Denon TU-680NAB Tuner
AS our regular readers probably know, AM performance is perhaps the least of the concerns of most tuner designers. Of the hundreds of tuners and receivers I have tested and used over the past thirty-five or more years, those with AM quality adequate for anything more than speech reproduction could easily be counted on the fingers of one hand. The frequency range of their audio output rarely extends as high as 3,000 Hz, and a more usual upper limit is in the range of 2,000 to 2,500 Hz.
In an effort to expand the audience for AM radio, the Electronic Industries Association (EIA) and National Association of Broadcasters (NAB) several years ago jointly established a committee (the NRSC) to develop standards for improved AM broadcast quality. To enable broadcasters to evaluate the success of the proposed AM broadcast standard, Denon designed and manufactured a limited number of tuners incorporating the committee's recommendations.
The Denon TU-680NAB is essentially that tuner, now available to the general public. Its stereo AM tuner section, using the widely accepted C-Quam system, meets the new AM broadcast standards (AMAX), and its FM tuner section is of comparable quality. A major goal of the AMAX standard was to increase the audio bandwidth of AM programs, as received by the listener, to a range of 50 to 7,500 Hz. To that end, it calls for a 75-microsecond pre-emphasis (the same characteristic used in FM transmission) to boost the level of the high frequencies in the audio program. A fully compatible receiver will have a complementary de-emphasis in its audio section, resulting in an overall response essentially flat over a range of 50 to 7,500 Hz or more (the specifications of the NRSC call for an AM response of 50 to 7,500 Hz +1.5, -3 dB). Since this pre-emphasis has a negligible effect below 2,000 or 3,000 Hz, it will not impair the quality of reception through existing receivers or tuners that do not pass the higher frequencies. If anything, it should slightly improve their sound.
The Denon TU-680NAB is a very conventional-looking component, with none of the "glamorizing" design or styling features often found on state-of-the-art products. It is small and light (less than 7 pounds), with a simple black panel and a conventional grouping of control buttons. A row of ten numbered buttons and an Enter button enable up to thirty preset AM or FM station frequencies to be memorized and recalled. A parallel row of buttons above them controls the tuning mode (auto-mute or manual), intermediate-frequency (IF) bandwidth (wide or narrow on both AM and FM bands), and FM and AM noise reduction (NR/NB). Two large tuning buttons scan up or down in frequency, one channel at a time or continuously until a signal is acquired, depending on the setting of the auto mute/manual switch. A large power button is located at the left end of the panel.
The display window shows the tuned frequency, band, and preset channel number (if applicable), plus legends identifying the operating mode (stereo, mono, auto, manual). A multisegment signal-strength indicator and identifying lights for the bandwidth and noise-reduction functions complete the display.
On the rear apron are the output phono jacks for left and right channels and three sets of antenna terminals. These include inputs for the furnished detachable and pivoting AM loop antenna, a separate powered AM antenna or long wire, and a coaxial 75-ohm FM antenna.
Operating the tuner is as simple as can be. Stereo/mono switching is automatic for both AM and FM (with a clear indication of stereo reception in the display window). The bandwidth selector offers narrow-band reception for best interference rejection or wideband reception for highest sound quality with clean signals. The noise-reduction system has distinctly different functions for the two bands. In stereo FM reception, it progressively blends the two channels at high frequencies as the signal weakens, reducing noise while maintaining useful stereo separation at middle frequencies. In stereo AM, it switches in a noise-blanking circuit designed to reduce impulse noise rather than the hiss that can disturb weak FM signal reception.
The tuner comes with a wireless remote control that duplicates the front-panel band-selector, tuningmode, and band-scanning functions as well as providing buttons that automatically scan the preset channels or an entire band.
The instruction "manual" is a single sheet, folded in quarters, explaining the tuner's operating controls and rear-apron terminals and their functions. Although it also includes the usual performance specifications, there is no information on the tuner's special features.
Our laboratory tests confirmed the TU-680NAB's outstanding performance on both the AM and FM bands. Lacking stereo test facilities for AM, we had to judge that aspect of the tuner's performance by listening to stereo AM broadcasts. The stereo indication in the display window lit on six AM channels, although the program material rarely gave any audible clues that it was broadcast in stereo.
The basic quality of its AM reception, however, in respect to both frequency response and noise, was clearly superior to what we are accustomed to hearing on that band. Subjectively, it came remarkably close to FM quality. Unfortunately, the only good music station in the New York area to have simultaneous outlets on the AM and FM bands recently changed the format of its AM facility, so we were unable to make a true A/B comparison between the two modes.
The measured AM tuner frequency response was flat within +/-1 dB from 100 to 8,800 Hz and down 3 dB at 57 and 10,000 Hz in the wide-bandwidth mode. In its narrow-band mode, the response was identical to the wideband measurements up to 3,000 Hz, down 1 dB at 6,500 Hz, and down 3 dB at 7,300 Hz. In either mode, the TU-680NAB was vastly superior to any other AM tuner we have seen in years.
A notice accompanying the tuner advised that its AM noise blanker is effective on impulse noise like that created by some automobile ignition systems but may have no effect on constant noise. We discovered this for ourselves: The noise blanker had no effect that we could discern (our major sources of AM noise are the fluorescent lamps in the lab, and only critical positioning of the AM loop antenna has any effect on that). But I must say that this was the quietest AM tuner we have used in many a year, in addition to its other admirable qualities.
The FM section was equally noteworthy. As with most good tuners, its frequency response was flat within better than +/-0.5 dB from 20 to 12,000 Hz, though it fell off sharply to -1.3 dB at 15,000 Hz (not an audibly significant loss). The stereo channel separation, as often happens, was slightly different from left to right than from right to left. The "poorer" of the two displayed 50 dB of separation (outstanding) from 300 to 3,000 Hz and was still a very good 43 dB at 12,000 Hz, above which the separation fell sharply. In the "better" direction, the separation was greater than 60 dB from 350 to 1,200 Hz, falling to about 50 dB between 3,000 and 12,000 Hz. In both cases, the separation fell at low frequencies, where it is subjectively less important, to about 33 dB at 20 Hz. The narrow-IF-bandwidth FM mode considerably reduced channel separation, although it always remained greater than required for subjectively complete stereo separation- about 35 dB across most of the audio range, falling to 25 dB at 14,000 Hz.
The FM 50-dB quieting sensitivity was good in mono and superb in stereo. The "usable sensitivity," which actually has little practical significance, matched the published specification. The other tuner specifications were also verified, and in some cases surpassed, in our tests, within the normal range of measurement error. Especially noteworthy were the 1.1-dB capture ratio, 80-dB AM rejection, and 80-dB image rejection. We verified the rated 75-dB alternate-channel selectivity with the narrow IF bandwidth, and in its wide mode our reading came very close to its 50-dB rating.
Good as these measurements are, the proof of the tuner's quality lies in the listening. Without checking back through years of test files, I cannot say with certainty whether this is the "best" tuner we have ever tested, but my strong impression is that it is. Totally lacking in glamour, it brings AM into almost the same league as FM insofar as audible qualities are concerned. I have no doubt that if AM and FM stations were transmitting the same program, it would be difficult to distinguish between them with much material. The Denon TU-680NAB is a winner if I ever saw and heard one, and a good value at its price.
