Crazy idea but might be brilliant...

[Govmnt_Lacky]

I was able to salvage (2) 27mm speakers from some old toys that my kids had. I was thinking that I could incorporate them into a Ruby amp to make an ultra-small practice amp (I am talking 1590A size  )

Each speaker is measuring out at 15 ohms. So here are my questions:

Could the speakers be connected together (in series or parallel) to BOTH be used in 1 Ruby circuit? Kind of like a stereo output.

Also, how would I match the circuit for the speaker(s) (impedance?) to get optimal sound?

Thanks for helping. 

[slacker]

Slap them in parallel that will give 7.5 Ohms, the Ruby will be happy with that. You could try series as well, people build head phone amps using LM386s and a lot of headphones are 32 Ohm.
It's not really a stereo output though it's just mono through 2 speakers.

[Govmnt_Lacky]

Thanks Ian. They are really just cheezy plastic toy speakers from Chinese made toys but I figured they MAY be good enough for a little pocket sized Ruby 

I am by no means a Tone master so the sound is not as big to me (for practice mind you) in this application.

[Mark Hammer]

I had a thread on the Lounge/OT forum about those little plastic speakers.  I salvaged one from a talking/musical birthday card my sister gave me that has some serious thump to it.  Actually, the "Smokey" practice amps use similar plastic-cone speakers.  I know the ones in the musical cards are fairly flat profile, but they have a lot of potential if placed in a suitable-sized cab.  Same thing for those little speakers that came in the old Mac Classics.  Those babies are either 32 or 64 ohms, but are really well-built for a 2" speaker; big magnets, foam surround for good bass response.

I've built a small 9v-powered 386-based amp into a small plastic box a little bigger, but somewhat flatter than a 1590B, Great for testing out things without having to fire up a bigger amp, and not half bad for low-level rocking out.

The little speakers and power-amp chips that come in cheap computer speakers (the ones that sell for $10 and say "180W PMP" on the box) can also be repurposed productively.  Those little power-amp chips are run in stereo, but can be easily bridged to feed a pair of speakers in parallel with 1-2W.  Put in a cab of some reasonable size (lunchpail?), and powered with 12V worth of AAs, you'd be surprised how loud those suckers can be.

[Govmnt_Lacky]

Mark... it is funny that you say this because it was that EXACT post that got me thinking about this project. I then salvaged some speakers from some cheap McD toys and away I go 

These speakers have small magnets as well. They are about 1" wide and measure out to be 15.2 ohms (16 ohm speakers I suspect) each.

This should be fun as I want to achieve the same thing as you. A small test amp for my projects. Nothing fancy but usable 

[edvard]

Hehe... I'm not good working with Thiel-Small parameters, but I wonder what might be a good ballpark air volume to put those things in?
Open back or sealed cab?
For sealed, maybe stick one in a cardboard or PVC tube with a plunger and adjust until it thumps.
For open, get a bunch of PVC reducers and stack them until it sounds nice.
Then it's as simple as pi*radius²*length to get cubic volume.

Or you could just leave them in the pipe...

[Govmnt_Lacky]

Actually, I am thinking about just slapping them into a 1590B so that it can be powered by battery. I don't know how the acoustics will be for these little speakers but like I mentioned above, it is just for practice at home. (As not to wake the preggo wife and kiddies  )

[runmikeyrun]

it's not going to be high fidelity by any means but you will get some sort of sound.  Stuff the box with some cotton batting or foam, that will help bass response a little, but i'm telling you those things are going to be t-h-i-n.

[Govmnt_Lacky]

I plan on doing the headphone mod as well so fidelity is low on the importance totem pole for me I suppose. I will mainly be using this for stompbox build testing and the occasional quiet moment of practice. Anything to keep the kids asleep at night! 

[PRR]

> tube with a plunger and adjust until it thumps

With not-large speakers: you get thump at a higher pitch than you might like (bump the second octave of guitar) or you get slowly falling no-thump in a large box.

> I'm not good working with Thiel-Small parameters, but I wonder what might be a good ballpark air volume to put those things in?

If they are fairly loud for the power:

Multiply nominal (frame) diameter by 1.1. Cube that number. That's a happy sealed-box volume for a 1% efficient driver. Double that volume will also be fine. Three times the volume may be satisfactory.

