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I have the following problem: I have two amplifiers, one is a mono amp for a subwoofer and the other is a stereo amp for two two-way speaker enclosures.

The subwoofer and two-way enclosures have a good overlap in the bass frequencies, however, due to the low-pass filter in the subwoofer amp, there's a phase shift ranging from 60º to 50º in the 0-50 Hz range, the subwoofer being the one "lagging".

And as all speakers are almost the same distance to me (and I can't change their positions), this causes the interference to be not completely constructive at the spot where I listen to them.

But all leading current circuits I see are also high-pass filters, which would be a problem for my sub.

So, how can I shift the phase without causing an dramatic change in frequency response?

Thanks!

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  • \$\begingroup\$ There is no good answer that is low cost. To get correct phase the midrange is delayed about 1mS, and the tweeters are delayed about 2mS. If it is not built into the audio gear, you could try baffles in front of the midrange and tweeters so the sound bounces off of walls before heading your way. Not ideal but... \$\endgroup\$ – Sparky256 Feb 10 '19 at 4:58
  • \$\begingroup\$ Are you sure it is not a mechanical delay of the subwoofer creating tens or even hundreds of milliseconds of delay? Perhaps an audio delay is more desired here, than any phase shift. \$\endgroup\$ – rdtsc Feb 10 '19 at 5:51
  • \$\begingroup\$ You can’t mean 50~60’ down to DC \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 10 '19 at 7:27
  • \$\begingroup\$ active All Pass delay line for time equivalent to 60 deg at 50 Hz but exact phase frequency amplitude measurements are needed \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 10 '19 at 7:37
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    \$\begingroup\$ The search term is 'all pass' filters, go search, here on wikipedia. \$\endgroup\$ – Neil_UK Feb 10 '19 at 7:52
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You want an audio delay line. These basically sample the incoming signal, remember it for a little while, and then send it back out again. As long as you sample at 2X the highest frequency you care about, then there should not be any changes to the spectrum across the delay (but you might need to low pass filter the input to avoid aliasing). The delay lines can be digital or analog, hardware and/or software.

The Teensy ecosystem has an amazing support for audio processing that would make building something like this as easy and connecting some processing blocks on the screen.

If you want to go old school, you could use this amazing chip that stores the samples as analog voltages. It can add a programmable delay between 5ms and 51ms.

If you want something more interesting, you might consider this approach that over-samples a single bit for its simplicity and elegance. I remember back in the day when single bit digital amplifiers first came out using this same trick.

If you just want to buy something, here is a delay board with audio connectors and potentiometers ready to go.

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