some practical tips for speakerbuilders
I am going to make a lot of enemies here, because crossover
supposed to be SCIENCE and contain a lot of theories, phisics and math.
I have been there and done that, and - I did read books, I did
softwares, I followed Thiele-Small parameters and Linkwitz, Bessel,
Chebychev formulas. I wasted 10 years for all that stuff fanatically.
You can see the proof in P0 to P10 designs.
In short (maybe I am simply too stupid, who knows?) all the science has
brought me to a corner, where I have done everything I could but
achieved nothing. No music. Just well measuring speakers.
The most deceiving part is chasing those measurement anomalities like
bumps, dips and peaks. Like cutting table's legs. You cut and equalize
and cut and you are left with a table without legs. With speakers - you
equalize and flatten the wrinkles and at the end everything just does
not sound like music at all.
Commercial makers MUST make flat speakers because A) they are afraid of
magazines catching them with abnormal responses and B) because they use
shitty plastic drivers for 1 euro from China.
There is only one way out of this corner. You throw away all these
theories and start building music making machines. LIVE sounding
speakers. I do NOT mean abnormal
measuring distortion generators but on contrary - speakers made with XO
being in tune with the driver own characted and intelligenly addressing
the issues to acheve speaker that both measures well and sounds
great. The theorical model, XO calculations and and
computer modelling are the WORST starting point. Listening to naked
drivers in their real room position is the BEST STARTING POINT.
First - get rid of all software. Best way is to format C:
I AM DEAD SERIOUS ABOUT IT. But I do not mean measurement software but
modeling and calculating software that produces ready recipes.
Second - choose drivers which are good. (As described in the
I will repeat again: starting point is broadband midbass. Size 20 cm,
(or oval close to 20 cm)
paper cone, alnico magnet , or ferrite if alnico is not possible. Best
source of such drivers is old german
made radios like Saba, Telefunken, Siemens, Philips and so on. They
usually have sensitivity or efficiency of circa 100 dB/W/m
The bass foundation must be done by a large driver. Minimum 12 inch,
better 15 inch, best - of course - you guessed right - 18 inch. The
drivers over 18 inch are too rare to mention. Japanese freaks bought
them all already and mounted in their attics.
"quality" of bass driver is not as important as many people
believe. The bass driver
is a big piston moving the most air volume relative to the volume of
room. Cubic centimetres versus cubic meters. Of course
baskets are good, but even stamped sheet baskets will do. Do not spend
2000 euro on super bass. The "quality" factor is important only for
commercial speakers which use 4 " or 6 inch bass driver imitations.
The top end must be reproduced by a driver which has sensitivity
similar to the midrange - and this is a difficult part. Most silk domes
are max. 93 dB. Thats too little. We need 97-98 dB. Paper alnicos have
that, some ribbons have that too. Paper cone tweeters should have at
the back of the cone an open basket not sealed.
Top end response of the broadband drivers , even the best ones like
like Lowthers or Fostex's is NOT ENOUGH. Our loudspeaker needs a real
tweeter - or call it - a super tweeter.
Now comes my philosophy - how to get the crossover (XO) from
scratch, without even fully understanding whats going on.
If we agree that the main part of the music is reproduced by the
broadband midbass, the two other drivers only augument the sound in
botom-most and top-most octaves.
We must LET THE MIDBASS PLAY. Not limit it, not flatten it, not use
notch filters or any manipulation by XO. Let's make it play the way it
plays. If you do not like the way it sounds "naked", please change it.
In order to protect it from damage from excursion, and audible
distortions from overloading and doppler's effect, we will restrict
slightly the bass signals. The antique alnico 20 cm drivers handle not
that 1 - 2 watts of musical signal without filters. Thats not enough
headroom. But thats enough volume !!! 106 dB of pure silk.
So I use a capacitor in series just to make sure that at 40 Hz I don't
get too much excursion - no more than 5 mm maximum. That's at the high
volume listening level.
In 99 % of cases a 100 uF MKP cap works great. Any non-electrolytic is
good. There are for example good MKP caps for passive power
compensation in home appliances.
If I listen to a programme via such cap and without (material is vocal
without bass content) I hear absolutely no sound degradation due to
this cap. It is transparent to the midrange content. I cant detect when
this cap is shorted in bypass. It is completely transparent. DONT EVER
SPEND $$$ on the exotic caps here !! Please !
So we have done the first element of the crossover (XO) : a first
order electrical high-pass filter superimposed over the natural
rolloff of the
driver itself and again superimposed over the open baffle effect (too
narrow baffle compared to infinite). These three elements add up to
circa second order accoustical rolloff at around 300-400 Hz. (If the
baffle is 40-50 cm wide).
The bass driver must only support the midrange where it is weak. The
two response curves must "meet" in the exact point of 400 Hz. Better
some overlap than a dip.
To do a bass response that crosses at say 400 Hz we must act strongly.
Otherwise the natural upper end rolloff of the bass happens at 2
or even 4 kHz.
So we need a second order filter - symmetrical accousticly to the
midbass response after filtration.
Because changing inductor coils is not practical (they are bulky,
hard to get) - we choose a given size that must do. Any size from 3 to
mH. DCR must be low (below 1 Ohm, that translates (for air-cores) to
the wire of 1,4 mm or up to 2 mm diameter. 0,5 Ohm is a beautiful
coil. Again - no flatfoil ribbon coils please.
We have the coils for bass. That is one kilo of
copper. With a dust iron core - we may get away with smaller physical
sizes. I believe we can use a core and get away with that compromise
because the core distortions happen only at 10 x or even 20 x the
current that is
practically usable. ( we VERY rarely exceed 1 ampere in peak demand).
