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LM386N-1 Guitar amp- get feedback/radio hum/crackle when run off AC/DC power converter...? Answered

Hi

Last year I built a mini guitar amp using the LM386N-1 IC and I followed these instructions/ schematic and modified it a little with the help of the author.

https://www.instructables.com/id/Make-a-Sweet-Portable-Guitar-Amp/

I integrated a switched DC plug so that I can run it from a 9V wall supply. When I use it with the wall wart I get a radio hum and crackle feedback that I do not get from the batteries. Is this because of the type of transformer? I've read something about audio grade power supplies before but cannot remember the ins and outs.

Does anyone have any insight into the cause of this humming?

Thanks

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steveastroukBest Answer (author)2012-07-18

Its coupling in noise from the mains. Put a proper regulator on the board, and a substantial capacitor across your input - a 1000uF, 2200uF kind-of-thing.

Start with that, it may be enough, it may not, but its an essential first step.

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mdog93 (author)steveastrouk2012-07-18

What kind of regulator do I need?

When you say a capacitor 'across' do you mean like this:

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steveastrouk (author)mdog932012-07-18

No, that's in series: I mean in PARALLEL.

An LM 7812 would work here as a reg.

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mdog93 (author)steveastrouk2012-07-18

Ok, wouldn't I use LM 7809 since its a 9V supply. Or would a LM317 work too?

What I'm worried about is firstly it's going to take up some space but also don't these need heatsinks?

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steveastrouk (author)mdog932012-07-18

The 386 will handle 12V easily, and the chip' s easier to find. If you have a 317, of course you can use that - put a capacitor across the higher valued resistor, to improve the noise rejection.

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mdog93 (author)steveastrouk2012-07-18

I've taken a look at the 317 datasheet and think I've worked it out right. Can you just check i've got it right:

A website states: R1 should be 100 to 1000 Ohms, they said 240 is typical but I don't have any in stock but do have 330, so I'll use that...I want roughly 9V ouptut ( or 12V you said, was that just because of ease of getting the other IC, or is there a benefit by using 12V instead of 9V)

Vout= 1.25 * ( 1 + R2/R1 )

therefore:
R2 = R1* ((1.25/ Vout) -1)
so: 330 * ((9.00/ 1.25 -1) = 2046
But since that's not a common resistor value I'll use the next highest one I have which is 2.2k.

Sub that back in: Vout= 1.25* (1+ 2200/330) = 9.58V, which for the LM386N-1 is well within it's range.
And you say put a cap across the 2.2k resistor. What value would you suggest for this? I've taken a blind guess at 0.1uF there...

Does everything look correct here in this diagram?

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steveastrouk (author)mdog932012-07-18

Looks good. You've done the math right, AND you've estimated the resistors correctly, AND you've got the cap across R2 in the right place. I'd make it bigger though, if I were you.

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mdog93 (author)steveastrouk2012-07-19

What sort of value? Ceramic or electrolytic ? And would the 1000uF cap out mentioned earlier be before the input or at the output of this part??

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steveastrouk (author)mdog932012-07-19

Use a ceramic, or better still a polyester.

Put the 1000uF on the INPUT to the 317.

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mdog93 (author)steveastrouk2012-07-19

so what value resistor are we looking at for the 317?

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steveastrouk (author)mdog932012-07-19

What you said. 330 Ohm, 2k2

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mdog93 (author)steveastrouk2012-07-19

sorry meant cap. what value cap should I try across the 2k2

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mdog93 (author)steveastrouk2012-07-19

Great, thanks. Does the lm317 need a heatsink?

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steveastrouk (author)mdog932012-07-19

Barely, for the load you're using - but yes, add some kind of aluminium plate.

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mdog93 (author)steveastrouk2012-07-20

On a side note, you'll be able to explain this i'm sure, I've been tryin to learn about audio electronics recently and now understand better what things are used. One thing i learnt is that potentiometers used in audio should be log, so why is a linear pot used here?

Are log pots only used for volume controls?

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steveastrouk (author)mdog932012-07-20

Yes. All our human senses have logarithmic sensitivity.

Looking at the circuit, THAT pot should be log too.

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mdog93 (author)steveastrouk2012-07-20

ok i may try changing that out then- the original instructable called for a linear pot- could explain why i hear a sudden burst of distortion as you turn it up.

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mpilchfamily (author)2012-07-18

Yes the problem your having is noise from the power source. You may want to try adding a filter capacitor to the power input to see if that helps. Adding a Toroid to the power cord may also help.

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mdog93 (author)mpilchfamily2012-07-18

What would the toroid do?

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mpilchfamily (author)mdog932012-07-18

Helps to shield any RF interference on the line.

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mdog93 (author)mpilchfamily2012-07-18

Can you put a link to a suitable example, i've never used one before. is it like a choke?

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mpilchfamily (author)mdog932012-07-18

Just pass the power cord through one of these and your all set. Put one turn through it is you want. This will filter out any outside RF interference that may be coming through on the power line.

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user

It sounds like he's getting mains, not RF though.

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