reverse phase

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by rreiser, Aug 30, 2005.


  1. rreiser

    rreiser Supporting Member

    Messages:
    1,756
    Joined:
    Dec 31, 2003
    Location:
    Wi
    I have my cab wired so the speakers move forward when a 9volt is applied. I experimented with a standard speaker cable and one reversed. I noticed two amps had more sustain and feedback with the reversed cable. A third amp sounded better with the standard cable. Is there a technical reason for this? Thanks, Russ
     
  2. billyguitar

    billyguitar Member

    Messages:
    4,327
    Joined:
    Aug 30, 2005
    Location:
    Kansas City
    Some amps, like a tweed bassman, invert the polarity from the input. I believe original Jensens were labeled with the plus and minus backwards also. So if you have an inverting amp (I don't know how to tell that though) you might have to wire the speakers backwards to get them to play forward. I believe Gerald Weber explains all of this in one of his books.
     
  3. hasserl

    hasserl Member

    Messages:
    4,734
    Joined:
    Oct 24, 2004
    Location:
    So Cal
    And of course, if you use any pedals that invert the signal it screws the whole thing,

    Guys, were talking about vibrations in the hundreds to thousands per second. You really think it makes a difference if the first vibration moves the speaker forward or backward? I say any differences you heard from your experiment above were due to other differences in the amp/speakers/placement/room etc.

    And like I mentioned above, sending the signal thru a pedal will usually invert it. Or it will invert it when it is on, then revert it when it is off. And if you have more than one pedal this is going on for each one.

    Then you have to consider your picking techniques. Upstrokes will start the string moving one direction, downstrokes start it the other. What about finger picking?

    Forget it! The variables are way too numerous to try to control so that the speaker would always move forward at the start of any note.

    In a blind test I do not believe you would hear any difference between the way the speakers are wired. And even if you could, how could you possibly control all the variables?
     
  4. scottl

    scottl Member

    Messages:
    17,042
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    I'll bet every penny in my name that I will immediately hear the difference using my gear. I plug straight in btw. The difference is exactly what the poster described. A VERY noticeable softening of the leading edge of the note, followed by greatly improved sustain and bloom. The feel is even different. Of course, I play a Dumble style amp that does invert. It is very touch sensative and has loads of bass. A less sensative or less bassy amp may not yield the same results.



    Please don't poopoo that which you have not experienced for yourself.

    :D

    Edit: Didn't mean to come off real aggressive.... Just certain it is 100% true. Also, are we all certain the absolute phase coming out of the guitar has anything to do with pick vs fingers vs pick direction?? I am not certain of this in context of this discussion.

     
  5. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Just try it.


    I think it does make a difference which direction the speaker first moves in - have a look at a DAW waveform of a guitar signal and see how uneven it is over the first few cycles especially, which is the bit your ear really focuses on. String picking direction does not reverse the phase because pickups are far more sensitive to up/down motion of the string relative to the polepiece, and very little side-to-side - and all notes start basically with the string moving away from the magnet when the pick comes off it (this was discussed in another thread).

    Amplifer distortion (especially preamp distortion, or single-ended output stages) is often quite asymetrical too, so whatever phase the input signal, the output tends to be asymetrical in the same way. So for distorted signals in particular, the final phase between the amp and speaker is likely to be more important than anything in front of the amp, either at the guitar or any pedals.

    Some pedals do reverse the phase, some don't. There's no hard and fast rule.

    The reason some amps (or pedals) are in opposite phase to others is because most gain stages are inverting, so the final phase depends on the number of stages. You can even have channel-switching amps where the channels are out of phase with each other - you might think that this would make it impossible to have both channels sounding good at the same time, but a clean sound doesn't necessarily need the same phase as a distorted one to sound good. There's no 'right' or 'wrong' phase, only one way or the other.

    It's certainly true that you have very little control over most of the phasing in a system, but that doesn't mean you can't hear it.

    The simplest thing to do is make a reversed speaker cable and just listen. If you don't hear any difference, fine. That doesn't prove that other people might not, especially with a different set-up.
     
  6. scottl

    scottl Member

    Messages:
    17,042
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    Hey John! Thanks for the string explanation. I was fairly certain it did not matter in my case , by just listening.

    Now to dig up that Mike Landau discography for ya..... LOL :eek:
     
  7. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,063
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Keep in mind there is a 90-degree phase shift when
    a signal moves through a cap.
     
  8. billyguitar

    billyguitar Member

    Messages:
    4,327
    Joined:
    Aug 30, 2005
    Location:
    Kansas City
    If you put a DC battery on a speaker it goes either in or out. This would seem to be the most simplified demonstration. Speakers are made to play forward to sound best, aren't they? If so that would be why the amp would sound better if the speaker pushes instead of pulls in.
     
  9. VaughnC

    VaughnC Supporting Member

    Messages:
    14,949
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    Speakers aren't made to "play foreward". In most applications, it doesn't matter which way the speaker moves in response to the input waveform as long as multiple speakers are in phase with each other. However, a guitar amp is a special application....and if the speaker moves foreward with the initial string attack, more initial presence will be felt with the resulting foreward air pressure. After the initial string attack, the rest of the note won't sound any different no matter what the phase. It's only that little thump at the very beginning of a note that makes the difference.
     
  10. VaughnC

    VaughnC Supporting Member

    Messages:
    14,949
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    When a signal moves through a cap, the voltage phase dosen't change....only the phase relationship between the voltage and current changes. If you put two caps in series, the signal voltage doesn't invert with 90 degress each from each cap, does it?
     
