Re-Engineering a Cheap Amplifier Module

How it started

I have a vintage Lafayette stereo amplifier from about 1969 (!) on my workbench for repair. It was manufactured at a time when transistor amps still were in their infancy; its predecessor, in an almost identical enclosure, was equipped with vacuum tubes (a.k.a. valves). In these olden times, germanium transistors were popular. Silicon transistors were hard to find, if at all, and expensive, at that. The other way round, germanium BJTs (bipolar junction transistors) are difficult to source nowadays.

Apart from that, I read somewhere that the early transistor amps were either really low-quality, or really expensive, or both. So I lost hope to bring it back to its original status but resorted to sympathetically modernising it instead.

You can find electronics parts and modules in China for very small money, and in several cases I found the quality to be ok. I did a search on the AliExpress homepage for something that might be useful. I found an already assembled stereo amplifier module, complete with soldered PCB, heat sink, and bass/treble/volume controls, having about the required (rather modest) output power, see the picture above. I guessed that it might even be supplied by the original mains transformer, which is one of the more expensive components in an audio amplifier, and I wanted to re-use it, if possible.

From time to time you also read about fake integrated circuits (ICs) a.k.a. 'chips', sold (or used) by Chinese sellers, among others. To do so, the original label on the chip is replaced by an inscription of a more expensive and/or higher-quality chip. I suspected that it was impossible to produce my amp module with the genuine chips as claimed by the seller for such low a price (CHF 14.27 at the time of purchase, shipping to my home address included). I already had the genuine chips purchased from serious sellers in my junk box, and when comparing their printed labels with the ones of the amp module, my suspicion was more or less confirmed.

I then made my standard mistake of modifying a circuit even before testing it in its original configuration. In this case, I replaced the original chips with the ones that I knew to be genuine. I also removed the rectifier and the smoothing capacitors because the supply circuit configuration did not match the mains transformer to be re-used.

The result of this operation was that the module didn't work as intended. So, what was wrong? We electronics tinkerers (if a little advanced, at least) are completely lost when dealing with a circuit for which we don't have a circuit diagram available, because understanding the function of a circuit from just watching its hardware is very difficult if not impossible.

To find out what was the problem, I had to 're-engineer' the circuit diagram of this amp module, because normally no paperwork at all is included when you buy from China. The AliExpress web site at least provided component layout and external wiring diagrams of the module, which helped a bit, even if the resolution of the component layout diagram was too low to be legible...

Fortunately, this circuit is not too complex, and the reconstruction of the diagram didn't take me an eternity, particularly because I found the same module in kit form with an unpopulated PCB.

In this Instructable I will show you how I proceeded, which may help you if you find yourself in a similar situation. Be warned that this process is usually not very fast, some patience is required.

The unassembled but otherwise identical module - from a different seller in China - was still cheaper (CHF 7.19 incl. shipping). The PCB (printed circuit board) was unpopulated, and the copper tracks on both sides were clearly visible, instead of being hidden by components. If I hadn't found that, I would have had to go the hard way, unsoldering all the components and identifying or even measuring them, if necessary. Thanks to this convenient fact, however, reconstructing the schematic diagram, based on the tracks visible on both sides of the PCB and the component layout silkscreen print, was much easier - but a bit of work nevertheless. See above the component and the solder sides of the PCB.

Here, as in my other Instructables, I can only show you how I did it - there are different ways of reconstructing a circuit diagram from the PCB for sure. If you know one that is easier as mine, please let me know in the comments section.

