SubWoofer2 GuitarAmp

[phatt]

Hi folks,
         My old Eagle eyes spotted this on a pile of junk and I could not help myself I had to see what was inside.
Lo and behold,, *An LM3886* complete with a big/reasonable PSU.

So after a few head scratching hours I disconnected all the fancy sub circuitry and I have myself a 50 Watt Powered Speaker.
Just finished adding a small input socket and preamp to the side panel,, works a treat with my other guitar preamps. <3)

I was not expecting much from the little 6 inch *Elcheapo* speaker but to my suprise it reproduces Guitar signal quite well.
Although the speaker is a little low in the SPL department it can still be loud enough to play over a Drummer.

The speaker looks funny from the front but a closer inspection of the rear shows it's just a normal cone with the concave dish added on the front.
So it's not a dedicated *long throw hifi type woofer* which of course would not work for guitar signals at all.

PSU delivers 36-0-36 VDC and drops via Resistor and zener for the 12-0-12 VDC to run the opamps ,,Oh how thoughtful they where.   Everything needed to make a simple cheap but gutsy little compact rig.
I'm stocked chaps.

I think this got dumped because the *Signal sensing circuitry* obviously was switching off but not switching back on when another signal came through.

So if you see one of these grab it,, just for the chip and complete PSU alone.
All the bits needed to make a 50 Watt Amp ,,,how kool.
Cheers Phil.

[J M Fahey]

Hi Phatt, great find !!!
It clearly is made out of MDF so you can easily grab your trusty old jigsaw and cut a hole for an 8" speaker (maybe even a 10") and blow away many "fancy" amps.
Absolutely LOVED your Proto-guitar-preamp and the "extra parts bin". :tu:
EDIT: I forgot: being a subwoofer amp I'm *sure* it has a limiter built in, as to avoid farting.
That must be the cause of the compression you hear.

[phatt]

Hi JMF,
         Well for the moment it works great for whatever reason.


I've read more than once where the LM3886 chip will clip hard and is apparently to be avoided but this just sounds gloreous when it distorts/compresses. (Ideal sound for the blues freaks)

My simulations tell me the signal from the preamp I built is bigger than needed but that is controlled via the Volume pot anyway. The harder you drive the volume the more it compresses, not as good as a tube amp but still very touch responsive,, ver-rhy Nice.

I wanted to be able to use it as a stand alone amp hence the simple 2 knob circuit.
Makes for a convenint little rig to pick up and go Jam.

The 150mm (or 6 inch,,, for you yankies, wink) speaker is not great in the SPL department but somehow this works well for my needs.
Remember it's the tone you want not so much the raw power.
Raw power is easy to get but IMHO a lot of them lack any real tone mojo.

Anyway I've added an extention spk socket to drive an external Spk box when needed and then it really starts to get LOUD.

I've got at my disposal;
2x 12" Eminance Legands in one box (loud but harsh).
A single 12" Fane in another (killer tone, just a little sharp)
Then 2 x 12" Peavey Black widows in an other box (about the loudest SPL on the planet, but useless for guitar)

And my trusty 15" in a sealed box (Devine tone, well balanced)
This one is a cheap Dia-achi (never can remember how to spell it but made in the SE Asia)

Anyway enough talk here are the schematics.
In time I will retrace the pwr chip (Again to be sure) and all the surrounding components then post my findings for better minds that maybe able to establish why it works so well.

The PreAmp schematic is very quite but
***don't delete the 1k R after the first stage as it will go crazy and squeal with to much treble.***

The tone pot should be 250k *Linier* and the Volume Pot 50k *Log* for best results.

The overall sound (using the internal 150mm driver) is just a touch bassy so I tested it in another box with no back and it lost something,,,
so sealed back is the better option.
Bare in mind I have no idea as to what is ideal here because this speaker has no name and it's one of those half baked designs that is not quite sure whether it's a woofer of a full range driver.

The local Muso's club metting tomorrow so next week I'll pull it apart (Again) and take some gut shots as I know you are into speakers and you may be able to shed some light.

