problem with a circuit using 741 and LM386

[drk]

Hi all, im trying to build a small amp using the dist+ circuit as the preamp and the LM386 as poweramp, but got some problems.

when i turn it on, i got alot of noise and some kind of feedback, i've tried using two power sources connected to the circuit, one to power the preamp and other the poweramp, and it kind of works, but the sounds comes really crappy and with lots of noise.

i tried to debug it by having just the preamp connected, and it worked 100%, tried the same thing with just the LM386 and again it worked nicely, it just doesnt work with both at the same time 
another thing, if i connect just the preamp to another LM386 amp that isnt in the same circuit and with a diferent powersupply, it works.

so, whats the problem? im thinking it has something to do with the powersupply, do i need a bipolar PS or something? or i just cant have these two chips on the same circuit?
thanks   

[GibsonGM]

Did you put a .1 uF cap from + to ground on the chips?  Sometimes that will stop the oscillation and noise.   Probably it's the layout...can you post a schematic?  Running long input/output and power supply leads will cause parasitic oscillations oftentimes. 

Also, using a 100ohm resistor in series from V+ to chips where the power enters the board, and having about 1000uF from V+ to ground there can help stop that kind of problem.

But I would suspect parasitics/ground loops.   Try to get a pic/schematic up!

FWIW, the Dist+ will probably swamp the 386, lol.  You can get drive by switching in a cap across pins 8 and 1 on the 386, about 10uF....

[Steben]

Try it with batteries. if the problem ceases, it's definitely a PS filter problem.

[drk]

tried your helps, but still didnt worked  im gonna try other circuit

thanks anyway

[ambulancevoice]

can you draw us your lm386 power amp schematic??? just to see if you have forgotten anything important

[drk]

here it is, from the left it comes the dist+ part, the right is the output


btw, while drawing the schematic i noticed that the 'IN' in the lm386 is minus and in the 741 is plus, probably a problem there??

[GibsonGM]

Nah, the + and - just refer to what you get as an output...if you use the + input, you get a larger signal in phase at the output.    The 386 will put out an inverted signal with that configuration, but that should make no difference.

Have you tried using the Dist + with its volume VERY low going into the 386 amp?  It could swamp the input very easily, I'd think.

Another thing that comes to mind ....are you running a separate wire from 9V to the voltage divider for the 741, and another to the 386 and its filter cap?  With all grounds going to the same place, for a star ground?
Sometimes putting a .1uF cap from the V+ at the chip to ground helps stabilize things, too.   
Could be a lot of reasons for the noise/crud issue there, these are the most common.  I think the 741 might be putting out too much voltage, for another.  Have you tried it with the gain on about nothing?
Keep poking with it, this is a great way to learn! 

[ItZaLLgOOd]

I've been playing around with circuits in front 386's.  Gibson has the right idea.  Even the beginner project NPN Boost kills the 386.  It makes all kinds of noises.  Try putting a Big Muff tonestack in between the two.  It might soak up some juice and make it useable.

[pjwhite]

Is the preamp output capacitively coupled to the power amp input?

[leigh]

so, whats the problem? im thinking it has something to do with the powersupply, do i need a bipolar PS or something? or i just cant have these two chips on the same circuit?

Your thinking was in the right direction here. You don't need a bipolar supply, since the 741 chip in the dist+ is set up to run off a single-ended supply (the input is biased to half-supply voltage).

However, as the previous poster suggested, you do need to AC-couple the 741 to the LM386 - in other words, you must pass the audio through a capacitor between the two chips. This will remove the half-supply DC offset voltage from the 741's output, and make the LM386 input's a lot happier.

This thread is a bit old, but you never posted if you got it working, so there it is.

cheers,
Leigh

[drk]

hi, i've tried for a bit but couldnt put it working

i will certainly try it again some time soon, i just decided i needed to understand a bit more about what i was doing, before trying again

thanks for the answers, when i decide to do it again it will be helpfull