NAD 6155 Cassette Deck Review

According to NAD, the Model 6155 is “the finest cassette deck NAD has ever designed.” The performance of this inexpensive two-head deck with a single-motor transport is said to rival that of much more expensive cassette recorders. That claim is based largely on three design elements: a unique feature called Play Trim, Dolby HX Pro headroom extension, and an amorphous record/playback head that is said to be exceptionally free of flux saturation even at high recording levels. The NAD 6155 includes both Dolby B and Dolby C noise reduction and has an instant-release pause control that simplifies precise editing.

NAD’s Play Trim circuit is designed to compensate during playback for the variations in high-frequency response among tapes recorded on other machines, whether the tapes were commercially prerecorded by different manufacturers or made on different home decks. These response variations can arise from small differences in equalization, biasing, head-azimuth alignment, and tape-saturation effects. Even minor response variations are magnified by the action of Dolby noise-reduction circuits and can thus become audible.

A special high-frequency play-back-equalization circuit, the Play Trim is located within the Dolby encode/decode loop, ahead of the Dolby playback decoding circuits. Since it is designed to restore a flat response before Dolby decoding occurs, a drooping high-frequency response can be corrected without increasing the playback noise level. In use, the Play Trim is adjusted by a front-panel knob for the most pleasing sound quality.

Although HX Pro control voltages are derived from the Dolby noise-reduction circuits, HX Pro is not a noise-reduction system. Rather, it adjusts the recording bias dynamically, in response to the high-frequency content of the program, so as to maintain a constant effective bias on the tape. Usually the total effective tape bias includes a portion contributed by the high-frequency energy in the signal, which can thus overbias the tape and cause dull highs in playback. By reducing the internal bias level when strong high frequencies are present in the program, HX Pro extends the high-frequency headroom of the recording. The HX Pro circuit is always operating while the NAD 6155 is recording, and no user action is required.

The styling of the NAD 6155 resembles that of other NAD electronic components. It is finished in metallic gray and has black knobs. The cassette is loaded into a door that hinges outward at its base when a button is pressed. The transport controls are mechanically operated, but the required pressure is very low. If the fast-forward or rewind button is held in while the deck is playing, recorded portions of a tape can be identified by the high-pitched playback sound (at a reduced level to prevent damaging delicate tweeters).

The level display consists of two horizontal rows of LED’s, green between — 20 and 0 dB, amber from 0 to +5 dB, and red from +5 to the maximum of + 8 dB. The indicators are spaced 3 to 5 dB apart at the lower end of the scale, but the interval narrows to 1 or 2 dB nearer the 0-dB calibration. A red light in the display window shows that the machine is in the recording mode, and colored lights at the left of the window show the status of the power, Dolby B, and Dolby C circuits.

Along the lower portion of the panel are knobs for the Play Trim and bias adjustments, tape selection (Types I, II, and IV), and Dolby-system selection. A filter button switches in a low-pass filter to prevent stereo MPX pilot-carrier leakage from affecting the Dolby operation when recording FM stereo transmissions. The NAD 6155 is 16-1/2 inches wide, 10-3/4 inches deep, and 4-3/4 inches high. It weighs 12-1/4 pounds. Price: $348.

Lab Tests

Our test sample of the NAD 6155 was supplied to us by the manufacturer along with the tapes used for its adjustments: Maxell UDXL I, UDXL II, and MX (all in C-90 lengths). We used the same tapes for our record-playback measurements. The playback response was measured with BASF IEC-standard calibration tapes. With the Play Trim control set to its center position, response above 8,000 to 10,000 Hz rolled off appreciably, but with the control at its maximum, the overall response was quite flat from 31.5 to 18,000 Hz (the test tape’s limits).

