Noob Alert - 386 Question

[beliefspace]

Hi Everyone,

I stumbled across this forum by chance and I've been reading it for quite a while.

I have some electronics experience, but have only just got into audio electronics, so I thought I'd throw togther a 386 circuit on breadboard to see what happened - and b***er me! it works!   Thanks to Dragonfly's 'Basic 386 Fuzz' layout.

I'm currently experimenting with gate control (feeding through a 100nF) at the moment.  I fried a 386 earlier feeding it through a 220uF - sounded smooth, but with a s**t load of squealy feedback for about 2 minutes before the 386 packed in .  I also melted a diode by feeding the output through it unprotected - also sounded sputtery and cool as it died horribly Still, I like to experiment and that costs components sometimes - nothing's gone bang with smoke ... yet 

So anyway, silly newbie mistakes aside, I have a couple of questions, if anyone can help it'd be really cool.

1) The 386 'Gain' connections ( pins 1 and 8 ), I read somewhere that if the pot is tuned all the way, i.e. the connections are shorted, the output can damage an amplifier?  Is that true? Should I place a resistor in series with the pot to limit the range?

2) I don't get what difference the diodes make in this circuit...

http://aronnelson.com/gallery/album17/386_FUZZ

...they almost seem to clean up the sound, not clip it.  I'm using the same diodes as stated 1N34A (I've also tried led's, 1N914 and 1N4148) doesn't seem to make any audible difference. (This is a new 386 I'm using now, not the one I destroyed in my earlier experiments)

3) Lastly, can anyone point me in the direction of a schematice involving a 741 and/or a 071.  I get the feeling making them work is a little bit more complex than the 386?

Thanks for any input, help is greatly received.  Sorry for the noob-style questions.

Stu

[petemoore]

  The 386's data sheet should tell all about that chip, except maybe the cap between pins 1 and 8 for a gain mod...one of those little LM386 amps at ROG shows this.

[beliefspace]

Thanks Pete, I've been studying the data sheet and was just thinking about experimenting with the capacitor between 1 and 8 for more gain.

I've figured putting a cap on the output of 100nF or higher will help protect the amp ... and filter some pretty high frequencies out.

As I said before I'm very new to the audio side of things, so any pointers much appreciated - thanks.

Stu

[Alex C]

Hello, and welcome to the forum.   

1) The 386 'Gain' connections (pins 1 and , I read somewhere that if the pot is tuned all the way, i.e. the connections are shorted, the output can damage an amplifier?  Is that true? Should I place a resistor in series with the pot to limit the range?

This should not be a problem whatsoever.  For example, on both the Smash Drive and the Ruby (and other Ruby variants like the Little Gem, and the Big Daddy I think), pins 1 and 8 on the 386 are involved in gain control, both involving the equivalent of a direct connection between pin 1 and pin 8.

In the Smash Drive pins 1 and 8 are connected together all the time, and gain is controlled via a variable resistor to ground (sorry for the big picture):



In the Ruby, there is a variable resistor between pins 1 and 8 of the 386 which controls the gain.  For "full gain," the two are connected directly (with the pot at its maximum rotation):



So you won't be doing any damage by connecting these pins.  There is a higher power chip, the LM3886, which is used for higher-power amplification purposes, and which has different internal circuitry and a different pinout; is it possible you got the two confused?

2) I don't get what difference the diodes make in this circuit... http://aronnelson.com/gallery/album17/386_FUZZ

Neither do I, and so I will leave it to someone else to answer this one.

Here is a PDF of the internal connections of the LM386: http://www.national.com/images/pf/LM386/00697601.pdf  . 
Pins 1 and 8 are not your typical feedback loop, and the diodes aren't placed between signal and ground, so I don't really know what their function is. 

3) Lastly, can anyone point me in the direction of a schematice involving a 741 and/or a 071.  I get the feeling making them work is a little bit more complex than the 386?

Here is a link to the MXR Distortion +, the classic 741 circuit:
http://www.generalguitargadgets.com/diagrams/dist_plus_sc.gif
Also here: http://www.christianmusicweb.com/schematics/mxr_distortion_plus.jpg
The DOD OD250 is essentially the same circuit.  Tonepad has this schematic and a layout for the MXR Dist+ or the OD250.  The ProCo Rat is a more complex 741 circuit (EDIT: OR SO I THOUGHT; LOOKS LIKE I WAS WRONG), also available at Tonepad.

I hope that helps. Do some reading, searching, and then ask any questions you might have.
Anything you ever wanted to know about effects can be found here:  www.geofex.com
And here: http://www.diystompboxes.com/cnews/FAQ.html
And here: http://www.geofex.com/effxfaq/fxfaq.htm
And here: http://www.diystompboxes.com/smfforum/index.php?action=search

Again, welcome, and have fun!

-Alex

[beliefspace]

Thanks for taking the time to post all that info Alex, it's really appreciated.

Looks like I've got some more reading and experimenting to do

Stu

[Peter Snowberg]

Welcome to the forum.

The path between pins 1 and 8 is meant for AC only so there should be a capacitor in series with any connection between these pins. There are several schematics that omit this cap, but the clean and distorted sounds are both much better to my ears when the cap is there. Any value over 10uF should be fine with 22uF being fairly ideal.

[beliefspace]

The path between pins 1 and 8 is meant for AC only so there should be a capacitor in series with any connection between these pins. There are several schematics that omit this cap, but the clean and distorted sounds are both much better to my ears when the cap is there. Any value over 10uF should be fine with 22uF being fairly ideal.

Ah, cool.  Thanks Pete

I noticed that if you short the pins 1 and 8 and add a gate control (pin 5 to 3 + 4 gnd through a pot and 100nF cap) , the IC starts to get quite hot, but with the cap between 1 and 8 it doesn't.  So I have a 10uF there at the moment, I'll give the 22uF a try later.

I also built an astable yesterday from a 555 and rigged it to the other inout of the Op Amp in the 386 creating a kind of distorted tremelo effect.  Was interesting, but not ideal as the 555 is so noisey. Just experimenting was good though.

Stu

[David]

Stu:

Although it's represented with the same graphic in a schematic, a 386 is not an op-amp.  It's actually a small audio amplifier.  The connections are vastly different from those on an op-amp.  You can only substitute in very limited situations. (You can stop twisting my arm now, Peter!)

[Peter Snowberg]

Actually, in some very limited situations you can sub a 386 in place of an opamp if you monkey with the bias. The overall topology is very similar.

[beliefspace]

Although it's represented with the same graphic in a schematic, a 386 is not an op-amp.  It's actually a small audio amplifier.  The connections are vastly different from those on an op-amp.  You can only substitute in very limited situations. (You can stop twisting my arm now, Peter!)

That makes sense.  That also explains why it's easy to connect - a very good place for me to start my quest.

I noticed the pin out on the 386 compared to the 741 or 071 is totally different.  I've been reading up on biasing.  I gather the 386 is already biased to ground

I'm coming to the end of my experiments with the 386 and am eager to learn more, I'd like to start building some circuits using the 741 or 071 next.  I think Dist + circuit that Alex linked me too might be a good place to start. I like to understand what's going on in each part of the circuit.  As opposed to just copying something, I think that's why I'm a fan of proto-board (breadborad) so I can experiment as I go.

Thanks for all your help and pointers so far, it's very, very much appreciated 

Stu