mwm@ucbtopaz.CC.Berkeley.ARPA (Praiser of Bob) (05/14/85)
[What's abelian and sounds great?] The addendum first. In an earlier review, I said that the AR System Remote Control was inaudible. Having just put in a better amp, I have to retract that statement. If you are listening to a quiet passage with much amplification, then the SRC adds a gentle "pop" to the music when the IR receiver kicks on. Since it only happens when you are changing the control settings, I still think the SRC is a worthwhile box. Now, on to the NAD. Appearance and Features: The NAD 2200 ("PowerTracker", for some reason) costs ~$450. It's a 100 W/ch amp that comes in a black aluminum cabinet measuring 42x12.3x37 cm, weighing 12.5 kgs. The front of the box has a power switch and green power led on the left, and a trio of led's in the center. The top led is red, and goes on when the amp decided that it's clipping. The second led is also red, and indicates that the amp has disconnected itself from the output for some reason. The final led is yellow, and is on when you have the NAD "soft clipping" enabled. When it comes on, the amp goes into protection mode (disconnecting the output lines) long enough to insure that it's transients have settled. This same feature comes on when it thinks it's being driven to overheating (trying to draw 200+ watts for a couple of seconds). The "soft clipping" causes the amp to lower the gain as it approaches clipping levels. A nice feature if you like to drive your amp very loud. I haven't turned it on, and have been told that the circuitry should be physically removed from the amp. The amp can be bridged, giving you a mono 400 W amp. The speakers posts on the back are color-coded for this, meaning that the negative post of the left channel is black instead of red. For fans of DC-light amps, there is a "lab in" that avoids the amps signal filtering (details later). Amp design and specs: The 2200 uses a couple of rather radical (to me, anyway) design technics. It's billed as a "commutating" A/B amp, meaning that there is a class A amp for low level use. When the higher power levels are needed, it switches a second A/B amp into the circuit. The second amp can deliver 500 W peak power. Note that this amp is added to the circuit; the low-power amp stays in circuit to prevent switching noise. The second radical feature is bass inversion in the right channel. The bass signal in that channel is inverted, amplified, then inverted again before it's output. This results in the bass drawing power out of phase with itself, so it needs less power - or, so you can get better bass with the same amount of power. The specs on all this are: Power: 100 W THD 0.03% IM distortion (both SMPTE and IHF): <0.03% Clips at: 140W (8 Ohms), 200W (4 Ohms) IHF headroom: +6db (8 ohms) IHF dynamic power: 400W (8 Ohms), 600W (4 Ohms), 800W (2 Ohms) Slew factor: > 50 Slew rate: > 35V/usec Damping: 100 Freq response, LAB input: 3Hz to 80kHz, +0, -3dB Infrasonic Filter, Normal input: -3dB at 15Hz, 12dB/octave Ultrasonic Filter, Normal input: -3db at 40kHz, 12 dB/octave How it sounds: The bottom line on the specs and techno-magic is, of course, how the amp sounds. A one-word description of the amp is "smooth." When running at normal levels (for me, that is), the amp doesn't seem to be working at all, just delivering sound. Even when driven to clipping, it doesn't sound like it's struggling. The next thing I noticed about the amp was the bass. It's very precise, without sounding peaky. If this is due to the inversion in the right channel, I expect it to appear in many amps in the future. After the bass, the clarity of the midrange comes to the fore (much to the detriment of my tuner :-). The image is large and stable, but will need some help with center fill. All in all, a very good sounding amp. [The rest of the system used: Magnavox FD 1000 CD player, Nak BX-100 cassette deck with unknown dbx unit (borrowed for the occasion) AR SRC subbing for a preamp, a pair of ITC 1 speakers, and a pair of little-known Canadian speakers (borrowed with the Nak), KimberCable speaker cable and Gold-Ens interconnects.] Summary: An excellent buy. When compared to amps that cost twice as much (Bryston, Tanberg), the 2200 is a clear winner - especially the clarity in the midrange. With amps in the same price range (Carver, Haffler), there isn't any contest. The 2200 sounds more natural, and better on all points. If you're looking for an amp in the < $1000 price range, you'll be doing yourself a disservice if you don't give this amp a listen. <mike
dmmartindale@watcgl.UUCP (Dave Martindale) (05/18/85)
I'd be very surprised if the amplifier inverted only the low frequencies in one channel. More likely, it inverts the channel entirely, and then "re-inverts" the signal simply by reversing the colour coding of the speaker binding posts on that channel. This does cause the two channels to load the power supply primarily out-of-phase, allowing the use of a smaller power supply for the same output power (clever) and also means that bridging the amp for mono requires nothing more than connecting both channels' inputs together. Unfortunately, it must mean that the amp is incompatible with headphones or anything else that requires a common speaker ground.
ben@moncol.UUCP (Bennett Broder) (05/18/85)
>The second radical feature is bass inversion in the right channel. The >bass signal in that channel is inverted, amplified, then inverted again >before it's output. This results in the bass drawing power out of phase >with itself, so it needs less power - or, so you can get better bass >with the same amount of power. Sorry, but this feature is neither new nor radical. It has been used in high powered tube equipment for years. Actually, I assume that the *entire* right channel is phase inverted. A circuit that only inverted the bass would be a sonic nightmare.
mwm@ucbtopaz.CC.Berkeley.ARPA (Mike (I'll be mellow when I'm dead) Meyer) (05/21/85)
In article <1859@watcgl.UUCP> dmmartindale@watcgl.UUCP (Dave Martindale) writes: >I'd be very surprised if the amplifier inverted only the low frequencies >in one channel. More likely, it inverts the channel entirely, and then >"re-inverts" the signal simply by reversing the colour coding of the >speaker binding posts on that channel. This does cause the two >channels to load the power supply primarily out-of-phase, allowing the use >of a smaller power supply for the same output power (clever) and also >means that bridging the amp for mono requires nothing more than connecting >both channels' inputs together. On closer reading, you're right. However, they leave the colour coding alone, and relabel the +/- posts on one channel. >Unfortunately, it must mean that the amp is incompatible with headphones >or anything else that requires a common speaker ground. Yup. In nice, bold letters above the diagrams in the owners manual it says "Caution: do not connect the output of this amplifier to any ... device that uses a common ground for left and right channels." However, at the price/performance ratio of the 2200, I'll live with it. <mike