Amp preamp impedance matching...can anyone explain?

Al, isn't it the case that ARC does not recommend less than a 30K load with their preamps? I know that is true of at least one of the recent Ref series.

Hi Ralph,

Yes, I seem to recall that being true of some of their later preamps and/or phono stages. For the Ref 3, though, the specs shown here, and also in the Stereophile review I linked to earlier, indicate 20K. Although the wording leaves it unclear whether that applies to the balanced or unbalanced outputs, or both.

In any case, given the 30K impedance of the balanced inputs on the Pass, and the measured worst case (20 Hz) output impedance of 1437 ohms for the Ref 3 (which it should be noted is only a bit more than a factor of 2 greater than the midrange output impedance), it would appear that there shouldn't be any frequency or phase response issues. And I'd be surprised if there were any audible distortion issues either, although you can speak to that possibility more knowledgeably than I can.

Best regards,
-- Al

Al and Ralph, both the ARC Ref 3 and 5 literature recommends that the minimum combined load on the Main outputs is 20K Ohms. My amp is the ARC VS-115 which has an input impedance of 300K Ohms (balanced) -- so no problem there. If you pull my threads, you'll see where I got into a problem was when I asymetrically loaded the 2nd Main output with a 20K Ohm load. Main 1 -- VS 115 (balanced) 300K Ohms; Main 2 --self powered subwoofer (SE) 20K Ohms.

As I mentioned in my old threads, the solution was that Tom Tutay designed and built an impedance buffer device that summed the left and right channels without shorting the Ref 3/5 output Mains and enabled me to load Main 2 (self powered woofer) with a balanced input having 330K Ohms impedance. So my overall preamp output impedance load is 157K Ohms -- well above the recommended minimum.

Result -- tighter and more extended bass. Possibly more open midrange. Per ARC, symetrically loading the Ref 3/5 will also extend tube life -- but what do I know???? FWIW.

Al, thanks so much for your explanation - I get it now, although coupling capacitors and such are still beyond the scope of understanding for me :)

The only regrettable part in all this is that it seems impossible to determine good impedance matches based on the nominal output impedance spec that the preamp manufacturer will show. Everyone would have to rely on magazines like Stereophile or audio websites to actually review a certain product and show readers the detailed impedance measurements across the whole spectrum in order to do the correct calculation. This is not a problem for popular brands like ARC or Cary, but the lesser known brands might not get a review, and then we audiophiles are taking an educated guess as to whether a preamp is a good match impedance-wise with a power amp.

Bifwynne, that is a common problem if you are trying to use a balanced preamp with a subwoofer. Many subs only have SE inputs.

Another solution is that Jensen transformers makes a transformer that is optimized for subwoofers. It has a fairly high impedance input that can be run balanced or SE, and has bandwidth to less than 2 Hz so it does not mess up the bass.

But I think you are lucky you ran into Tom Tutay. He does good work :)