I will try to get some film of this working but as I've never done the Utube thing it's all new to me and I don't know if the sound on my cheap camera is going to pick up a decent sound.
And understandably I don't wish to make a complete goose of myself doing these things.

Otherwise I'll become known as the Phatt U Goose.
Thanks for the interest.
Cheers, Phil.

[J M Fahey]

*Very* interesting and thanks for sharing (I'll Proto the preamp tonight).
The Power amp is classic, see nothing out of common there.
I'm still laughing at the "sophisticated subwoofer crossover": that 18K/68n filter amounts to incredibly sharp 6dB/octave above 130Hz, although it might be fine for its intended use.
I'm a minimalist myself and love simple solutions.
I guess the preamp may be close to clipping with a regular guitar signal, depending a lot on how hard you play, that would explain the perceived compression.
The "subwoofer" will certainly add "bass thump" and filter out the harshness; that might explain why it sounds better than those Legends.
The price paid is efficiency, although at home any power is too much power (just ask the Mizzuz and/or neighbours)
Mensur recorded his preamp into a very similar power amp driving a 10", heavy cone, rubber surround speaker, and the sound was killer.
We deserve some pictures of the "Nambour Blues Band" in action.
And some recording including that choir of howling dingoes backup vocals. 8|

[phatt]

Thanks for the interest JMF,

The Dingo couldn't make it but the Possum had good jam on Sunday. 

Sadly the hard hitting Wombat drummer turned up and He was just a little to much for the subwoofer possum amp.
(LM3886 through a 6 inch Speaker).

Not to worry a good experiment and it's all still quite useful.

One day I might be tempted to make a whole new Poweramp board and rebuild it all into a small Amp.
(I do have a 10" Guitar driver somewhere in my junk box)

also will have to look at rebuilding partly because Now that I've mounted the input socket to the Metal Back Panel that power cord has no EARTH 
so the Metal panel really needs to be Earthed to be Safe.

Meantime here's a few pictures of the internal bits.
Phil.

[phatt]

Hi folks,,
Been a while,, but I finally found a home for my LM3886 subwoofer circuit.

I Shoehorned it into an old UPS case. The case (like the subwoofer) was junked in a pile of unwanted electronic equipment heading for the dump.
Oh how I love junk.

Refined the one knob tone circuit only slightly.
So front panel is: Tone, Gain trim, Volume.
Only issue really was an Earth loop which kept eluding me for a day or two  :grr
but finally found the offending connection and now it's dead quite,
even when cranked.
The preamp has too much gain but I love what it does to the LM3886.
Through a 15 inch speaker it's a huge old 60's rock tone and it's loud.

An almost woody sound with the onboard controls but just adding the PhabbTone preamp in front delivers the more modern rock tones.

A whole Amp circuit in box for under $20.  I'm happy 0:)
Phil.

[spud]

Yeah, me too, but the wife....that's another matter!    :grr

Jim

[Roly]

{Hey Phil, looks like the film isn't winding on properly in your digital camera.  }

Great score.  I picked up a similar 5 way at the tip a few weeks ago in PWO(!).  I was also rummaging through a pile of TV's bound for recycling and grabbed a couple of chip amps on smashed PCB's.  Looking in my junk bin later realised that I have the makings for at least a couple of small "all junk" guitar amps; just gotta figure out some sort of packaging.

My experience of the Dai-1chi (berko name) 12's in PA & foldback has been all good, cheapish, sensitive, clean, robust.

Looking at the pre circuit that's a "Big Muff" type EQ (ref; Duncan's TSC).

Nice end result.   :dbtu:

[phatt]

Yeah,, the pics have lost the plot and I'm lost as to how or why?
but I think the other pics should give a fair indication of how it went together.

The tone was my attempt at a one pot control,, mainly just to see if I could improve on what has already been done.
Just lots of breadboard setups and dozens of simulations until I found something that worked for my ears.
A lot of SS stuff can easy get way too much top end for the rock sound I mainly use.
The units main use now is a driver for a small fold back, so the preamp section is bypassed and signal taken direct to the power stage as the preamp is too hot for such use.
Phil.