MCS 3125 Stereo Receiver Review

The Penney audio line, called the MCS (“Modular Component Systems”) series, is now headed by a more powerful receiver, the Model 3125. It closely resembles the earlier 3275 in styling and size. The most obvious specification improvement in the 3125 is its audio power rating: 125 watts per channel into 8 ohms from 20 to 20,000 Hz with no more than 0.05 per cent total harmonic distortion (the 3275 was rated at 75 watts with 0.25 per cent distortion). The integral five-band graphic equalizer, in lieu of tone controls, has been retained (with separate knobs for the two channels, making ten in all). The center frequencies of the equalizer sections are 60, 240, 1,000, 4,000, and 16,000 Hz. Each control has a nominal range of ±15 dB. Compared to the older model, the number of LEDs that show the equalizer settings has been doubled, so that each detented control setting has a corresponding and unambiguous light indication. Defeat buttons bypass the equalizer circuits.

Most of the other control functions of the Penney 3125 are conventional, except that LEDs are used for all the indications once handled by meters (including audio power output). The FM-tuning indicator of the 3125 is unusual and effective. In the upper right corner of the front panel, next to the dial scale, there is a compact array of pale white indicators. Two horizontal parallel lines are marked tune and sic. As a station frequency is approached, a double arrow appears above the tune line, indicating the direction in which the dial pointer should be moved for correct tuning. As the channel center is approached, the display changes to a single arrow, and at the center the arrows disappear (they reappear in the reverse sequence, pointing in the opposite direction, if the tuning continues through the channel center). When they are extinguished, the large tuning knob is released and a moment later a bright red locked light appears near the tuning knob. This shows that the receiver’s AFC system has accurately tuned it to the received frequency. The AFC is automatically disabled when the tuning knob is touched for ease in acquiring a weak signal close in frequency to a stronger one that might tend to “pull” the AFC circuits. To the left of the dial scales is a large digital frequency readout that operates only during FM reception.

Simultaneously with the tuning action, lighted bar segments appear below the sig line, progressively extending from left to right in proportion to signal strength. If the received signal contains a stereo pilot carrier, a small “st” appears to the right of the tune line. When tuning AM stations, only the SIG indicator is functional. At the lower center of the panel are two parallel rows of red LEDs that show the approximate power output into 8-ohm loads over a range of more than 50 dB from 5 milliwatts to more than 125 watts.

The function (input) selector has settings for FM, AM, two phono inputs, aux, and rec level check (this replaces the audio program with a 400-Hz tone whose level is equivalent to 50 per cent FM modulation and is used to set tape-recorder gain in advance of recording FM broadcasts). The Penney 3125 has switchable FM-multiplex filtering, FM muting, and loudness compensation, plus facilities for controlling two tape decks, including monitoring from either one or cross-connecting two for dubbing from either machine to the other. Either or both of two pairs of speakers can be driven, and the front-panel headphone jack is always active. There is also a front-panel microphone jack (mono) with a separate gain control that operates independently of the main volume control.

A display panel to the right of the graphic equalizer readout contains a vertical row of ten LEDs. They show the selected program source and which speaker outputs are being driven. If the output load impedance is too low (or is short-circuited), the outputs are cut off by a relay and a protector light comes on. The remaining light is labeled clipping, and it flashes when the output stages are driven to the clipping point. This light operates in conjunction with a front-panel switch marked impedance and is associated with the adjacent speaker selector. It is marked for 8 and 4 ohms, but the instruction manual makes no mention of its function. It does not seem to affect the amplifier operation, but merely changes the sensitivity of the clipping light to suit either 8- or 4-ohm loads.

The rear panel of the Penney 3125 contains, in addition to all the regular signal and antenna connectors, a hinged ferrite-rod AM antenna and separate pre out and main in jacks, normally joined by removable jumpers. A second set of input and output jacks marked DOLBY NR ADAPTER operates in conjunction with a front-panel Dolby nr pushbutton, which can insert an external Dolby-B decoder into the signal path for reception of Dolbyized FM transmissions. The instruction book does not indicate whether this action simultaneously changes the 75-microsecond FM de-emphasis to the 25 microseconds required with the Dolby system. Two of the three a.c. convenience outlets are switched.

The Penney 3125 measures 19-3/4 inches wide, 17-5/8 inches deep, and 8-1/8 inches high and weighs 55 pounds. Price: $899.95.

Laboratory Measurements

Preconditioning the receiver’s power amplifiers by operating them for an hour at one-third of rated power followed by 5 minutes at full power left the top and sides of the cabinet around the amplifier section quite warm, but the rest of the receiver was comfortable to the touch. The clipping power output into 8 ohms at 1,000 Hz was 145 watts per channel. Into 4- and 2-ohm loads, the clipping power was 159 and 80 watts, respectively. The clipping indicator light gave valid indications with either 4- or 8-ohm loads if the front-panel impedance switch was set appropriately. The measured IHF clipping headroom (8 ohms) was 0.6 dB.

Using the standard IHF tone-burst test signal, we measured the maximum dynamic power output of the receiver into 8-, 4-, and 2-ohm loads as 184, 151, and 79 watts. The IHF dynamic headroom (8 ohms) was 1.68 dB. The overload recovery time was too short to measure (less than 2 microseconds). The amplifier’s slew factor was greater than our measurement limit of 25, and its rise time was about 10 microseconds. Paralleling a 3-microfarad capacitor across a 4-ohm resistive load produced moderate ringing on a 10,000-Hz square wave but no signs of instability.