With the recording-level control at maximum, a 95-millivolt input signal produced a 0-dB indication. The playback output (into a standard load of 10,000 ohms in parallel with a 1,000-picofarad capacitor) was 0.51 volt with any of the tape formulations. A test tape recorded with 315 Hz at the standard IEC reference level of 250 nWb/m also gave a 0-dB reading and an output of about 0.6 volt. We measured the record-playback frequency response of each of the tapes at levels of 0 and — 20 dB, and other measurements were made to determine the effects of the bias control and the Play Trim adjustment.

Since the level indications of the NAD 6155 are only approximate, we referred our noise and distortion measurements to the level of the actual recording input. The third-harmonic playback distortion of a 1,000-Hz signal was measured at an indicated 0-dB recording level, and the signal level was increased until playback distortion reached the reference level of 3 percent.

We measured the tracking of the Dolby recording and playback circuits only for the Dolby C system, which is more critical in this respect than Dolby B. The Dolby tracking of the NAD 6155 was about as good as we have ever measured. The response curves with the Dolby C system on and off matched each other within 1 dB from 20 to 16,000 Hz, measured at 10-dB intervals from —10 to —40 dB. At the 0-dB level, where virtually all cassette decks show a high-frequency loss from tape saturation, the Dolby HX Pro circuit improved the response substantially above 8,000 Hz.

The three tape types behaved much as expected. At —20 dB the Maxell UDXL I (ferric) tape had the flattest response by a slight margin, and the response curves of the other tapes sloped down very slightly above 1,000 Hz. All three tapes held up well up to 18,000 Hz and dropped sharply in response between there and 20,000 Hz.

It is normal for a tape’s response at the 0-dB level to drop off sharply at high frequencies because of tape saturation. Metal tapes are usually distinctly superior in this respect, and in the NAD 6155 the Maxell MX showed negligible differences between its 0-dB and — 20-dB response curves. The performance of the Type I tape, however, was actually superior to that of the chrome-equivalent UDXL II (Type II), and both tapes had a better 0-dB output at 10,000 Hz than we have measured from most tape decks.

Comments

Our test results confirmed that the NAD 6155 met every one of its published specifications within normal measurement tolerances. Although the Play Trim control had to be set to its limit to obtain the flattest response from our IEC-standard tapes, it seems likely that a very small azimuth-alignment difference between the head and the tape was responsible for the high-frequency loss we measured initially. In practice, this is a very common condition and one that the Play Trim circuit was designed to correct. The fact that it did so with impressive accuracy testifies to the validity of the concept.

The Play Trim circuit also did a fine job of correcting a gently downward-sloping response in one of our record-playback measurements, which was apparently caused by a slight overbiasing. Ideally this correction should have been done with the bias control, but the Play Trim adjustment provides the option of correcting such a condition after a tape has been recorded, a significant advantage considering the awkwardness of making record-play-back measurements on a two-head recorder.

We made subjective tests of the NAD 6155’s performance by playing high-quality commercially recorded cassettes and by making recordings of FM broadcasts, tuner interstation hiss, and various CD’s. The latter two sources enabled us to compare the original signal to the recorder’s playback, which is probably the best way of judging the sound quality of a tape deck.

Our listening tests confirmed the high quality of the NAD 6155. The lessened tape-saturation effects we noted in our measurements could well be a result of the deck’s amorphous record/playback head, or the action of the Dolby HX Pro circuit, or both. In any case, the practical, audible benefit is to enable the recorder to handle high-level transient peaks without excessive compression.

When we taped interstation FM hiss (at about —20 dB), the playback was indistinguishable from the original signal. When we dubbed some CD’s with peaks of 0 dB (the maximum for a CD) using Dolby C, we could not detect any differences between the CD and the tape playback in A/B comparisons. One could hardly ask for more, especially from a $348 recorder!

It appears that NAD has once again managed to offer an unusual amount of value for a modest price, in this case by omitting costly logic circuits and solenoid-operated transport functions in favor of easily operated mechanical controls, and by providing effective useroperated adjustments instead of automatic tape-optimizing systems. NAD has created a deck that, even if not quite the equal of a $1,000 recorder, can make tapes that probably could not be distinguished by ear from those made on a machine costing three times as much.

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