you may remember this bookshelf speaker
right here
if not then let me tell you that I
turned it sibling into a portable
boombox during a previous project video
during that process I not only
temporarily removed the loudspeakers but
also got rid of the utilized audio
crossover in favor of a new one that I
got from eBay lots of viewers though
seem to disagree with this replacement
decision so in this video we will find
out how exactly Audio crossovers and the
specifially function and how to
theoretically calculate those in order
to find out whether it makes sense to
replace an existing audio crossover or
even create a DIY one let's get started
[Music]
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your Gerber files today before we have a
closer look at the old audio crossover
let's firstly have a look at a typical
audio signal this smartphone audio
signal would obviously need to get
amplified before entering the audio
crossover and finally the loudspeakers
but by having a look at the signal on
the oscilloscope we can already find out
that we are dealing with an AC signal
that features frequencies from 20-year
up to 20,000 Hertz this wide frequency
band is the reason why our bookshelf
speaker features one rufa which is the
big loudspeaker and one tweeter which is
the small out speaker the whoo far move
its big size comes with a lot more
inertia meaning it can move slower but
pushes more error
this makes it perfect for low frequency
40 and 2030s which it can play back
without any distortions of course it can
also play higher frequencies but volume
and quality reductions can be the
consequences that is where the small and
agile tweeter comes in which can play
back frequencies above 2,000 Hertz
without reduced volume or quality so
since the two loudspeakers are clearly
designed for specific frequency band the
job of the audio crossover is to
separate the frequencies of the
amplified audio signal into high and low
frequencies to do that it apparently
uses resistors capacitors and inductors
whose function we can experimentally
determine with the test setup which
connects my function generator in
serious to each component and the
loudspeaker as a substitute for the
loudspeaker however I will be using a 4
ohm resistor to match its resistance and
I will also be using an amplifier
between the function generator and loads
because the generator alone would not be
able to supply the required output power
first off let's connect a small 2 ohm
resistor which is close to what is used
for the actual audio crossover and let's
hook up the input and output voltage to
my oscilloscope by choosing a sine
voltage whose frequency we slowly
increase we can see that the output
voltage maximum pretty much stays
constant at a value of 3 point 4 volts
no matter the frequency this output
voltage level can easily be calculated
by Ohm's law which means a resistor does
damp our audio signal but sadly across
the whole frequency range so let's move
on by connecting a 150 micro Henry
inductor which is close to one of the
audio crossover inductor values by once
again increasing the frequency here we
can see a data maximum of the output
voltage slowly decreases
that means with a higher frequency the
resistance of the inductor needs to
increase which is actually correct
because the inductive reactance rises
proportional with the frequency and if
that confuses you right now then
definitely have a look at my video about
impedance anyway
this experiment shows us that we could
use an inductor to damp I frequent
Portage's which means the inductor alone
would act as a low-pass filter now if we
try the same experiment over 10 Mooka
farad capacitor which I stole from the
audio crossover then we can see that
with a higher frequency the maximum of
the output voltage stays pretty high and
only with lower frequencies the maximum
decreases once again the reactance in
this case the capacitive reactance is
responsible for the high resistance at
low frequencies that means a capacitor
alone would act as a high-pass filter
with this knowledge all we would have to
do is to connect a suitable inductor in
series to the woofer and the capacitor
in series to the tweeter in order to
create a simple crossover but since the
commercial crossover features a lot more
components this may not be the optimal
solution yet so let's move on to the
popular RC filter which like the name
implies consists of a resistor and
capacitor whose position can be swapped
to build up IVA high pass low pass
filter since the high pass filter is
pretty similar to the example we already
talked about let's for now focus on the
low pass filter where the output voltage
drops across the capacitor and the input
voltage across the resistor and
capacitor that means we pulled up a
voltage divider and by doing a little
mathematical magic we can find out that
the formula for the output voltage looks
like this as an example let's use a 10
mukarat capacitor 10 ohm resistor and
max input voltage of 5 volts and the
frequency in
5 kilohertz which should give us an
output voltage of 1.5 1 volts after
putting up the circuits and measuring
the output voltage we can see that our
equation was in fact correct another
important value for all filters is the
cutoff frequency which is the frequency
where the output voltage is 0.7 oh seven
times the input voltage which converted
to decibel is a damping of minus 3 dB
the formula for the cutoff frequency for
the RCA high-pass and low-pass filter is
the same and looks like this so with our
previous example values we would get a
cutoff frequency of 1591 hertz which by
adjusting the function generator to this
frequency seems to be correct above this
frequency the low pass filter attempts
the output signal with minus 20 DB per
decade which means 0.1 times the
previous input value per every 10 times
the frequency the high-pass filter
obviously attempts the output voltage
with minus 20 DB per decade underneath
its cutoff frequency now of course we
could also replace the capacitor with an
inductor and thus create an RL high-pass
and low-pass filter as well the formulas
for the cutoff frequencies are once
again identical for both filters and the
damping curve is also pretty much the
same in theory the reason why RC filters
are preferred though are the higher
costs and size of coils while also
featuring a higher dissipation factor in
comparison to capacitors which means
they feature more real resistance in
relation to the reactance which is not
ideal and for example causes more power
losses so would those basics out of the
way I finally started reverse
engineering the olds audio crossover and
found out that it uses a rather complex
combination of RLC components
to create two filters calculating those
can be quite challenging so let's
firstly find out why these circuits
partly only uses L and C components the
reason is that our RC and RL filters
only offer a damping of minus 20 DB
which means unwanted frequencies will
take a while to be completely filtered
out such filters are called first order
filters because they come with one
frequency dependent components so to get
a second order filter we can simply
combine an inductor and capacitor to
form an LC high pass or LC low-pass
filter this way the damping increases to
minus 40 DB per decade which removes
unwanted frequencies above or beneath
the cutoff frequencies much quicker and
speaking of cut-off frequency the
formula for the high-pass and low-pass
is like always the same and is for my
simple example here around four point
one kilohertz which by doing another
simple measurement seems to be correct
no sadly those simple formulas do not
work with such crazy filters but to make
our life easier we can simply download
the witwix cat 2 software in which we
can rebuild the old filter design to
find out that we got one low-pass filter
with a cut-off frequency of around 700
Hertz and one high-pass filter which
pretty much always features a damping of
underneath minus 3 dB
I also confirmed those values by doing a
couple of measurements with the
crossover
so after reverse engineering my
replacement crossover I found out that
it doesn't feature these same filter
curves which is bad because every
loudspeaker comes with its own frequency
response since a loudspeaker is partly a
frequency dependent inductor for which
the original manufacturer created a
precisely tuned audio crossover to get
the best sound quality so all in all we
learned that my audio crossover replay
was stupid and that dr wang and audio
crossover is definitely possible with
the witwix cat 2 software at least if
you got the frequency response file for
loudspeaker but don't think filters are
only useful for audio applications
because they are also used for mains
filters or for example to turn a high
frequent sinusoidal pwm signal into
proper sine wave I hope you enjoyed this
video and learned something new if so
don't forget to Like share subscribe and
hitting the notification bubble stay
creative and I will see you next time