Believe or not, never in my life had I the chance to use a solderless breadboard (like this). The reason – I learned to solder in age 7-8. And, if you follow some extra (well: normal) cautions at your DIY work like a) do measure and double check all the details you put on the PCB, b) turn trimmers to the proper side putting onto the PCB, then your designs mostly start working from scratch.
Now my life has completely changed – I have lost my solderless breadboard virginity:)
The story started from buying a laminator to establish a controlled process via Press’n’Peel transfer film. From my previous period of DIY activity, I had a quantity of Press’n’Peel films at the shelf. This way I can assure you, 12 years of storing does absolutely nothing to these blue films.
It is not so easy to explain what an unstable and uncontrollable PCB process is. It is much easier to show why one should have a repeatable process for the PCBs:
Well, after establishing a more or less stable process, there was a need in a design to be repeated. My first try was made by means of some very cheap devices (bimetal thermistors were later changed to more suitable ones):
Via this scientific experiment I got the impulse to obtain a normal laminator depicted above. These cheap laminators won’t work for two simple reasons:
- too small clearance between the rolls – 2.5mm PCB is already too much;
- unsufficient heat capacitance.
The box from Spain arrived unexpectedly fast:
As a test design I choose the headphone amplifier by Kevin Gilmore, ressurrected for us by Pavouk. These first PCB’s were not completely satisfactory (a little overexposition of heat) but I was able to normalize the situation by hand (manually painting, scratching, cleaning the letters with a needle :).
It took some 4-5 tests to establish the stable parameters for my laminator (a pre-heating empty run period, n forward-backward passes without the film, certain value of the temperature , x count of passes with the film, fast run to the tap water).
My readers have already seen a PCB of somewhat better quality.
Well, at some stage the details got mounted and the design even passed the smoke test:
However, I was not satisfied with the thermostability of the result. If not tuned and put into a closed box, thermal runaway will be inevitable. Thus a little bit about debugging and tuning.
In our days it is not so easy to build differential FET cascades. It got clear soon that the original Toshiba transistors should be changed to something else (in this case, I tried to use J112 and J175 that differ electrically much more than original 2SJ74 and 2SK170, not even speaking about the noise or distortion ). Low noise complementary audio FETs are not in fashion any more, thus speculants earn billions on eBay while idealists are burning FETs into the stardust 🙂
FETs are somewhat forgiving in such a schematic but possibly the nominals should be tuned. This way are we back to the solderless breadboards again:
Actually I was amazed that it was possible to put such a complex schematic onto a small breadboard with so many details involved. It is almost like chess to have the traces not crossed and the count of wires minimal. No need even to repeat – all transistors were matched and all other details were double-checked. Might be it’s even unethical not to show any measuring device on the picture (there were three testers there during the setup).
Some precautions to people trying to repeat Kevin’s/Pavouk’s design. Class A clearly means some heat:
- Do not even try this without a small heatsink (I haven’t yet b/c a PCB redesign is needed to drill 8xØ5 holes).
- Dynalo shows it’s positive side only after you put the diodes on the heatsink (I haven’t yet)
- While tuning/measuring, do not forget to ground the input via some 47K resistor (Pavouk omits this episode) … or your FETs will fly like hippies.
- Only add the opamp IC after everything else has become absolutely stable (no thermal runaway). As you see, mine is still missing.