Electronic – Help in designing a speaker driven by an LM386

amplifiermicrocontroller

I'm trying to design a circuit which produces a 17KHz pure sine wave from a speaker using the following components:

PIC16F1783 (microcontroller with DAC)
an LM386 amplifier and an 8-Ohm 1W speaker.

The power supply to the LM386 is 9v.

Since I have many constraints I'm not quiet sure about how to do it. I'm able to create the correct values in the DAC but then I'm not quite sure how to feed the LM386.

It says that the minimum gain is 20, does that mean I have to create a voltage divider before I feed the the amplifier? (1v*20=20 > 9v).
Is the network (0.05uF,10Ohm) at the LM386 output described in the typical application in the datasheet necessary?
Can the amplifier deal with 17Khz?
Should I put a decoupling capacitor in the output?
Should I put another resistor, or is the 8-Ohm speaker enough?

As you can see I'm pretty lost 🙁
thanks for the help!

Best Answer

First, you WILL need to put a divider, or an amplifier with a gain of .05 before the LM386.

The minimum parts schematic you reference in the datasheet will work for you. The part has a DC offset of Vcc/2 at the output. The purpose of that matching network is two-fold. First, it keeps the DC contained to the output pin, away from the speaker. The second function is matching. Follow that schematic and you should be fine. The only thing to add that isn't on the schematic is a decoupling capacitor at the power pin. If you don't add one, you will get some very weird artifacts at the output.

The part will do 17 kHz easily, and can go out over 100 kHz.

I would try to find 12V to power it though. The part was originally designed for automotive use.

Related Solutions

Electronic – Maximize performance LM386 and 4Ohm speaker

Filter the signal before the amplifier, not after it.
If you overdrive the signal, it will be distorted.
LM386 is good when you don't much care about audio quality or power or efficiency. If you do care about any of those things there are modern parts that do a lot better.

Electronic – Frequency shift

In principle, DDS should be easy to achieve in software - fractional accumulator, then lookup sine or cos for each tone you want to generate, then sum. On a PIC, that's going to be a tight loop, though...

I am assuming a 256 element LUT containing a complete cycle of a sine wave.

Each sample period , for each frequency, we add a fraction to the position of its pointer in the LUT, and take the sample at the (integer) part of the position. To illustrate how simple this can be, I played with a spreadsheet:

Fs  =   80000   
Fout=   16400   16700   17000
Fout/Fs 0.205   0.20875 0.2125
*256    52.48   53.44   54.4
integer 52      53      54
rem     0.48    0.44    0.4
*256    122.88  112.64  102.4
rounded 123     113     102
actual  16400.146   16700.439   16999.511

The actual DDS operation translates into pseudo-assembler (not PIC!) as

add acc1L,123
adc acc1H,52
lookup LUT,acc1h
mov sum, lookup result

add acc2L,113
adc acc2H,53
lookup LUT,acc2h
add sum, lookup result

add acc3L,102
adc acc3H,54
lookup LUT,acc3h
add sum, lookup result

out DAC,sum
wait for next sample period

which will expand considerably from what I remember of PIC assembly language, but is starting to look feasible.

Remember that the values in the lookup table must be scaled down so that the addition result in "sum" will not overflow.

Best Answer

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Electronic – Maximize performance LM386 and 4Ohm speaker

Electronic – Frequency shift

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