Preamp impedance mismatch with McCormack DNA2

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.

True enough, Art, but the RC networks result in bandwidth limitations I do not care for. I find I prefer the sonics with a 10K load and no RC termination.

Steve M