Summing up L & R line-level for dual subs

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

Some very low frequencies are, still, stereophonic in nature.
If you have two subs, why sum the channels to mono?
I guess I don't understand the theory behind this.

Al,

Thanks for the very thoughtful post. I'll do my best to answer:

1) input impedance of the line level inputs of the subs: Rythmik 12" with A370PEQ amps. Impedance is not listed. Input is through coax, though. Should this be a key data point I would ask the manufacturer - let me know.

2) input impedance of the main power amp: McIntosh MC275 mk IV. According to Stereophile measurements on the "RCA jacks was a high 86k ohms at low and middle frequencies, dropping slightly and inconsequentially to 62k ohms at 20kHz"

3) how long the interconnect cables would be, from the preamp outputs to each of the subs and to the main power amp: all of them are RCAs. From pre to amp: 4 feet of PUrist Audio Museaus - I don't know their impedance. From pre to subs: one coax is 10 feet, the other one is 17 feet of Neotech bulk OFC interconnect cable.

Thank you!

Based on that info I suspect that you would be marginally ok with respect to the effects I described.

Item 8 on this page appears to be applicable to the A370PEQ, and indicates an input impedance of 30K. The parallel combination of two 30K impedances and an 86K low frequency input impedance is 12.8K. That is not ideal in relation to the 3.3K output impedance of the preamp at 20 Hz (ideally you would want the combined load impedance to be 3.3 x 10 = 33K or more), but the result would be a rolloff of significantly less than 2 db at 20 Hz, and probably less than 1 db. There would also be some small phase shift effects. I suspect that neither of those effects would be great enough to be objectionable when you finish tweaking the sub adjustments.

I couldn't find capacitance numbers for the Neotech cabling, but I did find some anecdotal indications that it is on the low side. Between that and looking at diagrams of its construction I think it would be safe to assume that its capacitance is less than 50 pf/foot, which would mean that the total capacitance of the three cables is less than around (17 + 10 + 4) x 50 = 1550 pf. That represents a capacitive reactance (impedance) of about 5.1K at 20 kHz, which is reasonably high in relation to the 245 ohm output impedance of the preamp at high frequencies.

There is always, of course, the possibility that noise or hum problems can arise when long unbalanced cables are used, but that can't be predicted.

Also, fyi, member Bifwynne has mentioned in past threads that a gentleman named Tom Tutay, I believe of Transition Audio Design in Florida, has custom made for him at a reasonable price a combination mixer/buffer for addressing this kind of situation. It presents the preamp outputs with a high impedance, sums the two channels together, and provides a low output impedance.

Regards,
-- Al