Preamp impedance mismatch with McCormack DNA2

Ditto what Kal says, the SF Line 2 and 3 SE both sounded great with my DNA500, however they both have quite low output impedence for tube preamps, below 100 ohms in the case of the Line 3. Steve's answer is the conventional "amp input should be at least 10x the pre output" guideline, which I suspect will be fine.

Hello all -

Please note Darrin's reference to the DNA-500. He is correct when he says that it has the same 10K input impedance as the DNA-2 *after* adding the Jensen input transformer. This is also the same input circuit used in any of my SMc Audio monoblock amps (but *not* the same as the original McCormack Audio DNA-1 monoblocks) or any SMc Audio amp upgrade with balanced inputs. I believe there are several members here who have the DNA-500 (or my monoblocks, or my amps with balanced inputs) and their experience should be valuable.

I usually design my amps with a 100K input impedance for maximum compatibility, but using the Jensen (or Lundahl) input transformers forces a shift to 10K (at least for the 1:1 types I prefer). It would be nice if I could offer all of the advantages of the transformer input topology along with high input impedance, but I feel strongly that the strength of this approach more than outweighs this small disadvantage. Using these transformers allows me to simplify my input stage somewhat, and provides a “brickwall” filter against DC and RFI – without the need for blocking capacitors or additional filters. Like many audiophiles, I used to look askance at the use of transformers, but I must say that I have become very fond of them. Used properly, I feel they offer a lot of advantages, and their main drawback is high cost.

I know that many owners of these amps are using tube preamps happily. I haven’t made a list (apparently I should!) but I have heard quite a few mentioned: C-J, ARC, ART, Sonic Frontiers, VTL, Rogue Audio, just to name those that jump to mind. Most of these have output impedances in the several-hundred-Ohm range, but I’m pretty sure that none of them exceed 1KOhm. I have even heard one person say that they tried the DNA-500 with a Hovland HP-100 and liked what they heard, despite the Hovland’s high output impedance (over 2KOhm, I believe). While I would not recommend this combination as a good impedance match, it is interesting to hear that at least one person liked it. Of course, solid-state preamps should work fine, as well as buffered “passive” designs and some transformer-based volume controls (TVCs).

It would be great to hear from owners of these amps about their experience with various preamps. I’ll make a point of noting the models people have used successfully ;-)

Best regards,

Steve McCormack
www.SMcAudio.com

I have a Modwright SWL 9.0 and run it into a McCormack DNA 500.

It is a really fantastic combo. The Modwright has an output impedance of 600 ohms and it drives the 500 without any problem.

I had similar concerns when I first started looking at the McCormack amps. I've got an ARC SP9 preamp (tube hybrid) with GNSC ref mods that has a recommended load of 60k ohms, but it's output impedance is only 250 ohms. I was operating on the mistaken assumption that the ARC recommended load was reason enough to avoid 10k input impedance amps such as the DNA-500 or earlier DNA amps with Steve's balanced transformer upgrade. Bad mistake!

Fortunately for me, I sent Steve an email about this and received a similar response to the one he posted above. Since then I've replaced my Levinson 331 with a DNA-500 and it's working extremely well with the SP9. I am totally ecstatic about the changes the DNA-500 wrought in my system.

You can load the Jensen or Lundahl 1:1 transformers higher than 10K. You have to use a HF termination ("zobel") to get the FR correct. I do it all the time. The impedance is no longer flat if you do, but there is no such thing as a free lunch.