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I have built
speaker box for Critical Q subwoofer as per instructions. Am now at stage
of modifying the Jaycar AA 0508 amp. You now recommend the AA-0501?
The reason AA-0501 is preferred, it is easier to
modify, has also been more reliable that AA-0508. This model also has
variable phase control. The box construction can also be simplified and
the rear compartment of the box eliminated, making the sub also a bit more
compact. The new instructions for
AA-0501 are easier to follow, but if you are still using AA-0508,
click here for those (less clear)
instructions.
Does the critical Q
subwoofer need damping material ? You don't mention it in your website.
Use as little as possible. I use a little as lining
at the rear, and that's all. As there is no port for any peakiness to
escape (have you seen the amplitude vs. frequency response out of vented
outputs, then you know what I mean by "peaky"), there is IMHO less need
for stuffing. The nomex cone is very dense and unlike usual bass driver
diaphragm, much less radiates back through it. I am quite sure I am on
safe grounds when I point out that 'stuffing' is adding a non-linear
component - the conversion of mechanical energy to thermal mechanism. So
if you feel you must add a little more damping, then base so on your own
listening, but too much will seem to slow the sub down.
I padded the back of the
subs with open-cell foam and stuffed with about 20g very loosely plucked
Dacron. This sounded much better,.. but I'd like to know the ideal amount.
if you like it, then it's the ideal amount. That is not
too dissimilar to the amount I used in the subs I built, except I used
something called Innerbond.
I would prefer the 10 in. XLS driver, as the larger driver might overwhelm my smallish audio room.
Any ideas on modifying your box dimensions for this?
Don't worry re the size 12" versus 10", it doesn't
make any difference and the 12" is not going to be worse than 10".
Besides, asking me about using the 10" it is really now a totally
different design and I'm simply not able to cater for every alternative
and I get emails like "if I choose such and such
driver and amp combination, what changes do I need to make."
That's requiring a design service (it becomes a new design). That goes
beyond the original idea, which is FREE and supplied as is. It was
and is a GIFT.
I know a friend who designed the 10" XLS for commercial version of my sub.
It took him THREE MONTHS WORK! It is NOT a gift! You have to buy it.
"Thanks Joe for your Critical Q sub Article...,
but wondered why you did a closed cabinet instead of a ported one? What
would happen to your sub if you punched a hole and ported it ? "
Have you noticed that all high-performance are
expensive, these Subs are 1. Sealed and 2. Servo Controlled. This is no
coincidence. I am not anti ported alignments as some are.
It's horses for courses, but if you want a compact high-performance
Audiophile Sub, then sealed is the way to go. It requires a driver with
just the right parameters. Vented boxes are very complex, it is not just a
matter of adding a port. They need to be tuned to low Fb, they roll off at
24dB/Octave. This means finding a driver that can be tuned to very low Fb
and yet keep box size reasonable, a difficult feat. Vented alignments also
have poorer transient response. On the other hand, a sealed box using a
driver with large displacement capability, with a resonance of 40 Hertz
combined with Low Q of 0.5 will easily give flat response down to 20 Hertz
in a normal room, combined with much lower distortion at music levels. In
a few words, it is simple, needs minimal EQ and it WORKS. All this without
using a Servo, and thus a DIY dream project.
"Do
I need a special Sub Amp? Or can I drive it with a say Rega Brio amp, 35 W
per channel only, but very high in current?"
You could use any amplifier of choice, but you will
still need a Low Pass filter that reaches 3rd order by 200-250 Hertz. The
use of 35W would severely limit SPL but otherwise work. I would like to
develop a stand-alone unit that would allow the use of any power amplifier
of choice. This would contain the Low Pass Filter with its Variable
Frequency and Volume controls. I suppose it would look similar to a
Preamp. I am not putting any date on this, but it may happen.
"When making THD comparisons to the Velodyne sub,
there's no SPL reference mentioned for the Peerless bass unit. While I
would hope that you are using the same SPL baseline as the other test did,
it is quite possible to produce 'better' looking spec if one doesn't push
the driver as hard. As such, can you verify the actual SPL used as a
baseline for the THD figures that you published."
I
have no reason, certainly no financial incentive, to be misleading. This
project is my gift to the DIY fraternity.
Your idea regarding comparable SPL is a nice one, but difficult in
practice. How do you convert nearfield measurement into equivalent SPL @ 1
Metre? You cannot do THD measurement at one metre, one cannot edit out
reflections in the time domain as that also edits out LF information
required. What I did was to consider that the Velodyne was a 10 inch
driver and this is a 12 inch. This means a lot less excursion required for
the same SPL. I also would have
thought that the measurement on Velodyne was NOT done at maximum possible
excursion, but still at a signal requiring a fair bit of SPL. Evidentially
they, Hi Fidelity magazine of Denmark, also had a similar problem
converting into SPL, so they ESTIMATED it to be in the range of 100-105dB.
