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I trying to find an amp that I can shift the phase 90 degrees, or an add-on that'll allow me to shift the phase 90 degrees.

Does anyone have knowledge on a brand or model that has made such products in the past?

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    I think we need more information. Are you talking about a musical instrument amplifier, a home audio amplifier, a differential op-amp, a discrete circuit design, something else? Also I don't think questions about specific brands or models is encouraged here. Finally, it might help you get useful answers to talk about why you are looking for a phase shift that is not 0 or 180. – Todd Wilcox Mar 17 '15 at 21:57
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    A quick Google search of Bode plots seems to show that no amplifier design (regardless of the type of amp) is phase linear through its useful range. The linear portions are usually either around 0 or the design is inverting and the linear portion of the plot is around 180. I'm not sure how you can build an amp stage that shift around 90 and stays there without passing through 90 on its way to wilder skies. This is definitely where I'm wondering what you are trying to do with this unsual phase shift search. – Todd Wilcox Mar 17 '15 at 22:01
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    The only way I can think of doing a frequency-independent 90' (or anything other than 0 or 180) phase shift is through DSP, which is probably not what you're looking for and could result in some unexpected artifacts. I'm also wondering what you're trying to do. – biggvsdiccvs Mar 17 '15 at 23:31
  • Since the OP is keeping silent, I'm going to make a wild guess that this has something to do with dynamic speakers or magnetic pickups. – biggvsdiccvs Mar 19 '15 at 0:59
  • My guess is that OP is trying to get the phaser equivalent of that Roland stereo chorus amp... – player3 Mar 19 '15 at 18:21
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That's called a Hilbert transform filter. It's easy to do for a fixed frequency but can only reasonably approximated (you can't implement it "perfectly" as perfect output also "depends" on future input) using DSP if it is supposed to cover a larger frequency range.

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    Nice. Upvoted. Here's a link for the curious: iowahills.com/A5HilbertPhasePage.html. Now I REALLY want to know what the OP is trying to do. :-) – Todd Wilcox Mar 18 '15 at 12:08
  • I'm interested as well, I feel like OP is trying to get the phaser equivalent of that Roland stereo chorus amp which is in the vein of many of my experiments. I'll add that somewhere I saw someone take the approach of splitting the input signal into a few different frequency bands and using a DSP Hilbert filter on each to get a more even shift across the range of the instrument, if I find it I'll share the link. – player3 Mar 19 '15 at 18:20
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It is very easy to shift the phase by 180 degrees (essentially invert the signal "upside down"), so even very simple pre-amps may feature this (the signal may be inverted multiple times during amplification, the question is only odd or even number of times).

Shifting a realistic signal with complex spectrum by other angle is very difficult (simple delay is not enough as it must be frequency dependent). I suggest to look between sound processors and other effect tools.

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