|Scott LK-72 stereo integrated amplifier kit set|
AT A GLANCE: The Scott LK-72 is a high-powered, high quality stereo control amplifier available in kit form. United States Testing Co., Inc., points out that the unit is handsomely styled, relatively small, and weighs a mere 26 pounds. Nevertheless, it contains a full complement of controls, and will develop 40 watts of clean audio power per channel over most of the audio spectrum. Price: $159.95.
IN DETAIL: The LK-72 has separate bass and treble controls for each channel, a single volume control for both channels, a channel balance control, a seven- position function selector, a three-position input selector (phono or tape, tuner, and "extra" or auxiliary input), loudness switch, and scratch filter. A separate front panel switch is also provided for selecting either the NAB tape head input or the RIAA magnetic phono input. On the rear panel, terminals are provided for connecting either 4-, 8-, or 16-ohm speakers to the amplifier. Additionally, there are two low level input pairs, and two high level input pairs, as well as a high level tape recorder input which is connected when the tape monitor switch is on. A centre channel output jack is included, as well as tape recorder output jacks. An AC convenience outlet is also furnished on the rear panel. Provision is made for adjusting the DC balance of each channel without using test instruments.
In USTC's tests, a kit-built LK-72 developed 40 watts R.M.S. of clean audio power per channel at 1,000 cps with one channel operating, and 32 watts per channel with both channels operating. The decrease in power is due to the loading on the amplifier's power supply, which uses a GZ34/5AR4 rectifier tube and SRI selenium rectifier. At that, the power still available is, in USTC's opinion, quite ample for most installations.
The amplifier's power bandwidth extends from about 30 cps to about 20 kc. At the 1-watt output level, frequency response was found to be flat within +0.6 and -1.0 db from 16 cps to 29 kc. Response continued well beyond this point and was checked, in fact, out to 41 kc where it was down only 3 db.
Total harmonic distortion at 40 watts output was only 0.45% at 1,000 cps and remained below 2% from 30 cps to 18 kc. At half power (20 watts R.M.S.), the THD remained below 1% from 30 cps to 19 kc, which also is quite good considering the size and weight of the LK-72. Intermodulation distortion was very low on the LK-72, with only 0.73% IM at 40 watts R.M.S. output. At 10 watts, the IM distortion was less than 0.4%.
The sensitivity of the LK-72 for 40 watts output was 0.85 volts on the high level inputs, 16 mV on the RIAA phono input, and 7.6 mV on the NAB tape head input. In USTC's view, higher gain on the two low level inputs might be desirable to realize the full potential of some recent high quality, low-level pickups. This is not critical, however, for many other pickups. Signal-to-noise ratio was very good, being 79 db on the high level inputs and better than 60 db on the low level inputs at maximum gain. Channel separation was better than 50 db below 1 kc, and was 39 db at 10 kc.
RIAA equalization was generally accurate from 25 cps to 20 kc, varying no more than 1.0 db from the RIAA standard curve. NAB equalization was slightly poorer, but did remain within 2 db of the NAB standard curve from 35 cps to 20 kc. The tone control, loudness, and scratch filter all operated quite well and exhibited what one might term "tasteful characteristics" from a musical standpoint.
Square wave tests were made at 10 kc and at 50 cps. As shown in the accompanying photos of the square wave oscillograms, the 10-kc response is fairly representative of a good control amplifier, and relates actually to the high frequency roll-off measured in the extreme fringe area of the spectrum. The oscillogram also shows, by the way, fairly good transient response. The 50-cps response shows the effect of a "lagging" phase shift which relates to the steep drop in response below 20 cycles and which, again, seems fairly typical of many integrated amplifiers. The LK-72's damping factor was 14.6, indicating good stability with varying loads.
All indications are that the LK-72, with ample power output to drive virtually any speaker system, will per form very well in a home high-fidelity system. Its very low distortion makes for clean, sweet sounds. Not to be overlooked is its compactness, which enables it to be put on a bookshelf or in any cabinet. For those willing to spend the time building it, the LK-72 represents good value on today's market.
It Went Together
The instructions are prefaced with helpful hints on how to unpack the kit, what tools to select, correct soldering procedures, and so on. For those who are interested, there also is a section explaining how the amplifier operates, stage by stage. All told, this is a neat, attractive, very well-designed kit, and one which gives every assurance of successful completion even in the hands of the inexperienced or first-time kit builder.