Michael Rhodes
2008-06-08 13:17:24 UTC
Reading up on THX, I found this in Wikipedia...
"THX also provides certified theaters with a special crossover
circuit whose use is part of the standard."
Now what could that be, and why haven't we been told about it?
Couldn't that be the Zobel ciircuit? I was wondering where the Zobel
might be found commercially; and there it is, in THX. You might want
to install it yourself, without buying THX theaters. I have, and it
was quite an improvement to my system.
Though simple in itself, the problem with adding the Zobel is that the
speakers will probably sound bassy afterward, and should be rebalanced
afterwards. If that is not possible then a graphics equalizer might
correct that problem. The improvement in sound quality should be
worth it.
Before reading this, know that the solution is more simpler, and less
expensive, than the explanation below. The solution is at the end of
this post.
You can find information about the Zobel in other places on the
internet, but I will try to explain it here. Audio companies will not
give you this. They prefer you buy equalizers or more speakers to
ATTEMPT to correct the problem. They don't make much money off the
little Zobel. They won't even sell it to us.
The Zobel: -----/\/\/\-----| |------
Rz Cz
The simple Zobel circuit corrects the impedance of coil drivers that
are connected to passive crossovers. Passive crossovers, which are
the majority of crossovers used, require constant impedance at their
output to the driver.
Active crossovers, as opposed to passive, connect the driver directly
to the amp. They do not require Zobel-corrected drivers. The
amplifier, in such cases, is in direct control of the voltage across
the driver. The voltage across the driver is what is important.
Why is voltage important? Crossovers are voltage divider circuits,
with the driver as part of that circuit. And driver volume is voltage
dependent. So, as impedance of the driver increases with frequency
(with its coil acting as an inductor), then the voltage across the
driver likewise increases. Higher voltages increase volume at higher
frequencies.
The crossover can only try to reduce the voltage, but high impedance
prevented it. We want drivers with constant impedance, but the audio
industry is refusing to sell them to us. They could. Speaker
builders (including Bose) refuse to add this circuit to their systems.
It is a conspiracy; for the sake of a bloated and problematic audio
industry. I seriously believe this circuit is THE key to THX quality.
It is what they want, but bundle it with the rest to hide it.
The driver's coil acting as an inductor will negate the filtering of
the crossover at higher frequencies. So the woofer puts out midrange
it shouldn't. And the midrange is acting as a tweeter when it
shouldn't.
Expected Midrance Response Actual Response
___ __
/ \ / ----___
/ \ /
It is impossible to balance the speakers to a flat response with a
drivers acting this way. Distortion will also increase. Except for
THX theaters (unless you know others), this is the typical design of
the sytem sold to the public!
The Zobel is simply a resistor and capacitor in series. It is
similar to a hi-pass filter that is connected in parallel with a
driver that is acting as a low-pass filter, to ensure the driver's
actual impedance remains near its rated impedance.
The THX circuit probably includes a resonance compensator along with
the Zobel. Most all systems which vibrate, such as drivers, will also
resonate to some extent. When a speaker resonates its impedance also
increases. Resonance occurs at the low end of the frequency spectrum
(usually below the driver's normal operating range), as opposed to
inductor effects, which are at the high end.
Unless one is an electronics engineer, resonance compensation is
difficult to calculate. But it might not even be required in
well-made drivers that are well-damped. My Speakerlab midranges are
claimed to be well-damped. I've check its resonance. It peaked at 12
ohms at 200 hz. This is minimal, compared to some charts I've seen on
the internet. My Klipschorn tweeters are damped using magnetic fluid,
so I don't worry about those either. I could easily blow my tweeter
just by testing it, unless I had professional equipment. I do not.
From an impedance perspective woofer resonance is ignorable, since at
those low frequencies the crossover is essentially a wire to the
woofer. There is no L-pad. So the amp is direct control of its
voltage. Woofer resonance is ignorable, but not inductance!
Back to the Zobel...
There are standard equations for determining the values of the Zobel
resistor and capacitor. The standard values are Rz = 1.25 * driver dc
resistance, and Cz = driver inductance/(Rz^2). Another source I've
found suggests that Rz should equal driver resistance. BUT ANYTHING
HELPS! Seriously.
------->
Lacking information about driver characteristics (the inductance)
should not prevent you from installing something. I would suggest...
Rz = 8 ohms (or 4 for 4 ohm speakers.)
Cz = 30 uf, for the woofer.
Cz = 10 uf for the midrange.
For 4-ohm systems, the capacitor values should probably also be
reduced for the smaller coils. My speakers are all 8-ohm, so I have
no experience with 4-ohm, but the electronics is the same. You might
reduce capacitors to half the suggested values and see how that works.
------->
I consider tweeter Zobels only as an option. I added a 3 uf Zobel to
my Klipschorn tweeters. They are very efficient horns. But the
improvement wasn't noticeable as it was to the woofer and midrange.
Some tweeters may actually depend on impedance variations to produce
the ultra highs. It may be best not to install a Zobel to the tweeter.
That being said, speakers should be balanced. The purpose of the
Zobel is for a constant impedance at the high end. The 3 uf Zobel
added to the tweeter may not provide an 8-ohm impedance, but, if
balanced, it should flatten impedance (and therefore, hopefully,
response) at higher frequencies.
Speakers are not perfect inductors. The capacitor of the Zobel will
actually over-correct the inductor nature of the driver. But a slight
over-correction is much preferred over not having it installed at all.
