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How do you know if your preamp has inverted the phase?
How do you know if your preamp has inverted the phase?
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Old 20th February 2008, 11:39 AM   #11
lcsaszar is offline lcsaszar  Hungary
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True, but some (most?) acoustical instruments produce an asymmetric waveform. Think of a positive-ramp vs. a negative-ramp sawtooth as an analogy. We accustomed to one phase and we might find the opposite phase unnatural. I am not sure if this effect is detectable, because I never tried it.
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Old 20th February 2008, 11:50 AM   #12
AndrewT is offline AndrewT  Scotland
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Hi,
Some systems have a phase reversal switch to allow experimentation like this.
I have seen reports that some CDs suit phase reversal and other don't, kind of confirming EC8s assertion.
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Old 20th February 2008, 11:57 AM   #13
SY is offline SY  United States
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Thank you, EC8010, for using the proper term, "polarity," instead of the improper term "phase."
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Old 20th February 2008, 12:00 PM   #14
duderduderini is offline duderduderini  Australia
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Default polarity switches

Hi Sy
Point taken Sy. So Andrew, this switch you speak of.. Would it be feasible to install?
Thanks
Nick
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Old 20th February 2008, 12:15 PM   #15
AndrewT is offline AndrewT  Scotland
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Default Re: polarity switches

Quote:
Originally posted by duderduderini
this switch you speak of.. Would it be feasible to install?
Hi,
some amps may be very easy to modify like this, others may require an extra stage.
But in ALL cases extra switching and/or active stage will be required and the improvement you are looking for may be negated.
When it is already fitted then the possible deterioration will have occurred and comparison of phase/polarity is possible.
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Old 22nd July 2021, 08:41 AM   #16
alex961 is offline alex961  Slovenia
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To my oppinion, you should try with an osciloscope. Connect one channel to the input and another one to the output. (In responding ranges). Input vs. output signals should be in phase. If not, they are inverted.
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Old 22nd July 2021, 09:21 AM   #17
6A3sUMMER is online now 6A3sUMMER  United States
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What you can not be sure of is the music source signal:

Microphone, microphone transformers, preamp, recorder, then editing studio, etc.
Consider all of these processes, and the wiring of the interconnects, some balanced, some not, some with wire reversals, one more inverting stage, etc.

What you can be sure of:

You are sitting in a concert, and two Bass Drum heads are facing you.
One drum is struck by a Mallet on the front side of the drumhead that faces you (that transient starts with a Rarefaction of air).
The other drum is struck by a Mallet at the back side of the drumhead that does not face you (that transient starts with a Compression of air).
Those two drums are in different phase at your seating position.

An oscilloscope will not tell you which way the drum was oriented versus the position of the microphone, because there are too many circuits, wiring, parts, and processes between the original performance and the playback system.
Oh, and which side of an open baffle woofer are you sitting on?

Last edited by 6A3sUMMER; 22nd July 2021 at 09:28 AM.
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Old 23rd July 2021, 12:22 AM   #18
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by SY View Post
Thank you, EC8010, for using the proper term, "polarity," instead of the improper term "phase."
Alas its universally known as phase of phasing in recording studio equipment, as in phase inversion switch, etc.
Same with opamp datasheets - early one's had problems with "phase reversal" when driven to the rails, so it is common to see claims of "no phase reversal" in such datasheets now.


"Phasing" is very important in studio gear as signals are often sent to effects units and then the result summed with the "dry" signal
again, which sounds very strange if the polarity is different. Mic inputs always have phase switches in studio consoles as positioning of microphones varies and sometimes you want to play with phase to get specific response patterns.


You won't change this usage now

Last edited by Mark Tillotson; 23rd July 2021 at 12:25 AM.
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Old 23rd July 2021, 06:59 PM   #19
egellings is offline egellings  United States
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Only certain way I know is to hang a 2 channel scope on 'in' and 'out', feed in a sine wave, and take a look. Or if you can attenuate the amplifier's gain so that it is at unity gain, feed in a reasonable signal, say 100mV or so at a kHz or so, and differentially measure the input signal to output signal voltage. If it is very low, the amp inverts. If you see a large signal, the amp is non-inverting, and the input & output are summing constructively to give a larger voltage.
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Old 23rd July 2021, 07:27 PM   #20
6A3sUMMER is online now 6A3sUMMER  United States
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Usually, a struck bass drum waveform is not symmetrical.

No matter which side is struck, typically it will have a large initial excursion in one direction (no matter whether you are on the pressure side or the rarefaction side of the drum head), and then a smaller excursion in the other direction, and it continues to alternate, and collapse exponentially.

Next, let's take the upright acoustic bass.
What is the direction of the first acoustic pulse if the string is pulled away from the neck, and then released to strike the neck, resulting in a "whack" sound?
This is a sound effect, but it still is a purposeful part of the music.
I do not know the answer as to the 'polarity' of the initial impact, but I bet the direction of the first pressure or rarefaction is always consistent.

Music is a variable.
So are recording plus playback systems.

Live music is what it is, live.
But . . . are you sitting in the audience; in the middle of the performers; or at the back of the stage?
All those sound experiences are different.

So now, when that same performance is recorded, what are the locations of the multiple microphones?
Nothing like that is very simple.

Last edited by 6A3sUMMER; 23rd July 2021 at 07:29 PM.
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