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Phase Question

Discussion in 'Amps and Cabs [BG]' started by Chris Fitzgerald, Aug 6, 2004.


  1. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Hi, just a quick visit from the DB forums, where we have no techies! I'm using a (passive piezo) DB pickup which sounds best through a certain preamp which has a "Phase Reverse" switch. I have a new preamp which I'd like to use with no phase reverse function. Is switching the phase of the signal as simple as swapping the wires on one end of a patch cord, or is it more complicated than that? If it's that easy, all I need to do is alter a cord to use with that pre. If not, what next?

    Any and all replies appreciated. :)
     
  2. Hello Chris -

    I made a number of short (~1') XLR3F >>> XLR3M cables for that very purpose of doing a quick phase-change. Just swap pins 2 & 3 at one end. If you want a "ground-lift" cable, leave pin 1 disconnected. Check EIA Standard RS-297-A for more info. Take care, though if you are using phantom power through the cable.

    Here's a link for you…

    Regards -
    - Wil
     
  3. Tim__x

    Tim__x

    Aug 13, 2002
    Alberta, Canada
    Yes, but as I said in the other thread this will only work with balanced signals (most often XLR or TRS) so it won't work for your instrument cable.
    You could modify the XLR cable running from your preamp to your power amp. And then if you want to plug somthing into your XLR in you can engage the phase switch and then that signal will be in phase, since the inverse of the inverse of something is the same as the original.
     
  4. Passinwind

    Passinwind I Know Nothing Supporting Member

    Chris,

    First, I would try to determine why the phase reverse on the first preamp makes things sound better. Are you using a PA feed as well? Could that be out of phase with your stage sound? If that's the deal, reversing your onstage speaker lead would do the trick. If it's something else, it would be good to understand what it is. Not every preamp preserves absolute polarity, so your new one may not need to be reversed anyway.
     
  5. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Hi Wil, thanks for the link. What I don't understand at this point is how the XLR wired for phase reverse will help. Would it go between the output of the TubePre and the input of the PUB 280? I was hoping to be able to reverse the phase of the pickup signal before it entered the pre. It seems that this should be possible with a 1/4" signal, since my Raven Labs pre is able to reverse a 1/4" signal. I guess what I'm wondering is how the RL pre does that, and if it's doable inline before the pre in this case. But if an XLR on the out would give the same effect, I guess I'm all for it. :)

    Sorry I didn't make the circumstances clear...I've used the Raven Labs pre to listen to both phase settings through the TubePre, and the "inverted" phase setting is definitely cleaner. I'm not running anything into a sound system at this point, so it's just the sound of the bass and amp. Why does it sound better? It's less "tubby" and more natural sounding, I guess. This is the first pickup I've ever used where I thought that the Phase switch actually improved the sound in any way, but in this case it really did.
     
  6. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Interesting thread. I'm just parking here to get some new info. I must admit, my initial reaction was like Passinwind's. I dont think I've ever seen someone reverse the phase of a signal before the preamp. It's more common to see it happen after the amplification stage allowing you to reverse the movement of the speaker cone.
     
  7. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    That may well be what the switch on the Raven Labs unit is doing. Let's sit tight, and maybe we can both learn something. :)
     
  8. Passinwind

    Passinwind I Know Nothing Supporting Member

    Sorry I didn't make the circumstances clear...I've used the Raven Labs pre to listen to both phase settings through the TubePre, and the "inverted" phase setting is definitely cleaner. I'm not running anything into a sound system at this point, so it's just the sound of the bass and amp. Why does it sound better? It's less "tubby" and more natural sounding, I guess. This is the first pickup I've ever used where I thought that the Phase switch actually improved the sound in any way, but in this case it really did.

    I'm gonna take a wild ass guess on this. I build my own preamps, and I've had occasion to add an extra inverting stage to keep things in phase between the EFX loop and the rest of the preamp, as one example. The preamp might very well sound better without that extra stage. It could be that the "non-reversed" phase position uses one more (or maybe one less) amplifier stage, and thus sounds worse in some ways. There's another possible switching scheme not using an extra amplifier stage, but going to the alternate input (+ vs. - opamp input). Preamps can sound somewhat different depending on which input configuration the designer uses, but this seems like a long shot.

    But the real isssue is, would your new preamp benefit from such a switch? Does it already sound better to you than the one with the phase switch, or just have a more desirable feature set?

    Oh, yeah: if I absolutely had to polarity reverse a single ended 1/4" line, I'd either build/buy an inverting buffer amp (cheap if you roll your own), or get a Jensen transformer (not so cheap) that would float the ground, allowing you to reverse polarity. Or, I'd just rewire the whole pickup or preamp...but that's another story. :cool:

    Edit: I went to this thread:
    http://www.talkbass.com/forum/showthread.php?p=1593038
    and read up on the details of your current situation. I would just try swapping speaker polarity, or the pre-to-power connection, and see what you get. Have you done that yet? Did you ever try the Full Circle pickup in the reversed mounting?
     

  9. Mmmm - that's interesting… My comments were made from the point of view of using the phase-switch to change the phase of a microphone to avoid problems which might occur when the relative positions of more than one microphone causes cancellation of certain frequencies, leading to unwanted artifacts in the resulting sound. Your comment about the speaker cone also holds true, and I think in Chris's case, both considerations have to be taken into account.

    I used to use a K&K Bass Max with the Golden Trinity microphone, both going through a Raven Labs PMB-1 blender. I found that playing around with the phase switches would make a noticable difference to my sound from where I was listening (i.e. at the bass) - note: I think I could have achieved the same result by physically inverting the K&K Bass Max pickup.

