3-terminal loudspeaker

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joeymorin
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Posted by joeymorin: Fri. Oct 19, 2018 - 09:48 PM
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Remember Speak n Spell?  Well, I have to add an audio amplifier to one to make it louder (and likely add a beefier speaker while I'm at it).

 

So I crack it open, and find the speaker has not two, but three terminals, and three wires going to it.  Strange...

 

My best guess is that it's a separated push-pull arrangement, allowing a slightly simpler drive topology where both the push and the pull drive a positive voltage above ground i.e. two counter-wound coils sharing a terminal.  Placing a scope on the three terminals suggests the same:

 

Each channel shares what appears to be the common terminal on the speaker.

 

Quite noisy (and zooming it shows lots of inductive peaks), but notice that each channel seems to be in opposition to the other, while neither goes below ground.

 

Capturing the two opposing terminals on a single channel appears (skipping the common terminal) continues the suggestion:

 

What I'd like to do, is remove the 3-terminal speaker, and drive an amplifier with the opposing outputs of the onboard audio amp, but I suspect that the speaker-as-inductor is itself an integral part of the amplifier.  Removing the speaker from the circuit and looking at the three different signals, that is in fact what's happening.  Each drive (w.r.t. the common) plateaus at about 10V, with widely spaced decaying dips to 0V, while a look at the two drives together as one channel shows a nearly flat trace, with the occasional bump.

 

This is the kind of thing that really re-enforces the fact that my analog skills are pants :(  I'd rather not start hacking into this thing too much, lest I let the magic smoke out.  Is there a simple solution I'm overlooking?

 

I suppose I could leave the original speaker in a void within the enclosure, tie into it for the amp inputs, then feed that to a new beefier speaker installed behind the grille... but that sure seems ghetto.

 

 

 

This topic has a solution.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Learning and Information, General Electronics, solved
Last Edited: Sun. Oct 21, 2018 - 06:48 AM
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avr-mike
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Posted by avr-mike: Fri. Oct 19, 2018 - 10:00 PM
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It's easy to make fake inductors out of op-amps and capacitors.

Search for Gyrator on Wikipedia and it's about half way down

the page.

 

--Mike

 

EDIT: I have fond memories of the Speak and Spell!

Last Edited: Fri. Oct 19, 2018 - 10:03 PM
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joeymorin
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Posted by joeymorin: Sat. Oct 20, 2018 - 12:40 AM
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Well, I found this:

 

 

... at http://electro-music.com/forum/phpbb-files/speak_spell_schematic_211.gif

 

... but it differs somewhat from the unit I have.  Mine has no earphone jack, and as mentioned the speaker is actually a 3-terminal device.  The schematic seems to suggest a transformer, which of course immediately gives me an idea I'm surprised I didn't think of before.

 

I'd guess TI were shaving pennies by using a three-terminal speaker.  I imagine in the quantities they had these things made, that was cheaper than using a normal 2-terminal speaker with a center-tapped transformer.  In any event it seems clear that the synthesis chip (TMC0281NL) outputs separated push/pull drive on pins 11 and 12, sinking each to VDD on pin 5.

 

I'll try to dig up a 1:1 transformer with a center-tap on one side, but if anyone has any other ideas I'm all ears.  Mike thanks for the suggestion, but I expect that route would be more complicated than a small transformer.

 

EDIT: typo

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Sun. Oct 21, 2018 - 03:42 PM
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jgmdesign
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Posted by jgmdesign: Sat. Oct 20, 2018 - 12:45 AM
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joeymorin wrote:
Remember Speak n Spell? Well, I have to add an audio amplifier to one to make it louder (and likely add a beefier speaker while I'm at it).

 

Is E.T. trying to phone home again.....really long distance this time?

 

Maybe he can help you with the hack....he's got some experience:

 

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

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avr-mike
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Posted by avr-mike: Sat. Oct 20, 2018 - 02:48 AM
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joeymorin wrote:

I'll try to dig up a 1:1 transformer with a center-tap on one side... 

