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Author Topic: Build Your Own LM386 Preamp  (Read 29234 times)

willy1usa

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Re: Build Your Own LM386 Preamp
« Reply #15 on: October 04, 2006, 08:46:47 PM »
YES-stompin_T   "The Smokey" is the circuit I've been using. To get a little more bass response, I use a 220uc cap on the output. You can tailor this to suit your needs. The single coil pickups in my Strat needed a little more bass.

willy1usa

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Re: Build Your Own LM386 Preamp
« Reply #16 on: October 04, 2006, 08:52:41 PM »
Sorry Joecool, I skipped over your post.
YES, the circuit you got drawed out is the one I use.

joecool85

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Re: Build Your Own LM386 Preamp
« Reply #17 on: October 04, 2006, 09:46:01 PM »
Cool, now what about the bypass cap?  You said you used a .1uF between the +/-  Is that the +/- as in power, or signal?
Life is what you make it.
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willy1usa

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Re: Build Your Own LM386 Preamp
« Reply #18 on: October 08, 2006, 07:30:13 AM »
The .1, also numbered as a 104, bypass cap goes on the power lines as close the chip as you can get it. I usually run my +/- power lines on the back side of the circuit board (underneath, the chip). Or, if you're handy with the soldering iron, you can solder the little .1 ceramic cap also on the backside of the chip from + power pin to the - ground pin.

The little preamp boxes I built for some friends, don't even have a circuit board. The bypass cap is soldered on the backside of the chip,the output cap soldered directly onto pin 5, and with a small piece of solid wire, I soldered the input pin directly onto the 1/4" input jack.

Also, I use the 3 pin 1/4" jacks, wire the 9V battery to the 3rd pin, so when they plug in, it powers up. That eliminates the need for a switch. Most of your stomp boxes are wired this way.

joecool85

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Re: Build Your Own LM386 Preamp
« Reply #19 on: October 08, 2006, 08:06:12 AM »
Also, I use the 3 pin 1/4" jacks, wire the 9V battery to the 3rd pin, so when they plug in, it powers up. That eliminates the need for a switch. Most of your stomp boxes are wired this way.

Cool, thats how I always wire my stompboxes as well.  It just makes sense.
Life is what you make it.
Still rockin' the Dean Markley K-20X
"New" amp: Fender Frontman 25 DSP (FM25DSP)
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willy1usa

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Re: Build Your Own LM386 Preamp
« Reply #20 on: November 04, 2006, 10:14:58 AM »
Friend of mine had some LM380's, and I've been playing around with them. They are just as simple, and as low parts count as the LM386, but more power. And they accept the guitar with no problem. The data sheet I have says it's capable of 4 Watts. So it works as a preamp, and would also work as a small practice amp.There's a very noticable difference in power of the 386 and 380. I've just built one, and it really sounds good thru an 8" speaker. (in the tiny practice amp category of course)

My friend was a lucky dog - caught a Radio Shack moving to new location, and they sold their inventory to keep from moving it. He don't want to sell any of it, but he did give me a few pieces.

doug deeper

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Re: Build Your Own LM386 Preamp
« Reply #21 on: December 15, 2006, 01:07:16 PM »
if you use the 386 as a pre you might want to add a buffer as well,
i input inpedance is fairly low if i recall correctly.

ildar

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Re: Build Your Own LM386 Preamp
« Reply #22 on: December 15, 2006, 05:23:38 PM »
if you use the 386 as a pre you might want to add a buffer as well,
i input inpedance is fairly low if i recall correctly.


Actually, the circuit is almost a buffer already. It might be a good idea to put a pair of 1Mohm+ resistors at the input, (voltage divider?) one to ground from input and one to V+ from input. This is a Jack Orman idea, and will set input impedance.
« Last Edit: December 17, 2006, 12:42:57 AM by ildar »

teemuk

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Re: Build Your Own LM386 Preamp
« Reply #23 on: December 16, 2006, 02:01:55 AM »
Quite a lot of confusion here... At least for me...

I think it might not be a good idea to put a pair of those resistors to input: This is effectivily loading the supply with 2 ohm load! Besides, I can't understand who'd wish to have an input impedance of 0.5 ohms. I'm sure Jack Orman has meant 1 Megaohm resistors or something.

Internal input impedance of LM386 is 50 kilo-ohms. Any input resistor from signal path to ground is in parallel with it. The recommendation of using an external buffer is very wise.

The power of LM386 can't toast anything if you use the chip to drive high impedance loads. (P=UI) If the quoted output power of the chip is half watt to eight ohms then the output power is something pretty small for typical input impedances (few kilo-ohms and up).

ildar

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Re: Build Your Own LM386 Preamp
« Reply #24 on: December 17, 2006, 12:45:40 AM »
Yeah, I meant 1Mohm or higher, edited my post above to reflect that.
This circuit isn't being used to drive a speaker, in this discussion it's being used as a "preamp", but compare it to the buffers on Orman's site. In one case, the only things missing are the input resistors. If all you're looking to do is set input impedance, then that's all you should need to worry about. I would think that putting a buffer ahead of this would lead to the necessity of tweaking the circuit, there'd be a fair anount of gain on tap.
« Last Edit: December 17, 2006, 12:49:59 AM by ildar »

doug deeper

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Re: Build Your Own LM386 Preamp
« Reply #25 on: December 17, 2006, 02:51:50 PM »
i buffer wouldnt add any gain, just raise the input impedance, and make it a lot more pedal friendly (among other things).

teemuk

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Re: Build Your Own LM386 Preamp
« Reply #26 on: December 17, 2006, 06:35:32 PM »
I guess I have to repeat myself: "Internal input impedance of LM386 is 50 kilo-ohms. Any input resistor from signal path to ground is in parallel with it."
This is fixed input impedance. If you don't take my word for that see the datasheet. Putting more resistance to the input will just lower the input impedance even further. Thus need for external buffer.

Regarding those input resistors: The inputs of LM386 are internally fixed to about zero volt bias. If you try to tie the input to half supply potential externally you disrupt the operation of the chip. If I'm correct, in such occasion it will have a negative gain. LM386 is not an opamp!

I don't know why some people refer to LM386 as a buffer. This chip has an internal gain set minimum of 20. Buffers are unity gain.
« Last Edit: December 17, 2006, 06:46:12 PM by teemuk »

doug deeper

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Re: Build Your Own LM386 Preamp
« Reply #27 on: December 17, 2006, 07:17:22 PM »
well said!

 

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