Question About Audio Research Preamp Specs


I am trying to understand Audio Research terminology while looking at their product specs.

For example, for the LS28 OUTPUT IMPEDANCE, it reads "600 ohms Balanced, 300 ohms SE Main (2), 20K ohms minimum load and 2000pF maximum capacitance..."

What does the  "20K ohms minimum load" tell me?

I am running a McCormack DNA-500 and it has an input impedance of 10K Ohms. It would seem that the amp would be compatible with this preamp based on the 10X/15X minimum rule of thumb. 

My concern is that the AR spec is telling me use amps with input impedance of 20K ohms and the DNA would not work well.

Thanks for listening,

Yeah, that’s >33x more, I see no reason why they would state that. Hell, even 8x is debated as the recommended minimum.
You're reading it right and your 10 to 15 x is generally accepted. The world will not stop rotating if you use a 10k load.

I think where this goes is that the preamp may not meet full specs if you feed it into a 10k load. Output voltage may be compromised, THD may be a little higher and low frequency rolloff may be earlier. It will probably be fine and not hurt the preamp but may not sound the same in your system as in another one.

The Sterophile reviews for most of the ARC preamps I've seen seem to recommend a minimum of 10K in the test section, biggest concern being low frequency roll off. I've been feeding ARC preamps into 15K for years and they all sounded fine (Ref 2, LS26, LS 25).
I’d recommend against it. I have an LS-28 and a Rogue Stereo 100 which has 200k input impedance and this combination works well.  However I once tried to run a subwoofer with a 10k line level input impedance in parallel (causing the LS-28 to effectively see about 9.5k) and the neither the LS-28 nor the Rogue liked it a bit. Sound was distorted and the power tubes glowed red. The ARC recommendation of a minimum 20k load appears to be there for a reason.
What does the  "20K ohms minimum load" tell me?
Exactly what it says. Most ARC preamps are not recommended for amps with anything less than 30K but this one is good to 20Kohms. Your amps though have a 10K input impedance, which essentially means that the combination will be bass shy.
As others have pointed out, the output impedance of the LS28 **seems** sufficiently low, but that spec is done at 1KHz. At 20Hz, its output impedance is much higher owing to the size of the coupling caps at the output of the preamp. As a result, the 10K load of your amps will result in a bass rolloff.
So ARC is essentially telling you not to use this preamp with your amps.

We developed a direct-coupled output on our preamps so loads like the McCormick are no worries. At any rate, if you want to use a balanced tube preamp with your McCormick, ARC is not an option if you want it to sound right.