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Old 14th October 2003, 11:32 AM   #11
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Hi Steven and Nelson,
Yes you are both correct - those Cab Savs last night took my brain away.
I got it into my head that the - input was at virtual earth, but of course with the + input driven this is not the case.

Thankyou Steve, your circuit will find a good home.

Eric.
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Old 14th October 2003, 12:22 PM   #12
Steven is offline Steven  Netherlands
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You're welcome.

Steven
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Old 15th October 2003, 12:03 AM   #13
PRR is offline PRR  United States
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> I haven't seen this circuit before, so I think it is original

The circuit is very old.

The constant-impedance aspect may well be novel.

The opamp works differently + or -. It should have excellent CMMR or the "sound" will change just due to opamp nonlinearity, perhaps masking the phase/polarity-correction effects.

> walk to your loudspeakers and reverse the wires

You could add a DPDT switch on each speaker. You could use relay(s): one 4PDT behind the amp; or two DPDT, one per speaker, with 12VDC switching signal. Of course now we can talk about the effects of switch/relay contact distortion.

Do you hear a polarity difference on most records, or just a few? My guess is that most instruments are relatively insensitive to polarity, and that many recording sessions end up as random phase where neither polarity is "correct". Of course the classic example where polarity DOES matter: large bass drum facing the mike, beat from behind, the initial transient is positive pressure and large enough that you can tell. But how do you know which way is "right"? Maybe the drum was beat from the front, or faced away from the mike? Or maybe the artist wanted the sound of initial negative pressure?

BTW: I forget the implementation, but there were AM Broadcast limiters that would invert phase every few seconds. You see, in AM you can only go 99.9% negative modulation without gross distortion, but there is no physical limit on positive modulation. If all signals were symmetrical (on peaks over the long run), this fact would be useless. But Male Speech is often asymmetrical. These limiters would sense that and invert phase so the higher peaks were on the Positive Modulation side of the transmitter. If two men were talking, it would jump back and forth. On music, occasional small asymmetries would flip the polarity a few times a minute. Of course at the far end of an AM sky-wave, all this monkey business was inaudible, except in that you could get a LOUDER signal (the FCC eventually limited us to 125% positive modulation, about 1dB increase of loudness on the most asymmetrical signals; and my station choose to stay inside 100% both ways so no gain of loudness.) But even in the operations room on good direct monitoring, it was amazing how little effect changing-polarity had on the sound of the records of that day.
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Old 15th October 2003, 02:16 AM   #14
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Default Polarity Is Mission Critical............

"The circuit is very old.
The constant-impedance aspect may well be novel. "

The concept may be old.
AFAIK the constant impedence aspect IS novel - that makes this circuit novel.

"The opamp works differently + or -. It should have excellent CMMR or the "sound" will change just due to opamp nonlinearity, perhaps masking the phase/polarity-correction effects. "

With decent op-amps and standard line level this should be no problem at all.

"You could add a DPDT switch on each speaker. You could use relay(s): one 4PDT behind the amp; or two DPDT, one per speaker, with 12VDC switching signal. Of course now we can talk about the effects of switch/relay contact distortion. "

Yes a relay at the speakers would work.
However switching polarity upstream of the amplifier keeps the amplifier and speaker in the same polarity relationship, and this is important.

"Do you hear a polarity difference on most records, or just a few? My guess is that most instruments are relatively insensitive to polarity, and that many recording sessions end up as random phase where neither polarity is "correct". "

I hear differences on most recordings.
Electric instruments are least sensitive, acoustic are sensitive, vocals are mission critical.
Random polarity recording sessions indicates technical inability or just plain slackness.
On modern DAW software, a mic with inverted output is clearly visible, and audible.

"But even in the operations room on good direct monitoring, it was amazing how little effect changing-polarity had on the sound of the records of that day."

I suspect that your monitoring equipment was not up to the task, and that the monitoring room required acoustic treatment.
Crossovers causing phase anomilies masks polarity information.
On phase linear speakers, polarity is clearly audible, and when wrong is clearly inferior.

Eric.
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