thumbnail" knowledge that I like to have
(just enough to make me sound like an expert to my step son <G>).

....
Go ahead and do it. Try it - just hook up the speakers and receiver
with cables, with everything sitting in your living room, before you
install everything.

Odds are, it'll work just fine. The quality of the sound you get will
very probably depend mostly on whether the speakers you have salvaged
are decent, and will have very little to do with the fact that they
are "8 ohm nominal" impedance.
...

Great, I was doubtful that I'd find a set of 3 ohm speakers at the dump
& now I can just scratch "speakers" off of their "needed" list.

Thanks a bunch,
Bob
 
B

Bob Engelhardt

Jan 1, 1970
0
Bob said:
... Trouble
is the amp uses 3 ohm speakers and the ones that I found are 8 ohm.
....

Thanks to all reply-ers. We're going to go ahead and use them.

Bob
 
S

Sjouke Burry

Jan 1, 1970
0
My step son & family have bought a second home and are furnishing it
with 2nd hand stuff. My neighbor donated a Sony receiver/surround sound
without speakers. I found speakers at our dump's "swap shop". Trouble
is the amp uses 3 ohm speakers and the ones that I found are 8 ohm.

The speakers will need 1.4 times the voltage to output the same power,
but levels are non-linear between the dial & the output, so I'm not sure
if I should expect the amp to have enough power (voltage). In other
words, if a level of "5" would be normal for 3 ohm speakers, would a
level of "10" be enough to drive 8 ohm ones?

Fidelity is not a big concern - this is for a TV system. But if
distortion is going to be major, I could match impedance by adding a 5
ohm resistor in parallel with each speaker. But then the power demand
on the amp would be that much greater.

I could use some advice.

Thanks,
Bob

8 ohm is fine , you just get barely detectable less maximum power.
Most amplifiers have to much output power anyway.
 
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