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Vintage threads from the first ten years

386 Op-Amp

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5/24/2000 11:06 PM
Charlie	386 Op-Amp I was wondering if this op-amp is very useful in creating guitar circuits, and how much power it has. Charlie

5/25/2000 6:31 AM

According to one of those "Mini-Engineer" books from Radio Shack, it's an audio amp with a gain of 20, but shorting pins one and eight (the "gain" pins) with a cap will result in a gain of 200. The book also shows a circuit with a 741 as a preamp and a 386 as the output amp. Seems like it'd make a nice little headphone amp, and I keep meaning to try it.

The book is called "Op Amp Projects" or similar, and it's by Forrest Mims III, I believe. Two bucks, seems worth the money.

Larry

5/25/2000 9:22 AM

Charlie,

I built a little headphones amp with the 386, but with NO preamp as in the Mims Schematic. It sounds VERY good for as simple as it is. I was suprised how good is sounds:

Guitar into pin 3
+9v into pin 6
Pin 5 to a 330uF cap and then to headphones (+ side of cap towards opamp)
pin 2 and 4 to ground.

Use the volume control on your guitar, it distorts ugly on full volume. If you don't have a "treble bleed" cap on your guitar volume, it might not sound so good. The gain is 20, I've never tried it with a speaker. The Mims setup is better, but this is SO dog-gone simple, I had to try it! I think if you add a few more components for volume control and power protection, you'd end up with a very nice inexpensive Q&D headphone amp. I've been meaning to post a schematic of that...maybe one of these days I will.

The little smokey is the same except:

330uF cap is 47uF
Another 47uF cap between pins 7 and 4 (+ side to 7)
Pins 1 and 8 are connected. (This boosts the gain to 200. You can put resistance between to get variable gain.)

How much power does it have? Not much, good for headphones or maybe "bedroom" practice. Best advice is to go buy the chip and try it out, they're so cheap and easy you don't have much to lose...and its all good fun! If you want more power, take a step up to the 384 5w IC amp, or the 7w 383 (Anderton's choice) or the 60w 3886. The circuits get a little bit more complex, but not much.

JD

5/25/2000 3:07 PM
paul perry	Glad to hear people have had good results with the LM386! I must admit I havn't, I found it very unstable. Possibly the fact that I was building it on veroboard was the problem. A lot of tears.

5/25/2000 6:36 PM
Charlie	Thanks very much for both of your replies. I do actually own that Forrest Mims book, but haven't built anything in it yet. I'm at school irght now, but I'm very eager to get home so I can proto a headphone amplifier w/it. Thanks again, Charlie

5/25/2000 6:52 PM
Stuart	I was about to try one of these. I've traced out the Marshall MS-2 circuit, but I can't find a source for the chip, so I was going to hack in an LM386. The Marshall relies on the IC alone in the "ON" mode; switching to "OD" adds a transistor in front of the IC. Very simple.

5/26/2000 3:38 AM
Charlie	Re: 386 Op-Amp --- OSCILLATION? I just built a prototype, with a 10uf between the 1 & 8 pins, and a 220uf after pin 5. It sounds cool, real in your face distorted sound, and it can power a small speaker fairly well, but it sometimes breaks into oscillations, or something. It sounds really funky, anyways----is there any way to tame this beast? Charlie

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