Absolute Phase/Polarity?

[jordan86]

THERE ARE NO ABSOLUTES!!! OR ARE THERE?

I know there is much banter on this subject from a tone standpoint on lesser forums :) So I ask not for opinions on the tone front, but more so a technical/function perspective.

Is it safe to assume that an amp always put out a signal in absolute phase/polarity (I will use those interchangeably, possibly incorrectly). Meaning does the amps OT always "push out" the speaker on initial attack? Is there a way to test or verify this?  Would the number of gain stages in the preamp have a factor on this?

Out of necessity (lots of gear not at home) I grabbed my reverse polarity speaker cable that I made to fiddle around for a few yesterday. I have two DR Z amps not in phase so I modified a cable to allow me to run those in stereo (easier than digging in the back and pulling tabs).  I REALLY liked what the REV polarity cable did to the tone. Seemed to fatten up things in the mids and soften the highs just a touch with my Princeton/Jupiter 12LA speaker.

All my cabs (whether 1x12 or 2x12) are in absolute phase. Confirmed with the old 9v battery trick. However in theory, that is only "valuable" if the amp is in phase - pushing out on first cycle of the wave, correct? The inquisitive mind in me wonders if I could test that by putting a battery to the guitar cable leads, plugged into the amp. Albeit turned way down. And maybe just a 1.5v battery, not a 9v. Assuming that wouldn't kill me? :)

Am I overthinking this? I have heard of certain amp builders like Dumble and TwoRock have intentionally reversed leads on speakers. Reportedly to get the output back in absolute phase with the guitar signal? <<<Possibly only for feedback/sustain purposes (interaction with the speaker cabinet)

[shooter]

Am I overthinking this

phase is important with NFB
it's NOT for anything else
I do "phase" my amps with a scope, not for any sonic reason, just quirky dave 

[jordan86]

Shooter...When "phasing" I presume the signal may or may not show to be in absolute phase with the guitar input. But would always be in correct phase coming off the OT to the speaker?

[jordan86]

Also, I understand that many overdrives, modulation effects and compressors flip the phase of your signal... so its a crapshoot trying to nail down phase even before it hits the amps input. So again, it may not matter at all.

I did enjoy the rev polarity tone on my princeton though, so wondering if what I enjoyed was actually absolute phase or opposite.

[jordan86]

Soooo....I just got a little crafty. Connected some basic wire leads from a half dead AA battery to a 1/4" cable plugged into the amps input. Battery measured .75 volts. Connected as follows...

- Battery positive lead connected to cable tip
- Battery negative lead connected to cable sleeve
- With a standard speaker cable the speaker cone appears to jump backward.
- With my flipped polarity cable the speaker cone jumps forward.

The movement is subtle. I guess I could turn the amp up more or try a fresher battery to get more measurable results, but it definitely seems to be putting out a reverse polarity signal from the amp, which would validate why I do perceive a different tone (and like that tone more). 

 

[shooter]

in order for phase to be "a thing" it needs to be referenced to something so your ears or eyes can note the difference.  Guitar audio (in most cases) have NO reference.
guitar sound out the speaker are complex waveforms with many, many phases, bouncing off the walls, the drummer, getting absorbed.


the ONLY place phase makes sense is in multi-cab array where most of us want all the speakers to do the same thing, push or pull the same way on all speakers.

I make my in and out "in-phase" simply so I don't have to mess with scope knobs 

[jordan86]

the ONLY place phase makes sense is in multi-cab array where most of us want all the speakers to do the same thing, push or pull the same way on all speakers.

Yes, that's been my standard practice thus far.

in order for phase to be "a thing" it needs to be referenced to something so your ears or eyes can note the difference. 

Well now that my eyes have referenced it, my ears can't unreference it 

Appreciate the time Shooter. I take this all to mean very little in real life practice except that I do now feel more OK experimenting with running speakers in different polarity to see which I like more. Enlightening to know that many amps out there are actually not in absolute phase with the input.

Also, there's some peace of mind to me in knowing which of my amps are in vs out. I can now flip them accordingly and the universe is back in order :)

[shooter]

I'm not a musician, so I'm the guy at the table, I've talked to lots of "those guys" not one ever mentioned , "hey, is that guitar players speakers outta phase?" 

If you're interested in phase look at hobbies like pulsed Doppler, laser measurement...... 

