Lowpass for subwoofer -project

[vainajala]

http://www.electronics-lab.com/projects/audio/008/index.html

I might start building a small active subwoofer, so I have some questions about this circuit. :

1: my HT amplifier has a line-level mono sub-out connector. Can I replace the filter's stereo RCA input jacks with single jack, halving the R1 value?

2: This project needs +-12 Volts. What's the best and simplest way to make +-12V from +-42V, which is the operating voltage of the amplifier i'm planning to use. Standard zener regulation perhaps?

3 Is the lowpass slope 12dB/oct.?

I'm planning to put the filter and amplifier inside the subwoofer's ported enclosure, resulting a compact "bass-cube"...

 

[audioguru2]

Hi Vainajala,
Nice project, but use a TL072 dual opamp. The TL062 has low gain and high distortion.
1) That's right. Halving the value of R1 for a single input will keep the gain the same as for 2 inputs.
2) If you have 10mA in each 12V zener diode then they will each dissipate only 120mW. Use 400mW ones.
Each voltage dropping resistor will have about 12mA, and will dissipate 360mW. 1/2 Watt resistors will get quite hot so use 1W ones.
3) Yes, with 2 RC networks, the slope is 12dB/octave.

Check that your source does not have a DC voltage on it since this circuit is DC-coupled throughout.

Consider adding highpass filters to your main speakers' amplifiers, to avoid acoustical phase-shift cancellations (and help them avoid "doppler" distortion).

 

[GPG1]

Frequency response

View attachment 35640

 

[vainajala]

GPG:
thanks for that nice plot! 8)

Audioguru:
Also thanks to you. I'll use the TL072 in this circuit, especially if they're pin-compatible!

I've got just one thing to ask:
If I decide to place the amp+filter into the subwoofer-enclosure, is it necessary to "isolate" the circuit boards and transformer from the subwoofer's airspace? At least special attention is required to proper mounting, because of vibration...

And what do you think about this simple power amp, look at the "Figure 2 - The "New Improved" El-Cheapo" http://sound.westhost.com/project12.htm

edit: I chose this amp, because I have almost all of its parts lying in junkbox Perhaps going to use MJ15003/MJ15004 as output transistors, and going to drive 8 ohm load.

 

[audioguru2]

Hi Vainajala,
Yes, the TL072 has the same pinout as the TL062.

Don't worry about vibration, except don't use an IC socket.

That's a nice amp, but as the author says,"... and it might be necessary to connect a low value (47pF ?) capacitor between collector and base of Q2" and "In reality, it is almost certain that a small value cap will be of benefit to ensure stability with difficult loads." Check his forum for any discussion.

How "little" is your base-cube's speaker? When I fed 60W to a ported 4 inch woofer, it let me know that it reached the limit of its excursion during very low frequencies, where its port provides no support. And its wind through the port at its tuned frequency scared my dog and made quite a racket. I made its port a small diameter so that it didn't use too much space in the box, and even so, it was long enough to require a 90 degree bend.

 

[vainajala]

Audioguru:

The speaker I'm currently planning to use is Peerless SLS 213 / 830667. Its diameter is 8", and is supposed to work in small(ish) sub-woofers...
http://www.d-s-t.com/peerless/data/830667.htm

Computer simulation shows quite good results in a 35 liter "classic" bass-reflex box tuned to 27Hz. The port needed is quite long, most propabaly i'll have to bend it, like you had too. Other option is to bring the port outwards from the enclosure...

Woofer's over-excursion is the worst thing in reflex boxes. As you said, there's almost nothing to control the cone movement below the tuning.

There exists a variation of bass-reflex called "Dual chamber bass-reflex", which should help a bit with this matter. It should offer a bit better mechanical power handling but still it doesn't help with 10-20Hz rumbles

I'll propably try out Dual Tuning in my sub if it's not too complicated to get working...

http://www.diysubwoofers.org/prt/dual_chamber.htm

View attachment 35647

 

[audioguru2]

Hi Vainajala,
Your Peerless 8" woofer certainly isn't very small. It has a very long throw for its size and has a big magnet. Although its spec's don't mention power, its 1.5" coil should be able to handle plenty.

Your transfer function curve shows a dropping response due to its very low Q. That low Q will make it sound nice and tight. Are you going to EQ it to flatten it out, or just stick it in a corner? You might need a lot more than only 60W because of its low sensitivity and EQ requirement.

I have some cheap Pioneer 8" woofers that must be a copies of your Peerless (or visa-versa). Same big magnet and low Q. Their ported (tuned too low) boxes are too small but sound awesome with a bit of EQ: for the bass-boosting I use simple 2nd-order high-pass filters with a Q (adjustable) that is too high. They peak a little above the ports' frequency, then sharply cut the extreme low end. Therefore flat response and no excursion problems.
Visitors often ask, "Where's your sub?". I don't have one. ;D

The dual-chamber enclosure probably isn't worth the trouble since the different tuning frequencies are nearly the same.

 

[vainajala]

In my opinion, 8" woofer is rather large for main speaker use, but quite small for sub-woofer use. But bigger cone size doesn't always mean better sound quality.


About EQ:
I think room acoustics will help somewhat with lowest frequencies. If it doesn't work that way then I might consider some sort of sub-bass boosting...

60W isn't very much power, but as the amp designer says:

Additional output transistors can be connected in parallel to allow for 4 Ohm loads, where 100W should be readily achieved.

I have five of both MJ15003 and MJ15004 power transistors in my component-box... :

 

[audioguru2]

Hi Vainajala,
To get more subwoofer power, does the Peerless driver come as a 4 ohms model?
He, He. If not, are you going to wind your own voice-coil?
Or build another one and parallel them?
The small difference of about 2dB between 60W and 100W probably isn't worth it.
The sub will gain 6dB in output (like using a 240W amp) if it is in a corner (solid walls), compared to being away from any wall. But its frequency response will still be tilted.

 

[vainajala]

No, the SLS line is mostly meant for indoor-use so only 8 ohm version is available. and I'm not gonna wind new coil into it. ;D

You're right, 2dB difference might not be worth that increased complexity at output stage...

 

[vainajala]

I'm 99% certain, that I'll be using this woofer. I got it from friend for 35 euros!
Now I need only to measure its parameters with Speaker Workshop...

View attachment 35679

 

[audioguru2]

Hi Vainajala,
How are you able to lift that thing?
Also, its rubber surround is huge so it must have a long excusion.
Let's see your sim of it.

 

[vainajala]

Audioguru:

I don't know how long Xmax it has, because I don't have any spec-sheets or instructions. This is most propably an automotive 4ohm woofer, so I guess the manufacturer has concentrated a bit more on the glamorous appearance, than the sound itself. It is almost too "pretty" to hide in box... ;D

However, it weighs about 6,6 kilograms, has a ventilated 2" aluminium (i guess) voice coil, huge double-magnet, screw wire connectors and 4cm wide surround. Unfortunately, it is foam surround. :-X But considering the 35 euros I paid for it, I don't complain!

These parameters are somewhat approximate, because Speaker workshop can't calculate the BL value. So I just approximated a value of 11 into it. I must measure the correct BL value sometime with my school's variable power supplies before making any real decisions for an enclosure.

(Approximate) parameters I measured:
Fs: 29,8 Hz
Qts: 0,617
Qms: 3,617
Qes: 0,743

According to these measurements, this needs a little bigger enclosure than the Peerless woofer. But anyway, this is a 10" woofer, so that doesn't surprise.

View attachment 35682

 

[audioguru2]

Hi Vainajala,
I'm disappointed by your estimated response curve for the new Focus woofer. Since the Qts is almost perfect, I expected its response to be very flat down to about 40Hz. Maybe its resonance frequency is too high for a sub.
Are you ready to make awesome bass? You've probably seen it before, but this woofer is ideal for a Linkwitz Transform! The link is here:
http://sound.westhost.com/linkwitz-transform.htm
Of course, then it will need a smaller sealed box and lots of power that it can probably handle. You can parallel your transistors to give enough current to its 4 ohms. The bass will be very tight, powerful and reach very low frequencies.
In a car, the small cabin peforms the "transform" when the woofer is in a sealed box, due to "room gain".

 

[vainajala]

I sim'ed the sealed+linkwitz alignment and a normal vented box with WinISD.
Both use 50L box and the Linkwitz transform circuit's -3dB point is set to 28Hz. That way it goes quite low and has maximum boost of only 5,5dB. Both use 2nd order Butterworth 65Hz lowpass filter.

Since I don't know the woofer's Xmax, I (again) approximated 6mm one-way.

For Cone excursion plot: As you can see, sealed+Lw hits the excursion limit at 20Hz with 9W of input power. Reflex hits the limit at ~40Hz with 50W of power, but has higher mechanical power handling at 21-30Hz range. Below 20Hz, it drops very steeply.

SPL graph is then simulated by the maximum mechanical power handling.

Both are pretty much as (in)effiecient, but bass-reflex can stand more power.

Only drawback with reflex is the worse group delay... (the maximum is fortunately below 20-25Hz).

The Linkwitz circuit is indeed very good idea, but it could work better with a very long-throw 12"-15" sub. such as some of these:

http://www.adireaudio.com/TextPages/TumultPageFrameText.htm
http://www.adireaudio.com/TextPages/TempestPageFrameText.htm
http://www.adireaudio.com/TextPages/ShivaPageFrameText.htm

View attachment 35691

 

[audioguru2]

Hi Vainajala,
Yeah, It seems to be best to use your new woofer in a big, ported enclosure with a bit of EQ.
Your posts of high-quality woofers reminded me that I have a very high-quality, very high-power, very long-excursion, very.... etc. 18 inch woofer still in its packing case. It weighs a ton (tonne?). I'll transform it with Linkwitz and shake my neighbourhood, or sell it at the next local car-audio SPL contest.

 

[vainajala]

Hi Vainajala,
Yeah, It seems to be best to use your new woofer in a big, ported enclosure with a bit of EQ.
Your posts of high-quality woofers reminded me that I have a very high-quality, very high-power, very long-excursion, very.... etc. 18 inch woofer still in its packing case. It weighs a ton (tonne?). I'll transform it with Linkwitz and shake my neighbourhood, or sell it at the next local car-audio SPL contest.

Wow! Be sure to hold your hat while powering those ;D

 

[audioguru2]

Hi Vainajala,
I don't care about my old hat. What about the new windows in my home? And my hearing? And my wife? And my dog? ;D
The cars in the SPL contest have Lexan instead of glass, since at nearly 150dB the actual air pressure is almost +,- 15lbs/square inch. Of course, nobody is inside the cars that are blasting that much sound level. They destroy many speakers and amplifiers. ;D

 

[vainajala]

I don't care about my old hat. What about the new windows in my home? And my hearing? And my wife? And my dog?

Forget the hat, those are way more important things!
But is it really necessary to operate your woofer at its maximum SPL level...? I'm sure it will play gently too, just watch out for over-turning that volume knob!
~~~~
I came to think about using 24dB filters for both lowpass & subsonic. Because I heard from my electronics-teacher that steeper-slope filter works generally better with sub-woofers.
I made a quick (and really messy) draft, which uses TL074 and TL071 op-amps...

Highpass frequency is 15 Hz and lowpass 50 Hz.

Propblem is that it needs quad pot to adust the lowpass frequency. and there's no EQ (if that is needed anyway) View attachment 35713