NAD 6155 brochure Dolby HX PRO Headroom Extension, Amorphous Head

These errors arise from tape saturation at high frequencies, from biasing that didn’t match the tape when the recording was made, and from differences in equalisation and head azimuth among recorders. Such errors are magnified by Dolby NR decoding because they cause the Dolby NR circuits to mistrack, often resulting in dull sound.

Play Trim is a special high-frequency playback equalisation circuit, located ahead of the Dolby NR circuit so that it can be used to restore accurate playback response before Dolby NR decoding takes place. As a result the playback of all cassettes can now approach the high-fidelity ideal: accurate reproduction of every original sound, with all of its brilliance and clarity preserved

These several causes of variable high-frequency playback response are significant by themselves, but when decoded by the noise-reduction circuits, these errors are greatly magnified. For example, mild tape saturation or skewing that rolls off the highs above 8kHz will cause a slight loss of crispness and “air” in the sound. But if that rolloff alters the tracking of the Dolby noise-reduction circuit, it will depress response at lower frequencies as well, so that the sound becomes dull in the mid-range too. Since the playback error occurred ahead of the noise-reduction decoding, this problem cannot be solved simply by adjusting amplifier tone controls.

The solution: Play Trim. The new Play Trim control not only corrects these common high-frequency aberrations but also prevents the consequent mistracking of the noise-reduction circuits.

The Play Trim control is easy to use: you simply adjust it in playback to obtain musically correct tonal balance, removing any false brightening or dulling of the sound. The Play Trim circuit is a narrow-band equaliser that operates in the top two octaves of the audio spectrum, providing an adjustable boost or cut of up to ±3 dB at 10 kHz and 6 dB at 20 kHz. The key to its success is its location in the signal path, ahead of the Dolby NR decoder. Since any high-frequency errors are corrected before Dolby NR decoding, playback fidelity can be as excellent in everyday use as it is in idealised lab tests.

2. DOLBY HX PRO HEADROOM EXTENSION

At high recording levels, overbiasing rolls off high frequencies and causes the tape to saturate at a reduced level. This yields a dull sound which is commonly assumed to be an inevitable characteristic of cassette recordings. The 6155 includes the Dolby HX Pro headroom extension circuit, which constantly monitors the level and frequency content of the input signal during recording, and it automatically varies the amount of ultrasonic bias so that the total effective bias (including the effect of the high audio frequencies) is optimum at all times. Consequently, when you are recording a strong low-frequency signal, the bias level increases to an unusually high value. providing remarkably low distortion at high recording levels. When the input signal has strong highs, the bias decreases to provide a dramatic improvement in high-frequency saturation level, preventing any dulling of the highs.

As a result, the useful dynamic range of the 6155 at both low and high frequencies is greater with ferric and chrome tapes than the range many other decks achieve with costly metal tape. Best of all, the operation of the Dolby HX Pro circuit is totally automatic. And since it works only during recording, the superior quality of the resultant recording will be evident wherever the tape is played - in the car, in a personal portable player with headphones, or in the 6155 itself.

3. AMORPHOUS HEAD

The NAD 6155 contains a costly amorphous record/play head that is remarkably free of flux saturation even at the highest recording levels. In many cassette recorders, at low and middle frequencies the head saturates before the tape does, and that is what limits the useful dynamic range of the recorder. With the amorphous head you can record musical peaks at levels substantially higher than 0 dB (Dolby NR reference level), confident that the sound will remain distortion-free.

The ability of the amorphous head to handle extra-high signal levels without saturating is especially important with high-bias (chrome) and metal-particle tape formulations. These tapes excel in high- frequency dynamic range, allowing you to capture the full brilliance and power of cymbals, bells, brass, and synthesizers in clear, airy sound. Play Trim, Dolby HX Pro, and the amorphous head add up to a level of performance, in both recording and playback, that normally is found only in tape decks that cost far more than the 6155.

While the 6155 may be a mid-priced machine, it was designed to work at its best with premium-quality tapes, producing first-class recordings. In addition, its wide-range response, low flutter, and very quiet electronics provide a superb play-back environment for the many high-quality pre-recorded music cassettes that are available today.