Free (OpenSource) SubWoofer audio processor project.

[BrianHG]

Though I mentioned this project in the new beginners 'The "I made this!" thread - Beginners achievements with pride.', this dedicated thread is for questions on how to build and use and modify the device.

Hi everyone.  Got a strong amp with low frequency capability, but no sub-woofer, or need a batter active filter for your sub-woofer?  I'm posting my full Subwoff project, one of my earliest home projects from the late 90s.

This was my first project using AdvancePCB/Schematic, which became Protel98/99se, and now is Altium, back in 1997.

It is a sub-woofer enhancement processor I called Subwoff.  It takes line level in, allows you to boost 25/20/15hz and below to ridiculous gains with an inaudible soft smooth rounding clip function allowing you to boost your speakers/amp subsonic rumble for movies.  Yes, the filter is so perfectly tuned that no bass will be added to the voice or music, just the rumbling effect.  It is stereo and allows you the feature to turn off the main audio so you can also use it as a active sub-sonic filter for a dedicated subwoffer instead.  Documentation and full schematic is in the .pdfs.
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I've included schematics & CAD CAM GERBER files with BOM.  It is a 2 layer board under 10cm X 10cm allowing for cheap 5$ prototypes for anyone who wants to build it.  The .pdf files have hi-res versions of the full schematic and PCB.  It requires a 9vac or 12vac output wallwart to opperate, not DC.  All the 10uf and 1uf caps should be non-polarized.
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Major update - Feb 14, 2018.  PCB and BOM redone with modern available components.
- BOMs now includes Digikey links and part numbers for everything except resistors.
- PCB is 10cm by 7cm.  Now has mounting holes.  10mil space, 10 mil gap, 2 layers.  All components on top.
- Schematic has been cleaned up with some additional descriptions.
- Except for 4 green leds at 0603, all other SMD parts are 0805 or larger. Easy to hand assemble.

- Dirt cheap to make them in China, PCBway.com quote on this PCB is 5$ us for 10 PCBs + shipping, 2-3 day shipping.
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2 units built and tested - March 13, 2018 - Some minor patches.
- Switched 2 opamp gain resistors from 51k to 4.7k, WAY too much subsonic gain, this fixes it.
- Tented all vias
- Enlarged and relocated silkscreen labels for RCA inputs and 12VAc input.
- Recommend now at least 12vac, 14vac max.
- Added silkscreen description for the two 3 position switches.  (Sorry, the only room was under the switch itself..)
- Regenerated Gerbers, schematic, previews.
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Works amazingly good.

[BrianHG]

Updated 2018-03-13.

BOM files, .xls digikey order to build 1 unit and the project file in Protel99se Project format.
If you need any other files, just ask.
And for heavens sake when ordering at digikey, up the resistors to multiples of 10 so you have a few extra.

Warning, when inserting the green LEDs, you must get the polarity right, they are crucial in protecting your speakers from firing out of their sockets.

To test this, before wiring anything up, power up the unit, turn the volume knob to the max and touch the RCA L & R input center pin with 1 finger, and, tap the AC power input diodes #1&2 with your other finger.  Testing 1 channel at a time, every time you tap the power coming into the diodes on the AC input side, either one or the other LED will blink green.  This means you did it right.  If both LEDs come on simultaneously, you need to reverse the polarity of on of them.  If only 1 LED or neither lights up, you need to re-solder the LEDs.

Enjoy!

[Audioguru]

Your schematic with the opamps as a layout drawing instead of being a schematic with the input on the left and the output on the right is extremely difficult to follow. Abbreviations are all over the place and there is not a single GND symbol, just those 3 letters all over the place.

Are SLIN and SRIN the inputs? The schematic doesn't even show input terminals, just these letters on connecting wires.
Why are the gain controls separate from the schematic?

[BrianHG]

Yup, the full schematic with the power rectifier and RCA jacks is shown in the .pdf of the schematic.

