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Originally Posted by Hi-Finthen /img/forum/go_quote.gif
As has been said: Why would R19 and R20 exist (again, in identical fashion to the Porta Corda, in Jan's assessment, which probably few, if any, who know him doubt) if only to short them with wire?
This IS the "smoking gun" IMO, needing to be verified by any owner for the debate to be settled.
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The following might not be the reason why it was done, but...
This could be done because of a trick I've learned early on in an internship consisting of developing a certain piece of hardware from design to prototype to finish.
Making PCB's can get very expensive, especially in small orders. So, instead of ordering a bunch of small orders, tweak with them, and then go for a big order for the "production" run, small manufacterers often order a large number of PCB's right away to lower the price per PCB. Now because there is still prototyping done with these PCB's, 0 ohm resistors are placed at certain places and in certain lines where there might be changes later on.
This is to be able to just take it out, place a resistor if needed, or jumpers to other places, and not having to buy a new batch of PCB's because the lines are fixed. It can very well be that early produced items (or, if it is a small scale item, such as headphone amps would be) have these jumpers or 0 ohm resistors in place because they are on the PCB boards that were ordered. This might seem unlogical when looking back at a finished consumer item, but it is critical during prototyping and because of the lower price for large orders of PCB's, can extend to finished designs even after the prototyping stage. This is comon procedure and pretty much all engineers will use this trick during the first batch of a product, unless they are very sure of their work (and which engineer is?
).
That said, I find it unlikely that someone else would come up with the same schematic and values that Jan has came up with. It's a fairly straight forward amplifier, and as said before, the crossover is (partially?) public, but there are so many ways to do it that I find it unlikely to be coincidence. Not impossible, just very unlikely.