Why is it that I was taught that when the impedence halves, power output
doubles yet some amplifiers don't follow this rule?

For example, an amplifier is rated at 200w x1 @ 4ohms, but only 300w x1 @ 2
ohms (as opposed to 400w @ 2 ohms)?

Is it a power supply issue? Something else?

Tony


--
2001 Nissan Maxima SE Anniversary Edition
Eclipse CD8454 Head Unit, Phoenix Gold ZX475ti, ZX450 and ZX500 Amplifiers,
Phoenix Gold EQ-232 30-Band EQ, Dynaudio System 360 Tri-Amped In Front and
Focal 130HCs For Rear Fill, 2 Soundstream EXACT10s In Aperiodic Enclosure

2001 Chevy S10 ZR2
Pioneer DEH-P9600MP (Just gettin' started)

Why is it that I was taught that when the impedence halves, power output
doubles yet some amplifiers don't follow this rule?

For example, an amplifier is rated at 200w x1 @ 4ohms, but only 300w x1 @
2 ohms (as opposed to 400w @ 2 ohms)?

Is it a power supply issue? Something else?


This is due to the output impedance of the amplifier. Put simply, the
internal resistance of the output contributes to added losses. This effect
is often modelled by putting a resistor in series with an "ideal" (lossless)
amplifier, before you attach the load. And yes, it's partly a power supply
issue. It lumps in all the effects that contribute to the output
impedance - power supply, output stage, and so forth.

Actually, you should be able to calculate this number if you could trust the
damping factor ratings of the amplifier.

On 4/9/2005 9:55:43 AM, "MZ" wrote:
Why is it that I was taught that when the impedence halves, power output
doubles yet some amplifiers don't follow this rule?

For example, an amplifier is rated at 200w x1 @ 4ohms, but only 300w x1 @
2 ohms (as opposed to 400w @ 2 ohms)?

Is it a power supply issue? Something else?


This is due to the output impedance of the amplifier. Put simply, the
internal resistance of the output contributes to added losses. This effect
is often modelled by putting a resistor in series with an "ideal" (lossless)
amplifier, before you attach the load. And yes, it's partly a power supply
issue. It lumps in all the effects that contribute to the output
impedance - power supply, output stage, and so forth.

Actually, you should be able to calculate this number if you could trust the
damping factor ratings of the amplifier.



I have to disagree. I think the drop in calculated power output into a two
ohm load is due entirely to power supply current output limitations and has
nothing to do with output impedance.

I have to disagree. I think the drop in calculated power output into a two
ohm load is due entirely to power supply current output limitations and
has
nothing to do with output impedance.

Power supply output limitations contribute to output impedance. Output
impedance isn't just an OPS measurement. It treats the amplifier as a
"black box", and all the losses are factored in to a final output Z value.

Thanks, Mark.

Tony


--
2001 Nissan Maxima SE Anniversary Edition
Eclipse CD8454 Head Unit, Phoenix Gold ZX475ti, ZX450 and ZX500 Amplifiers,
Phoenix Gold EQ-232 30-Band EQ, Dynaudio System 360 Tri-Amped In Front and
Focal 130HCs For Rear Fill, 2 Soundstream EXACT10s In Aperiodic Enclosure

2001 Chevy S10 ZR2
Pioneer DEH-P9600MP (Just gettin' started)

SO in laymans terms, why do some amps double power output, and others
don't? Is it more costly to build an amp that would double its power
into 2 ohms?

SO in laymans terms, why do some amps double power output, and others
don't? Is it more costly to build an amp that would double its power
into 2 ohms?

Yes. It means a more robust power supply and fewer losses.

Then there are amps of outstanding build quality that rate output the same
at all impedances. Not sure if this is just to be conservative or if it is
attributable to the type of design. McIntosh comes to mind, I believe they
do not indicate any difference in output for different load impedances.

