Best way to connect a sub to a GC?

[sounder]

I'm building a powered sub for my GC, my KEF's just don't do it for me on the very low end. I know the GC has plenty of low bass because it drives my AV system sub well.

I have two options, one is a plate amp; and the other to use a mono GC. If I use the GC, how would I connect it to the main GC amp? Do I just split the input with a "Y" connector and if so, how would I attenuate all three channels together. It would have to split after the pot I suppose. Almost like a pre out. It seems simpler, operationally, to run the L and R speaker channels through the sub GC but is electronically more complicated.

What type of circuit would be needed to combine the speaker outputs and reduce the signal to input level or even to combine the low level signal int one.

The plate amp seems the simplest, a no brainer, but the 3 GC channels seem a much more elegant. solution

[Lligior]

my best guess is to first buffer the left and right input, then put the signals into a summing opamp, then into another opamp with gain control and finally a low pass filter. The low pass filter can be passive, 1st order made from a cap which would be possible because the gainclone input has a known impeadance. you are looking at four opamps, a few resistors, a capacitor, and a pot. Add in two regulators for the power supply and put it in the gc case and you should be good to go.

[Immo_G]

Yeah, i'd go with Lligior's suggestion, i simulated this for the same purpose (sub gainclone). I had a summer op amp circuit, then a low pass op amp circuit, then a volume.

[sounder]

Thanks for the suggestions, I'm going to give it a try

[ir]

what i did when builinng my sub amp was feed a l/r line out from the main amp then at the crossover, put them thru a 220K resistor (each) then into summing opamp. seems to work fine. the individual channels still maintain their seperation and the sub amp get it's mono feed

[sounder]

Ok, I've built a bridged 2x inverted LM3875 sub amp following National's tech notes for bridged amps . It works OK on the bench w/ a junky speaker I had lying around, can't really crank it up because the speaker is not a woofer and it distorts like crazy .

My problem is I have a 315 mV offset voltage Ouch!!

Question, how does this manifest itself in the audio, and

Which values would I have to tweak to get rid of it? I followed the values shown within about 5%. ( the toleance of my ca$75 multimeter) Can I place a trim pot in series w/ one feedback resistor and adjust?

Thanks in advance.