Monday, 5 May 2014

LM386 Stereo Amplifier

I have etched the PCB for the LM386 Ruby amplifier which I talked about in the previous post.  I haven't however populated the PCB as I got distracted!  That happens a lot...The reason I designed and built the amplifier was to provide a simple amplifier for a friend's project....turns out he wants to drive two speakers (Stereo) with mono sounds....and not a single speaker as I had originally understood.  Not to worry all of the previous work won't go to waste and it was development time well spent.

All we have to do is double up the circuit and combine the volume control and gain controls and we have a stereo amplifier.  It will only be capable of 1W output but that should be more than enough for our purposes.  So in order to combine the volume and gain controls we can use dual ganged potentiometers or variable resistors.

Dual gang Potentiometer or variable resistor
 I also decided that if we aren't going to use the amplifier as a practice guitar amplifier then the gain control potentiometer can be removed to save costs - being able to manipulate the gain and overdrive the speaker is only really something guitar players like - MP3s sound horrible if the speakers are being overdriven.  I replaced it with a 10uF capacitor which fixes the gain of the amplifier at 200 times whatever the input is up to 1W Pk to Pk or 0.707 r.m.s.   It will be loud enough....I have tested it!!!

Here is the new Schematic:

Stereo LM386 Amplifier
Next I designed and laid out another PCB...I seem to be doing that a great deal these days....Hopefully I'm getting better at it.  I must use some surface mount components sometime to get with the times although I find through hole manufacturing much easier.  If I were getting PCBS professionally made I may well swap to surface mount.  Here is the new PCB layout:

Stereo LM386 Amplifier top layer

Stereo LM386 Amplifier bottom Layer



I also rendered the design to show how the populated PCB will look when it's been finished.  Here is how it might turn out....I left the dual gang potentiometer off as I intend to solder those on with wires. Here is the render:

Isometric view of how the PCB will look when populated

Top View of the populated PCB
I then went ahead and etched the PCB using the toner transfer method:
  • Print out the reverse image not scaled of the bottom layer on 120 GSM matt paper with a laserjet printer.
  • Using a suitable piece of copper clad PCB and a clothes iron transfer the PCB image to the copper clad PCB.
  • Soak off the paper using water and an old toothbrush to complete the transfer.
  • Etch off the excess copper using a suitable etchant (ferric chloride) in a tank.
  • Clean off the excess ink and lacquer the PCB if available (helps protect the PCB)
  • Drill the holes for the components using a 0.8mm and where necessary a 1mm drill bit and a dremel.
I then populated the PCB with the components in the following order:
  1. Solder the Audio jack connector in the holes indicated by the label U1
  2. Solder the 1M5 resistors (Brown, Green, Green, Gold) in the holes indicated by R2 and R5
  3. Solder the 3k9 resistors (Orange, White, Red, Gold) in the holes indicated by R3 and R6
  4. Solder the 2N5457 Field Effect Transistors (T0-92 three D shaped black component) in the holes indicated by U$1 and U$2.
  5. Solder the 470nF capacitors (yellow component with No.473) in the holes indicated by C1 and C9.
  6. Solder in two wire links in the holes where the two red lines are on the top layer diagram
  7. Solder in the two 10uF capacitors in the holes marked C4 and C10 - check polarity.
  8. Solder in the two 8 pin IC holders in the holes marked IC1 and IC2.
  9. Solder in the two 100nF capacitors (yellow component with No.104) in the holes indicated by C2 and C6. 
  10. Solder in the two resistors (Brown, Black, Brown, Gold) in the holes indicated by R1 and R4
  11. Solder in the two 47nF capacitors (yellow component with No.473) in the holes indicated by C3 and C7.
  12. Solder in the two 220uF capacitors in the holes indicated by C5 and C8 - check polarity.
  13. Solder in the three 5mm screw terminal connectors indicated by JP1, JP2 and JP3.
  14. Solder in the 100uF capacitor in the holes indicated by by C11 - check polarity.
  15. Solder in the DC Jack connector in the holes indicated by CN1.
  16. Solder wires to the dual gang potentiometer (use a different colour for each pin)
  17. Solder the potentiometer wires to the holes indicated by JP4 and JP5.  Ensure the middle pin of the potentiometer goes to the middle hole, and top to the top and bottom to the bottom.  Do not crosswire the connections!
Once all the components have been soldered in correctly before fitting the LM386 integrated circuits it is a good idea to apply power and check that the correct voltages are present on the correct pins. Apply 9V -12V from a suitable power supply or battery using the screw terminals on JP3 or via the DC Jack connector and ensure that this voltage can be measured on pin 6 of both integrated circuit holders using a multimeter set to measure DC voltage.  If the voltages are present then the circuit should work as anticipated.  If not check the soldering and check again.

Finally insert the LM386 integrated circuits with the pin 1 facing the audio jack socket.  Now it's time to connect some speakers.  Any 8 ohm speakers will be suitable for use with this circuit.  Connect the +VE speaker wire to the inner connection on the 5mm Screw terminals JP1 and JP2 and the -VE connection goes to the outer connection.

Finally attach a suitable audio source (MP3 player, mobile phone or Raspberry PI) to the audio jack input.  Connect a suitable power source as before and play a sound file or an MP3 of some kind...If nothing is heard increase the volume using the dual gang potentiometer.     

Hopefully you will now be enjoying some sweet amplified sounds!  Here are some pictures of my version fully populated:

The circuit fully populated and ready to go!

Fully populated PCB!

Here is a short video showing the Stereo Amplifier in action - Enjoy:


Well that is about it for this post - Next post I think will be on designing an enclosure for this amplifier and testing the LM386 Mono amplifier, take care people - Langster!