Date: Thu, 28 Apr 2011 08:41:00

Author: Zani, Gerald

Subject: Re: speakers

Post:

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Wow.

I think this Hyperphysics has a Hyperpuzzling explanation that reveals a
misunderstanding?

It shows an incorrect model for an the audio bridge and includes the
disclaimer that "impedance matching is no longer considered best practice".


Huh?

What are they saying?

Someone help and clear this up.

How is the output impedance of an audio amplifier designed to meet the input
impedance of a speaker?

My feeble and basic understanding from reading audio engineering handbooks
is that an 8 Ohm output of an amplifier does not have anywhere near an 8 Ohm
impedance.
Although I certainly agree that it is labeled as such. But the label
implies that it wants to drive an 8 Ohm speaker load. To give low audio
distortion it is designed to deliver near zero output impedance not 8 Ohms
impedance.

The amp output wants a power loss to the load.

Not unlike an ideal battery model that delivers any current required to
maintain a constant voltage to any load. An ideal battery has no internal
resistance and a real battery hopes to have a very low ir to maintain the
voltage. - Jerry

On Wed, Apr 27, 2011 at 4:59 PM, Andrew Morrison wrote:

> George,
>
> I could be wrong, it wouldn't be the first time today that has happened.
>
> My understanding of the power delivered to the speaker is summarized on
> HyperPhysics:
>
> http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html#c2
>
> Maybe they
> are wrong, too. Or I just misinterpreted what they were saying.
>
> Andrew
>
> On Wed, Apr 27, 2011 at 1:51 PM, George Herold wrote:
>
>> Hi Andrew, I don't know where the idea of matching the amp output
>> impedance to the load came from. I suspect it was that simple freshman
>> physics problem we all did. A battery with internal impedance R and a load
>> of variable impedance.... at what load impedance is the power delivered from
>> the battery max? equal R's. You should observe that in this case 1/2 the
>> power is dissipated in the battery. A better question to ask might be, "For
>> a given load resistance R, what source impedance gives you the most power in
>> the load?"
>>
>> (Oh, when I said the output impedance of an audio amp was low I meant some
>> fraction of an ohm. I don't really know what these numbers are for home
>> stereo systems, but I've built a lot of amps for driving other loads.) You
>> can measure the output impedance of an amplifier. Just measure how much the
>> voltage drops when you decrease the load impedance. (assuming you stay
>> within the current limit of the amplifier)
>>
>> I'm not sure what the 4/6/8 ohm switches are on a stereo amplifier.
>> Perhaps they are changing the DC voltage that the amp is supplied from. (If
>> you load an amp with less than it's 'rated' impedance then at the maximum
>> output current there is less than the full voltage across the load.. the
>> extra voltage has to fall across the pass element (big transistor) in the
>> amp and this dissipates more power.
>>
>> George H.
>>
>> (PS. one place where impedance matching is important is when driving a
>> transmission line. (coax cable) Then you need to match the transmission
>> line impedance.)
>>
>> ------------------------------
>> *From:* tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] *On
>> Behalf Of *Andrew Morrison
>> *Sent:* Wednesday, April 27, 2011 12:00 PM
>>
>> *To:* tap-l@lists.ncsu.edu
>> *Subject:* Re: [tap-l] speakers
>>
>>
>> But the way that the amp delivers the max power to the speaker is if the
>> impedance of the speaker matches the output impedance of the amp. You're
>> right about audio amps generally having low output impedances, which are
>> close to the impedance of a speaker. That is why, in general, most speakers
>> can connect to most amps with little problem.
>>
>> Usually you don't run into any problems if the impedance is not perfectly
>> matched. Some amps have a switch on them to change from 4 ohm to 8 ohm
>> (I've even seen 6 ohm options on some). If you connect an 8 ohm speaker to
>> the 4 ohm output, it doesn't really sound much different or perform much
>> differently over much of the range of the amplifiers capability. You're
>> right about possibly getting distortion at the high end of the amp's output
>> on mismatched impedances, but you can also get distortion if the speaker is
>> not rated to handle the total power that an amp can deliver.
>>
>> --
>> Andrew Morrison
>> Department of Physics
>> DePaul University
>>
>> On Wed, Apr 27, 2011 at 9:35 AM, George Herold wrote:
>>
>>>
>>> Dear tapers, I'd like to try and correct what appears to be a common
>>> misconception.
>>> Audio amplifiers will be 'rated' by what impedance they are designed to
>>> drive. This has to do with the maximum current/voltage that the
>>> amplifier can produce. This 'rated' impedance has nothing to do with
>>> the output impedance of the amplifier itself. Typically audio
>>> amplifiers will have a very low output impedance. (After all you don't
>>> want to be wasting a lot of power in the amplifier, you want it all
>>> dissipated in the speaker or load.)
>>>
>>> Now the only time you will find distortion problems when using speakers
>>> that have a different impedance than the 'rated' impedance of the
>>> amplifier is when you are driving them really hard. When you start to
>>> push either the maximum current or voltage that the amp can produce. (Of
>>> course even when the speaker is matched to the rating of the amp, if you
>>> drive it hard you can still get distortion.)
>>>
>>> George H.
>>>
>>>
>>> > -----Original Message-----
>>> > From: tap-l-owner@lists.ncsu.edu
>>> > [mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of Jerry DiMarco
>>> > Sent: Tuesday, April 26, 2011 2:12 PM
>>> > To: tap-l@lists.ncsu.edu
>>> > Subject: Re: [tap-l] speakers
>>> >
>>> > At 4/26/2011 10:30 AM, you wrote:
>>> > >......
>>> > >1. Does the impedance of a speaker affect the sound quality/loudness?
>>> >
>>> > The sound quality may be affected indirectly. One
>>> > example is the case of an amplifier with an 8 ohm output
>>> > impedance connected to speakers with 4 ohm input impedance.
>>> > If the amplifier cannot handle the increase in current drawn
>>> > by the lower impedance speakers, the music signal peaks may
>>> > get clipped and sound distorted. A 16 ohm speaker in the
>>> > same situation would not cause signal distortion since it
>>> > draws less current from the amplifier.
>>>
>>>
>>>
>>>
>>
>


