开云体育

Radio shack used to sell a board that they called a "Stereo Headphone Amplifier", which basically supported a pair of LM386 chips and supporting components. Some years back I found one at a hamfest for some really low price and snagged it. Built the board, put it in a minibox with both RCA and quarter-inch connectors at inputs and outputs, and found a suitable wall wart to power it with (also originally from Radio Shack, as it turns out). There were a few different options for things like gain, rolloff, etc. that you could configure on the board with jumpers, I elected to route these to mini toggle switches.

Sometimes when I power this thing up it'll break out into oscillation. Turning the volume control all the way down will usually put a stop to that, but I'd like to find out what's going on and put a stop to it.

*ALL* of the connectors are grounded to the box. Is it likely that this has something to do with it? Something I was reading not too long ago had input connectors isolated from the physical enclosure, due to "ground loops" or something like that. Could this be the cause?


--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
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Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin

On Thursday 09 January 2025 03:17:17 pm wn4isx via groups.io wrote:

Add a 1000uF (or higher) capacitor across the V+ rail to ground.

Yeah, I should probably try some of that stuff. It's been so long since I built that I don't recall what's in there. But looking in my files I found the booklet that came with it. It's their catalog number 277-128. Apparently each chip is decoupled with 10 ohms and 100 uF.

Look up the spec sheet Different manufacturers might have minor differences.

The zobel network, low value capacitor in series with a low value resistor is critical and the specified values might not be correct depending on layout.

They're using 10 ohms (again) and 47nF.

Sometimes you need a 1uF cap directly across the IC V+ to ground, on the pins.

They've got a good bit more than that in there. But what they're showing for powering the thing is 4 AA cells. The wall wart I'm using puts out a bit more, so I figured I'm getting about 1W/channel out of it.

Sometimes the minimum gain of 20 is too much and you need a resistor across AF +/- in directly across the pins. 10K worked for me.

I don't see the gain as being a problem. One of the jumpers they suggest (which I have wired to a switch) is to boost the gain to a higher level.

Could the original electrolytics have dried out? Rat Shack wasn't known for milspec quality.

They didn't supply parts for this kit, I did. But it might be worth looking at, I suppose...

--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin