Denon PMA-1520 Integrated Amplifier Review

Denon’S PMA-1520 is one of a growing number of integrated amplifiers with built-in digital-to-analog (d/a) converters that enable them to accept digital output signals from a CD player or digital audio tape (DAT) deck and convert them to analog form. The main benefit claimed for this feature is the elimination of any signal degradation, such as noise, hum, or crosstalk, in the transfer from the source to the amplifier.

In addition to electrical and optical digital input connections for a CD player or DBS (direct broadcast satellite) source, the PMA-1520 has both kinds of digital input/output jacks for use with a DAT recorder. It also has a full complement of analog signal-processing and control features. Flat pushbuttons on the front panel, which operate with almost no physical travel, select the input source. Along with the digital inputs and a phono input, there are provisions for four high-level analog sources—Aux(iliary) l, aux 2, tuner, and CD—and inputs and outputs for two analog tape decks.

A large volume knob is at the right of the panel, with smaller knobs used for the balance, bass, and treble controls. Bar switches control speaker selection and tape monitor/copy, which connects the analog decks for dubbing in either direction or selects the output of the d/a converters or another analog source for recording. A third bar switch is marked source selector. Actually an adjunct to the in-put-selector buttons, it can insert a subsonic filter into the signal path or put the amplifier into its CD direct mode, in which the input-selector buttons and the tape-monitor loops are bypassed and only the source connected to the CD inputs is heard.

Small pushbuttons place the tone controls into the signal path, drop the audio level by 20 dB, activate the loudness-compensation circuit, and switch the phono preamplifier for either a moving-coil (mc) or moving-magnet (MM) cartridge. A digital direct button connects the internal D/A converters to either the electrical or optical digital inputs and connects the converter’s output to the analog amplifier stages. Like CD direct, the digital direct button bypasses the normal input switches and tape loops.

The PMA-1520 features Denon’s “Optical Class A” technology, a form of variable-class circuit design. The bias of the output transistors is switched between two levels by an extremely fast peak-detection circuit that monitors the input signal’s waveform. The actual switching is done through an optical link that isolates the signal-carrying circuits from any spurious noise arising from the digital control signals. At low signal levels the output transistors are biased close to cutoff, like a Class AB amplifier, but because of the small signal amplitude they are always conducting some current and in effect are operating as a low-distortion Class A amplifier. At higher signal levels the bias shifts to a normal (fully on) Class A level, again providing the low distortion of a Class A amplifier in this important part of the program range. At still higher levels the bias returns to its lower value, but because of the high signal amplitude the amplifier now operates in Class AB, which gives it lower heat dissipation and higher power output than is possible in Class A.

The power-amplifier section of the Denon PMA-1520 is rated to deliver up to 130 watts per channel into 8-ohm loads, from 20 to 20,000 Hz, with no more than 0.005 percent total harmonic distortion. It also carries dynamic power ratings of 300 and 400 watts per channel into loads of 4 and 2 ohms, respectively. The rear apron has gold-plated RCA-type jacks for all signal connections. The PRE OUT/MAIN IN jacks are normally joined by jumpers, which can be removed for inserting a signal-processing accessory into the signal path. Insulated multiway binding posts are used for the outputs to two pairs of speakers. There are three AC convenience outlets, two of them switched.

The PMA-1520 is a large, heavy amplifier, measuring 17 inches wide, 13-3/4 deep, and 6-1/4 inches high (including its large mounting feet). Its weight is 28 pounds. Price: $1,000.

Lab Tests

At its clipping point, the Denon PMA-1520 delivered 155 watts per channel into 8-ohm loads, 218 watts into 4 ohms, and 271 watts into 2 ohms. The distortion was in the vicinity of 0.004 percent for power outputs up to 150 watts (8 ohms). It was only slightly higher with 4- and 2-ohm loads. At the rated 130-watt output, the distortion was typically 0.004 percent, reaching 0.008 percent at 20,000 Hz. Lower power levels always produced lower distortion readings. Our dynamic power readings surpassed Denon’s specifications by a comfortable margin, with respective outputs of 196, 338, and 528 watts into loads of 8, 4, and 2 ohms.

The bass tone control shifted the turnover frequency between 100 and 500 Hz, and the treble curves were hinged at about 2,000 Hz. The loudness compensation boosted both low and high frequencies as the volume was reduced. The response with the subsonic filter engaged was down 2.5 dB at 20 Hz. The RIAA phono equalization was accurate within +0.15, —0.35 dB from 20 to 20,000 Hz.

