Question about phase and pots.....

[StratTone]

Ok I know that a pot has a right and a left. I had to go and do it to believe it but can someone explain to me why you can't just wire pots oposite and it not work? How does this mess up the sound. I would seem that it should only change whether you would turn the pot clockwise or counterclockwise but it for sure changes the sound.

Also about Phase. On an amp like a Vibro Champ where the phase is opposite to the output jack as compaired to other amps like the twin, super, bassman. I'm guessing that there is a right way but how do you know the output phase of an amp without using test equipment. Would you just look at how many gain stages it has and figure the up down up down with the signal to see the final phase? Sorry for the stupid questions but I've gotten pretty far with amps and there is still a few basics that I really need to "know" the answer too and not just be able to figure it out with trail and error. Thanks for all the help. Especially you ENZO.

[Chuck H]

Originally Posted by StratTone

It would seem that it should only change whether you would turn the pot clockwise or counterclockwise but it for sure changes the sound

No, it doesn't change the sound. But it may change how fast the gain increases per digit depending on the "taper" of the pot. An audio taper pot wired backwards would have a much more rapid increase in gain as the knob is turned than it would wired correctly. All the same settings will be there, but at different amounts of rotation. It won't behave like a mirror image unless you were using a linear taper pot to begin with. All the same tones will be there, and they will sound exactly the same.

Originally Posted by StratTone

Would you just look at how many gain stages it has and figure the up down up down with the signal to see the final phase?

Yes... Sorta. It depends on how much tone shaping you do in between stages also. RC networks also change the phase, though any one circuit won't change it much. But if you have alot of them the overall shift can be considerable. This is how a tube tremolo circuit works. A series of RC circuits flips the phase around 180* to create a positive feedback loop.

But overall your OK just counting stages. Better still, just listen to the amp normal and then wire the speaker in reverse and decide which you like better. The difference will seem very subtle. You won't even notice on very clean settings. But the harmonic order will change when overdriving the amp. Also, if you plan to run two amps at the same time and you find they are out of phase, you can try switching the speaker leads on one of them to see if you get a fuller tone. If you do, the amps were out of phase with respect to each other.

IME an amp that plays "foreward" will have better fundamental note sustain and an amp that plays "backward" will go to a sympathetic note instead.

Chuck

[SWEINC]

"IME an amp that plays "foreward" will have better fundamental note sustain and an amp that plays "backward" will go to a sympathetic note instead."


Can you explain to me why this is so?

[Chuck H]

I have a guess... I figure that when the amp is playing foreward the fundamental doesn't have a tendancy to cancel as a result of acustic feedback. This doesn't mean an amp that plays out of phase won't sustain the fundamental. In fact most high gainers will. But for that just over the edge cleanish fat distortion that seems to sustain the fundamental well even though it's not that far overdriven I get better results with the amp playing in phase. Of course when the amp is playing backward there would be different harmonics that are accentuated. And it's not as if good design dictates that an amp be designed to play in phase. In fact the basic Marshall jtm45, Plexi, Super Lead and JMP designs all play out of phase with the input.

Chuck

[Enzo]

It would be very simple to wire in a reversing switch on your speaker leads and listen to the amount of difference it does and does not make.

Your strings react to the sound coming from the speakers. Very hard to say what the phase relationship between your strings and the amp circuits is, one reason being, you could be standing three feet away or 40 feet away. That distance will affect the sound in the delay of propagation time.

But whatever acoustic coupling the sound has with your speakers, if the speaker phase is reversed, then so is the polarity of the acoustic waveform. SO that wavefront that used to reinforce some overtone is now reversed so it bucks that overtone. And the reverse - wavefronts that attenuated some overtone now would reinforce it.

When you add them all together, who knows what the net result would be.

In some way it is a little like reversing the wires on your pickup.