For a reference output of 1 watt, the aux input was 14 millivolts at maximum gain, and the A-weighted noise level was 85 dB below 1 watt with IHF-standard gain settings. The phono sensitivity was 0.2 millivolt with a noise level of —83.7 dB. The microphone input sensitivity was 0.4 millivolt, and its noise level was —83.2 dB. The minimum phono-overload input level between 20 and 20,000 Hz was 209 millivolts (at 20.000 Hz, converted to the equivalent 1,000-Hz level). The microphone input overloaded at a very high 275 millivolts. The measured phono-input impedance was 50.000 ohms in parallel with a capacitance of 175 picofarads.

The graphic equalizer had the indicated effect on the frequency response, and in spite of having considerable interaction between adjacent controls it was able to create a nearly infinite—and generally useful and effective—variety of response curves. The RIAA phono equalization was within ±0.5 dB of the ideal response from 30 to 20,000 Hz and was boosted by no more than 1 dB between 5,000 and 20,000 Hz when measured through the inductance of a typical phono cartridge. The microphone frequency response was down 3 dB at 130 and 3,500 Hz and fell at a 6-dB-per-octave rate beyond those limits.

One major change in the specifications of the Model 3125, compared with the earlier Model 3275, is its much lower audio distortion. The improvement was dramatically confirmed by our measurements. At 1,000 Hz the distortion was less than 0.002 per cent below 10 watts output, reaching 0.01 per cent at about 135 watts into 8 ohms. The 4-ohm distortion was slightly higher, falling from 0.007 to 0.003 per cent as the output rose from 1 to 100 watts and reaching 0.01 per cent at 150 watts. Even a 2-ohm load, for which the amplifier is not rated, yielded distortion readings of 0.01 per cent up to 10 watts, falling to 0.007 per cent just before clipping occurred at 80 watts. The intermodulation distortion was less than 0.1 per cent at almost all power outputs up to the rated 125 watts.

At the rated power level into 8-ohm loads the distortion of the amplifier was typically between 0.006 and 0.01 per cent from 40 to nearly 20,000 Hz. It reached a maximum of 0.016 per cent at 20 and 20,000 Hz. At reduced power outputs the distortion was even lower. The IHF IM distortion was measured with equal-amplitude input signals— at 19,000 and 20,000 Hz—whose equivalent sine-wave peak output was 125 watts. Referred to that power level, the third-order distortion at 18,000 Hz was —66 dB (0.05 per cent) and the second-order component at 1,000 Hz was —88 dB (0.004 per cent).

The FM-tuner section had a usable sensitivity of 11 dBf (2 microvolts, or μV) in mono and 17 dBf (4 μV) in stereo. The respective 50-dB quieting sensitivities for mono and stereo were 14 dBf (2.7 μV) and 38 dBf (43 μV). At 65 dBf (1,000 μV) input the distortion was 0.6 per cent in mono and 0.2 per cent in stereo, and the respective noise levels were —72 and —67.5 dB. The stereo frequency response was essentially flat up to nearly 7,000 Hz and rose at higher frequencies to +1 dB at 10,000 Hz and +1.6 dB at 1 5,000 Hz. The channel separation was very uniform at 34 dB throughout most of the audible range, and it was 9 dB at the extremes of 30 and 15,000 Hz.

The capture ratio of the FM tuner was 1.4 dB, and its AM rejection was a rather low 47 dB at 45 dBf (100 μV) input. Increasing the input-signal level made a considerable improvement in AM rejection, which measured a very good 67 dB at 65 dBf. The selectivity characteristics of the tuner were very good, particularly its 97-dB image rejection. The alternate- and adjacent-channel selectivities were 73.5 and 5.5 dB, respectively. The various operating thresholds of the FM tuner (muting, lock, and stereo switching) were all about 13.5 dBf (2.5 μV). The 19-kHz pilot carrier leakage was —66 dB. The IHF IM distortion of the tuner was measured with a modulating signal of two equal-amplitude tones—at 1,000 and 15,000 Hz—whose combined peak corresponded to 100 per cent FM modulation. The third-order IM products at 13,000 and 16,000 Hz were at — 45 dB relative to the modulating tones, and the second-order distortion at 1,000 Hz was — 47 dB referred to 100 per cent modulation at that frequency. The only measurement made on the AM tuner section was of its frequency response, which was extremely restricted. The response was down 6 dB at 1,700 Hz.

Comment

When we compared the results of our test of the new Penney 3125 with those from the earlier 3275, it was apparent that most of our earlier criticisms no longer apply. The Penney 3125 has an absolutely first-rate audio section, very powerful and with extremely low distortion, yet thoroughly protected (we shorted the outputs accidentally and the protective system shut the amplifier off with no damage).

The FM tuning system is very unambiguous and easy to use. Unfortunately, however, one must be directly in front of the receiver and on a level with the panel to see the tuning indications, which are deeply recessed behind the front panel. The lock (or AFC) system worked very well, always tuning the receiver accurately for minimum distortion. The digital frequency readout on our test sample had a systematic calibration error, reading 0.1 MHz lower than the station frequency.

The AM quality was so bad that we cannot understand the rationale for its inclusion. This is the first AM section of a high-fidelity receiver we have used whose bandwidth was so narrow that intelligibility of human voices was very seriously impaired.

In spite of these criticisms, some of which are probably due to the fact that the unit tested was a very early production unit, we continue to be favorably impressed with Penney’s approach to hi-fi component design. The styling is still very different from that of most of the products one might find in a typical audio showroom, but perhaps it will have greater appeal to a mass market that associates technological sophistication with a plethora of lights and controls (the front-panel displays employ a total of 153 LEDs by our count!). Nevertheless, it is important to realize that this is a real “hi-fi” receiver whose performance is light years beyond that of mass-market “stereos.”

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