  11. 908SSP

    908SSP Member

    Messages:
    5,794
    Joined:
    Jun 22, 2004
    Location:
    Michigan
    I made a phase reversal foot switch for testing and I can put it between the amp and speaker. It even has an LED on it. Anyway you hit the switch if the LED is on the speaker is in phase if it is off it is out of phase and yes I can hear a difference. All my amps sounded best in normal phase.


    Just don't hit the switch while playing it might not be safe for the amp.
     
  12. hasserl

    hasserl Member

    Messages:
    4,734
    Joined:
    Oct 24, 2004
    Location:
    So Cal
    John I have tried it, there is no difference. I don't believe you or anyone else could accurately with repeatability and reproduceability identify in a blind test which phase was which.

    It would be cool if someone here set up such a test so we could resolve this once and for all.

    BTW, how do you know that all notes start with the string moving away from the pickup? Where did this come from? It seems reasonable that when plucking the strings with fingertips (with a pulling motion) the string moves toward the pickup first, not away from it. The same with an upswept pick using a flat pick. What about finger picks? Each of these would seem to have the string moving first toward the pickup, while picking down would likely have it moving the opposite way. And strumming would seem to have the string moving side to side.

    But that is just how it would seem. Without some type of measurement it is only a guess. So what measurement has been done so you say it moves away from the pup?
     
  13. trdlasvegas

    trdlasvegas Member

    Messages:
    168
    Joined:
    Aug 30, 2005
    Location:
    Las Vegas
    I made a inline speaker phase reverse with a switch for a very skeptical customer with excellent ears. He took it home and came back very sheepishly a few days later saying he could not tell the difference.

    If you got two amps the phase is obvious... but with a single amp there is NO WAY you can hear the difference.

    -Tony
     
  14. hasserl

    hasserl Member

    Messages:
    4,734
    Joined:
    Oct 24, 2004
    Location:
    So Cal
    A battery is DC, the signal from an amp to the speaker is AC. When you put a battery to the leads of a speaker it moves one direction as far as it can and stops. With an amp the speaker will vibrate back and forth from ~80 to several thousand times per second. It really doesn't matter which direction the speaker moves first. And you couldn't control it if it did.
     
  15. rreiser

    rreiser Supporting Member

    Messages:
    1,756
    Joined:
    Dec 31, 2003
    Location:
    Wi
    I sat in various positions and played the same note with full volume on the guitar with the same amp settings. The only thing I changed was the speaker cable. Each time there was a noticeable increase in the length of the sustain and the note went more easily into feedback. I did this numerous times to see if it was a fluke. Everyone has a right to their opinion. I just know what I heard.
     
  16. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Very simple....have a look at the waveform in a DAW/computer recording software. I've done this (for many purposes concerned with phasing, latency etc) and it's not a guess.

    Certainly the initial peak can be made to be in the opposite direction if you deliberately pull the string up and let go... that wasn't the question, you asked about upstrokes and downstokes. In normal picking the pick (or fingertip) pushes the string sideways and downward, so when it slips off and forms the initial attack the string is always moving sideways and away from the pickup. Same with strumming, in either direction.

    As I said, the effects of phase are also more noticeable (and also different - not necessarily a clear case of one particular phase always being better) on some types of tone than others. I don't doubt that there are some where the phase is virtually irrelevant. But there are some where it's very clearly audible.

    If you don't want to believe it, don't. I'm a skeptic by nature too BTW.
     
  17. billyguitar

    billyguitar Member

    Messages:
    4,327
    Joined:
    Aug 30, 2005
    Location:
    Kansas City
    I'm satified that phase doesn't matter except when multiple amps or speakers are involved. Probably not a subject worth this much discussion but since we're all apparently minutiae freaks there it is! ha ha
     
  18. scottl

    scottl Member

    Messages:
    17,042
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    Thank you. I agree. With my particular amp, there is an immediate and big difference in tone. The sustain and bloom is better. The notes feedback easier as well.

    Anyone who says otherwise and that I am mistaken is insulting me. I hear what I hear and I have the ears and experience to back up my claims. We are not talking minutia. At least with a Dumble style circuit, it is a noticeable phenomena. Maybe in a Marshall style amp with less output section bass, etc... The effect is less noticeable. But until you listen to my setup, A/B, please don't insinuate I am a moron who thinks he hears something that is not there.

    It is real. End of story.
     
  19. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Ever use a phase reversal switch on a DI for an acoustic? They can be quite effective for reducing or eliminating feedback between a monitor and an acoustic guitar.

    I really think that the same physics that account for that effect come into play with an electric.....with the exception that (unlike the acoustic) we always *like* the phase that results in guitar-amp interaction that results in easier feedback. It's a natural speaker to guitar body vibration interaction that is either resulting in more frequencies in phase (and additive), or out of phase (and subtractive).

    I'm as skeptical as anyone when it comes to stuff like this, and for all the same reasons others have already given. The fact is, I tried it and heard a difference quite easily. More so with some amps than others, and most often at higher volumes, but I did hear it.

    For my particular tests, I just used an inverting buffer at the input of the amp so I could easily go back and forth while playing. I'm not convinced that there is any difference with an inversion introduced before the amp vs after the amp, asymmetrical distortion or not. I heard the same difference reversing the speaker wires that I did invering the buffer. But I am convinced that there is a difference with phase between guitar and speaker.
     
  20. scottl

    scottl Member

    Messages:
    17,042
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    Thank you Mark for so eloquently putting it!! Sorry if I pissed anybody off but I know it to be true.

    Scott

     

Share This Page