Scott Mayo wrote: "Hardwired designs present the problem that when the parts go out of true - and all parts do in time - you get to rebuild the crossover if you want it fixed"
I've been following this thread with some amusement, but as an electrical engineer just couldn't let this comment pass. I think the makers of all those mil-spec resistors and caps would be really surprised to know they go "out of true".
They aren't the least bit surprised. Circuit boards in mil radar arrays have expected lifetimes and it's because components drift and fail. Ask me how I know. And those are massively conservative designs, employing self test, self correction, and no-expense-spared custom parts. Continuous self-test, right down to voltage monitoring of rails, is a design requirement for a reason. It's not cheap - but it's necessary. It would be nice if it was not.
I've had amps blow up (after years) because resistors decided to change values, sometimes by emitting smoke, but not always. Old caps change value in temperature shifts. Commercial amp manufacturers rarely warrant their products for a decade; it's because they have failure modes, not all of which involve catastrophic smokeballs. I've had diodes on my workbench become temperature sensitive when they didn't start that way. Copper oxidises. Tubes gas. Discrete transistors change gain. Even ICs degrade. Anyone here ever have a PC fail?
It's fair to say that components don't change values *within their expected lifetimes*, which explains why inexpensive electronics generally don't fail until after the warranty period is over, but do nonetheless fail.
Yeah, stuff drifts. You can't beat entropy.
And yes, sure, pots drift too. But you can adjust a pot. Pots are good.
[Edit, added:]
To my amusement, John Casler just pointed to the new VMPS Europe site. I lifted this text from it just now; I hadn't seen it before:
We use pots because they allow a 1/20th of 1dB change in level to be effected by the user. Often such a change is all that is needed to tame hot trebles or a forward midrange. A fixed value does not allow the user to adapt the music to his tastes. Also, resistors are not made with sufficient tolerances to give me the precision I feel necessary for level changes. Also, fixed parts change value with age and there's nothing you can do about that. The pots change too, but all you have to do is move the wiper over one winding and you're back in business. Fixed values are invariably determined by measurement, i.e. whatever meassures flattest with your mic and test gear and environment. However, bass below 200Hz is boundary-dependent; fixed values do not allow for changes in room or placement. You're going to be wrong virtually 100% of the time, no matter what you think your measurements tell you is best.
Brian knows rather more about crossover design than I ever will, so I'm gratified to see I was in agreement with him.
