Background
I have a Denon 5.1 av receiver and have bought some new speakers. The only restriction (from my wife) was on size so I have managed to get hold of a set of speakers for the Sony DAV-IS10 (http://www.trustedreviews.com/Sony-DAV-IS10-DVD-Home-Cinema-System-review) which should work. Five 10ohm satellites and a mid/bass dual driving woofer. The dual driving woofer is required as the satellites are very small and don't output a full range. What I planned to do was have the dual driver run off the receivers sub out. I built a lm3886 dual mono amp from chipamp.com driving each woofer as the old amp was fully integrated and needed to work with the original Sony DVD/receiver. I now realise that the Denon receiver sub out is set to low pass at around 100-250hz which is no good for the dual driver.
What I have at the moment is 5 satellites and the sub working with the receiver but the mid-woofer is not really receiving any signal. What I want to do is to use a line level adapter on the speaker outs to draw a 1v line (speakers are around 27v) to the input of the mid-woofer amp similar to http://www.dougcircuits.com/lineout.html. I believe this is simple enough for a single speaker circuit but then I would have to choose which circuit to use. The left and the right channels clearly have different signals at different times. The center speaker is where the bulk of the dialogue sits but I don't know if it contains all the range and effects the left and right get. Ideally pulling one line off the center speaker would be easiest (if that works then great) but I am considering whether I need an output from the left AND right or even all three to get the full range through to the mid-woofer.
Question
What I have not been able to work out is: can I combine these line level signals into one RCA input for the mid-woofer amp? It would be a separate line level adapter at each speaker output but then combined into the mid amp. Is this even necessary… does the center channel put out enough range to cover the mid for the left and the right?
I know that you cannot do this for speaker level outputs as there will be interference and likely shorting of the amp. I believe that the Denon amps use common grounds so that side of things should be okay.
Best Answer
With a bit more searching I think I may have come across the solutions.
https://sound-au.com/articles/audio-mixing.htm
It looks like, for a simple combination of inputs a simple resistive circuit as shown in the question with simple mixing of the outputs is fine, if gain control is required on the individual inputs then it is better to use a virtual earth mixer is best.... so I think what I propose should work.
I'll leave the question open for a couple more days to see if anyone can add any further inputs.
update:
Yes that solved it. I have three pots dividing the voltage across the three speakers (left, right, centre) at the speaker outs on the receiver. The outputs of the pots are combined (mixed) into one input for the mid-woofer crossover and then through to the amp... pretty much perfect now... one day (when we get a bigger home) ill be able to get proper speakers and get the best sound out but this is a great workaround for a small house.