--
Gerald Zani
Demonstration Manager
Physics
Brown University
(401) 863-3964

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Content-Transfer-Encoding: quoted-printable

Wow.=A0 I think this Hyperphysics has a Hyperpuzzling explanati=
on that reveals a misunderstanding? It shows an incorrect model for=
an the audio bridge and includes the disclaimer that "impedance match=
ing is no longer considered best practice".=A0

Huh?=A0 What are they saying?Someone help and clear thi=
s up. How is the output impedance of an audio amplifier designed to=
meet the input impedance of a speaker?My feeble and basic understa=
nding from reading audio engineering handbooks is that an 8 Ohm output of a=
n amplifier does not have anywhere near an 8 Ohm impedance.=A0

Although I certainly agree that it is labeled as such.=A0 But the label imp=
lies that it wants to drive an 8 Ohm speaker load.=A0 To give low audio dis=
tortion it is designed to deliver near zero output impedance not 8 Ohms imp=
edance.

The amp output wants a power loss to the load.Not unlike an ide=
al battery model that delivers any current required to maintain a constant =
voltage to any load.=A0 An ideal battery has no internal resistance and a r=
eal battery hopes to have a very low ir to maintain the voltage. - Jerry

On Wed, Apr 27, 2011 at 4:59 PM, Andrew Morr=
ison depaul.edu> wrote:

George,I could be wrong, it wouldn't be the first t=
ime today that has happened. =A0My understanding =
of the power delivered to the speaker is summarized on HyperPhysics:


http://hyperphysics.phy-astr.gsu.edu/=
hbase/audio/imped.html#c2
Maybe they are wrong, too. =A0Or =
I just misinterpreted what they were saying.Andre=
w


On Wed, Apr 27, 2011 at 1:51 PM, George Herold /a>> wrote:







Hi An=
drew,=A0 I don't know where the idea of matching the amp=20
output impedance to the load came from.=A0 I suspect it was that simple=20
freshman physics problem we all did.=A0 A battery with internal impedance R=
=20
and a load of variable impedance.... at what load impedance is the power=20
delivered from the battery max?=A0 equal R's.=A0 You should observe tha=
t=20
in this case 1/2 the power is dissipated in the battery.=A0=A0A better=20
question to ask might be, "For a given load=A0resistance R, what sourc=
e=20
impedance gives you the most power in the load?"=A0=A0
=A0
(Oh, =
when I said the output impedance of an audio amp was low I meant=20
some fraction of an ohm.=A0 I don't really know what these numbers are =
for=20
home stereo systems, but I've built a lot of amps for driving other=20
loads.)=A0 You can measure the output impedance of an amplifier.=A0 Just=20
measure how much the voltage drops when you decrease the load impedance.=A0=
=20
(assuming you stay within the current limit of the amplifier)=A0=20

