Now that you've installed the Zobel circuit on your drivers, you may
want to attempt to balance the speakers' output for a flat response.
My system was a collection of unmatched drivers and crossover, so I
had to develop my own method. I have one, and it is simple--darned
near obvious--though a bit tedious.
What we want is the sound ouputs of the drivers to have the same
graphical shape, and relative amplitude, as their filter's voltage
output. The Zobel circuit should correct for most of the shape error.
As for amplitude, resistor attenuation networks are used to level the
differences in efficiencies between drivers.
But how much attenuation? Do we guess by just listening for balance?
I do not have confidence in that method, and it's the only one I've
heard suggested to those who do not have access to oscilloscopes and
good quality microphones.
The Method: What we know about crossovers is that the voltage outputs
of adjacent filters are equal at their crossover frequencies. It is
logical that the amplitude of the drivers at those frequencies should
also be equal. Equal voltages, and equal volumes (at the same
frequency) are not so difficult to detect and correct.
You will need a signal generator for the test. I used the WinISD
program (internet, free, at
http://www.linearteam.dk/default.aspx?pageid=winisd
in my computer,
which was connected to the amp.
We want to know where the voltages are equal, with accuracy. The
rated crossover frequencies provided by the manufacturer are not be
sufficient, which are probably calculated into a resistive load, and
not your particular driver/Zobel combination.
To make crossover outputs accessible so I could easily measure
voltages, I installed taps to each filter output, at the point before
attenuation. A tap is a single wire attached to one filter, and the
other end attached to a screw which is visible from the back of the
speaker so it could be touched by a meter's probe. There is one tap
for each filter. Only one speaker needs the taps. Only one speaker
is tested throughout, while the other is disconnected.
The AC voltage of each filter is measured by touching the meter's
probes to its particular screw, and the other probe to the convenient
cable ground--with the amplifier's volume set sufficiently high to get
a reasonable voltage reading. Say 3 volts or more as measured from
the speaker cable itself. Set amp volume by measuring voltage across
the main speaker cable. At some frequencies a particular tap will
have zero volts. You may be surprised how loud 3 volts RMS can be.
But, at this point, we don't care how loud, as long as you and the
equipment can stand it. Generally, the higher the voltage then the
more accurate the test.
Select the Signal Generator function of WinISD. Vary the frequency
from the signal generator until you find that at which the bass and
midrange voltages are equal. This is the first crossover frequency.
If you have a 3-way, then again vary the frequency until you find that
at which the midrange and tweeter voltages are equal. This is the
second crossover frequency. WinISD increments in 100 hz. But you can
type in any particular frequency you want, such as 650 hz.
Now that you know what frequencies to test, the next thing is to
install switches to one of your speakers, which are connect in series
with the drivers. The switches allow you to listen to each driver at
the crossover frequency by turning the other off so that you can
determine if they are at equal volume. The switches are simple
on-off, and are connected externally by using long wiring (~10 ft) so
you can walk around while alternately switching each driver. Drilling
a small hole in the back of the speaker for the wiring should not
cause much damage, and easily filled after you are done.
Standing waves occur when listening to a single tone, and they will
****ft position when changing speakers, or just when moving your head.
So you will have to determine an average volume by walking around with
the switches.
WARNING: Higher order crossovers (than 1st-order) can resonate if
they are disconnected; such as by switching, or connected to a
burned-out driver. The amplifier itself may burn up under such
conditions. So you might use an inexpensive audio source. You will
not require a loud signal in this test. Power consumption will be
low.
You can speed up the procedure by installing the test attenuation
resistors to the external wires, instead of reopening the speaker box
for each change. So you wll also need to bring a ground wire through
the hole. When you have found the right combination then they can be
installed permanently inside the speaker.
WinISD's signal generator includes a built-in attenuator, apparently
rated in db, though it is not marked. So you can adjust it to
determine how many decibels attenuation will be required to equalize.
Since it is not marked, I did not think to use it in my tests. It
sure would'be been easier. But it does seem to be in decibels. What
else? I could not find it in the Help section.
First, adjust the midrange loudness to that of the woofer. You cannot
attenuate the woofer, but reduce that of the midrange if possible. You
may need to reduce the current attenuation built into the speaker.
When they are equal then adjust the tweeter to the midrange, by
adjusting the tweeter's attenuation only. Before testing the tweeter,
remove the switch from the woofer circuit and splice it in series with
the wire to the tweeter.
The Resistor Attenuation Network:
(from crossover) •------•------Rs----• (to driver)
|
Rp
|
• (to ground)
db Rs (ohms) Rp (ohms) Impedance (~8)
---------------------------------------------------
-1.9 2 40 8
-2.8 3 29 7.97
-3.5 4 24 8
-4.2 5 21 8.02
-4.9 6 19 8.06
-5.5 7 17 7.96
-6.0 8 16 8
-7.0 10 14 7.87
-8.0 12 13 7.87
-9.2 15 12 7.88
-9.9 17 12 8.1
-11.8 23 11 8.11
-14 32 10 8
-14.8 36 10 8.14
-20 72 9 8.08
-24.1 120 8 7.53
-29.8 240 8 7.75
-35.5 470 8 7.87
The db attenuation rating for an 8-ohm network is...
db = 20 * log(8/(Rs+8)) (with log to the base 10)
Replace 8 with 4 for 4-ohm speakers if necessary.
Rp is calculated so that impedance always remains near the rated
value, 8 ohms in this case.
Rp = 1/(1/8 - 1/(8+Rs))
Good luck, and thanks for reading this. Hope it was worth it. The
method did smooth out my speaker response, and it was an old
_complaint_.
--
Michael
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