Poor Man's Demo Board

parts-man73

There was an earlier thread where I had brought up a little project I had going on with one of the new Proto Boards. I called it the Poor Man's Demo Board.

Over the weekend I managed to finish the audio jack. and here are some pictures, and well as a schematic of the audio section.

I had tried for awhile to use a couple of LM386 to amplify the output of the Prop. But all I got was horrible distortion and clipping. When I get a chance, I will post the circuit I was trying so that you can pick it apart and tell me where the problem lies.

For now, I have it set up like the second image attached. When my amplified speakers that I use with my computer are connected, I get very nice, distortion free sound.

So now I have connections for VGA, Mouse, Keyboard, Composite Video, and Stereo Sound. The sound only works with amplified speakers though. There's no Mic like the demo board, but I don't have anything planned that uses the mic, so I'll keep those 2 IO's open for future use. I'll solder on a female header for quick connection to P0-P7.

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Brian Meade

"They who dream by day are cognizant of many things which escape those who dream only by night" - Edgar Poe

Forrest

Just curious why you deviated from the unamplified sound output circuit posted by Parallax's Paul Baker and used on the Hydra here http://forums.parallax.com/showthread.php?p=606294

parts-man73

I actually looked at the Demo board schematic for inspiration. I was trying to feed a LM386 from the 1 uF capacitor, But no Joy. There was too much boost, and thus clipping and distortion. I then substituted a known good amplifier(my computers amplified speakers) and they sounded great. So I thought why change a good thing, and soldered it as is.

BTW...I was not trying to drive the amplified speakers from the LM386. I had another speaker I was using for testing. Good thing I didn't use headphones though! I would've blown my eardrums out!

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Brian Meade

"They who dream by day are cognizant of many things which escape those who dream only by night" - Edgar Poe

parts-man73

Here is the circuit I was trying to use for amplified output. This produced extremely loud, distorted sound. Now that I've had a chance to draw it out, I see I may want to use a fixed value or potentiometer between the 1 uF and pin 3 LM386. If a volume control is created using a potentiometer, is there a way to adjust both channels with one pot ?

I have show only the right channel for clarity, the left channel would be a mirror image.

Also I did not have a 0.05 uF cap on hand. Correct me if this is wrong, but two 0.1 uF caps in series would be equivalent in a pinch?

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Brian Meade

"They who dream by day are cognizant of many things which escape those who dream only by night" - Edgar Poe

Post Edited (parts-man73) : 3/21/2007 1:09:02 AM GMT