BASS Low Pass filter
After adding the coil in
series with the bass driver, we add a paralell
capacitor after it. 100 uF is a good ballpark value. This cap directly
shorts the driver terminals. Then we measure
the whole thing with a microphone callibrated in dB. And a true RMS
meter or better - laptop with TRUE RTA software
(www.trueaudio.com) (note that this is NOT the software that
designs speakers, it just changes a laptop to a measuring device)
If the bass curve
reaches too far - it is time to increase the cap. Otherwise - decrease
it. Make incremental changes of 50 % not less. The bass and mid drivers
(with the mid filter installed) must be
measured first separately and then - together. If the "together"
response has a dip in the XO region compared to the individual
components - it means we must reverse one driver's polarity (any one of
the two). Then we measure again - the polarity with higher response in
XO region is the correct one.
me and my true spectrum analyser
and pink noise
That means we completed the second stage - the bass to mid XO.
Below the picture illustrates "my mother's little helper" - universal
XO board. I
used a piece of particleboard with clip-spring terminals which can form
any 1st, 2nd or 3rd order XO, with zobels, filters, resistors, polarity
swap and so on. I can compare results in seconds just by changing the
parts. And I can also compare parts with same value but different brand
If we use a good test signal and we have big component drawer
do, we can create a perfect XO in less than an hour. Like on the
picture - after 2 years of pain - I finally found a great XO for the
Raven2 ribbon. Without the "board" it wouldn't be possible at all!!
To do the mid to tweeter XO we act similarily.
First - we measure the midrange in its top region - 2 to 10 kHz.
Normally - it has a rising response - a peak near 5-8 K before the
sharp rolloff at 10KHz. Now if we add a small series coil like
0,2 - to 0,5 mH - it will smoothen out the peak and roll off nicely at
Just what we need - eliminate the peak AND the uppermost octave. Two
birds in one shot and by one component only.
This will result with a rolloff at around 6 kHz or something
similar. We are perfectly happy because the top octave from the
midbass sounds very dirty anyway.
This is a good moment to mention the capacitor quality dilemma.
Is the cap quality important in this circuit? 100 uF of foil is VERY
COSTLY in premium brand.
Well - forget good caps. The capacitor "quality" in audiophile terms
means low inductance and low resistance and low leakage current. And
low tangent of current to voltage phase shift. So in this case,
the cap will
be in series with a coil which has 10 000 times bigger inductance than
the cap. And resistance too. So who cares if the capacitor is a
Hovland, Jensen or no name. I tried them all and they are inaudible
for midrange if they are working in series with the coul or even
Now the last part is the tweeter - first we add a cap in series to
protect it. Start with 3 uF MKP and work from here. Make the
measurements "meet" and match the midbass. If 3 uF is too big, - reduce
it to 2 or 1,5. If 1,5 is too big - go for second order with a paralell
coil after the cap. 0,5 mH air core with 1 mm wire is ok.
In the parallel tweeter application the coil quality and the capacitor
quality are both very important. The capacitor works ONLY with the
tweeter without series parts. And the coil directly affect the treble
signal being parallel to the tweeter. So here I say YES, Go and spend
but not too much.
OHM's law is very handy in making cross-over circuits.
The simple formulas for XO part choice is that caps in series increase
impedance with frequency the smaller they get. The rule of thumb is
that a 4,7 uF equals 8 Ohm at 3 kHz. At 6 kHz it will
double, at 1,5 kHz it will halve. And from Ohm's law we can see that at
3kHz the cap equals the driver impedance (8 Ohms) So the cap AT
THIS FREQUENCY is simply an 8 Ohms resistor. Then it is easier to
understand it's role. It will
absorb half the energy. And so on. Simple, isn't it ? Coils are the
opposite - series impedance will increase with value. X=1/jwL remember ?
At the end we measure tweeter plus midbass in the XO region from 2 to
10 kHz, then we change tweeter polarity and choose the higher curve as
Important - we place the speaker in its future position, mike in the
listening position at ear's height. We DO NOT MEASURE CLOSE TO THE
DRIVER'S CONE OR AT 1m DISTANCE. That produces "shop" speakers that
sound like shit.
At the end we add bi- or tri-wiring terminals, wire everything with a
decent wire (not necessarily all runs with the same one - use solid
core for bass and stranded individually insulated for tweeter, any good
one for midrange). Same wire theory in bi-wiring, in triwiring
or inside speakers is a stupid myth. Please forget it.
Place the speakers symmetrically in the room at a perfect unilateral
listening chair, move speaker's base line 1 m from back wall, toe in
10-15 degrees, sit back and enjoy.
This approach produces the best sound achievable from any set of 3
drivers in 100 % of cases out of all I ever tried. And no maths is
required because ALL elements are already built in the measuring
process. The driver spacing, the baffle shift, the wall refflections,
the carpet, the sofa, the cones, the domes, the interferences, the
refflections, diffractions, and the wire influence. The part quality,
the flowers in the corner, the MDF vibrations and early floor
reflections. ITS ALL IN THE RESULT. Just imagine a mathematical or PC
software model which envelopes ALL THESE FACTORS. DO YOU GET IT
Please note that I do not mention resistors. I believe in
crossovers. I try to match the drivers so I don't need resistors
consume the signal energy and reduce the pleasure. Resistors are an
evil necessity like a condom. If the drivers sensitivity does not match
significantly, sometimes I'll use a series resistor to the
midrange or a tweeter, never
to the bass. The value should not be higher than single ohms. Four- I
say - is the maximum value.
If I only can, I "build-in" the resistance to the coils. I just choose
thinner wire and it increases DCR just rightly. And the coil is cheaper
and smaller. What a stupidity is to put a resistor in series and then
buy EXPENCEIVE thick wire coil with very macho ribbon or something.
This is like driving with the handbrake applied.
driver catalog with measurements