EX: 4" driver. Times 1.1 is 4.4". Cube is 4.4*4.4*4.4 which is 85 cubic inches. Literally using a 4.4" cube begs resonances, make depth less and height higher. 3"x5"x6" is the same volume. 4"x5"x7" will also be fine.

As an extreme: a 4" speaker in 8 cubic inches (basically: seal the basket) will give a BIG bump at 600Hz. I have used that, with heavy EQ, in a communications system. Terrible gitar thumper.

Teeny-speakers tend to be low efficiency, can benefit from a larger box. Change "1.1" to "1.3".

> Open back or sealed cab?

Vent design is too darn hard. Use the guide above but make the box volume 2x to 8x the minimum. Chop a hole, cover with board, slide around to hear various too-boomy and gut-less responses. Then fill the hole, run sealed.

> (2) 27mm speakers ... just slapping them into a 1590B

A Ten is too big for a 1590B.... if it could fit, the resonance would be high midrange.

A 27mm in a footlocker may work about as well as a smaller box, meaning the box is annoyingly large and wasteful.

Two 27mm in a 1590B may not be Small-optimum, probably on the large size, but ain't far off, and the excess size is not a storage/transport problem. Don't think, drill.

Two 15-ohm in parallel is near-optimum loading for '386 at 9V. Two 15 in series will be less-loud (midnight practice) and give 4X the battery life. Easy to try both ways.

[Govmnt_Lacky]

Tried the circuit last night on breadboard. Its a no go.

I wired the speakers in series AND parallel. They definitely sounded LOUDER in parallel however, I had to have the GAIN and VOLUME maxed out just to get an audible level out of the tiny plastic speakers. Plus, when the controls were maxed, the signal was distorted (as expected) and I could not shape an "audible" clean signal out of it.

Tried it at 12V as well with no measurable changes.

[ItZaLLgOOd]

I ordered some 50mm speakers from Mouser. They fit perfectly in the top of a 12oz. beer can     Sounded like a speaker mounted in a beer can though.

[edvard]

Multiply nominal (frame) diameter by 1.1. Cube that number. That's a happy sealed-box volume for a 1% efficient driver. Double that volume will also be fine. Three times the volume may be satisfactory.

I've been searching for that information for I can't tell you how long.
Ask the hi-fi guys and they tell me I don't need a ballpark, I need dead center of the pitcher's mound.
... and then try and sell me tickets.   

Vent design is too darn hard. Use the guide above but make the box volume 2x to 8x the minimum. Chop a hole, cover with board, slide around to hear various too-boomy and gut-less responses. Then fill the hole, run sealed.

Does that go for open-back amps as well?
I've got a Peavey Bandit 65 I'd like to whip into shape, although that might take more than a hunk of plywood...
The '53 Fender Deluxe can keep it's open back, I'd rather let the tubes breathe.

[PRR]

Well, 27mm drivers are more common in headphones, not as speakers.

Or, as you know, as "TOY" speakers. Kid's head close to the toy, mother not going crazy from obnoxious loud toy.

Also toys may choose to use high pitches and "voices", to put the sound above 700Hz where a 27mm diaphragm can get a 1% grasp on thin air. On the other hand you have choosen to use a full-size geetar that starts at 82Hz (8 times or 3 full octaves lower).

There's a wide range of 27mm drivers. Some will be "flat" to 200Hz but at only 0.1% efficiency. So you get "some bass" but hardly loud enuff to hear. OTOH hooters can be designed to be LOUD above 1.5KHz with nothing lower.

Strap it to your shoulder?


> 50mm speakers from Mouser. They fit perfectly in the top of a 12oz. beer can. Sounded like a speaker mounted in a beer can though.

The 4" Fostex FE-103 is a perfect fit in a 1-pound coffee-can.

Yes, your ear knows the sound of a can-shape. Boxes are less distinctive. Even so, there are examples where a listener could describe the size/shape of a speaker he could not see.


Small open-back boxes are another Black Art. And often no happy solution.

Large open-back is less cloudy and has significant benefits. For guitar, "large" is over 80 inches square, or 80"w 40"H set firmly on floor. A popular short-form is the large Two-Twelve open cab, 30"W 24"H. It gets full baffle support only down to 170Hz, then it falls, but at "only" 6dB/oct, -6dB at 90Hz. A cone driver tuned to 90Hz with very small damping will bump-up 6dB or 9dB at 90Hz. The result is fairly-flat from around 80Hz to well over 500Hz, to 5KHz _ON_ axis.