hey pal what u wasting ur time at just get a TDA2002 and with less components and complication u can have 5 watts or even 6 and an output current of up to 3.5 amps so quit the crazy figuring and use it on something like bridging two TDA2002 or ever the TDA2050 that i thinks is 18 watts(per chip). another method is to use power transistors for amp boost but i don't know how to figure the component values but i supose it is basically a voltage folower configuration (also known as common emmiter)
p.s. i thout u meant 80386 processors at first. try to be plainer especialy in the thread tittle tkx

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Wow...no offense you guys, but you don't know much about bridging the lm386's eh?

It's been done, many a time, and done very well.

http://runoffgroove.com/littlegem.html

It works real well as a practice amp for guitar.

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Yeah man,
We don't know much about bridging two puny little LM386 amps.
I figure that a 4 ohm speaker like in the article will represent the same current as a 2 ohm speaker with a single IC. The amps will both be seriously overloaded and produce a voltage swing of about only 1.5Vp-p from one, or 3Vp-p when bridged. 3Vp-p is only 0.28W into the 4 ohm speaker. A single LM386 will produce 0.245W in a 4 ohm speaker. You won't hear the small difference of only 1dB.

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bridged_lm386.png (25.6 KB, 237 views)

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Uncle $crooge

 

Regardless of hearing a difference or not, its totally possible and they won't overheat driving a 4ohm load when bridged. That was my point.

**EDIT**
And besides that, a lm386 chip will out out around 500mw running at 9volts and pushing an 8ohm load.

http://downloads.solarbotics.com/PDF/LM386.pdf

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The bridged circuit WILL overheat feeding a 4 ohm load, unless it's only run at low volume - in a bridge circuit each amp effectively sees half the load - so with a 4 ohm speaker each amp is feeding 2 ohms, which is too low for a 386.

Also, as there's no feedback in the design, the two halves of the amps aren't balanced - so distortion will be far higher than a single amp, however that's probably not a concern for a guitar?.

With a minimum 8 ohm load, and if you're not bothered about high distortion, then it is possible - but it's a pretty poor idea!.

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Oh yeah? They are seriously overloaded with the poorly recommended 4 ohm speaker. The very high supply voltage of 12V makes their heating even worse. They each produce an output of only 1.5Vp-p. Where does the remaining 10.5V go? HEAT! Try it. Feed them a sustain and turn up the volume to just below clipping. I figure that each LM386 will try to dissipate a whopping 2.8W and they will smoke.

It is designed for a single (not bridged) IC driving an 8 ohm load and a 6V or 9V battery. Look again at the datasheet and you will see its output is a min of 500 milli-whats. "Whats" are square-wave output with a horrible-sounding 10% distortion. It is a min of only 350mW to 400mW at clipping. Flea power.

A TDA1554 (and many others) bridged power amplifier IC is used in car radios and gives a low distortion output of 14 real watts to a 4 ohm speaker with a charging 12V battery. Enough to blow your ears off if you are close to the speaker!

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Uncle $crooge

 

Actually, the LM386 does have built-in negative feedback. Its gain is 20 without the capacitor from pin 1 to pin 8 and 200 with the capacitor. The bridged circuit has the caps so has the high gain and the balance isn't bad, a gain of 200 on one of the amps and "only" 199 on the inverting side.

Without a load, the distortion of an LM386 is nearly unmeasurable. It is used in "headbanger" headphones amps driving 32 ohm or 600 ohm headphones with very low distortion. In this circuit that is bridged and with its dead-short of a 4 ohm speaker, its distortion is probably pretty high.

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Ok, I'm not saying this thing is a killer amp, but it works fine to practice guitar on. And personally I've only built the little gem which uses a single 386, but I know several others that have built the bridged model. Maybe when you are only playing guitar on it, it doesn't matter? But from what I understand, its harder on a chip using it for guitar than for regular am/fm etc. I've run mine on 12volts at 8ohms for hours with little/no heat out of the 386, NO heatsink at all. It could have to do with the fact that the incoming signal is only something like 0.4volts or so out of humbucker pickups. Not sure, I'm not an electrical engineer or anything. And I didn't mean to come off as an ass when I first posted saying that you guys don't know about bridging the lm386, I was amazed at how many people were saying that it couldn't/shouldn't be done, when I know at least 5-6 people that have done it with no problem. Most of them use their Little Gem MKIIs to drive 2x12 guitar cabs no less!

Don't believe me, check out diystompboxes.com

Also, I know about guitar amps more than it seems you guys think. My own regular practice amp is a Dean Markley K-20, a 15wRMS amp with an 8 inch speaker. But I've played a Peavey 212 with 250wRMS and built in 2x12. I'm not saying I'm any guitar amp guru either, but I do know a fair amount. I'm planning on building a lm3886 based chipamp for the poweramp section of my next guitar rig.

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But that's a single ended amp, feeding a sensible impedance, so no problem - try adding three more 8 ohm speakers in parallel with your existing one and see the difference - THAT'S what the bridge circuit is doing with a 4 ohm load.

Guitar isn't that hard on an amp, there's no real low frequencies (bass is fairly hard on amps), and it obviously depends on what's been played and what style. If you're overdriving a power amp it's bad regardless of the instrument, and you should look at valve amps for that.

The worst instrument is probably keyboard?, because you can get loud sustained single notes - which can mess up the power distribution of your cross-overs.

Nothing to disbelieve - I've no problem feeding a 2x12 from an LM386. the size of the speakers doesn't matter, it's their impedance that does! - an 8 or 16 ohm 2x12 will be fine (obviously 16 ohm is preferable though). Large speakers will most probably be louder as well!.

As for "shouldn't" be bridged, I would certainly say not, it's a really poor idea, and very low quality - you could easily build something much better with no more effort.

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Didn't you try my "smoke test" with a continuous sustain at clipping?

The graph for the voltage output of an LM386 that I posted before shows that it produces 660mW at clipping into an 8 ohm load with a 12V supply. The power dissipation graph I am posting now shows 820mW of heat.
Its spec's show a thermal resistance of 107 degrees per watt so its chip temp with a 25 degrees ambient is 112.7 degrees C. Its max internal temp is 150 degrees C.
YOU ARE RIGHT, it doesn't get too hot in your application! :lol:

EDIT: With a 4 ohm load, your LM386 with a 12V supply will produce only 450mW at clipping, and 1.3W of heat, way off the end of its graph.

Attached Images

lm386_469.png (26.2 KB, 194 views)

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Uncle $crooge

 

Well, its good to hear all this stuff from you guys. I really hadn't talked to any electronics people really, only guitarists that their hobby is electronics on the side (like me.) I make overdrive/distortion pedals when I get bored lol.

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Hi Joe,
We never get bored because we are always analysing an building stuff.
Post the schematic of one of your overdrive/distortion circuits and we will simulate it to show how good/bad it looks on a 'scope.
Here is a sim of a Hendricks circuit with a pure sine-wave input:

Attached Images

sim-_fuzz_face_133.png (22.5 KB, 179 views)

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Uncle $crooge

 

This is one that I designed myself.



And the first one I built was actually very similar to what you just showed for a fuzz face. Mine used germanium trannys instead of 2n3904s. Along with a couple different resistor values.[/img]

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What's the point of two identical circuits in parallel?

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Ron

 

One uses LEDs as diodes, the other uses 1n914s. It sounds *almost* the same using one or the other, but I found that I like it with both.

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