Phase check

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454Casull

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Joined 2004
#1
Hello everyone. Hope you are all well.

I have something like an integrated amp that I am working on, and I would like to be able to check that I have not accidentally inverted any channels due to haphazard or simply ignorant wiring. What would be the best way to make sure that everything stays + and - from pre-amp inputs to amp outputs?

I don't think a "battery test" at pre-amp inputs will work since I know for sure there are coupling capacitors at pre-amp inputs.

Also, it will be extremely inconvenient for me to procure an oscilloscope, sadly.
 
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#2
If it's a stereo preamp, you could play monophonic music through both channels and notice the bass level with just one channel playing and then both channels playing. If the bass increases when the 2nd channel is added, then the channels are in phase. If the bass disappears, then an out of phase condition exists.
 
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#4
egellings said:
If it's a stereo preamp, you could play monophonic music through both channels and notice the bass level with just one channel playing and then both channels playing. If the bass increases when the 2nd channel is added, then the channels are in phase. If the bass disappears, then an out of phase condition exists.
Click to expand...
Thanks, yes, this makes a lot of sense.

How can I check if both channels were accidentally inverted (relative to the source)?
 
#5
That s much harder without a scope, if a battery cannot be used so you can see the cones move out.
Use a scope and compare the input and output waveforms. Both should be of the same polarity at all times.
 
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#6
If it is an amplifier with only unbalanced inputs, the only place where you could swap the polarity without getting other trouble (such as lots of hum) would be at the output terminals.

Instead of an oscilloscope, you could use a voltage divider and a computer with a sound card. You can obtain a suitable test signal by saying oooo... (as in book) in a microphone and recording it. Play it back over the amplifier and record its input and output waveforms, and use an audio editing program such as GoldWave or Audacity to compare them.
 
#10
If you have another amp (that you trust for correct phase), play a mono signal with each channel through one amp. Adjust levels to match and then check for eventual lacking bass.

Or you could look for one of those people who can hear absolute phase ... 😊
 
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#12
Make just one phase shifter: assuming XLR for the balanced input, take a male and female, pin 1 to 1 (ground), pin 2 to 3 and pin 3 to 2.
Apply this phase shifter to one channel and follow the various tests above.
Without a scope, you will never know if the 'absolute' phase (input to output) is non-' or inverted, only the relative phases between the two channels.
Does it really matter, do you think you can hear it?
It's only valid for live stage performances with monitors and the FOH. In general, all pro equipment is absolute phase flat (zero degrees I to O) for this purpose only, Or we're generating squeeks all the time. (save intended time delay's)
 
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#13
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#14
It takes a bit of exercise, but ones you learn to hear out of phase it can be pretty obvious.
In phase and from the listening position, most music will have the vocals in the middle and a well-defined sound stage.
Out of phase will have a 'hole' in the middle and a sound stage that seems to extend left and right out of the speakers.
From that same listening position, moving your head over from left to right will clearly reveal the above.
Practice and experiment by changing the polarity of one speaker.
Hearing reversed polarity from both channels is another matter and far more difficult, as Marcel explained. Personally, I never managed to hear the difference.

Hugo
 
#15
MarcelvdG said:
There have been double-blind tests done showing that there are people who can hear polarity, at least when listening to monaural recordings of asymmetrical waveforms over headphones or in acoustically dead rooms.
Click to expand...
Most interesting! A necessary longread.
Would those people who can hear this phase polarity be musicians, piano tuners and/or with absolute hearing?
Instead of my 'phase plug' (#12), a switch in a small box (xlr-in, xlr-out, straight or crossing 2/3) but with no indication on the outside can reveal ones sensitivity to phase errors. Hugo (#14) gives the guide.
Also to keep in mind, the distance between our ears, shifting from amplitude directional sensing to phase directional sensing, somewhere around 2 - 3 kHz. I've never understood why loudspeaker manufacturers are designing two way systems with the crossover in that range... or maybe just because? It is so delicate there.
 
#16
Indeed, very interesting.

I was ones invited to listen to a 3-way system with passive crossovers lying behind the speakers.
The man had quickly connected the speakers, but at the very first moment of listening, I told him that the one tweeter had been wired with the wrong polarity.
He didn't believe me and refused to take a look. But I insisted, and finely he had to admit the mistake.
This was long ago, when my ears were far better.

Here's a nice test: https://www.audiocheck.net/audiotests_index.php
In particular, the Stereo Imaging Tests are relevant to this thread.

Hugo
 
#17
454Casull said:
Thanks, yes, this makes a lot of sense.

How can I check if both channels were accidentally inverted (relative to the source)?
Click to expand...
That would be difficult to do without the use of test equipment, such as an oscilloscope. You might try a recording that has a good bass drum passage in it. This might occur: if both channels are correct in polarity, you would experience a pressurization effect when the big drum was struck with the beater. If both channels were out of phase together, then you'd notice a rarefaction, rather than a pressurization on the initial strike of the drum. I noticed this listening to Telarc's dreaded bass drum!
 
#18
egellings said:
...you would experience a pressurization effect when the big drum was struck with the beater. If both channels were out of phase together, then you'd notice a rarefaction, rather than a pressurization on the initial strike of the drum.
Click to expand...
I assume the beater is outside the drum, so when that very drum is hit, the rarefaction will happen first.
Being a fencer myself years ago, the reaction speed of humans is limited down to 50 milliseconds at best. For fencing, it needs training for one year at least to become near this time par. Ok, the ears are closer to the brain then the limbs, but how good are the ears trained? 50 milliseconds require a (sub) 20Hz drum beat. A large drum is needed... with plenty of space inside to envelope a beater if choosen.
 
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