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 Post subject: 2:1 Speaker System
PostPosted: 29 Dec 2017, 13:19 
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Joined: 16 Jan 2015, 16:41
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I am new in this captivating field of DIY amp and speaker building hobby. I have a question that I could not find answer anywhere. My question is as following:

In 2:1 Speaker system there are two main speakers and one Woofer. I know the system uses active crossover before amp section. In stereo mode, there are two channels (from the music source), one for right, and another for left. For right channel, active crossover makes two lines- one "High" one "low" at the same time for left channel active crossover makes two lines- one "High" and another "Low" (all lines have common ground). So, we have two "High(s)" that we can amplify and send them to two midrange speakers.
But, how can we make two "low(s)" in one channel and amplify and send it to one woofer? My question is how can we convert two "lows" into one channel? So that we can amplify and send it to one woofer speaker???
I request you to help me understand the above problem.


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 Post subject: Re: 2:1 Speaker System
PostPosted: 30 Dec 2017, 14:56 
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Joined: 06 Apr 2009, 10:08
Posts: 1420
Location: US Pacific Northwest
jhorapalok wrote:
My question is how can we convert two "lows" into one channel? So that we can amplify and send it to one woofer speaker???
The answer is actually quite simple. In an amplifier chain, all stages except the last are voltage amplifiers. Only the last stage is a power amp to drive speakers. This simplest way to do the summing is to take a voltage tap from each channel, isolate them so that whats on the output can't get back to the main signal chain (this can be done with emitter or cathode follower stages, but many different topologies are possible), sum the two signals by tying them together, feed the result through the desired low pass filter, and then feed the resultant signal into a driver for the combined "low frequency" center channel.

This is ok to do for low frequencies because it is almost impossible for your ears to localize the direction from which a low frequency sound is coming. The audio wavelength is just too long in relation to the distance between your ears. What was discovered, is that if your ears still can localize the higher frequency sounds, then your brain will "infer" the direction of the low frequency sound from the higher frequency components. It's a neat trick that your brain does to overcome the physical shortfall in the hearing system.

This fact allowed Professor Amar Bose to produce his first "2.1" system with a single center channel low frequency driver. This significantly reduced the size and cost of the main satellite mid and high frequency speakers and resulted in only a single high cost low frequency unit.

There is ongoing debate as to whether the combination of the low frequencies is a good or bad thing for listening. It is almost universally understood that the 2.1 topology is superior to a simple "stereo" topology for audio/visual and gaming systems. In these instances there can be significant content at 20Hz and even as low as 15Hz (which can be felt if not heard). Having a single high power driver to effectively cover these frequencies can greatly enhance the viewing and/or gaming experience. However, for general musical listening where there is very little content below 30Hz, and the content at the low frequencies generally has significant harmonics at the higher frequencies, the topology has it's problems. First, it can confuse the musical soundstage because instruments get split between the center and left/right channels. The second is that even minor mismatch of amplification between the center and left/right channels can severely color the music.

In the end, it is mostly a matter of personal taste which topology is preferred. But here are opinions on both sides of the aisle.

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