Audio Research LS 26

Must have remote and 2 sets of pre outs since i have 4 mono blocks.

Like most ARC line stages and preamps the LS26 has a recommended **minimum** load impedance of 20K, although it is not made clear in its documentation if that applies to the balanced or unbalanced outputs or both.

Therefore a point to keep in mind, given that you are using two amps per channel, is that as with some other ARC line stages the two outputs of the LS26 for each channel are most likely driven by a single output stage, with the two connectors for each channel (both balanced and unbalanced) simply being jumpered together internally. The same goes for many preamps and line stages from other manufacturers which provide two pairs of outputs, BTW. Which means that if you connect two monoblocks on each channel the output stage will see a load impedance equal to half of the input impedance of each amp (assuming the amps are identical), just as it would if you used a splitter externally to the preamp. And with results that would could very possibly be worse than using a splitter, because if a short splitter is placed near the two amps for each channel the cable capacitance seen by the preamp’s output stage would be essentially just the capacitance of one cable rather than two.

Since the output impedance of the LS26 almost certainly rises to much higher levels in the deep bass region than its specified values of 350 ohms unbalanced and 700 ohms balanced (that rise probably being the reason for the 20K minimum load recommendation), if your amps do not have input impedances of 40K or more the result is likely to include further degradation of the bass weakness you referred to.

Good luck. Regards,
-- Al

Hi Al

Thanks for the info, I´m not the most technical person in the world but i was able to follow what you mean.  

I`ve just changed out two of my monoblock so that i now have a pair of Vincent sp991 plus on the bass tower  ( Input inpedances 47K) and a Audio research HD220 on the panels  ( 300K ohms Balanced, 150k ohms SE)     Big of a difference between the Vincent and AR HD220, I could sell the Vincent and see if I can grab another HD220 or something simular??  

Which outputs would be best on which??   RCA to Vincent and XLR to HD220 or??

Cheers
William

Good! You won’t have an impedance compatibility issue using the LS26 (or most other preamps) with that combination of amps, or with a pair of HD220s if you go that way.

Which outputs would be best on which?? RCA to Vincent and XLR to HD220 or??

Just a guess, but based on my general knowledge of the two brands I suspect that XLR to the HD220 and RCA to the Vincent would turn out to be preferable. I can’t say that with any certainty, though.

Big of a difference between the Vincent and AR HD220, I could sell the Vincent and see if I can grab another HD220 or something simular??

You’ve probably seen Stereophile’s review of the IRS Beta in the past, but I’ll note some comments in it that I think are particularly important:

First, based on the impedance characteristics and the related comments shown in the measurements section I would not necessarily expect much similarity between how a given amplifier would perform on the woofer towers and how that same amplifier would perform on the mid/treble section. The impedance characteristics of the two sections are drastically different, and are unusually challenging on the mid/treble section. And as noted in JA’s comments:

The impedance of the mid/treble panel drops significantly below 4 ohms for much of the time, reaching minima of 3 ohms at 150Hz, 2.6 ohms at 720Hz, 1.5 ohms at 2750Hz, and approximately 1 ohm at 23kHz. It will therefore present any driving amplifier with an extremely demanding task in terms of being able to deliver current, particularly in the lower midrange where the bulk of musical energy lies. On the positive side, the extremely low moving masses involved in Infinity’s EMI drive-units and the well-damped nature of the fundamental resonance of each unit will not give the driving amplifier any problems with back EMFs, as is the case with some conventional dynamic drivers.

An important point that should be noted is that the overall low nature of the impedance means that the speaker’s frequency balance will be much more amplifier-sensitive than normal. Certainly, a traditional tube amplifier with a rising output impedance with frequency will produce a sound that is much mellower in the treble than will a modern high-performance solid-state design. This was certainly the case in my experience.

And as noted earlier in the review:

When Nudell arrived, he insisted that we try using tube electronics. He even brought along one of his own Audio Research SP11 preamps (the man owns five of them!).... Nudell confirmed that the Betas, like Infinity’s other top systems, were designed in conjunction with Audio Research tube electronics, so it was hardly surprising that they sounded a little less decent with solid-state electronics, no matter how "accurate" these may have been. This report, then, applies only to the sound of the Betas with some of the best tubed electronics available. I think I can state with confidence that the system must be so used in order to fully exploit its performance capabilities.

Finally, if you experiment with amplification be sure to keep this in mind:

Because the servo control involves a large amount of negative feedback, signal polarity is crucially important. The servo connecting plugs are polarized, and can only be connected one way, but if the bass amplifier is polarity-inverting, or if the woofer cables are reversed, plus for minus, the negative feedback becomes positive and the system will go into violent full-power oscillation at around 35Hz, which is (I can assure you!) one of the most frightening (and potentially destructive) sounds you will have ever heard! This can also happen if everything is phased properly but the bass amplifier has very high gain, which explains the "Open Loop Gain Comp" switch. So, regardless of how absolutely certain you may be that your bass amp is noninverting and has the right gain and the speaker cables are properly polarized, you should always set the crossover’s Woofer Level control all the way down before turning everything on the first time, and raise the level slowly to ascertain that everything is okay.

That’s about all I can offer at this point. Good luck. Regards,

-- Al