"why it's not so easy
to turn a mini into a 3-way"
Copyright © Troels Gravesen
have the question if one of my smaller minis
can't be made into a 3-way from using 6-10" bass
drivers by adding an e.g. simple 1st order high-pass
filter to the mini and "some" low-pass filter
to the bass driver. What's it going to look like?
(The same goes for removing the
low-pass section of a 3-way speaker to make it suitable for e.g.
electronic crossover between bass and mid+tweeter).
There are a number of problems
in this and I'll try to list a few.
First of all a small mini usually makes
82-83 dB/2.8 volt, where the suggested bass driver may
produce significantly higher output, like 86-90 dB/2.8
volts, thus a major discrepancy in sensitivity, and for a
number of reasons we just never want to reduce a bass
driver's output by series resistors. I won't go into
detail, but it's a no-no.
The mini is usually tuned to make some bass output down
to 60-70 Hz depending on driver size, i.e. cone area, the
mass of moving parts, magnet size, voice coil impedance,
The small 4-5" bass driver may have a sensitivity
around 85-87 dB/2.8 volts in the 500-1500 Hz region, but
this doesn't help if the bass region can't produce the
same output, thus we add a series coil (we equalise) to
tame the midrange and to compensate for baffle step loss,
and we end up with a system sensitivity around 82-83
The same driver may very well be suitable as a midrange
driver in true 3-way system, maybe even tuned to 90
dB/2.8V system sensitivity despite only having 86-87 dB
sensitivity in mid/upper-mid, but more on this later.
Let's take a look at Eekels'
Above the predicted response from LspCAD.
System sensitivity around 83 dB/2.8V. Now, if we add a
simple 1st order filter @ e.g. 300 Hz (100 uF for 5 ohms
impedance at 300 Hz) we get this:
This does look a bit weird making a major
bump at 800 Hz and anything but an ideal lower roll-off
profile. Adding a 2nd order filter at 300 Hz looks even
To cut a long story short,
it doesn't work. Making a crossover for a 3-way system is
an integrated process taking into account all drivers at
once and we can't just add passive high-pass filters to
an existing filter without seriously compromising
Second option is using an electronic crossover
and no objections here, but the mini will still only
produce 82-83 dB/2.8 volts and it's never going to be a
"big" system. We're into a sub-sat situation
and we may like the mini for what it does so well, but it
will never produce the kind of sound level we expect from
a large speaker system.
And I have to emphasize here that buying a big amplifier
doesn't turn a low-efficiency mini into a big speaker. I
often have that question too. Even the TJL3-way isn't
particularly sensitive - or big - and a 200 watts
amplifier will never make this speaker into a really big
speaker due to the cone size of the bass driver. Look at
membrane size when it comes to acoustic output. Read more here.
Next we pick an 8" bass driver,
e.g. SEAS W22NY001, and we start modelling and maybe we
get something like this:
Now this is a completely different
story. The W22NY001 makes a decent midrange and
works all the way up to 800 Hz. The W12, now working as
an upper mid driver, only covers the 800-3200 Hz range.
No system optimisation applied here. Just an example.
What we see is a small 4" midrange being used in a
larger system tuned to 90 dB! Some of the upper mid
actually comes from both the bass driver and tweeter due
to overlap. You can see the red graph being below summed
If we wanted a lower point of crossover between the bass
and mid, we had to pick a larger and more sensitive
midrange, e.g. M15CH002. It may look like this:
The M15CH002 is a bigger speaker
with higher sensitivity and can handle most of the
midrange and we can move point of crossover down to 350
Hz or even lover. This is just not possible with the
small W12 driver due to its low sensitivity.
So, if you enjoy your
minis and think of keeping the qualities of these minis,
buy a 24 dB/octave electronic crossover plus a second amp
and experiment with points of crossover between 80-160
Hz. This will relieve the mini of having to pump bass and
you can play a little louder.
If you buy a sub with integrated amp and in-built
low-pass crossover and have the minis run fullrange, the
minis will still have to cope with the bass signal and
not be able to play any louder than before.
Turning the drivers of a
your mini into a 3-way system is starting all over and we
need to do what's described here.