SPON pin inside Game Gear (doing audio upgrade hack)

Hey folks,
So I've decided to upgrade my GG audio circuitry since what is in there is an ancient TDA2822 which is inefficient and hissy as hell. Today we have much better Class-D amps and I'm thinking of trying this one:

http://www.ebay.com/itm/281595730350?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

A little weary of the no-name brand but it will still beat the 25 year old GG one.
ANYWAYS, the circuit is pretty straight forward aside from this mystery SPON pin (Speaker On?) coming from the GG. It is a detect pin of sorts from what I can tell, and since the output to the amp is stereo (not BTL) my guess is it makes the L channel into a mono channel when it's active (ungrounded from what I can tell). Anyone else have any idea what this could be for or do?

The only other hiccup will be configuring the new amp to work with the stock jack and switch appropriately. It is not ground-common like the TDA2822, so it might get a little tricky (unless this SPON pin does what I'm guessing). Any help much appreciated!

P.S. here's a schematic o the PAM8403 circuit I'd be replacing it with: http://circuits.datasheetdir.com/271/PAM8403-circuits.jpg

-Segasonicfan

 

OK, the first thing is that you won't be able to get stereo using that chip - a 3.5mm stereo jack has the low side of the two speakers connected together, and attempting to connect that to dual channel amp chip that's designed to work in BTL mode (like that one...) will certainly sound terrible, and would rather quickly blow the chip up if you carry on operating it in that mode.

Your understanding of the wiring in the amp board is correct - there is a SPDT swtich buiilt into the jack that changes over when the headphone plug is inserted. In the normal (I.E. no headphones) position, the switch connects the low end of the speaker to ground and the SPON pin pulls high - this tells the sound logic to output mono on the L channel which is connected to the high side of the speaker.

When you plug in headphones, the switch changes over and the low side of the speaker is disconnected. At the same time, SPON is grounded, which tells the audio circuts to output in stereo mode for the headphones. As a result, the speaker (and hence the headphone) common pin has to be wired to ground for the SPON switch to operate correctly.

So, basically you have two choices:

1) Use a different chip with outputs referenced to ground
2) Run the amp in mono mode even with headphones - leave SPON high, run everything off the L channel and common the L&R on the headphones thenuse the switch to disconnect the speaker when you plug the phones in.

 

Thanks for the reply!!

Glad to know my hunch was correct about the SPON pin. What a strange (and kinda cool) way to switch between the two. There is only one other headphone amp SMD board on ebay right now and I think I might go with it instead:
http://www.ebay.com/itm/131542044241?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

It's a mono circuit that sums audio L and R with 10k resistors. What I might do is use this for the speaker, use the heaphone switch built in to active shutdown mode (pin 1) and then use the headphone jack as a line output, switching audio when you plug it in with the HXJ8002 inputs. If needbe I can build a dual opamp for more amplification (like the 6db gain that the 10k resistors take out). I rather have the headphone jack only work as line output (which is what I really want anyway), than to make it mono.
I think finding an analog chip with outputs referenced to ground that is also Class-D would be difficult.

-Segasonicfan