McCormack DNA-2 DLX balanced input impedance

Hi Bundy -

The amp should be off for this test. Set your multimeter to read resistance in Kohms, and set the range (if it is not an autoranging meter) to 20K or 200K full scale. The XLR connector should have the pins labeled on its face, but just in case, it's laid-out like this: pin 1 - top right, pin 2 - top left, pin 3 - bottom center. Put the black probe in pin 1 and leave it there. Put the red probe in pin 2 and check the reading, then move it to pin 3. You will get readings that are either beyond full-scale (no reading), or under 10Kohm. If the reading is high (beyond full-scale, or no reading) then you will need to add the loading resistors. Otherwise, you are ready to go.

I hope it works-out well for you.

Best regards,

Steve McCormack
www.SMcAudio.com

Thanks Steve for the great information. I am a very clueless electrician but have access to a multimeter. Could you (or anyone else) describe with a little more detail on how to test the XLR input's resistance? For instance: Should the amp be on or off? Do you check pins 2 & 3 together or individually to pin 1? That kind of stuff.

Hello AudioGonners! Here is the story...

The DNA-2 has both balanced and unbalanced inputs but is not a "true" balanced amplifier, so there is a high-quality summing amplifier following the XLR balanced input. This sums the balanced input signal and feeds the unbalanced result to the normal input. This summing amp has a very high natural input impedance (multi-Megohm), so you normally use some loading resistors to tailor the input load to whatever you like. Well, given that life isn't perfect, some of the early DNA-2s left the factory without these load resistors installed. This lead to various performance problems when the balanced input was used, from hard, glassy sound, to instability and low-level oscillation. The problem was quickly identified and the resistors added, but that still left a few DNA-2s without their input load resistors. Sorry, but I do not have any serial number data.

It is easy enough to check your DNA-2 for these resistors. The load value chosen was 10Kohm, so you simply have to use a multimeter to measure from pins 2 & 3 to pin 1 to see if the resistance measures around 10Kohm, or is way up in the Megohm range. Please keep in mind that this relates only to the XLR inputs - the RCA unbalanced inputs are fine.

If you find that your DNA-2 is lacking these loading resistors, you have a few options. You could ship the amp to McCormack Audio in Virginia (www.McCormackAudio.com) for service, but this is time-consuming and involves the all-too-real risk of shipping damage. You could have a local technician attempt to do the work, but this involves partly disassembling the amp, and the DNA-2 is not exactly simple. My suggestion is to install a pair of small 100Kohm load resistors inside each male (amp end) XLR connector. A good technician (or your cable supplier) should be able to do this without too much trouble, in my opinion. How difficult this job may be depends on the specific connector used. Many cable companies use Neutrik connectors, and they are pretty easy to work with. Connect the resistors from pins 2 & 3 to pin 1 in both XLR male connectors. (I happen to be partial to the 100K load value, but the actual resistor value is not terribly critical, and values in the 10K - 500K range will work. Use a good quality resistor, though. The Vishay VSH series are ideal, but any good 1/4W metal-film will work.)

Finally, you can always send the amp to me for upgrade, and I will be happy to do it for you ;-)

Best regards,

Steve McCormack
www.SMcAudio.com

SMc changed the balanced and unbalanced inputs on my DNA-2 to Jensen transformers when I had it there for upgrading. I'm not sure what changes resulted for input impedance, but it eliminated the circuit boards and jumper switches. I'd be interested to know what McCormack Audio of Virginia is doing to mod the inputs on these amps.

Call Mccormack @ smcaudio.com and ask him. He will tell you what you need to do and go from there.

I am aware of the SMC upgrades. This was an inexpensive mod to alter the input impedance only. They seemed to indicate that I should not use the XLR inputs until doing this.

I have not heard that McCormack of Virginia was offering upgrades of any kind to the DNA-2. What you may have heard is that Steve McCormack's own firm is offering such (and they are worhtwhile). smcaudio.com is where you might look.

Also there is a related thread currently open discussing the general changes visa vis McCormack (Steve McCormack) and Conrad-Johnson (McCormack of Virginia).