This work was done 20 years ago in my early days.  What I did was each op-amp dedicated for 1 channel, left/right.  The top opamp is all left and the bottom is all right.  When testing this on a breadboard, I only did 1 channel.  The purpose of the visual layout of the opamps was to create 2 layer PCB with minimal vias and signal crossing by planning the layout right at the beginning of the schematic stage.

Give me a sec and I'll add some annotations to the schematic .png.

[BrianHG]

I've improved the schematic description by each amp and color coded net names + the full left side of the schematic:
As usual, the .pdf version is much higher resolution...

Check the First Opening Post, it has the new updated files.

[BrianHG]

I'm cleaning up the PCB and BOM, just for the hell of it, give me 2 to 3 days...

[Audioguru]

Thanks for labelling your schematic but since its filter opamps are backwards then I cannot see if they are the Sallen-Key highpass with a high Q for a low frequency boost that I use or if they are Multiple Feedback Bandpass Filters.

I expect a schematic to look something like this:

[BrianHG]

Thanks for labelling your schematic but since its filter opamps are backwards then I cannot see if they are the Sallen-Key highpass with a high Q for a low frequency boost that I use or if they are Multiple Feedback Bandpass Filters.

I expect a schematic to look something like this:

That circuit of yours is not a smart multiband limiter.  It also doesn't give the realtime whopping 1 through 100x gain at 3 different bands with V-VLF below 5hz protection like my circuit.

Working backwards:
     Take a careful look at the trick I'm performing.  The green led create a very soft knee VS current limiter, with a smoothing 100nf cap (C14), before the LEDs, (looking at the left channel), I have the Subsonic gain 10k pot fed in series with 10uf (C22), which removes the very low frequencies and will rapidly charge and discharge in the event of the volume being up too high through the leds.  But you need to go further back to see that there is a 1Kohm series feed to a 1uf to GND, (R10 & C16), then go back further, there is another 1Kohm series and 10uf to GND (R5 and C20).  What I have done here, is there are across these 3 LPF filters, at 3 bands + the 4th super low cut filter (C22), a different level of current being allowed through to push the green led slightly further and further.  It is something akin to using multiple resistor and diode steps to convert a triangle wave into a sine wave design you see in some function generators, but, with the LPF in my design, even if you force a square wave through, it will still come out rounded like a sine wave.

     Remember, this board is NOT a simple equalizer with a very low frequency volume knob.  You can get that effect just by turning up the VLF channel on you DVD player/Sound system.  What my device does is separate out the VLF, auto volume it up to the max continuously, referencing only to the VLF content and adjusting on 3 different VLF bands, then re-mix that back onto the normal audio without altering the volume of the normal audio.

     It is an impressive effect which freaks out many who have experienced is at my old house.  No added bass, no signal clipping, yet you feel the breeze in the movies, even in stereo content, like when I used to watch Dr. Who, the sound when inside the Tardis would quietly shake/vibrate my entire house, yet the actors voices and actions and background music wasn't affected at all.  This was the effect I was after, not just a simple over-glorified bass knob.  (In my current apartment, my neighbours would think there were numerous mild earthquakes going on every other day...)

     Think of this soft over volume rounding of the VLF audio function like a vacuum tube amp with it's comforting soft response to overdrive, but just for the VLF portion of the sound.

[Audioguru]

I hate the sound of overdrive distortion. I also hate the gain pumping caused by an audio compressor.
Instead I like to have enough headroom to eliminate these problems.

[BrianHG]

I hate the sound of overdrive distortion. I also hate the gain pumping caused by an audio compressor.
Instead I like to have enough headroom to eliminate these problems.

I know about headroom.  Remember we are talking about boosting everything between 5hz and 20hz, nothing audible.  I'm creating and managing an inaudible effect.  So many different movies have different levels of flatness and inclusion of the serious VLF I'm dealing with here.  I can make a 90's film like Terminator2 perform with the sensations of a modern 2015 film.  Even an 80s film like one of the early StarWars will have VLF signals of a modern movie.  While at the same time, if you were to play the modern 2015 film which was deliberately recorded flat below 5hz, if played through my box, it wont overshoot and blow you amp's input with a 10 volt signal.  This was another reason for such a separation of VLF and normal audio in my audio path using the extra op-amp stages and multiple RC filter passes.