 

The Radio Shack 150-in-1 (and other) electronic kits with the spring

connectors included an audio transformer with a center tap on the

input side.  Some kits are listed on eBay at around $40 US for a beat

up one.  More 70's-era nostalgia....

 

--Mike

 

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joeymorin
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Posted by joeymorin: Sat. Oct 20, 2018 - 03:27 AM
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avr-mike wrote:
The Radio Shack 150-in-1 (and other) electronic kits with the spring connectors included an audio transformer with a center tap on the input side. Some kits are listed on eBay at around $40 US for a beat up one. More 70's-era nostalgia....
I know it well!  My brother had one when we were kids, although we both got a lot of use out of it.

 

I don't think he has it anymore, and if he did I wouldn't want to cannibalise it, my nephew will need it soon ;-)

 

There are a couple of shops not far from me which have transformers, I'll try them next.

 

jgmdesign wrote:
Is E.T. trying to phone home again.....really long distance this time?
Something like that... Stage prop.  Either I make it louder, or they live with recorded cues from the stage's sound system.  It's hard to hide the fact that the sound will be coming from a booming PA system, so they've asked me to take a kick at the can.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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DocJC
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Posted by DocJC: Sat. Oct 20, 2018 - 04:47 AM
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How about a 600 ohm to 600 ohm transformer, as was often used in phone interfaces.

If you are going to drive a small audio amp, then does it really make any difference if you have the push pull or only one half of it?

 

I'd try 600:600 transformer, connected on 1/2 of the driver and the ground, the output of which fed a small class D audio amp, and thence onto the speaker.

 

JC

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joeymorin
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Posted by joeymorin: Sat. Oct 20, 2018 - 04:55 AM
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Taking only one half of the driver would be like putting a diode in series with a normal audio source i.e. serious distortion, but I could certainly give it a try.  Some 600:600 have a centre tap on one side anyway.  And I just sent a bunch of ISA and PCI modem cards to the scrap yard :( ... I might have one hiding around there anyway...

 

The amp I'm planning on using is in fact a class D, based on the TPA2005D1 (appropriate, yes? ;-)

 

Heading off to bed.  Thanks gents.  I'll keep you posted, and keep the ideas coming :)

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Sat. Oct 20, 2018 - 04:58 AM
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joeymorin
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Posted by joeymorin: Sat. Oct 20, 2018 - 04:55 AM
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[Oops.]

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Sat. Oct 20, 2018 - 04:57 AM
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avr-mike
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Posted by avr-mike: Sat. Oct 20, 2018 - 05:22 AM
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This may be too involved for your project, but I'll throw it out there anyway....

 

Microchip used to make a chip called the SP0256 (that's a zero, not the letter O)

which is a speech synthesizer.  You feed it a stream of phonemes and it uses a

filtered PWM output to pronounce the words.  I'm working with one now as part

of our Halloween candy display.  You can still find them on eBay, $5 from China

or $15 from N. America.  Cool synthesized speech sound, not exactly like the

Speak 'n Spell though.  Twenty-eight pin DIP, needs a 3.12 MHz crystal.

 

--Mike

 

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js
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Posted by js: Sat. Oct 20, 2018 - 05:42 AM
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Some speakers have 2 coils to cater for 8 Ohms and 4Ohms circuits.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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joeymorin
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Posted by joeymorin: Sat. Oct 20, 2018 - 01:42 PM
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js wrote:
Some speakers have 2 coils to cater for 8 Ohms and 4Ohms circuits.
That was my first thought.  Or, that two different volume settings were effected by this arrangement.  However, test with a scope show this not to be the case, and the schematic I found supports the separate push-pull action of the speaker outputs on from TI's custom speech synthesizer chip, TMC0281.  I also found this article which explains the theory of operation, but doesn't touch on the speaker output of the chip.

 

avr-mike wrote:
Microchip used to make a chip called the SP0256
I recall Radio Shack carrying something similar (Or perhaps that very device).  IIRC there was an article in CoCo Magazine (or perhaps Popular Science?) which examined it, and showed how to construct a speech module to connect to a computer.  I never built one :(

 

But you're right, that's too involved for this project.  I'd like this off my plate before the weekend is up, as I have 3 other more pressing projects for the same show.

