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OP-Amp Troubles Answered

hey everyone I was hoping someone here could help me with a problem in having. I've been tinkering with some op-amp circuits with the LM386N-1 namely the one I have pictured. I first assemble them on my prototyping station and they work well using the signal generator but when I connect them to a audio line from my computer nothing comes out the speaker I have tested the cable and speaker and they both work so I am at a loss as to what's not working. I would appreciate any help you guys can give me thanks

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user
PKM

10 years ago

I've never seen an Op-Amp used like this- I thought they were pretty much always used in negative feedback setups, especially for stuff like audio amplification. Check out this WP article or this page for more about negative feedback. IIRC negative feedback is a lot more stable and predictable in its response than just potential-dividing the signal voltage down as it appears you are doing. I don't know this particular chip so can't comment on your exact circuit.

Remove the 10uF from pins 1 & 8. Gain of 200 is waaay too much for a line level input to this chip. See if you can get it to work with gain at 20 (pins 1 & 8 open). If you should need to increase the gain you can tweak that after the circuit is working. Put that 10uF cap ahead of the volume control at Vin; this will protect your sound card and LM386 circuit from any DC potential between the two. The cap on pin 7 and the snubber circuit on pin 5 can be left alone - they're either helpful or harmless as they are just there to provide stability. I'm not sure if you're dealing with stereo signals (virtually all sound cards are), but in any event make sure that your connections are correct from the sound card.

k thanks I just took out my electronics stuff to build a small lm 317 based bread board power supplie ill have to try and put this circuit together ill have to do some research though because I have hundreds of different op-amps and audio amps thanks to free samples from ti, fairchild and the like

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user
gmoon

10 years ago

After you recheck everything based on the good advice offered by the others (and your component values and capacitor polarities), then... -- What's your power supply? 5 or 6V isn't gonna cut it, probably. 9V maybe, 12V better. -- Try adding an input coupling cap, between the volume POT and the non-inverting input (pin 3.) The value could be anything from 2.2uF to 0.01uF. -- Remove the 'tone control' (cap and resistor connected to the output), or substitute a 0.01 or 0.005 uF. The results might be too 'bright,' but it's worth a try.... -- If you have anything connected to pin 7, remove it. -- If all else fails, compare the datasheet circuit to some practical ones on the web.

i got a lot of dip8 op-amp and audio amp sample chips in the mail today to play around with on this ill remove my cap from 7 (10pf i just stuck on a random disk to ground like in the pic)and try a couple disk and electrolitics on mu input and i have my power supply at about 10 volts i dident want to go up to 12 because its the chips maximum and i want to avoid frying it thanks for all your help guys ill let you know if i get it to work

Oh, and substitute a 100K or 250K pot for the 10K at the input. Higher input impedance and less attenuation might help....

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user
VIRON

10 years ago

It should work, except you need a 10uf cap in the input.

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user
westfw

10 years ago

Did you connect the audio ground from the computer to the ground of the amplifier circuit? BTW, the 386 is an audio amp, not an op-amp. Although I'm not sure how they'd be different for this case.

I tried it both ways with the input and output gounds conected to the chip and straight from my comp to the speaker

. hmmmmmm If it worked with the signal generator and you have the grounds right, then all I can think of is the input level is way too low.