LM386 amp circuits

[Martin Bush]

Not sure if this is in the right place, do move it if not.

Following a discussion on another thread I bought a couple of LM386 amplifier kits for about £1 each. I managed to assemble them without instructions (for me that's an achievement) and worked first time. So, all good.

However the volume seems to be all at one end of the pot, what you might call towards the 0 end. So I am curious as to why. Go beyond a certain point and it's all distortion. I may give one to a friend with a guitar to see what it's like as an effect.

But, my main question is whether people have any favourite circuits for using the LM386? I want to build an amplifier on veroboard that will give pleasing results using a CD phono in.

I know there's many circuits out there on the web, but I value your views.

[Orakle42]

There are many published circuits out there for the 386 including those in manufacturers data sheet, however, a quick solution to your current problem would be either a log pot or change the value and add an input series resistor but take care with the layout if you are using a high gain configuration...

Orakle42

[paulsherwin]

Yes, I think your source is overdriving your amp given the gain configuration. The manufacturers' datasheets explain how to adjust the gain (very easy) or as said you could attenuate the input with a resistor before the volume control.

Any amp should normally use a log pot for a volume control. A linear pot will work, but will produce some of the symptoms you have described.

The LM386 is a useful little chip but will only produce a few hundred mW of output (it depends on the supply voltage). If you drive it above this it will indeed clip and distort.

[David G4EBT]

I've used a number of these little amplifier units - so cheap that it rather deters from home-brewing, other than for interest. I think a possible reason for the volume control behaving as it does, is that for some reason, a lot of these units use a linear pot for the volume control when it should of course be logarithmic. The same is so for that very useful Velleman MK7000 signal Tracer/Injector kit, which uses two linear pots for both the injector level and the tracer volume.

As to distortion, I've attached some traces of a cheapo Chinese 386 amp module - not the SMD type with a miniature pre-set pot for volume control - the one with a volume control as shown in the third pic. The first pic shows a 20kHz sine wave with an input of 0.1V p-p and an output of 4V5 p-p and no distortion - what's not to like about that? The second pic shows that at 6V p-p output it's driven into distortion. Well it would do, wouldn't it?

As to suitable circuits for simple audio amps and pre-amps, I think the definitive article was that published in EPE magazine in 2002 by R.F. Haig (Of R.B. Wobbulator fame). A splendid ten-page article with simple audio circuits and PCB layouts (could be made on strip-board for those without PCB facilities). The circuits are explained, and there are tips and diagrams on how to connect the stereo output of MP3 players etc.

EPE have now kindly placed these circuits in the public domain. I built some of them back in 2002 and they worked fine. The article can be downloaded from here:

http://www.epemag3.com/lib/free_proj...20Part%202.pdf

A lot's happened since 2002, with e-bay and cheap well-made electronics from China, so that apart from the sheer enjoyment derived from building such projects, and the skill and experience gained, it makes no economic sense, except for old buffers like me with a loft full of bits and bobs squirreled away over five decades, so it's more of less a no-cost option.

Hope that's of interest.

[60 oldjohn]

Try adding a resistor, about half value of the track from the centre slider to the earthy end of the track. This should turn the Lin into a Log pot and works for a lot of circuits.

John.

[julie_m]

If there is a capacitor across pins 1 and 8, you can disconnect this to reduce the gain. If you then don't have enough gain, adding a resistor in series with it will give you an intermediate amount of gain. Moving the negative end of the capacitor from pin 8 to pin 4 will give you even less gain.

[Martin Bush]

Thanks all.

All useful stuff. I think, as long as I can work it out from the PCB, Julie's tips will be my first port of call simply because I can visualise it most readily. However these kits are so cheap, and I have a pile of other components, so i will be able to try all of what you've suggested.

This is great fun and at about my level

[paulsherwin]

You can learn a lot by playing around with these things, and probably more importantly gain confidence.

[Restoration73]

1nF cap across pin 3 & 4 stops RF breakthrough etc. You can obtain more gain using
positive feedback without making it into an oscillator. LM386N-4 has a higher output.

[Martin Bush]

Thanks again for the feedback.

Is there a 'good' source for these ICs? I have been buying from eBay And the prices are almost pence per item. But I did wonder about the quality - is this an issue?

Not that I am trying to make. Hi-fi but at the stage I am at how would I know a duffer?

[paulsherwin]

Fake or substandard components are a problem, but commodity parts like these are rarely affected because they aren't worth faking. The cheap Chinese ones on eBay will be locally manufactured and any manufacturer's logo should be treated with scepticism, but they are generally fine for hobby use. I would be a bit more careful about using them in a commercial product, and certainly in anything medical or otherwise safety critical.

Always check the eBay seller's feedback, reading the comments as well as the bare ratings.

Always use IC sockets for homebrew projects so that you can swap parts easily.

You are less likely to buy fakes if you buy from the big UK or US distributers like CPC or Mouser, or even Maplins for the common stuff, but it's not unknown.

[Valvepower]

Hello,

I recently designed a commercial product using the low power JRC NJM2073D audio amplifier. I was pleasantly surprised just how good it was.

The NJM2073D is available from Rapid Electronics.

I’ve seen complete projects on eBay using this IC http://www.ebay.co.uk/itm/Stereo-0-7...-/150665395757

Terry.

[MrElectronicman]

I recently made a couple of amps with these IC's. You really need a log pot for the volume control. As already stated a 10uF cap between pins 1 and 8 will give you max gain, 200 as I recall, but adding a 5K preset pot in series with the capacitor allows you to adjust the gain to suit.

[Martin Bush]

So, l may as well confirm my ignorance here, how do I tell if the pot is log or Lin?

Is it as simple as it being marked on the component? I've seen that on pots in my radios, but with the kits I've had from China I'm never sure everything is fully and accurately marked up.

[paulsherwin]

They are usually marked on the side. You can also tell with a meter - if you position the pot at the halfway position, the two resistances will be roughly equal for a linear pot, but very different for a log pot.

[Martin Bush]

Hope this isn't too far off topic, but I also wanted to ask about combining L and R phone in to make one mono signal for the LM386.

What is the best way of doing that?

My old 'twist both wires together' technique won't cut it here I don't think...

[mhennessy]

Two resistors between the L and R signals and the top of the volume control. Value isn't critical - 1k or more, I'd say.

These resistors will form a potential divider with the volume control track, meaning an amount of signal will be lost. This could range from "insignificant" to "useful", depending on the value of resistors you pick. From what you said earlier, some loss might be useful? Though it's better to reduce the gain of the LM386 rather that "attenuate then amplify" as a broad principle, sometimes it's easiest to be pragmatic

[Martin Bush]

Thanks.

I am learning as I go. Back when I played guitar and employed the crudest form of 'electronics' - ie converting stereo to mono by cutting off the stereo jack, bunging the two signal wires together and blobbing it onto a mono jack - I'd never considered there was more to it than that.

Those were great days, but I'm trying to do things the right way.

So, if I've got it right, the resistor method is the right and proper way regardless of the need to sort the amp out. Is that right?

My idea is to work with a standard input and speaker then work on the amp - trying some different designs until I get an acceptable combination.

[mhennessy]

There's a diagram on this page: http://electronics.stackexchange.com...-mono-with-aop

That should get you started. Though R4 isn't needed in your setup - R3 is the existing volume control.

Give it a go - best way to learn

[MrElectronicman]

Some of the pots have a label with either an "A" for Audio (Log) or "B" for linear