--
Michael
"THX also provides certified theaters with a special crossover
circuit whose use is part of the standard."
Now what could that be, and why haven't we been told about it?
Couldn't that be the Zobel ciircuit? I was wondering where the Zobel
might be found commercially; and there it is, in THX. You might want
to install it yourself, without buying THX theaters. I have, and it
was quite an improvement to my system.
Though simple in itself, the problem with adding the Zobel is that the
speakers will probably sound bassy afterward, and should be rebalanced
afterwards. If that is not possible then a graphics equalizer might
correct that problem. The improvement in sound quality should be
worth it.
Before reading this, know that the solution is more simpler, and less
expensive, than the explanation below. The solution is at the end of
this post.
You can find information about the Zobel in other places on the
internet, but I will try to explain it here. Audio companies will not
give you this. They prefer you buy equalizers or more speakers to
ATTEMPT to correct the problem. They don't make much money off the
little Zobel. They won't even sell it to us.
The Zobel: -----/\/\/\-----| |------
Rz Cz
The simple Zobel circuit corrects the impedance of coil drivers that
are connected to passive crossovers. Passive crossovers, which are
the majority of crossovers used, require constant impedance at their
output to the driver.
Active crossovers, as opposed to passive, connect the driver directly
to the amp. They do not require Zobel-corrected drivers. The
amplifier, in such cases, is in direct control of the voltage across
the driver. The voltage across the driver is what is important.
Why is voltage important? Crossovers are voltage divider circuits,
with the driver as part of that circuit. And driver volume is voltage
dependent. So, as impedance of the driver increases with frequency
(with its coil acting as an inductor), then the voltage across the
driver likewise increases. Higher voltages increase volume at higher
frequencies.
The crossover can only try to reduce the voltage, but high impedance
prevented it. We want drivers with constant impedance, but the audio
industry is refusing to sell them to us. They could. Speaker
builders (including Bose) refuse to add this circuit to their systems.
It is a conspiracy; for the sake of a bloated and problematic audio
industry. I seriously believe this circuit is THE key to THX quality.
It is what they want, but bundle it with the rest to hide it.
The driver's coil acting as an inductor will negate the filtering of
the crossover at higher frequencies. So the woofer puts out midrange
it shouldn't. And the midrange is acting as a tweeter when it
shouldn't.
Expected Midrance Response Actual Response
___ __
/ \ / ----___
/ \ /
It is impossible to balance the speakers to a flat response with a
drivers acting this way. Distortion will also increase. Except for
THX theaters (unless you know others), this is the typical design of
the sytem sold to the public!
The Zobel is simply a resistor and capacitor in series. It is
similar to a hi-pass filter that is connected in parallel with a
driver that is acting as a low-pass filter, to ensure the driver's
actual impedance remains near its rated impedance.
The THX circuit probably includes a resonance compensator along with
the Zobel. Most all systems which vibrate, such as drivers, will also
resonate to some extent. When a speaker resonates its impedance also
increases. Resonance occurs at the low end of the frequency spectrum
(usually below the driver's normal operating range), as opposed to
inductor effects, which are at the high end.
Unless one is an electronics engineer, resonance compensation is
difficult to calculate. But it might not even be required in
well-made drivers that are well-damped. My Speakerlab midranges are
claimed to be well-damped. I've check its resonance. It peaked at 12
ohms at 200 hz. This is minimal, compared to some charts I've seen on
the internet. My Klipschorn tweeters are damped using magnetic fluid,
so I don't worry about those either. I could easily blow my tweeter
just by testing it, unless I had professional equipment. I do not.
From an impedance perspective woofer resonance is ignorable, since at
those low frequencies the crossover is essentially a wire to the
woofer. There is no L-pad. So the amp is direct control of its
voltage. Woofer resonance is ignorable, but not inductance!
Back to the Zobel...
There are standard equations for determining the values of the Zobel
resistor and capacitor. The standard values are Rz = 1.25 * driver dc
resistance, and Cz = driver inductance/(Rz^2). Another source I've
found suggests that Rz should equal driver resistance. BUT ANYTHING
HELPS! Seriously.
------->
Lacking information about driver characteristics (the inductance)
should not prevent you from installing something. I would suggest...
Rz = 8 ohms (or 4 for 4 ohm speakers.)
Cz = 30 uf, for the woofer.
Cz = 10 uf for the midrange.
For 4-ohm systems, the capacitor values should probably also be
reduced for the smaller coils. My speakers are all 8-ohm, so I have
no experience with 4-ohm, but the electronics is the same. You might
reduce capacitors to half the suggested values and see how that works.
------->
I consider tweeter Zobels only as an option. I added a 3 uf Zobel to
my Klipschorn tweeters. They are very efficient horns. But the
improvement wasn't noticeable as it was to the woofer and midrange.
Some tweeters may actually depend on impedance variations to produce
the ultra highs. It may be best not to install a Zobel to the tweeter.
That being said, speakers should be balanced. The purpose of the
Zobel is for a constant impedance at the high end. The 3 uf Zobel
added to the tweeter may not provide an 8-ohm impedance, but, if
balanced, it should flatten impedance (and therefore, hopefully,
response) at higher frequencies.
Speakers are not perfect inductors. The capacitor of the Zobel will
actually over-correct the inductor nature of the driver. But a slight
over-correction is much preferred over not having it installed at all.
--
Michael