 

If I wasn't worried about letting the magic smoke out of this important show prop (more can be had on eBay, but the show loads into the theatre in 9 days), I would dig deeper into how the TMC0281 speaker outputs actually work, and then maybe come up with a more elegant plan than shoving a transformer in there, but since the schematic show that this is very likely how some units were made, it will be the first thing I try.

 

EDIT:  Ah yes, this is the datasheet I remember:

https://web.archive.org/web/20111004123416/http://courses.cit.cornell.edu/ee476/Speech/SPO256-AL2.pdf

And in fact, according to the Wikipedia page, this is the very chip that ended up in the speech/sound cartridge add-on for the CoCo.

/EDIT

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Sat. Oct 20, 2018 - 01:58 PM
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joeymorin
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Posted by joeymorin: Sat. Oct 20, 2018 - 11:09 PM
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Long day today, but took a dinner break to go track down a transformer.  Struck out at the first two shops.  Tried a third, a surplus store, and started opening up gear (I've known the owner for 20 years) until I found something suitable.  It was this:

 

 

It contains what appears to be a 2:1 transformer, with a center-tap on the 2: side (75ohm-75ohm : 75ohm).  I'll put a heat gun to the PCB (or maybe cut traces and tie in directly as a test) tonight or tomorrow and see what's what.

 

I took apart about a half-dozen pieces of gear which looked promising, including a few other modems, but they either had no transformers, or had non-center-tap transformers.

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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DocJC
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Posted by DocJC: Sun. Oct 21, 2018 - 01:36 AM
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Two more thoughts. Just read an article on how trivial it is to play wave files on an RPi, although I bet you may have already done that before on an avr. Concept is you record the sounds you want and play them back spoofing the speak and spell. Option two, not needed as you have access to both push and pull outputs, was to read the push output via adc and then recreate it

through an OP amp for the opposing 1/2 cycle of audio, if your testing shows you need both halfs

JC

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joeymorin
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This reply has been marked as the solution. 
Posted by joeymorin: Sun. Oct 21, 2018 - 06:46 AM
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DocJC wrote:
Just read an article on how trivial it is to play wave files on an RPi, although I bet you may have already done that before on an avr. Concept is you record the sounds you want and play them back spoofing the speak and spell.
Something similar is usually done in cases like this.  We just install a wireless in-ear monitor with a little speaker (with amp if necessary) into the prop in question, and then run sound from the console in the control booth.  This is often the preferred method, and I'd suggested it to the production manager for this show, but the requirement is that the actor be able to use the toy as is.

 

DocJC wrote:
Option two, not needed as you have access to both push and pull outputs, was to read the push output via adc and then recreate it through an OP amp for the opposing 1/2 cycle of audio, if your testing shows you need both halfs
Also a good idea.  However the push- and pull-halves don't match as it's a fairly complex sound being reproduced, although in the final analysis the audience probably wouldn't be able to tell the difference.

 

The good news is that the tranny I pulled from that old modem in #13 worked a treat.  I added a 200R resistor as a load on the secondary, and the waveform looks very clean on the scope.  I then fed that into this amplifier.

 

The original 3-terminal speaker has a resistance of 96 ohms, 48 at the centre tap.  It worked well on the output from the amplifier, but was scarcely louder than the original toy.  I snarfed the 8R (probably 1/4W or less) somewhat smaller speaker from the same modem, and tried it.  My ears bled.  Looks like I'll need to add the optional 10K pot to the amplifier.  I'll also pick up a 1/2W speaker to fit the enclosure.

 

I discovered that the pitch of the voice can be changed without changing the speed, just by turning a trimmer on the PCB.  I've offered to break that out to the edge of the case so the actor can play with it, although I haven't read the script or seen a rehearsal so I have no idea if that would even be appropriate.

 

Thanks for all the feedback gents.

 

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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