This box has no effect on bass whatsoever.  Sensations and vibrations are enhanced exclusively.  If all you want is more bass, you do not need this device.  If you are sick of bass and want more sensation below the audible range, this is the perfect thing to use.

I'm cleaning up the schematic and PCB now.  The newer schematic is even better labeled, with GND symbols as well.  The new BOM has modern digikey links and full part numbers for the dedicated switches/buttons/connectors/ceramic caps.  Better PCB layout and a size of 10cmx7cm so you can get 5pcbs for 5$ at a Chinese fab house.

[BrianHG]

I hate the sound of overdrive distortion. I also hate the gain pumping caused by an audio compressor.
Instead I like to have enough headroom to eliminate these problems.

Let me put is to you this way, in the original Terminator movie, there subsonic effects at 10hz is -24db while in a modern counterpart, the same effect level at 10hz would have been 4db.  With your filter and headroom, if you are playing your system at full volume, boosting the 10hz by 20db, the old movie should sound good.  What would happen when you play a modern movie next with it's 10hz signal level of -4db.  How would adding 20db to that signal sound with your filter scheme.  I know from experience on my Subwoff, not only would there would be no clipping or damage on my equipment, it would still sound perfect.

[Audioguru]

Where did you find a speaker that plays frequencies below audio frequencies? Is it 3 meters in diameter and weighs a few hundred kgms?

[BrianHG]

Where did you find a speaker that plays frequencies below audio frequencies? Is it 3 meters in diameter and weighs a few hundred kgms?

My speaker is only 7 inch woofer.  It plays flat to 25Hz, -3db at 20hz, -10db at 15hz.  (Clements RT-7, patented "R-HD Compression Line Bass Filter"  Yes, 25hz truly flat with only a 7 inch woofer...)  On these super efficient speakers, the effect is unbelievable, my whole house shakes, the neighbours too, with only 100w RMS stereo, however, it is pitiful next to my friends 12Hz active 1kw sub-woofer.  With my box on his system, you can feel a breeze in the room.

[Audioguru]

I could not find a datasheet for the Clements RT-7, patented "R-HD Compression Line Bass Filter" speaker, but many people in Google are selling the ones they have.

[BrianHG]

I could not find a datasheet for the Clements RT-7, patented "R-HD Compression Line Bass Filter" speaker, but many people in Google are selling the ones they have.

They are 25 years old...
You better believe I'm keeping mine...

Clements/Solus Audio has just created a new smaller tower called 'David Tower'
https://www.solusloudpeakers.com/h-pas-products
No detailed datasheet yet, but it's 75% the size of my RT7 and the woofer is a little smaller & speced from 30hz to 22khz.  I waiting for my local store to get a pair for audition.

Watch this video starting at 16 minutes in.  Philip Clements explains the mechanism of how he achieves the super low frequency response using a carefully tuned inverted horn/whistle.  The ass the under-powered weak low frequency air coming out of the bottom of the speaker, their is an additional chamber similar to a gigantic low frequency whistle.  The the air is blown through that whistle, the VLF is amplified like blowing through a giant pipe organ tube.



Believe when I say my speakers, only simple stereo 2, driving them to 200 watts, they can not only kick you chest in with properly recorded full orchestra/of a good movie recording, if you we sitting on a stool, they can knock you over.

[BrianHG]

Major update - Feb 14, 2018.  PCB and BOM redone with modern available components.
- BOMs now includes Digikey links and part numbers for everything except resistors.
- PCB is 10cm by 7cm.  Now has mounting holes.  10mil space, 10 mil gap, 2 layers.  All components on top.
- Schematic has been cleaned up with some additional descriptions.
- Except for 4 green leds at 0603, all other SMD parts are 0805 or larger. Easy to hand assemble.

- Dirt cheap to make them in China, PCBway.com quote on this PCB is 5$ us for 10 PCBs + shipping, 2-3 day shipping.

Check my opening post #1, all the files have just been updated.