1. Take photographs of both sides ('solder side' and 'component side') of the PCB (see Step 1). You might also scan them with a flat bed scanner - but in my experience the photographs are nicer to process, even if they tend to be somewhat distorted.
2. Import them into a picture editing software (such as Photoshop).
3. Individually align them so that the edges are exactly horizontal and vertical, respectively.
4. Import (place) these pictures on two different layers in a vector graphics software (such as Adobe Illustrator or Corel Draw); you may want to open two different drawings for the two sides.
5. Re-size these pictures so that their dimensions correspond to the ones measured form the real PCB.
6. Lock these layers.
7. Draw the donuts in black on a next layer, according to the ones of the picture of the component side (1st picture above). As the PCB is double-sided and plated-through, the donuts for the solder and component sides are identical, and they must be drawn only once, since on the solder side they simply are a mirror image of the ones on the component side.
8. Draw the tracks of the component side (2nd picture) on a different layer in a colour different from the donuts. If your PCB has a copper-covered ground plane area on one or on both sides, just draw plain tracks and ignore the large ground plane areas.
9. Duplicate the layer with the donuts, reflect it and align it, on a different layer, with the picture of the solder side.
10. Draw the tracks of the solder side, on a different layer again, in a third colour (3rd picture).
11. You can now hide the layers with the PCB photographs.
12. Reflect the drawing of the solder side donuts and tracks.
13. Place the reflected drawing of the solder-side tracks (with the donuts) on a layer below the drawing of the component side, so that the donuts match (4th picture).
14. Make the tracks transparent by changing the opacity of both track layers to 50% or less, so that you can exactly see and follow each track.
15. You might want to change the colour of the donuts that are connected to the ground plane for improved 'legibility'.
16. Now it would be a good time for checking all tracks - once, or even several times - for completeness.
17. Draw the component overlay on a next layer. In case the PCB silkscreen print is nice enough, you might process it in your picture editor in such a way that you get a black drawing with transparent background that can be placed on a next layer above your PCB drawing. I used the drawing I found on the AliExpress homepage.

In my actual case, no component designators are printed on the PCB, but only the component values. The designators are a convenient means to link the components on the silkscreen print with the ones in the circuit diagram, and in fact no electronics designer does without them, as they will be required for the bill of materials, too (5th picture).

1. Print your PCB drawing as large as possible. It can also be printed in black and white if no colour printer is at hand, because you have the colour picture on your monitor in front of you (1st picture).
2. Trace the PCB tracks using differently coloured pens for component and solder sides, one after the other, and at the same time draw a rough sketch of the schematic diagram, perhaps in the same colours as the tracks (2nd picture). Don't forget the component designators corresponding to the ones in the PCB drawing.
3. Needless to say: If you have more than one channel in your hardware (e.g. a stereo amplifier containing two identical circuits), it is sufficient to draw the diagram of one channel only and then duplicate it; only the component designators need to be updated for the second channel.
4. As you can see from the examples given above, several iterations are necessary for this process (3rd and 4th picture), using a pencil and a lot of eraser - repeat this as long and as many times as it takes to get a nice schematic diagram draft that makes sense to you. Don't forget to cross-check with the PCB drawing again and again.

Once this stage is reached, you have completed the basic tasks - in sports, such as in figure skating, this would be called the 'compulsory' program.

1. From here on it is the 'freestyle’ program, for the sake of completeness.
2. Use a PCB design application (KiCad, in my example) to produce an aesthetically pleasing :-) circuit diagram. Who would have thought that my first sketch might look so nice a while later? (see above).
3. The second channel can then, of course, be (re-)produced by copying and pasting; only the component designators need to be updated.

After having studied my re-created circuit diagram, I got the confirmation that, at least, the label of the 'NE5532' operational amplifier cannot be genuine. A real NE5532 chip consumes a rather high quiescent current of around 8-10 mA; the resistors (R28 and R29) between the positive and negative supply lines and the chip's supply pins are too high to allow such a current to pass. This also is the reason why I measured a supply voltage of only about ±2 V on the chip's supply pins, after substituting it by a real NE5532 from my junk box. This definitely is too low a supply voltage for this opamp. Apart from that, I never saw the 'Texas Instruments' logo look like the one in the first photograph above.

I'm not totally sure whether the power amp chip in the left part of the second photograph is genuine - but there is a distinct difference between that and the real thing on the right...

I also referred to the data sheet of the LM1875 power amp chips. There I found that two diodes are recommended per channel, as a protection against voltage peaks produced by the inductive load some loudspeakers present to an amp; these diodes are included in both the circuit diagram of step 4 (D1-D4) and in the component layout (step 2, 5th picture); they will be added to the solder side of the PCB.

How to Proceed From Here?

I will have to give one or even more thoughts about how the circuit could be modified in order to work correctly with a genuine NE5532 opamp.

If I should manage to make this amplifier module work, together with a modified power supply circuit, in the vintage amplifier mentioned in the introduction, I think I might provide a follow-up Instructable about this next adventure...