Popping in lm1875 circuit

Joecool85 · March 13, 2006, 05:58:29 PM

I built a 2 channel lm1875 amp and when I turn it off it makes a popping sound. I've tried a bunch of different grounding schemes to no avail (currently star grounded.) It makes no difference as to what the volume is set at as to how loud the popping is.

RDV · March 13, 2006, 08:42:37 PM

My chipamps all pop to varying degrees when turned off. A relay circuit can be built for a soft turn-off I believe.
Is it speaker destroying loud? Or just irritating?.

RDV

Joecool85 · March 13, 2006, 09:07:05 PM

Barely irritating. Here is a more important question. I left it on for an hour (by accident actually) and came back to it and the heatsink was pretty much just as hot as when I'm using it...is that normal?

**edit**
I should note that my lm3886 chipamp doesn't do this. Maybe the transformer is too small. I know you've used smaller trannies, and this one is one I salvaged out of a CB so I'm not sure its VA rating...

RDV · March 13, 2006, 09:13:01 PM

Quote from: Joecool85 on March 13, 2006, 09:07:05 PM
Barely irritating. Here is a more important question. I left it on for an hour (by accident actually) and came back to it and the heatsink was pretty much just as hot as when I'm using it...is that normal?

**edit**
I should note that my lm3886 chipamp doesn't do this. Maybe the transformer is too small. I know you've used smaller trannies, and this one is one I salvaged out of a CB so I'm not sure its VA rating...

Are you using a dual or single supply?
Is the tranny very heavy(since we don't know the amperage)?
How hot is it getting when it's playing?
Is it getting to a good undistorted volume?
Does the tranny get hot itself?

My subamp runs a little warm at idle, but that's at 4 ohms. What ohmage are the loads you're running?

RDV

RDV · March 13, 2006, 09:21:20 PM

Make sure your .1µF bypass caps are really close to the chip to help prevent oscillations that will make the chips run hot. If you've done this already disregard.

RDV

Joecool85 · March 13, 2006, 09:22:43 PM

Center tapped transformer, power supply very similar to the snubberized one for the lm3886. Pretty heavy transformer, about 4lbs or so, maybe 5lbs. The transformer gets warm and thats it, the heat sink is too small and gets really freakin hot. Hasn't shut down yet though. And I can play it at almost full volume with no distortion. The load is 8ohms per channel.

Joecool85 · March 13, 2006, 09:24:21 PM

Yeah, the .1 cap is right next to the chip.

RDV · March 13, 2006, 09:27:52 PM

It sounds like you just need a bit more heatsink to me then.

RDV

Joecool85 · March 13, 2006, 10:07:31 PM

Well, I do need more heatsink. But, I've managed to fix most of it. I had some ambient hum, got rid of that by re-doing how I was grounding my volume pot. Then it was still getting pretty hot. I realized that every now and then when I touched the sink, I got a little static through the speakers, so I grounded the heatsink (it wasn't grounded before because this is all in a wooden box), and sure enough, no more noise when touching the heatsink, and now it only gets warm. Granted, it gets pretty warm, but its cool enough I can hold onto it without letting go. Well, I just tried it, after about 10 seconds you really want to let go lol. But before it was like 1 second and you'd have to let go. And btw, my "heatsink" is the old steel bracket that use to hold the transformer in the old CB chassis. If flattened it would be about 4x3 and 1/8" thick. I bolted on a 1x5 aluminum heatsink (again off the CB, this was originally used for regulators), and thats it. I'm getting a real heatsink soon, but it hasn't come in yet, and I wanted to do some testing :-)

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Popping in lm1875 circuit