Amp preamp impedance matching...can anyone explain?

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 :)

I don't know if this thread is still active (if not I'll have to find another forum).

I'm driving a Quad 303 power amp from the headphone socket of my Yamaha Amp.

From the published specs:

The Quad Power Amp i/p is 0.5V @ 22K ohms impedance

The Yamaha Headphones o/p is 0.51V @ 330 ohms

It looks like the impedance match is ok, but I had to attenuate the signal with 1MΩ resistors which I soldered into the DIN plug (Quad i/p).

What I want to do now is connect to the aux i/p of a Cambridge amp; the impedance is 47KΩ (the signal requirements aren't mentioned in the specs, but I would think it'll be around 15omV). From what I read here this will be a mismatch one option is to solder 47KΩ resistors across the i/p's (sig to scr) in the phono/rca plug; I don't really want to go inside the amp and change the resistors to lower values.

I'll also need to attenuate the signals perhaps with higher values than 1MΩ.

I wonder what the experts think.

Regards:
-Pete