[sluckey]

A simple way to "see" the phase relationship between amp input and amp output is to inject a small 500Hz sine wave in the input jack. Connect CH1 of a scope to the input jack and trigger the scope on CH1. Now connect scope CH2 to the speaker (one side of speaker must be connected to ground). It should be obvious if they are in phase or out of phase.

BTW, some amps have a DPDT switch connected to the speaker output just for swapping phase. It does make a difference. Maybe not in all amps.

[pdf64]

Tube amps pass ac only, Vdc at the input won’t result in Vdc at the speaker output.
It will cause an almighty thump though, and probably put the input stage in a kerfuffle (shouldn’t cause damage though).

[jordan86]

I did notice that thumpity thump.  LOUD AND CLEAR :)

And it did occur to me that the battery was DC. So would that invalidate my experiment? Or would the amp somehow pass the battery's DC coming in through the input jack as AC?  The speaker definitely pushed and pull depending on the polarity of the speaker cable.

[shooter]

did occur to me that the battery was DC

the slam alone to the tubes grid will produce enough arcing AC to validate your test and invade the tubes life 

go steal a scope, do it correctly otherwise like computers junk in = junk!

[brewdude]

I an no expert, however...
My thoughts on guitar amp speaker phase are that the difference in response is likely relative to the guitar’s pick up’s physical distance  from the speaker and the wavelength of the specific frequency which is transmitted through the pick up.  This relationship could be in phase, out of phase or anywhere in between.  Some notes will go into feedback if you stand at a particular distance from the cab.  I suspect that there may be other contributing factors, such as gain, tonal response of the guitar, amp, cab or room, etc. 

[jjasilli]

There is such a thing as absolute phase.  It can be seen on a scope as sluckey says.  The input signal has phase.  Each gain stage reverses phase.  But a cathode follower does not reverse phase.  The OT itself does not reverse phase. But reversing the leads will invert phase.


Also, filters -- combinations of R, C & L's -- alter phase, often in 90 degree increments.   It it depends on the specific complexity of the filter.


There is no reason for the phase of the audible speaker output signal to match that of the amp's input signal.  But 2 or more out of phase signals can cause issues within the electronic components, or as soudwaves in the air. 


Speaker placement affects the relative  phase of physical soundwaves in the air.

[tubeswell]

The OT itself does not reverse phase. ...

Also, filters -- combinations of R, C & L's -- alter phase, often in 90 degree increments.   It it depends on the specific complexity of the filter.

Inter-winding capacitance in interleaved OT's can result in some phase shift/lag at higher frequencies, which can result in positive feedback at HF from the global feedback loop (necessitating some HF slugging)

[finkaudio]

Hi,

in HiFi, the absolute phase has to be taken into account, because several speakers would play together and that only works with all of them @ the same phase. In theory, this would be valid for musical instruments if they would reproduce the same signal. In reality, the signal is different, the position on the stage is different, the distance to the listener is different, so the result is more or less an accident.

Maybe it's a good idea to add a phase switch to the amplifier and check on the stage if there is an improvement when playing together.....

ATB KH

[pdf64]

...a signal in absolute phase/polarity (I will use those interchangeably, possibly incorrectly)...

Signal phase and polarity are different. Circuits apply an integer number of signal inversions; that may or may not result in the output being in the same polarity as the input. Generally, eg if there's a negative feedback loop, a circuit's polarity response can be determined from its schematic. There's only 2 possible results; either the signal output is in the same polarity as the input, or it's inverted.
Whereas there's an infinite range of possibilities for the amount of phase shift between input and output; for tube guitar amps, signal phase will be all over the place; even before the significant phase shifts introduced by tone controls and speaker/s is considered, it will be massively shifted at either end of the audio band. Usually the phase response of speakers completely flips, from lagging to leading (or visa versa, can't remember which) somewhere around the midrange.
However, I think you are asking about signal polarity. Generally, if one signal source is used with two parallel signal paths simultaneously, it is usually preferred if those paths result in the same overall signal polarity.
With regard to absolute polarity, I'm not aware of a standard or accepted 'correct' response for guitar pickups. Without that, I'm not sure that the concept of an 'absolute' polarity for guitar rigs can be valid, in the way that it is for systems that use signal sources for which there is a standard, eg microphones