Audio amplifier: LM386

[zachtheterrible]

Hi there. I'm planning on building this receiver here: **broken link removed**. It doesn't have enough power to drive headphones so ive gotta add an audio amp. i have one question about this though: Why is there a - and a + input?

Also, is this a good audio amp for this circuit?

 

[zachtheterrible]

Please?? Someone??

 

[plot]

OpAmps can take a positive and a negative power supply, so they can run at an AC level, not just a DC level like normal IC chips. In other words, instead of putting out a 0 or 5 volts like normal IC's, an OpAmp can put out a positive 15 volts and negative 15 volts if it needs to. In order to do this however, it needs a negative power supply to drive the negative side.

Now... just becuase it has the option to put in a negative voltage, you can just stick a ground there... for audio, this isnt recommended, becuase for audio, you do have negative voltages.

There are plenty of ways to use the same power supply to supply both a positive and a negative voltage.

 

[zachtheterrible]

I thought that ground is the same as the negative side of a battery. So you put the signal into where it says + on the opamp and the negative where it says - on the opamp.

 

[plot]

Yes, you can do it like that.

The output range of the OpAmp with that settup will be 0-12 volts. So hook the + of the opamp to the + of the battery, and the negative of the opamp to the - of the battery.

Now... if you are asking about the signs on the inputs of the OpAmp... well, OpAmp's are funny little things, and kinda hard to understand unless you know alot of electronics. Normally EE's take a class solely dedicated to OpAmp's, so i can't really explain specifically to you how they work in the short period of time I have, but basically, the + and the - are the inputs, just hook them up where the schematic says too.

 

[Oznog]

Speakers require AC, they can't handle DC in the signal.

Most simple circuits just use a capacitor to filter out the DC in the signal. So you may have a 12V supply, the audio output waveform might be 5v to 7v, but the cap will filter it to the speaker sees +1v to -1v.

Understanding op amps or the LM386 (which is somewhat different than normal op amps) is good but you can also rip off plenty of schematics from the web. There's hundreds for the LM386 alone.

 

[zachtheterrible]

Hit me with it!! I know quite a bit about electronics, ive been studying and reading books and stuff on them for about 2 years. What I read is that one input has a phase shift of 180 degrees and the other doesn't. Is that true? If it's too hard to explain maybe someone knows of a good site that explains them. I fully understand a simple transistor op-amp, so I know about how the cap filters the DC out of the signal and all that.

Also, is this a good amplifier for this circuit? **broken link removed**

Thanx :lol:

 

[zachtheterrible]

I went here **broken link removed** and read up about opamps. Pretty simple. The only thing that I don't get is why is the - and + called inverting and non-inverting?

 

[plot]

I went here **broken link removed** and read up about opamps. Pretty simple. The only thing that I don't get is why is the - and + called inverting and non-inverting?

well, OpAmps run off of feedback, and negative feedback (-) is called inverting becuase it flips the signal, or does that 180 degree's phase shift. With positive feedback (+), you get non-inverting, in otherwords, there is no phase shift.

 

[samcheetah]

well zach, basically an operational amplifier is a differential input, single ended output high gain DC amplifier. that means it has two differential inputs and a single output. one of the input is inverting, which means that the it produces an inverted output. and the other is the non-inverting input, which means that the output produced due to it is of the same phase. the input stage of every op amp is a differential amplifier succeeded by another one or two differential stages and amplification stages. an ideal op amp has infinite input resistance and zero output resistance. it has a high CMRR (common mode rejection ratio). the opamp is used with feedback to give a number of useful circuits.

the best reference for opamps on the web is "Opamps for everyone" on TI's website **broken link removed** there are many documents for opamps on TI's website. i would recommend that u read them.

 

[audioguru]

Hi Zach,
The LM386 is not an opamp, but is an audio power amplifier with built-in biasing and feedback. It is designed to operate with a single supply voltage (negative is not required) and with its inputs referenced to its ground voltage.

The inverting (-) input is normally grounded, but is available so that you can add a resistor from it to ground to minimize offset voltage if the non-inverting input has a very high source resistance.

Its datasheet (search Google) has many circuits and spec's about min and max supply voltages, etc.

It is perfect to drive a small speaker or your headphones when operating from a 5V to about 16V supply, and its gain is adjustable from 20 to 200.

For applications such as yours, I have also used (clock radio, speaker instead of piezo beeper in alarm clock and headphones amp) an MC34119 bridged audio power amplifier IC, because it doesn't need the large output coupling cap that is needed with the LM386. The MC34119 works on a supply voltage as low as 2.0V up to about 16V.

 

[zachtheterrible]

thanx, ill look into that. the lm386 was gonna be really good cuz it was easily accessible @ radioshack but ah well :roll:

 

[plot]

thanx, ill look into that. the lm386 was gonna be really good cuz it was easily accessible @ radioshack but ah well :roll:

they work basically the same, and will for this application as far as i can tell.

Don't let radioshack components limit you... goto www.jameco.com, www.digikey.com, www.mouser.com, or www.newark.com, and search for something like an LM324 OpAmp.

 

[zachtheterrible]

oh yeah, i hate radioshack now. its just @ least i dont have 2 pay shipping if i go there.

couldnt believe it, u buy two .1 uf caps there for 1$ . . . you buy 150 caps on the net for $8. radioshack is making a KILLING!!!

 

[audioguru]

Hi Plot,
Hi Zach,
Don't use an old LM324 opamp nor its baby sister LM358. It is the very first low-power opamp ever made. To reduce current, its output transistors run without bias, so it produces terrible crossover distortion! Its high frequency bandwidth is very poor and it certainly doesn't have enough output current to drive headphones.
For years I have been using low-noise, low-distortion and FET-input TL071 single, 072 dual and 074 quad opamps for audio circuits. But they need more supply voltage than a 9V battery.
For battery operation from 3V and more, I use MC33171 single, 172 dual and 174 quad low-power opamps. They are pretty good for line-level audio circuits. Their inputs operate down to ground, just like the old LM324.

If you buy 10 or more pieces of parts from those suppliers, you'll get exactly what you need and the savings over the Radio Shack prices add-up to much more than just for shipping. Newark-In-One promises next-day delivery on orders placed before 8:00PM. You click "Send" on your order and hear the ding-dong at your door just a few hours later.

 

[zachtheterrible]

Why is it that i once ordered 100 transistors from someone off ebay, and they came within 2 days, and the shipping was only $1.50? Are there any places that make shipments like this for realy cheap?

I just ordered 3 TDA7000 chips from newark.com. I got charged $6.00 for S&H, and then i got charged like another $6.00 bucks for freight. In all i ended up paying $25.00!! I dunno if i wanna use newark again :?

 

[John Sorensen]

Newark (and Allied) is not really geared towards the hobbiest. Think Mouser or better yet DigiKey.

j.

 

[Roff]

Zach, I'd like to see the receiver you plan you build, but the URL you posted takes me to Harry's home page, and he has a bunch of receivers. Which one is it?

 

[zachtheterrible]

This be it: **broken link removed**

the TDA7000. It's supposed to be really sensitive

 

[zachtheterrible]

Well, now im gettin down to getting all the parts to build my receiver and amplifier . . . but the thing is, i can't find a suitable amplifier using the lm386 (I'm set on using the lm386 cuz i already bought one ). Can anyone recomend a circuit that's really small? the smaller the beter :lol: . I've found plenty that are rather complicated

Thanx