Regards,
Steve

Then there are amps of outstanding build quality that rate output the same
at all impedances. Not sure if this is just to be conservative or if it
is
attributable to the type of design. McIntosh comes to mind, I believe
they
do not indicate any difference in output for different load impedances.

There are some amplifiers that essentially switch modes based on impedance.
JL Audio comes to mind. I have no idea how this could be considered
beneficial...

MZ wrote:
Then there are amps of outstanding build quality that rate output
the same at all impedances. Not sure if this is just to be
conservative or if it is
attributable to the type of design. McIntosh comes to mind, I
believe they
do not indicate any difference in output for different load
impedances.

There are some amplifiers that essentially switch modes based on
impedance. JL Audio comes to mind. I have no idea how this could be
considered beneficial...

I wondered that myself when I first saw it ... why would I want an amp that
gives me 500W @ 6ohm and still only 500W @ 2 ohm?

--
Dan Snooks

There are some amplifiers that essentially switch modes based on
impedance. JL Audio comes to mind. I have no idea how this could be
considered beneficial...

I wondered that myself when I first saw it ... why would I want an amp
that
gives me 500W @ 6ohm and still only 500W @ 2 ohm?

Traditionally, amps that offered this sort of thing used some sort of switch
that would allow you to "get the most out of" your amplifier. Sony and ESX
come to mind, but I know there were others. They worked by basically
stepping the rail voltage up when you were driving high impedance loads, and
bringing it back down for low impedance loads. I assumed that JL was doing
something similar, but of course they have no switch. So, unless they're
autodetecting impedance on the fly, I'm thinking they're doing something
else. But autodetecting impedance on the fly is tricky business because if
your voice coil is hot, the resistance can be several times what it would be
when cool. Or maybe it's just autodetecting at startup? Basically, I have
no idea how JL is doing it, so I'll shut my big mouth now.

In article ,
"Daniel Snooks" wrote:

MZ wrote:
Then there are amps of outstanding build quality that rate output
the same at all impedances. Not sure if this is just to be
conservative or if it is
attributable to the type of design. McIntosh comes to mind, I
believe they
do not indicate any difference in output for different load
impedances.

There are some amplifiers that essentially switch modes based on
impedance. JL Audio comes to mind. I have no idea how this could be
considered beneficial...

I wondered that myself when I first saw it ... why would I want an amp that
gives me 500W @ 6ohm and still only 500W @ 2 ohm?

Its best output would be 500W at a single impedance if it weren't for
auto adjustment. It's a gain, not a loss.

In article ,
"MZ" wrote:

There are some amplifiers that essentially switch modes based on
impedance. JL Audio comes to mind. I have no idea how this could be
considered beneficial...

I wondered that myself when I first saw it ... why would I want an amp
that
gives me 500W @ 6ohm and still only 500W @ 2 ohm?

Traditionally, amps that offered this sort of thing used some sort of switch
that would allow you to "get the most out of" your amplifier. Sony and ESX
come to mind, but I know there were others. They worked by basically
stepping the rail voltage up when you were driving high impedance loads, and
bringing it back down for low impedance loads. I assumed that JL was doing
something similar, but of course they have no switch. So, unless they're
autodetecting impedance on the fly, I'm thinking they're doing something
else. But autodetecting impedance on the fly is tricky business because if
your voice coil is hot, the resistance can be several times what it would be
when cool. Or maybe it's just autodetecting at startup? Basically, I have
no idea how JL is doing it, so I'll shut my big mouth now.

There are two simple ways I can think of -

Use a more sophisticated version of the circuit that protects the output
transistors from overload. It doesn't know the speaker impedance but it
knows the safe operating conditions of the transistors and it can adjust
the power supply.

Use a power supply with a wattage limit. When A x V is too high, drop
the voltage.


Even both of those could be used together. I doubt that any amp
directly measures the speaker impedance. Speakers don't have a single
impedance and it's not what really matters anyway. What matters is
keeping the circuit from going up in smoke.