That's an inaccurate range and not helpful. I would also be inclined think
it more likely at 100dB than 105dB. In fact I have my doubts.
It's really the excursion we need to consider. I played a variety of
music including organ and in my fairly well sized room I saw not much more than
4mm P/P excursion. At that level I know that this Sub measures about 0.6%,
so my aim has been achieved. I tried 10mm P/P and that gave 1.6% - still
very respectable and considering larger cone area, I suspect this at least
as good as the Velodyne which would need near 15mm P/P or more to achieve similar
equivalent SPL.
Above that, at more extreme excursions, the distortion does rise.
This is because the volume (of box) to displacement ratio becomes much
higher. I do admit this is an area that Servo Control may have an
advantage. At audiophile SPLs as used in my room, the performance is
clearly very low THD.
"Very impressed with your subwoofer project and
site, but it is not exactly clear to me which amplifier you are using. Do
you recommend to only use the US Parts Express amplifier. Can you confirm
that modifications you have detailed on the website are for the Parts
Express amp."
The 'modifications'
are for both. As shown by the measurements, the Jaycar is set up with a
boost around 30Hz, whereas the Parts Express is more flat. They have
chosen different values for R26 and R27 and that causes the differences.
But replacing these with values I have designated, they should end up
exactly the same.
"Does the sub-amplifier changes that you
recommend in your excellent pages applies to the Jaycar, Parts Express or
both versions? It is not very clear for me. Thank you for your support."
The answer is 'yes' to both versions! The 'modifications'
page Instructions are likewise the same for both.
"I have read with interest your sub-woofer
design web pages, but have not been able to follow how you decided on a
Vb of 42L. I have used both WinISD and Unibox to determine Vb for a Qtc
of 0.5 and both programmes suggest around 20L would be required. This is
of course for the Peerless 830500 speaker in a sealed box using
manufacturer’s T-S parameters or your measured values. Can you throw any
light on this matter for me?"
I am familiar with WinISD and entered the Basic Fs, Qt and Vas. With this
'quick and dirty' method got the value of 0.46 - not bad. so you must have
entered the wrong values. May I make it very clear that the
0.5 Q result came NOT from using computer modelling, but actually building
the box and then measuring it. There is no substitute for the REAL world
and the simulated one.
"Hi my name is Dxxxx. Could you please send
me the Critical Q Subwoofer Project steps on how to make it? I looked at
the project on the website and it looks great and I really want to make it
because I am into electronics."
The changes required to the Sub Plate Amp is now virtually step-by-step
instructions and provided you
have good soldering skills should not prove difficult. As for the box,
there are so many construction methods that I could possibly cover. As
long as 25mm MDF and the final dimensions are met, then it will work. Make
sure all joints are air tight.
"I was very impressed with your Critical Q
Sub, after finding it on the net, and thinking, this looks like a good
thing... I have a question.. Is this sub suitable for use in the down
firing position? if fitted with feet to raise it off the floor...... I
look forward to your reply."
That is not something I've seriously considered. Provided there is
significant clearance below, it should work OK. The exposed area around
the down-firing panel and the floor must be several times that of the
driver. I suggest 5 inches above the floor. The sub's upward bandwidth
will be less now. This is because the directionality are impaired and
effectively adding to the low pass filtering. So I would use this sub with
main speakers able to reach down to 50 Hertz.
"What if I want to use the Critical Q Sub
in my home theatre, can I still do that?"
Yes, but with great care. By looking at the cone excursions you can
get an indication of how loud you can play your Home Theatre setup. Trust
your eyes and keep excursions down and you will be OK. Do not expect to be
able to play as loud as commercial Home Theatre Subs, but you may well
find that the quality is better. The in-box Q of 0.5 means that the
roll-off is much slower than usual and this means that cone excursions are
magnified as they are not limited. This is good for high quality music,
for which it is intended. The new recommended Sub Plate Amp does have some
sub-sonic filtering and thus a bit better than the previous recommended
amps. But be careful.
"What effect do the baffles
in your box design have other than support? Do they have a significant
effect on the sound? Can I use a different box shape provided I still keep
the same internal volume?"
In normal practise, when making loudspeakers, the
baffle shape and size matters a lot. This because the diffraction loss of
the baffle will affect the frequency response, the crossover requirements
etc. At low frequencies, generally below 200 Hertz, the baffle dimensions
are less important as they are swamped by the dimensions of the room
boundaries and Sub's location within the room. So keep the internal volume
to 42 Litres and you can choose a different box dimension.
More of these will be posted
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