    The point about the phase of the speaker is well taken - and I think it's this, in conjunction with the distance of the speaker from the bass (position), which will affect the overall sound of the bass from a distance, i.e not right next to the bass.

    Sorry if this is confusing - I've not had my coffee yet…

    - Wil

    BTW - I'm guessing that in the PMB-1, the (unbalanced) ¼" TS input is buffered with an op. amp and the phase is changed by selecting the appropriate output…
     
  10. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    This is where I get confused. I used to use a Double Bass Max as my main pickup. (All Hail) Bob Gollihur told me at one point that if it didn't sound as good as I might like, I might try switching the leads of one of the pickups to avoid phasing issues (I didn't yet have my PMB-1). I never had to do it, but for some reason I've never forgotten it either.

    Personally, I get confused between "Phase cancellation" between two sound sources and "Phase inversion" of a single source, which is what I think I'm dealing with here. Hopefully, a kindhearted techie will stopp by soon and enlighten us all. :)
     
  11. Phase cancellation implies there are two (or more) sources. Phase inversion (of a signal) is the reversal of phase of a signal and may be used as an attempt to avoid (or alleviate) phase cancellation.

    If you only have one sound source, say just the bass (no amp), the issue doesn't arise, although the bass might sound different according to where you are in the room (reflections etc.) - OK, so you put your bass in an anechoic chamber, or play in the middle of a field… (just joking - although if you've ever listened to a concert from Tanglewood you will notice that there is abolutely no "room resonance" as the performances take place in the Koussevitzky Music "Shed" (!) which is a listening area with a roof, and no sides - the sound just expands out, and keep going… Concerts from Tanglewood have that unmistakeable "deadness" of sound which is quite characteristic.)

    If you add an amp, then you have the bass and the amp, so there is a potential now for some frequencies (depending on the distance between the amp and the bass) to cancel each other out. If you now add a microphone to the bass, and still have the input from the pickup, there is a possibility for the pickup and the microphone to be out of phase with each other (for certain frequencies, depending on the distance between the microphone and the bass), resulting in some cancellation… See how quickly it starts to get complicated? Er, what was your original question?

    - Wil
     
  12. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Not to run this completely into the ground, but here's what's going on with the Full Circle:

    When I plug it into a preamp and keep the "phase" switch disengaged, there is a bass-heavy "tubbiness" which unnaturally accentuates some frequencies in the lows and low mids, which makes the bass sound "electric" and/or "compressed". When I engage the phase switch, the sound "thins out" a lot and sounds more natural, like the bass does acoustically. When the switch is enabled, I'm able to get a lot more volume out of the PUB, as the low frequencies don't overload the speaker.

    I thought at first that I could solve the problem with a simple high pass filter, but even with that engaged, there is a real "unnaturalness" to the sound that goes away the second I hit the "phase" switch. It's maddening, but I'm sure I'll resolve it one way or another. If the PMB-1 had more gain and a high pass filter, I'd be all set. :)
     

  13. The changes you're hearing - do you notice this when you're next to the bass, or from the middle of the room?

    - Wil
     
  14. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Yes. :)
     
  15. Ah!!!! :)

    Actually, the Presonus Tube Pre is a tube amp, and will tend to "warm-up" the sound (it's to do with how the tube amp handles distortion). Have you tried doing the same test with a solid-state pre-amp (like the PMB)?

    - Wil

    BTW (looking through your earlier posts) if you're using both the Presonus and the PMB, then you're effectively using two preamps in series… (not a Good Thing…) (try Googling "gain-staging"…)
     
  16. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    Unfortunately, the PMB and the TubePre sound pretty much identical when the "Phase" switch on the PMB isn't engaged. I don't use the "toob gain" stage on the Tube Pre at all. What's attractive about the TubePre is the amount of clean headroom is has even without the Tube stage. It's easily twice as loud as the PMB through the PUB. IMO. YMMV. ***? LOL! :D
     
  17. Passinwind

    Passinwind I Know Nothing Supporting Member

    From the Fishman website:

    Mount the Full Circle “threads down”, and the hot side of the pickup faces the strings. This translates to more defined articulation and attack, with a slight emphasis on “string” sound. When mounted “threads up”, the hot side of the pickup faces the body of the bass. You’ll get a more “wooden” acoustic tone this way, with a more rounded attack and less “edge”. Which way is better? It’s too subjective a decision to say one is better than the other, but our local bass repairman tells us that most of his customers prefer the sound of the pickup in the “threads up” position.

    Does this more or less describe the tonal differences you're hearing from using the polarity reversal switch?
     
  18. (thanks for the RAPCO link, by the way…)

    Well, I just don't know…

    Oh wow! I've just been listening to a recording† by Sonny Criss which features Larry Gales on bass - man, what a fabulous sound! I really love his tone - it suddenly occured to me that it sounded just like the bassist on Monk's "Monk In Olso" DVD - I checked, and sure enough he's on there also, and on further hunting around, I found that he did quite a lot of work with Monk. God! I'm so ignorant!

    - Wil

    † "Criss Craft"
     
  19. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    A "phase reverse" switch is actually a polarity reversing switch, and what it does is just like swapping the two signal wires in a balanced connection.

    In itself, a polarity reversal won't produce any audible effect. But with an acoustical instrument that is in its amplifier's sound field, it may be useful for helping to suppress feedback and resonances. That's because there may be some frequencies at which the sound from the amp reinforces the vibration of the instrument, resulting in feedback, tubbiness, or other frequency response problems. Reversing the polarity will reduce or eliminate that reinforcement, although it may shift the problems to other frequencies instead. However, one polarity setting or the other should prove easier to EQ; it may be the straight setting, or it may be the inverted one. For this purpose, it makes better sense to have a switch than to have to insert a polarity inverting thing in a signal cable.