Product Review -
NAD 208 THX Power Amplifier - May, 1996
By John E. Johnson, Jr.
NAD 208 THX Power Amplifier; Two channel power amplifier, power output 250 w rms/ch into 8 Ohms; Frequency response 20 Hz - 20 kHz + 0.3 dB; THD at rated output < 0.03%; Input impedance 20 kOhm; Slew rate >100 v/microsec; Damping factor >200; Size 6 3/4"H x 19"W x 14 1/2"D; Weight 40 pounds; $1,649; UK: Phone 011-44-181-349-4034; Fax 011-44-181-343-3871; USA: NAD Electronics USA, 89 Doug Brown Way, Holliston, Massachusetts 01746; Phone 800-356-9470; Fax 508-429-2426.
The typical home theater amplifier these days has bipolar output devices. This results in the ability to deliver high current, which is a nice advantage for today's very demanding movie sound tracks. So, it is a bit unusual to see a MOSFET power amplifier designed for home theater, especially one that is THX certified. The NAD 208 is just that. It uses 8 MOSFETs per channel, with a rail of 87 volts (peak to peak) to deliver 250 watts rms/ch into 8 Ohms. However, there are an additional 6 MOSFETs per channel for use during high level transient periods, i.e., short duration. The rail switches to 120 volts peak-to-peak during these periods, and the amp can deliver 600 watts rms/ch into 8 Ohms, 750 w rms/ch into 4 Ohms, and 900 w rms/ch into 2 Ohms. The 208 is one of very few MOSFET power amps rated into 2 Ohms. This takes a big power supply, and the 208 utilizes 100,000 uF of capacitance, arranged in a bank of small capacitors rather than two large ones, and a 1.8 kVa toroidal transformer. The bank of capacitors gives a lower equivalent series resistance and equivalent series inductance, as well as keeping the recovery period down. The design also makes the amplifier very fast, with a slew rate of better than 100 v/microsec.
We tested the 208 with many of our reference pieces. Although it is designed as a home theater amplifier, it performed remarkably with CD stereo as well. Using Osborn Eclipse floor standing speakers (8 Ohm nominal impedance, 5.8 Ohms minimum), we could easily achieve SPL in the mid-90 dB range, and peaks at 105 dB, with no harshness or glare to the sound (both channels driven into the pair of speakers, microphone 5 feet away). We used the CD soundtrack of Gettysburg which has a musical spectrum of 25 Hz - 16 kHz for the SPL measurements. Other CDs included Dukas' La Peri, Copland's Fanfare for the Common Man, Enya - Watermark, Mary-Chapin Carpenter - Come on, Come on, and The Great Fantasy Adventure Album. Our LD tests included "Jurassic Park", "Return of the Jedi", "The Scarlet Letter", and others. This amplifier is something else! Bottomless bass. Thunder in a 40 pound chassis. That's what it is. The high end is very clean and detailed, courtesy of MOSFETs and the high slew rate. The mid-range is slightly laid back. All in all, a wonderful sound. It takes a good 30 min to 1 hr for the amp to stretch its limbs and warm up to full capability, so this may be one of those items that you leave on all the time. The heat sinks are massive, but they never got hot, even after extended use at high power. No corners were cut here, or anywhere for that matter from what we could see (and hear). You will want to use full range speakers with this honey. No bookshelf or mini-monitors allowed (feed the lion with a shank, not giblets). Good cables are in order, and also a high quality front end. The 208 would have the $199 CD player or "on sale" surround sound receiver for breakfast, even with its pre-outs. This amp will stand toe-to-toe with any other component you can match it with. So give it some respect, sit back, and enjoy.
The 208 appears to have been beautifully engineered. It is built in the UK, where it was designed. The chassis is black metal with a sputter finish. It has rack mount handles and rubber feet. The front panel has, besides the NAD logo, a push button on/off switch which turns red upon activation, and then green a few seconds later. The delay is from a solenoid which eliminates turn on thump and protects home circuit breakers. The rear panel has the AC receptacle for the detachable grounded power cord, balanced XLR and unbalanced RCA input jacks, a selector slide switch for balanced/unbalanced input usage, a slide switch for soft clipping mode (reduces peak signals when they would otherwise cause clipping), a slide switch for bridging mode (500 w rms into 8 Ohms), and a recessed slide switch for 8 Ohms/4 Ohms use. There are also two sets of five-way binding posts for multiple speaker setups and bi-wiring. Each set of binding posts is protected by a hinged cover. Since putting two sets of speakers in parallel results in high current demands, this is testimony to the heavy duty design of the 208. We found no difference in sound quality with the soft clipping switch on vs. off. The amplifier can produce so much unclipped power, the soft clipping just never seemed to be activated. We would recommend leaving it on all the time, just in case, since high power clipped signals can easily damage speakers, and you never know when someone might accidentally turn the volume control all the way up. The impedance selector is for use with speakers where the impedance never goes below 6 Ohms, in which one would use the 8 Ohm setting "for maximum power delivery". In all other cases, the 4 Ohm setting is used. Since this amplifier produces plenty of muscle at the 4 Ohm setting, we would suggest leaving it set at 4 Ohms regardless of the speakers. We feel that the bridging slider switch should not be in between the soft clipping switch and balanced/unbalanced switch. If the amplifier is connected to anything that shares a common ground between the two channels, and then one accidentally slides the bridging switch instead of the soft clipping switch or the balanced/unbalances switch, a short could occur. The bridging switch should be located somewhere else on the back panel, or at least, recessed.
In summary, the NAD 208 THX two channel power amp represents a new plateau in home theater MOSFET packages. The model has been around for a little while (winning accolades along the way), and if there were ever any bugs, they aren't there now. Practice makes perfect. We hope NAD will produce a three channel version, and soon. 250 w/ch all the way around the home theater. Now that's a five star picture worth dreaming about.
John E. Johnson, Jr.
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