"Bob-Stanton" wrote in message
om...
"Arny Krueger" wrote in message

Actually, I did take out the first filter, when I inserted the second
(active) filter in front of the amplifier. So, the response remained
rolled off by the same 24dB/octave.

Providing that the passive filter had the identical same response, which
I
will doubt until measurements are shown.

You don't need measurements if you know filter theory. Both the active
and passive versions of the 24 db/octave Linkwitz will have the same
response, except for two factors:

1) The finite Q of the coils and inductors.
2) The impedance change of the speaker load.

right. Hold that thought about the tweeter.

Zobels and other networks can make the tweeter impedance constant.

Not necessarily perfectly constant in both magnitude and phase at all
frequencies without a ton of work. For example, modelling the inductance of
a tweeter as being a pure inductance, which is essentially what simple zobel
calcuations do, is not a particularly good approximation for most tweeters.

See http://users.ece.gatech.edu/~mleach/...oads/zobel.pdf for
additional caveats. Notice that his plots only show the magnitude of the
impedance and not its phase angle. There's a slight dip and a downward slope
in most of his magnitude plots, and that suggests non-zero phase.

So, the only difference in the filter response will be due to the
finite Q of the inductors and capacitors.

Hopefully.

Below is a chart of an ideal filter response, for a 24 dB/octave
Linkwitz high pass, and of the calculated actual response of a passive
Linkwitz:

The calculated actual response assumes you use Madisound Air-Core
inductors.


Frequency Ideal Attenuation Pratical Attenuation
in Hz. in dB in dB

2000 -6.02 -6.71
1750 -8.64 -9.31
1500 -12.38 -12.98
1250 -17.56 -18.09
1000 -24.61 -25.12
750 -34.25 -34.76
500 -48.20 -48.62
250 -72.25 -71.83

You can see the rolloff curve of the passive filter, is the same as
the ideal filter's, but with a added 0.7 insertion loss.





---broadband sound------ High pass filter------CD
player------amplifier----- full range speaker

The above not really necessary, since we are looking for a *change* in
audible distortion.

It is necessary because we're looking for a change that we can attribute
to
some UUT, either amplifier, CD player or both.


No, the distortion of the CD player and the tweeter stay the same
during the test. The only thing that changes is the distortion of the
amplifier.


Also, I doubt that the CD-player contributes very
much distortion.

I doubt that either a good amp or a CD player contributes much
distortion
since I've done the experiment outlined above with several amps
including an
OEM car audio system.


Well, that is why we do the tests, to find out if the amplifier has
audible distortion. I'm glad your amplifiers did well.



If the amount of audible distortion is dependent on amplifier volume
control settings, and actual volume levels remain high enough that
Fletcher-Munson effects remain constant, then the distortion is in
the amplifier.

That is why I suggested an using and (optional) L-pad in front of the
tweeter. It's a little bit more work, but the level out of the tweeter
can be kept constant as the volume of the amplifier increases.

The L-pad changes the impedance of the load on the amplifier, greatly.

No. L-pads don't change the impedance at all. Here are the values for
8 Ohm L-pads, for various attenuations.

Attenuation R(series) R(parallel)
in dB in Ohms in Ohms

1 0.87 65.0
3 2.34 19.4
6 4.0 8.0
10 5.47 3.7

But the load isn't a pure 8 ohms resistive. It's a tweeter with variable
impedance magnitude and phase, even with a zobel. The L-Pad isolates the
input impedance of the tweeter from the amplifier. This in turn causes the
attenuation of the filter to wander around.

If you check this out, you will see the impedance stays 8 Ohms.

Presuming a pure 8 ohm resistive load, which is not what a practical tweeter
will be at all frequencies, zobel notwithstanding.

No, I say that TIM is an obscure way to talk about high frequency
nonlinear
distortion.

That is because, it is an obscure form of distortion. :-)

It's actually very common and easy to understand if you call it by its
conventional name.