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December 15, 2019, 12:21:11 AM

Author Topic: Phase inverting preamp  (Read 251 times)

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Offline ejb14

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Phase inverting preamp
« on: August 09, 2019, 08:12:03 AM »
Hello -

I recently acquired a Croft Micro 25 preamp, which I understand is phase inverting. When I switch speaker wires (red wire to black input, black wire to red input, etc), I can definitely hear a difference with a phase test track on the HiFi News test LP.

So that solves the speakers problem, but what about headphones?

To get the best performance from this preamp, could I reverse the plus and minus on the headphone jack (akin to reversing the speaker terminals, just doing it at the headphone jack).

For a more flexible solution, would adding a phase inverting switch be possible? If so, any recommendations on a switch? I am willing to pay more for quality.

Thanks for your help - you guys have always been great.
-Ed

Offline Paul Birkeland

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Re: Phase inverting preamp
« Reply #1 on: August 09, 2019, 11:51:56 AM »
If you're talking about track number 2, that is reversing the phase in one channel, but not the other.  This is not the same as an inverting preamplifier/amplifier, which would invert phase in both channels.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline ejb14

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Re: Phase inverting preamp
« Reply #2 on: August 09, 2019, 02:30:46 PM »
Hi Paul, thanks for the reply!

Yes I was talking about track number 2 - the description for the track describes that if you have the speaker terminals reversed (among other things reversed) that this is the track to use to determine if you system is wired out of phase.

So, is it not a good test then?

Is the pre-amp being phase inverted an issue?

Thanks
-Ed

Offline Paul Birkeland

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Re: Phase inverting preamp
« Reply #3 on: August 09, 2019, 04:38:50 PM »
There have been some threads on other forums about how vague the description is on this track, but playing this track is equivalent to switching the positive and negative connections at one speaker, not both. To hear what an inverting amp or preamp would sound like, you would want to switch the red and black wires at both speakers, though this is not always super audible, and the configuration that you like more may change between albums/tracks.

The SEX amp is non-inverting BTW.  If you'd like to invert the output of the SEX amp for headphones, you could switch around some wiring on each output transformer to easily do so.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline ejb14

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Re: Phase inverting preamp
« Reply #4 on: August 09, 2019, 05:59:02 PM »
Ok - sounds good Paul - thanks for the education!
-Ed

Offline kgoss

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Re: Phase inverting preamp
« Reply #5 on: August 10, 2019, 03:53:48 AM »
For what it’s worth I had one speaker out of phase by mistake one time after swapping gear. The audible clue was that the center image was shifted toward one speaker. Paul is right the effect is subtle and it took me some time to notice it and find it the cause.
Ken Goss