[Hextejas]

If anyone is going to order the 10 boards, I will buy some off of you. 

[BrianHG]

If anyone is going to order the 10 boards, I will buy some off of you. 

At a price of 5$, for real PCBs, with soldermask and silkscreen, you should just order them yourself directly, and you get 10 of them.

Send this 'CAM for Subwoff_PCB_2018.zip' file to the PCB maker: https://www.eevblog.com/forum/projects/free-subwoofer-audio-processor-project-for-forum-members/?action=dlattach;attach=398453

It has everything they need.

This PCB is 2layer, 10cm x 7cm.  10mil trace, 10mil space, 20mil drill.

Inside the 'CAM for Subwoff_PCB_2018.zip', in the Gerber output folder, the files has the layer description on the films.
In the NC Drill Output, you have the drill files.

Got to https://pcbshopper.com/ to get quotes and delivery times from multiple PCB vendors.

[BrianHG]

I just ordered 10 PCBs from 'JLCPCB.COM'  They charged me including shipping 16$us for 10 day delivery to Canada.  They took my 'CAM for Subwoff_PCB_2018.zip' automatically and filled in the rest.  They also accept Visa payment instead of Paypal.

Here is their onscreen render of my 'CAM for Subwoff_PCB_2018.zip':



Now, should I order components for 2, or order for 10 and offer to ship out kits with parts for users to assemble?

[BrianHG]

I can offer up to 8 Subwoff PCB boards with all parts included for 40$usd plus shipping.  You still need to assemble them.  All parts are new from Digikey, no Chinese knockoffs.  I will not do my Digikey order until I get 5 requests.

[BrianHG]

Finally, built 2 Subwoff PCBs.  Audio samples coming in 2 days.  I'll be looking through Youtube movie previews & use one as an audio reference & provide 3 samples of original, Subwoff enhanced mixed audio and subsonic only at full blast.



Holy S--t, this sounds insane with modern recordings which have VLF all over them.  I'm adjusting 1 gain resistor value, otherwise, it all rumble and nothing else...

[Audioguru]

Holy S--t, this sounds insane with modern recordings which have VLF all over them.  I'm adjusting 1 gain resistor value, otherwise, it all rumble and nothing else...

I have noticed that TV sound "engineers" have a sound frequency response down to earthquake rumbling frequencies when recording a reporter on the street. The reporter is speaking, not playing music.

[BrianHG]

Holy S--t, this sounds insane with modern recordings which have VLF all over them.  I'm adjusting 1 gain resistor value, otherwise, it all rumble and nothing else...

I have noticed that TV sound "engineers" have a sound frequency response down to earthquake rumbling frequencies when recording a reporter on the street. The reporter is speaking, not playing music.

Watching the 2 green protection/signal limiting LEDs during such a broadcast, you will see the wind blowing in each direction, or, slowing down and accelerating.

This device doesn't do as much for older music than newer rumbling tunes, however, it was designed for movies, not music.  For music, you are better off with a simple bass knob.

If I raised the frequency of the 25hz LPF, you would hear the heart beats of the studio shots of the microphones work on the host's ties.  Though, the beginning of the lower frequency male's voice will begin to be amplified.

[Bassman59]

Holy S--t, this sounds insane with modern recordings which have VLF all over them.  I'm adjusting 1 gain resistor value, otherwise, it all rumble and nothing else...

I have noticed that TV sound "engineers" have a sound frequency response down to earthquake rumbling frequencies when recording a reporter on the street. The reporter is speaking, not playing music.

You would think their field recorders have a high-pass filter on the microphone inputs!

[Audioguru]

Holy S--t, this sounds insane with modern recordings which have VLF all over them.  I'm adjusting 1 gain resistor value, otherwise, it all rumble and nothing else...

I have noticed that TV sound "engineers" have a sound frequency response down to earthquake rumbling frequencies when recording a reporter on the street. The reporter is speaking, not playing music.

You would think their field recorders have a high-pass filter on the microphone inputs!

Of course the field recorders have filters but the engineers don't know anything about it.