Question About Audio Research Preamp Specs

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.

Hi All,

Thanks for the responses.

I have a CJ 17LS2 that bumps up close to the 10X rule with the DNA-500 and I can hear the LF rolloff compared to other preamps.

Knowing that AR really wants 20K, I will look elsewhere.

Dsper

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

As mzkmxcv stated 8 x output to input ratio is even fine.

But in this case the AR preamp would be capacitor coupled output, and the roll off in the low frequency could be too high if the partnering power amp is less than 20kohm input, with the size of the AR’s coupling cap. And so you may miss out on the low frequencies.

There are two way this can be fixed, don't get fooled into needing another preamp.  
dsper
1: If you own the AR pre, tell me the size (uF) of the output coupling cap, (it will be the big ones near each output) and what the poweramp input impedance is you want to use it with, and i’ll calculate at what low frequency it’s at -3db for you.
It’s a simple to correct if too small, with a $20 larger coupling cap, don’t go buying another preamp because of it.

2: Or you can go the other way and get the input impedance resistor (20cents) raised to 50kohm on the poweramp, if it’s fet input no problem, even a 100kohm

Cheers George