The McCormack DNA HT-5
The McCormack multichannel amp is an impressively built amplifier offering five channels at 125 watts each into 8 ohms. Built in the Conrad-Johnson factory in Virginia, it shows the exact attention to manufacturing detail that CJ puts into every one of its own products - which is to say that they are all first-rate. Measuring 19" x 7.43" x 16" WxHxD and 65 lbs, this amp is a real handful. While the plastic nut binding posts don't exactly exude class and elegance, they in fact gave me no problems at all and everything else about the amplifier including incidental hardware spells high-quality and ruggedness. McCormack specifies a 100 kOhm input impedance and a high 1-volt input sensitivity. The IEC power receptacle around back invites power cord experimentation and the 12-volt trigger facilitates remote turn-on.

Getting back to basic design and build quality, a couple of things impressed me about the HT-5. First, despite a lot of heat sinking, there were no sharp or jagged edges to be found. More than just an aesthetic issue, this becomes very important once you start hoisting around this sizeable and heavy piece. Sixty-five pounds of mass and a sharp edge can spell disaster. Second, once I opened it up for a snapshot, I was struck by something else. When you open up many 5-channel amplifiers, what you find is often a card-based modular design enabling an economy of manufacturing costs. Each pre-fabricated channel card just plugs into the chassis. The problem is that these vertically arrayed "cards" often provide inadequate ventilation to have a propensity to overheat and shut down when driven hard. Not so with the HT-5. In fact, some may mistake its insides for a two-channel amplifier. There's lots of ventilation room inside and all the heat is dissipated by a wealth of mass and heat sinking along the sides of the amplifier. Lots of open space and lots of mass mean great heat dissipation. Looking at the picture of the open HT-5, you can see two thermal sensors, one midway on each of the amplifier's sides. Though the HT-5 idles at comfortable warmth, it never got all that hot in use to force those sensors into action.

The DNA-HT5 is based on the same circuit design as the DNA-125, which has received enthusiastic accolades from reviewers and owners alike. Backed by a massive power supply, the HT5 is said to be capable of handling the most demanding speakers with ease. I asked McCormack's Steve McCormack to share with us his design philosophy and goals. Here's what he told me:

"The 'DNA' portion of the name stands for 'Distributed Node Amplifier'. This concept is based on the idea of distributing the high-current power supply storage capacitance as a series of nodes, or storage points, located as close as physically practical to the output transistors. This allows current to be stored local to the transistors that demand it, without the usual intervening impedance buildup of wiring, connectors etc. In addition, the capacitors employed may be physically smaller than is typical, and may be chosen for the superior sonic performance often available in smaller electrolytic capacitors. It is my experience that this technique results in a persuasive improvement of sonic performance. To the best of my knowledge, the DNA-1 amplifier (designed in 1989) was the first to employ this technique. This idea has subsequently been used in many other amplifier brands - the sincerest form of flattery, as we know :-)

The DNA amps use a J-FET/MOSFET driver topology with no local feedback and only a very small amount of global feedback (on the order of 6dB). This driver circuit is arranged to clip before the bipolar output stage so that the MOSFET clipping characteristic dominates the sonic character. The result is a bipolar output amplifier with a more graceful overload character than you normally hear.

I have been in this business for more years than I care to count, and I discovered long ago that I preferred the sense of bass firmness and overall authority I hear from bipolar output stages over MOSFETs. (While it is very difficult to arrange a true 'all-other-things-being-equal' test of the two transistor types, I felt that I kept hearing these preferred qualities in bipolar output designs although they often had other failings.) By combining the FET front-end with a bipolar output, I feel that I have been able to achieve the balance of sonic character I had in mind.

The original DNA-1 and DNA-0.5 amplifiers enjoyed an unusually long and successful run in the marketplace, but have been replaced by the new DNA-225 and DNA-125 respectively, and now the DNA-500 and the DNA-HT5 have joined the ranks. These new amplifiers incorporate a number of innovations and refinements that push their performance beyond the original designs.

Back when I did the original design work on the DNA-1, I felt that I did not want to use any coupling capacitors at all. In addition, I let the response of the amp extend to DC so it became necessary to use a servo amplifier to control DC offset and drift. A protection circuit was added to monitor DC levels (among other things) and shut the amp down if a potentially dangerous situation was encountered. I always felt though that it would be desirable to eliminate the servo amplifier altogether if possible.

Recent advances in capacitor design have provided us with some truly outstanding caps, and the new amplifiers incorporate excellent DC blocking caps at the input. In addition, I have limited the amplifier's gain at DC and this has made it possible to eliminate both the DC servo (replaced with a static DC offset control) and the protection circuit (now unnecessary). I have also refined the input circuit topology and changed the bias scheme a bit.

These changes -- along with a host of smaller refinements -- have resulted in the DNA amplifiers exhibiting greater coherence, better bass definition and a greater sense of pace and rhythmic control. The soundstage is more 3-dimensional and images are more confident and palpably real. The DNA amps exhibit exceptional stability and reliability and are capable of driving a wide range of loudspeaker types including 'difficult' low-impedance models. This has all been accomplished at reasonable cost and with improved cosmetics, placing the DNA series amplifiers (I believe) among the real bargains in high performance audio today."