=A0
I'm not sure=20
what the 4/6/8 ohm switches are on a stereo amplifier.=A0 Perhaps they are=
=20
changing the DC voltage that the amp is supplied from.=A0 (If you load an a=
mp=20
with less than it's 'rated' impedance then at the maximum outpu=
t=A0current=20
there is less than the full voltage across the load.. the extra voltage has=
to=20
fall across the pass element (big transistor) in the amp and this dissipate=
s=20
more power.=A0=A0
=A0
George=20
H.
=A0
(PS.=A0one place whe=
re impedance matching is important is when driving=20
a transmission line. (coax cable)=A0 Then you need to match the transmissio=
n=20
line impedance.)=A0



From: tap-l-owner@lists.ncsu.edu=20
[mailto:t=
ap-l-owner@lists.ncsu.edu] On Behalf Of Andrew=20
MorrisonSent: Wednesday, April 27, 2011 12:00 PMTo:=20
tap-l@lists.ncs=
u.eduSubject: Re: [tap-l]=20
speakers

But the way that the amp delivers the max power to the spe=
aker=20
is if the impedance of the speaker matches the output impedance of the am=
p.=20
=A0You're right about audio amps generally having low output impedanc=
es,=20
which are close to the impedance of a speaker. =A0That is why, in general=
,=20
most speakers can connect to most amps with little problem.=A0

Usually you don't run into any problems if the impedance is not =
perfectly=20
matched. =A0Some amps have a switch on them to change from 4 ohm to 8 ohm=
=20
(I've even seen 6 ohm options on some). =A0If you connect an 8 ohm sp=
eaker=20
to the 4 ohm output, it doesn't really sound much different or perfor=
m much=20
differently over much of the range of the amplifiers capability. =A0You&#=
39;re=20
right about possibly getting distortion at the high end of the amp's =
output on=20
mismatched impedances, but you can also get distortion if the speaker is =
not=20
rated to handle the total power that an amp can deliver.

=A0--=A0
Andrew Morrison
Department of Physics
DePaul University=A0
On Wed, Apr 27, 2011 at 9:35 AM, George Herold=
=20
wrote:
=A0Dear tap=
ers, =A0I'd like to try and correct=20
what appears to be a commonmisconception.Audio amplifiers will =
be=20
'rated' by what impedance they are designed todrive. =A0Thi=
s has to=20
do with the maximum current/voltage that theamplifier can produce.=
=20
=A0This 'rated' impedance has nothing to do withthe output =
impedance=20
of the amplifier itself. =A0Typically audioamplifiers will have a=
=20
very low output impedance. =A0 (After all you don'twant to be w=
asting=20
a lot of power in the amplifier, you want it alldissipated in the=
=20
speaker =A0or load.)Now the only time you will find distortion=
=20
problems when using speakersthat have a different impedance than th=
e=20
'rated' impedance of theamplifier is when you are driving t=
hem really=20
hard. =A0When you start topush either the maximum current or voltag=
e=20
that the amp can produce. (Ofcourse even when the speaker is matche=
d to=20
the rating of the amp, if youdrive it hard you can still get=20
distortion.)George H.> -----Original=20
Message-----> From: tap-l-owner@lists.ncsu.edu>=20
[mailto:tap-l-owner@lists.ncsu.edu] On=20
Behalf Of Jerry DiMarco> Sent: Tuesday, April 26, 2011 2:12=20
PM> To: tap-l@lists.ncsu.edu> Subject:=20
Re: [tap-l] speakers
>> At 4/26/2011 10:30 AM, you wrote:>=20
>......> >1. Does the impedance of a speaker affect the so=
und=20
quality/loudness?>> =A0 =A0 =A0 The sound quality may=20
be affected indirectly. =A0One> example is the case of an=20
amplifier with an 8 ohm output> impedance connected to speakers =
with=20
4 ohm input impedance.> If the amplifier cannot handle the incre=
ase=20
in current drawn> by the lower impedance speakers, the music sig=
nal=20
peaks may> get clipped and sound distorted. =A0A 16 ohm speaker =
in=20
the> same situation would not cause signal distortion since=20
it> draws less current from the amplifier. =20
other stuff>

-- Gerald ZaniDemonstr=
ation ManagerPhysicsBrown University(401) 863-3964

--bcaec5555206db1f9604a1f9df4e--

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