It's important to save the best for last.
Myth: Replacing the resistors and capacitors in preamps and power amps with higher quality units can improve the sound of a system.
Fact: Unless your capacitors are defective (they allow DC current to pass through them), or have changed their value over time due to heat and other environmental factors, you are not likely to improve anything by replacing them. The same goes for replacement metal film resistors. It's true that metal film resistors have lower noise than other types, but that makes a difference only in certain critical circuits, such as the input stage of a high-gain mike preamp. It's also true that different types of capacitors are more or less suitable for different types of circuits. But if you think the designers of your amplifier or mixer are too stupid to have used appropriate components in the first place, why would the rest of the design be good enough to warrant the cost of improved parts? In fairness, extremely old gear often employs carbon composition resistors, and replacing them can make a difference in many audio circuits. But anything manufactured in the past 20 years or so will use carbon film resistors and decent capacitors.
Yee haw! So our friend Ethan, who, BTW, is a very smart, experienced and knowledgeable fellow, suggests that if I were to use a 1mF $0.34 electrolytic capacitor, vs. a $10 1mF Film and Foil Rel Cap on the input of a high performance preamp or power amp, we'd hear no difference?
Really? I can tell you from personal experience this is simply untrue. It may be true that the author of the original post cannot hear a difference, but I assure you most people reading this blog would pick it out in a matter of seconds.
If it were not true, then we'd be idiots to use the $1.00 each PRP resistors and $10 Rel Caps in our designs. We'd be money ahead by simply using good old carbon film resistors, electrolytic or cheap film capacitors in the signal path. If it were a matter of perceived value, we could pot the whole mess and claim there was "secret sauce" inside. Instead, we just use the best sounding parts we can manage - and they are chosen for their sonic attributes by (gasp) people who listen.
And I can already read the response to this. "Prove it! Because my measurements show there to be no difference." A reasonable response since it
can be shown through the limited tests we engineers routinely perform, that there are no differences. And this is where the problem lies.
When we use limited testing methods to "prove" something, it doesn't suggest what we hear isn't valid if those limited tests don't reveal any differences. In fact, we know (and Ethan should know too) that simple sine wave, THD, IM, and impulse response testing barely scratch the surface of what makes one product sound differently than the other.
The only test I have ever seen that came even remotely close to displaying differences was Bob Carver's null test between two amplifiers. Using a sensitive differential scope and meter setup, two gain matched power amplifiers were fed the same musical signal. The differences between the two were immediately obvious. Bob was able to tweak his amp to more closely match the other and the two sounded very much closer after he did that (though not identical).
It is important to remember that between the two schools of thought:
if it can't be measured it doesn't exist, and,
if you can hear it it is real, there's the truth of the matter.
We do not know how to measure all that we hear, just as physicists can't explain all that they see.
