Tube Amps / Music Electronics
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|previous: jacques here we go again.||View Thread|
|9/6/2000 8:38 PM|
|R.G.||Re: circuit data|
OK, what I think you said is that two otherwise identical circuits can differ in sound because of
(2) thermal dissipation - that is, the parts get too hot and change value
(3)parasitic capacitance can be different
(4)electromagnetic shielding can let radio waves in and cause interference.
One at a time:
(2) if internal power dissipation causes changes in the sound, in my opinion it's a poor design. Even so, two identical circuits should have identical thermal drifts unless the packaging keeps some parts cooler than others.
If the packaging is such that parts drift differently, this is the same thing as making two circuits, each with a variable resistor, and then turning the resistors to two different values. In both cases, the maker has knowingly allowed the originally identical circuits to become different. Of course they would sound different. If the maker didn't know that two different packages caused different drifts, they did not put enough effort or understanding into the design. This is not some new principal of design, it's normal old sloppy work. This is not a characteristic of the circuit at all, but an external effect, and certainly not magic at all.
(3) parasitic capacitance exists, and it can be calculated for certain setups. In my article on PCB versus point to point wired amplifiers, I addressed just this point. When you calculate the parasitic capacitances, they are dramatically too small to make an audible difference in vacuum tube circuits in almost all cases, and certainly too small to make a difference in semiconductor circuits with only few exceptions. It is possible that parasitic capacitances make a difference in some circuits, but the conditions are so rare that I'd have to try to design one where it *did* make a difference. To all practical purposes, parasitic capacitances don't make an audible difference EXCEPT in cases where they cause a circuit to oscillate at RF. When that happens, it affects the sound, all right. I contend that if this happens, the circuit was never what you thought it was anyway.
Parasitic capacitances are usually 0.1 to about 5pf. Most audio circuits never get that low.
(4)if electromagnetic shielding makes a difference in sound, the packaging is not excluding radiation from the circuit. The radiation is an unwanted input, not a characteristic of the circuit at all.
I believe that (2) and (4) are not properly parts of the circuit at all, but rather consequences of poor packaging design. All good packaging designs will sound the same, while poor ones may vary in how much they degrade the sound.
(3) is a valid technical point, but actual measurement and calculation indicate that it is not significant in the vast majority of circuits.
So if (1) is true, and the circuit is protected from overheating (2) and RF (4), parasitic capacitance(3) will not make a difference in the gigantic majority of circuits, by both calculation and experience. And it *can* be calculated, and the design reworked so it will not matter.
I don't think the point is proven.
|jacques The old English way. -- 9/7/2000 7:53 AM|