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Bi-Amping Discussion

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Modern DSP software can make it incredibly easy to bi-amp or tri-amp with little error on the behalf of the user if you possess some basic knowledge. I have written a number of articles of doing just that with both Acourate and Audiolense. I have indeed ripped out passive XO's in numerous speakers and achieved better than the passive XO that was in them. This is coming from a diyAudio enthusiast, who spent a great deal of time designing and building passive XO's for years.


The reason is that modern DSP software uses linear phase digital XO's. Linear phase XO's sum perfectly in both the frequency and time domains. This short article explains the advantage: http://www.acourate.com/XOWhitePaper.pdf


Further, the XO slopes are steep, so the drivers acoustic bandpass is "convolved" with the digital XO slope and therefore the digital XO slope becomes the acoustic slope. Here is a quick example to illustrate the point:


bandpass xo example.png


While this is a 3-way digital XO'd system, here were are looking at a band-pass digital XO filter (light blue trace) that is set for 45 Hz and 630 Hz. The drivers are JBL 15" woofers and the blue and red traces are the acoustic measurements of both left and right drivers. What you see is that the drivers acoustic response is greater than the digital XO bandpass at both ends of the spectrum. So when the drivers acoustic response is "convolved" with the digital XO bandpass filter, it is a) within the operational limits of the driver and b) the acoustic response is now the digital XO bandpass, which if you have read the linked article above on "thoughts about crossovers", sum perfectly in the frequency and time domain. Both on and off axis from an acoustic perspective.


For sure, it take some know how, but for me I would never go back to passive XO's. Aside from the description above, with digital XO's one can then time align the drivers so all of the direct sound from the speaker is arriving at your ears all at the same time. You can also linearize each driver and convolve the linearization into the digital XO before applying an overall frequency response adjustment. This also allows one to put big solid state amps directly connected to the woofer for ultimate low frequency control, but choose a small wattage tube of Class A amp of your choice on the mids and/or tweeter.


This is all done in the digital domain and requires multi-channel DAC's depending on bi-amp or tri-amp setup. I currently run a tri-amp setup with dual Rythmik F18 subs crossed to dual 15" JBL woofers per cab and then crossed to a compression driver/waveguide combo using a flea watt Class A Pass amp. So the convolution filter is hosted in JRiver or Roon's convolution engine and I use all 3 stereo analog outputs of my Lynx Hilo connected directly to 6 amps and control the volume inside JRiver and Roon.


With some finesse, one can achieve the ideal minimum phase response of a loudspeaker at the listening position with a frequency response tailored to your preference, time aligned drivers, including subs, reduce group delay to below audibility, no extra AD/DA conversion, and flat phase response at the listening position, with both channels perfectly matched in both the frequency and time domains to give you a perfect listening window into one's music. Personally, I love it! Lots of fun.


Happy 2020!

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6 hours ago, firedog said:

Much of this discussion is a good argument for internally powered speakers where each driver has it's own amp, and crossovers are digital and designed for the individual drivers. 


I agree with the first bit--my 4-way active speakers are all analog.😀

The designer was not happy with the sound of doing all the work digitally.

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