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RE: non-inverting preamp w/schematic ...

"I appreciate the concept that what comes out of a preamp is exactly what goes in. IMO, that should include the input-output phase relationship."

OK, I'll take the bait on this one, and ask another question or two.

I used to be very skeptical about the notion of absolute phase mattering, until I read up a bit about the waveforms of musical instruments. And in fact some can really be quite asymmetric between the positive portion and the negative portion of the waveform. This can mean that if there is some other amplifier or transducer in the chain that has an asymmetric response, then it can matter what the absolute phase of the signal that is driving it has. That is to say, the perceived sound could be different depending on whether the asymmetry in the sound source is in phase or out of phase with the asymmetry in the amplifier or transducer.

I suppose one example of a potentially asymmetric transducer is the human ear. Thus it can, I suppose, matter whether the phase-sense of the original sound source is the same, or the opposite, of what the human in the room where the sound is being produced would experience.

Another potentially asymmetric transducer is the loudspeaker. And apparently loudspeakers can really introduce a very significant amount of distortion, which is very likely asymmetric.

Normally these days, an amplifier would not be an appreciable source of asymmetric distortion. But perhaps an SET amplifier could introduce enough (largely second harmonic, and so asymmetric) distortion to matter.

On top of all that, from what I have read, there is no attempt made by most recording studios to preserve any definite absolute phase of the audio signal picked up by the studio microphones.

In the light of all that, I wonder where this leaves someone who cares about absolute phase? If the home stereo amplifier is of low distortion I would have thought that everything that could be possibly be under the listener's control, as far as absolute phase were concerned, could surely be achievable by choosing the polarity with which the speaker leads are connected to the amplifier. (Of course, obviously, left and right channels would be flipped, or not, in unison.)

If there is concern about asymmetric distortion in the amplifier too, then one might want to be able to flip the phase going into the distorting amplifier as well as the phasing of the speaker leads.

But I don't quite understand why one would want a hard-and-fast rule that the preamp should not be phase inverting. If absolute phase is important to the listener, and if there is concern that the power amplifier amplifier has enough asymmetric distortion that it needs to be taken into account, then it seems to me the listener would want to have the option to be able to flip the phase going into the power amplifier.

If, on the other hand, the power amplifier is not considered to be a significant source of asymmetric distortion then everything that the listener could control, as far as absolute phase were concerned, could be done by flipping the wires on the speaker leads.

But I don't really understand, given the recording studios' alleged cavalier attitude towards maintaining absolute phase, why one would consider it important to have one's preamp preserve a fixed, unswitchable phase choice of +1 rather than -1. If absolute phase going into the power amplifier mattered, then you would want to be able to switch it to fit with the recording studio's random choice. If absolute phase into the amplifier didn't matter, then it wouldn't matter whether the preamp was inverting or not; any desired switch of absolute phase could be achieved by flipping the speaker polarity.

Sorry if this went on a bit. I'm just trying to understand the issues here.


Edits: 06/19/17

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  • RE: non-inverting preamp w/schematic ... - cpotl 05:34:26 06/19/17 (0)


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