Using T-amp to drive LEDs

[joe carrow]

Hey all, I'm looking for a cheap and easy way to drive some LEDs to flash along with the music. Does anybody know if it's an especially bad idea to drive LEDs with a Sonic Impact T-amp?

I am planning on using a series resistor to limit current to the LEDs, and wiring all of the LEDs in parallel.

Any thoughts, opinions? Anything's appreciated.

[Pano]

Hi Joe! Nice to see you over here.

Interesting idea. I would think as long as your LED load doesn't go below 4 ohms, you're OK. Your balast resistor should take care of that.

Since LEDs only conduct in one direction, will you put 1/2 of them in reverse? Or may go from one leg to ground? I think the Lepai amp does that on the left channel.

I've never run LEDs off a T-Amp. Want me to try? I have plenty of LEDS and amps.

[joe carrow]

Hey, it's been a while since I've been here! That other place is where I go for what (I think) are the best discussions of theory, measurement, and crossovers- but you can't bet DIYaudio for variety and being host to a huge international community.

Yes, I was planning on running half of the LEDs in reverse polarity. I was thinking of having different colors for different polarities, since it will give really subtly changing colors based on certain waveforms.

This is going to be a valentine's gift for my girlfriend. She always said that she wanted the lighted sign from Weezer concerts, like this:



So I got some veneered 1/4 inch ply, and tomorrow when she goes out I'm cutting the pattern. It's my only opportunity to do that and keep it secret before the big day, but I should be able to quietly get some lights in it some time after she's asleep.

There's no need to try the LEDs just for me, since I think i can figure out the impedance to ensure that I don't short out the amp. I do have a couple of questions about the T-amp, if you don't mind.

First, what do you use to power them? I bought one a long time ago, and I fried it. I think it was from the unregulated supply I gave it, since it went up past 15 volts with no load.

Second, do you know if there's a big problem running the amp with an open circuit? If it is, I'll consider bypassing the LEDs with a resistor, so that there's not an open circuit during the 0.6 v before the LEDs start conducting.

Joe

[Pano]

As long as you have DC over 8V and under 13.5V, you're OK. I like the line lump SMPS style. But all my sources for cheap ones have dried up. I understand that laptop or LCD monitor supplies work well.

Since you are not interested in sound quality, no need for anythng fancy. Just find 12V.

So just how many LEDs are you planning to use? That Weezer sign has a lot of lights on it!

For different colors, this year we had LED Christmas tree lights that changed color. The bulbs contained just 2 colors, blue and yellow. But the cross fade between the 2 was beautiful. All sorts of shades of purple and lavender. Nice.

[cpemma]

Originally posted by joe carrow
I am planning on using a series resistor to limit current to the LEDs, and wiring all of the LEDs in parallel.

That's a bad idea with LEDs, especially if you're after high brightness with forward current near the max. One resistor per parallel string prevents current hogging.

Ohm's Law is is fine for resistors, but a LED "resistance" varies tremendously with current, plus you get a thermal runaway effect as the hog warms up.

A simple way is to use an NPN transistor switch to turn the LEDs on at some settable voltage level, though the T-amp's bridged output makes this more tricky than usual.

[joe carrow]

Originally posted by panomaniac
As long as you have DC over 8V and under 13.5V, you're OK. I like the line lump SMPS style. But all my sources for cheap ones have dried up. I understand that laptop or LCD monitor supplies work well.

Since you are not interested in sound quality, no need for anythng fancy. Just find 12V.

So just how many LEDs are you planning to use? That Weezer sign has a lot of lights on it!

For different colors, this year we had LED Christmas tree lights that changed color. The bulbs contained just 2 colors, blue and yellow. But the cross fade between the 2 was beautiful. All sorts of shades of purple and lavender. Nice.

Thanks- I'll give it a shot, last time I "meant to" add a regulator to the power supply, perhaps I'll just re-use the thing, but add the regulator. That should be as easy as a trip to Radio Shack.

I'm hoping to get something much nicer interaction with the music eventually, but it's beyond my time constraints of keeping it a surprise for now.

Originally posted by cpemma


That's a bad idea with LEDs, especially if you're after high brightness with forward current near the max. One resistor per parallel string prevents current hogging.

Ohm's Law is is fine for resistors, but a LED "resistance" varies tremendously with current, plus you get a thermal runaway effect as the hog warms up.

A simple way is to use an NPN transistor switch to turn the LEDs on at some settable voltage level, though the T-amp's bridged output makes this more tricky than usual.

Cpemma, thanks- I guess my results will be kind of unpredictable. It looks like (for the time being) it will be better just to throw some christmas lights in there. It will serve the purpose, and buy me some time.

Thanks, both of you guys. I appreciate it- this is going to be a good one

[joe carrow]

Actually- Panomaniac- I take it back, would you mind giving the LED thing a shot to see if it actually looks good? I don't think I'd do more than 20 or 40 LEDs. If you could do just a couple in forward and reverse, taking care not to blow anything up, I'd really appreciate that!

Joe

[Pano]

Will do! I have a bunch of "12V LEDs" - in other words they have a resistor in the lead already. I'll hook them up and have look.

EDIT: Quick try shows that ONLY the reverse LEDs light. But they flash nicely. Volume contols flashing.

[Pano]

OK, back from lunch. Cleaned up connections. Now have forward and reverse working. That's better!

The forward reversr thing is cool, because the flashing is noticable different. So is left/right.

However - bad news. Now I've lost the left channel completely, and right is working only half. Don't know if I've killed the chip.

[joe carrow]

I'm sorry to hear it I was kind of worried- these things don't seem terribly hearty. After I fried mine, I opened it up and couldn't even find any parts that were really worth salvaging. I guess that's what we get for $29 retail...

If you want me to help share the cost of this little experiment, send me an email. I do feel bad if you've fried yours.

Joe