Passive summing of L&R for subwoofer signal using resistors... - diyAudio
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Old 24th October 2015, 11:38 PM   #1
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Arrow Passive summing of L&R for subwoofer signal using resistors...

Hi Y'all, I've seen this subject come up here a few times so I figured I would add another post about it in case it helps anyone.

For folks who don't know: if you use only a Y-jack to grab signal from left and right channels and feed that into a mono subwoofer amp you will lose your stereo image to your main speakers -it will turn your stereo into an unimpressive mono. The cheapest and nastiest way to get around this is to put a series resistor in line with the L&R wires before you combine them into 1 jack and send that to your sub. This preserves the stereo image to your main speakers.

I used 4.7k and 3.9k R's and measured how much signal loss there was when compared with 1 channel. I also measured how much crosstalk there was on the left channel while the right channel was given a full strength signal:
Click the image to open in full size.
-As far as the -3.1dB and -3.7dB numbers, I was comparing that to only 1 channel of output. If you compare those numbers to an active summer, you would get -9.1dB and -9.7dB of loss in signal. Pretty significant. Also, ~40dB of crosstalk is going to affect the stereo image of your source material negatively. I think most listeners would be able to detect this in a blind test.

Something that might come in useful for the people who aren't using plate amps with a built in low pass filter for their sub amp is a simple 1st order passive low pass filter using a capacitor after the in line resistors:
Click the image to open in full size.

Here is the rolloff you get for 2 different values of caps in the above schematic using 2-4.7k resistors:
Click the image to open in full size.
-You can choose which one curve best blends with your mains. I think the cutoff points might change somewhat depending on your signal chain.

In all, this is the most simple solution but you do sacrifice some audio quality when doing this compared with going active.

Last edited by shredhead; 24th October 2015 at 11:51 PM.
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Old 25th October 2015, 01:34 AM   #2
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I don't think you need the capacitor if the woofer will have its own crossover.
That RC combination has a breakpoint of 72Hz.
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Old 25th October 2015, 01:50 AM   #3
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Quote:
Originally Posted by shredhead View Post
Something that might come in useful for the people who aren't using plate amps with a built in low pass filter for their sub amp is a simple 1st order passive low pass filter using a capacitor after the in line resistors
You're right Nigel, I was just referring to people who might be using a full range amp into a raw subwoofer driver in a box. I usually cross my sub over at 100Hz with a 2nd order and the 1uF, .47uF option I showed with my signal chain gave a -3dB cutoff point of about 100Hz and 200Hz.

Last edited by shredhead; 25th October 2015 at 01:59 AM.
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Old 25th October 2015, 02:01 AM   #4
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I must admit I dont bother with crossovers in my subs.
The reactance of the sub's inductance is quite high above the subs normal range.

I quite often just use a sub with a tweeter box.
Here is a 1000WRMS 21 inch sub with Pyle tweeter box on top.
Click the image to open in full size.
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Old 25th October 2015, 02:20 AM   #5
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Cool. I usually end up messing with low efficiency, high excursion drivers. They sound pretty crap on the upper end. Is that the Pyle 21"? How does it do on the low end?
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Old 25th October 2015, 10:06 AM   #6
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If you know the output impedance of your source then you don't need to measure the crosstalk with resistors as you can calculate it.
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Old 25th October 2015, 02:46 PM   #7
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Quote:
Originally Posted by shredhead View Post
Cool. I usually end up messing with low efficiency, high excursion drivers. They sound pretty crap on the upper end. Is that the Pyle 21"? How does it do on the low end?
It sounds good. You can really pick up bass guitar notes with it.
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Old 27th October 2015, 05:58 PM   #8
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If you know the output impedance of your source then you don't need to measure the crosstalk with resistors as you can calculate it.
Will you explain how please? I don't know what the source's output impedance in this case so how can I figure that out with the measured values?
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Old 27th October 2015, 06:48 PM   #9
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If you use two resistors R to sum two channels with source impedance Rs, then assuming that the load impedance is high compared to R (which will usually be the case) and Rs is low compared with R (also usually the case) then crosstalk = Rs/(2R+Rs) which can be approximated as Rs/(2R). This is because the summer acts as a potential divider and feeds some signal across to the other side.
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