To evaluate the d/a converters of the PMA-1520, we used a Denon DCD-1500 II CD player, which has both electrical (coaxial) and optical digital outputs as well as conventional analog audio outputs. First we measured frequency response, noise, distortion, channel separation, phase shift, and low-level d/a converter error directly from the CD player. Then we connected the player to the PMA-1520 at the optical digital input and repeated all the measurements using the amplifier’s d/a converters and measuring at its preamplifier outputs.

Although most of the differences we found were slight, almost all of them favored the conversion circuits in the CD player. For example, the player’s own frequency response was very flat, varying only 0.2 dB overall from 20 to 20,000 Hz. Through the amplifier’s digital circuits, the response was flat within 0.05 dB up to 6,000 Hz, but it rose to a 0.5-dB peak at 17,000 Hz and dropped off sharply above 18,000 Hz. The channel separation of the CD player was 20 to 30 dB better than the values measured through the amplifier, greater than 110 dB at 1,000 Hz and 87 dB at 20,000 Hz compared with corresponding readings through the amplifier of 88 and 66 dB.

On the other hand, the low-level linearity of the converters in the amplifier was extraordinarily good, by far the best we have yet seen, with errors not exceeding 0.5 dB from —70 to — 100 dB. In contrast, the CD player’s errors were 3 to 6 dB in this range, which is still better than average for the CD players we have tested. The interchannel phase shift of the player at 20,000 Hz was a barely measurable 0.36 degree, but the amplifier’s circuits showed a 3.6-degree shift—still much better than most CD players or amplifiers and entirely negligible. Measurements using the amplifier’s electrical instead of optical digital connections did not disclose any differences between them.

Comments

Considered only as an audio amplifier, the Denon PMA-1520 is an excellent product. It is among the more powerful integrated amplifiers currently available, it has all the expected control features, and it provides excellent performance in every respect. Although fairly expensive, it is priced competitively with similarly rated amplifiers.

Removing the top cover of the amplifier revealed the separately shielded compartments that house its critical digital and analog circuits. The power supply, with its massive toroidal transformer, is on the opposite side of the chassis from the sensitive circuitry. The PMA-1520 is built the way one expects a $1,000 amplifier to be built—with no apparent compromises and an obvious attention to detail.

Once we learned the functions of some of the less conventional controls, we were able to appreciate the full capabilities of the PMA-1520. The control nomenclature was not always helpful, since similar names mean different things in other products. For instance, in other amplifiers we have used CD direct bypasses all tone controls and filters, and sometimes even the balance control, leaving only the volume control and amplifier circuits in the signal path. In the PMA-1520, it replaces any other selected program source with the analog CD input signal and bypasses the tape-monitoring loops. The tone controls are still active, however (they can be bypassed by a button between their knobs). Since tape connections are often used for inserting a signal-processing device, such as an equalizer, into the signal path, the design of the PMA-1520’s CD direct switch is apparently intended to insure minimal modification of the CD signal before it reaches the speakers.

Since the digital inputs do not pass through the normal signal path, the digital direct button has a function for those sources similar to the role of CD direct in playing CD programs in their analog form. It, too, replaces the normal signal input with the CD player’s output, received in digital form and converted to analog by the amplifier’s d/a circuits. It also bypasses the tape loops but leaves the tone-control functions intact.

When we discovered the frequency-response difference between the d/a converters in the amplifier and the CD player, we listened carefully to see if it could be heard. In general, it could not, although with programs having a lot of content at the uppermost audio frequencies the difference was audible.

Our “bottom line” on the Denon PMA-1520 is that it is a fine amplifier equipped with a very good d/a converter section. As with similar amplifiers we have tested, we are not yet convinced of the practical benefits of a built-in converter. Even where such converters measure better than those of a given CD player, which is rarely the case, whatever differences exist have been audibly insignificant. Perhaps the benefit would be clearer if an inexpensive CD player were used with an amplifier like the PMA-1520, though at present most of the less expensive CD players do not have digital outputs. In any case, there is certainly no disadvantage to the inclusion of this feature, and the PMA-1520 is well worth consideration whether or not you expect to use its D/a conversion circuitry.

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