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Internal speaker hack


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Thought I'd share a high-level view of a hack I had some fun with.  If it's of interest to anyone, I can post specific parts I used and detailed instructions.  If it's *really* of interest, I would consider turning the design into a PCB that others can copy. 

 

The short summary is that this is a way to add an internal speaker to a TI-99/4A.

 

image.thumb.png.a998c0f2317e32fd617ea2eefb106c8f.png

 

I'm just getting into Ti-99, and there seems to be rather a dearth of external power supplies.  That meant that I had a couple of reasons for doing this:

  • I was already doing an upgrade to using a 12V power brick for power
  • The monitor I was using with the system didn't have sound.

 

The main question was, where to put the volume control knob?  I didn't want to drill holes in the case.  That meant options were limited.  Either an external knob dangling out the back of some port (yuck), or perhaps repurposing the hole where the "power on" LED comes out.  I looked at the latter, but it was difficult to find a potentiometer with the right dimensions, and it would be awkward to have a knob sticking out the front.  I looked at thumbwheels, but they were also dimensionally challenged.

 

Finally I settled on putting the volume knob on the body of an Atari joystick adapter that I bought from seller techdungeon on ebay.  (High quality unit, recommended.)

image.thumb.png.ffdb420036afe3eb0ae754b0500b124a.png

 

This takes advantage of unused pins 1 and 6 on the joystick connector.  The audio signal goes out of the port, and the attenuated signal goes back into the computer.

image.thumb.png.5f57d1559d503b31d313cfd47f5b26d3.png

 

But you need a third wire - you need GND.  And the joystick connector does not have GND in it.  But it turns out that GND is available in the shield, and the techdungeon adapter nicely picks up the shield.  In my case, the shield's contact with the motherboard had grown flaky over the years, so I made myself a small clip out of 0.010" sheet metal that provides a small amount of spring force, not unlike the larger clips that are part of the stock unit.

image.thumb.png.a348d8fadb6d67527f9ebd0811333d3c.png

And with that, it works like a charm.  The speaker I chose can actually get overbearingly loud if turned up all the way.  But it's always good to be able to go to 11.

 

Brad

 

 

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That sounds like it could be a nice option as well, particularly if you prefer to keep the original power supply.  

 

The speaker I used was pretty beefy, likely wouldn't have fit up there.  But it's perhaps more capable than required, a smaller one would probably be fine.

image.png.fdefa8f86be020e3d79d7d50421e06df.png

I do like how this one turned out.  The speaker sounds come out from the air vent below the power supply and after reflections, you get a sort of nice 360-degree sound effect, unlocalized - at first I actually had to check to verify that I still didn't have my external speaker plugged in.  Nope, it was coming from the new speaker.  

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24 minutes ago, PasadenaBrad said:

That sounds like it could be a nice option as well, particularly if you prefer to keep the original power supply.  

 

The speaker I used was pretty beefy, likely wouldn't have fit up there.  But it's perhaps more capable than required, a smaller one would probably be fine.

image.png.fdefa8f86be020e3d79d7d50421e06df.png

I do like how this one turned out.  The speaker sounds come out from the air vent below the power supply and after reflections, you get a sort of nice 360-degree sound effect, unlocalized - at first I actually had to check to verify that I still didn't have my external speaker plugged in.  Nope, it was coming from the new speaker.  

the original /4 (not the A) has a headphone jack already installed, and TI had planned to have internal speaker, but it was scrapped, but the plastic thingy if you look at it still has the place for it, and the "solid state cartridge" placeholder on /4A systems (not all them but the early ones), was were the volume control was mounted, allowing the user to adjust the volume

Edited by Gary from OPA
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On 4/20/2024 at 1:27 PM, Gary from OPA said:

The better way is to use the original design for internal speaker, in the plastic cartridge holder thingy under the grille on the top, is designed infact for small speaker.

The Micro-Mark web-site sells model-building supplies and tools.  A few years ago I ran across some tiny speakers for installing into model trains and figured I could mount one inside my console.  Those cooling slots looked just right for one of those speakers.

After buying a postage-stamp sized amplifier module and knob from All Electronics before they folded, "life" intervened and it never got installed.  Found the module and knob last week while cleaning up.  I'll have to look at those speakers again.

K-R.

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