Summing up L & R line-level for dual subs

Probably too well. Combining the L/R out of the Lamm will probably sum the output to the main amp/speakers. Result: No stereo.

If your subs have L/R inputs, you should connect to them independently and let each sub do the summing. Otherwise, you need an isolated or buffered Y connection.

Why would you not want your subs to be in stereo as extensions of L/R main speakers?

If the second set of outputs on the Lamm LL2 are buffered than you can use a Y cord. If not you will have to add a small buffer stage.

You could always try a Y cord (as it will not hurt anything) and if the main system ends up in mono you will know that the Lamm does not have separate buffered outputs.

Thanks for the inputs. I'll ask Lamm if the outputs are buffered or not. Good tip!

Karl, you wrote "If your subs have L/R inputs, you should connect to them independently and let each sub do the summing". Yes, my subs have L & R inputs, but how should I connect them independently? From preamp left output #2 have a Y connector to go to sub # 1 and to sub #2 connecting to left input in each one, and do the same on the right?

Csontos, the reason being Mr Geddes has put much more time than me into this and that's his prescribed way...so if I want to give the approach a fair shot...it follows I should follow the prescription. It is indeed an intriguing approach.

Thanks all for the input!

My subs have L & R inputs, but how should I connect them independently? From preamp left output #2 have a Y connector to go to sub # 1 and to sub #2 connecting to left input in each one, and do the same on the right?

What is the input impedance of the line level inputs of the subs? Or, if you can't readily determine that, what model subs are they?

It is common for the line level inputs of powered subs to have input impedances of as little as 10K, or even 5K in a few cases. Driving two of them in parallel would cut that value in half, from the perspective of the preamp.

According to JA's measurements in Stereophile the specified 250 ohm output impedance of the LL2 rises to 3.3K at 20 Hz, which is too high to drive that kind of a load without significant rolloff of the deep bass occurring.

Also, if the LL2's two sets of outputs are not independently buffered, which I suspect is probable, and you use a y-adapter to connect each of the two channels independently to both of the subs (letting the subs do the summing), the load impedance seen by the preamp's output stage would be reduced even further by the input impedance of the main power amp. In other words, in that arrangement the left and right channel output stages of the preamp would each be driving three loads, the two subs and the main power amp.

Let us know what the input impedance of the main power amp is, as well as the input impedance of the subs.

Also let us know how long the interconnect cables would be, from the preamp outputs to each of the subs and to the main power amp. If you were to drive all three cables from the same preamp output stage, the combined capacitance of all three cables would affect the signal that is seen by the main power amp and speakers, potentially resulting in perceptible rolloff of the upper treble if any or all of the three cables are particularly long and do not have low capacitance per unit length.

Regards,
-- Al