Testbench - DJ PowerMaster Stereo 200
Test Report on: D.J. PowerMaster Stereo 200
Date: May 1975, £120 Ex VAT.
The D.J. Stereo 200 is a dual slave amplifier rated at 100W + 100W. A feature which is concealed by the word 'stereo' in the title is that the two channels are independent, even to the extent of having separate off/on switches, and may be used as separate amplifiers. There is nothing in the engineering which commits the two channels to being used for stereo signals only.
The 'Power Master' range also includes a single channel 100W unit and dual and single channel 150W versions. Controls on slave amplifiers are usually kept to a minimum: just volume controls, input jacks, output jacks and an off/on switch. However, on the D.J.200 there are also buffered line output sockets with level controls for driving extra slave amplifiers; output meters and separate illuminated off/on switches for each channel. All unexpected luxuries.
The unit is housed in a black painted metal case 22¼ in. long, 5 inches high and 10¼ inches deep. A silver satin finished control panel sloping at about 45 degrees is mounted behind an aperture along the top front edge of the outer metal case. All the working parts, including the control panel, are mounted on a 'U' shaped aluminium chassis, which is about 1/8th inch thick. This forms the bottom and back of the case and is also the heat sink for the power transistors. This permits good accessibility for servicing and a tidy layout. In appearance it is a bit of an 'ugly duckling'; rather large and awkward but very strong. The large size does, however, mean that it is cool when running.
The electronics are clearly divided into two separate amplifiers; mains transformers at the ends and glass fibre printed circuit boards towards the centre. Wiring is very short and neat and held in place with plastic clips. The standard of soldering, care in assembly and attention to detail are good and should contribute to above usual reliability.
The measured performance of the unit provided for test was better that the manufacturers specification in every respect. In particular the signal to noise ratio was 87dB compared with the manufacturer's claim of only 70dB and the distortion measurements were about 50 times better than claimed.
The overload protection caused surprise. It was tested by trying to drive full power into a 0.1 ohm load. The protection circuit turned off the output 'short circuit'. This test condition was maintained for 2 mins, and then the load was restored to normal. The output only turned on again when the input type of overload protection, but it clearly works well.
The following table sums up other aspects of performance:-
|Power Output||157W RMS||@ 10% total harmonic distortion|
|into 8 ohms||107W RMS||@ 1% total harmonic distortion|
|Distortion||0.023%||@ 90% rated power)||Very much better than the|
|(Total Harmonic)||0.02%||@ 50% rated power) 1 KHz||'less than 1 %' claimed by the|
|0.014%||@ 1W RMS||manufacturer.|
|Sensitivity||660mV RMS||for full output||Voltage gain of 43 compatible with 0dBm signal levels.|
|Bandwidth||49 KHz||Upper -3dB point||Wider range than required for|
|22 Hz||Lower -3dB point||audio signals.|
|Square Wave||Good||Clean response to 1 KHz & 10 KHz square waves.|
|Capacitive Load Test||Good||1uF and 8 load.||No overshoot or ringing, therefore good stability margin.|
|Noise Level||0.2uW RMS||Wide band measurement||—87dB below 100W good.|
|Slave OP level||-1.4dBm||660mV RMS at full output||This output impedance is about 270 ohms|
|DC offset at Output||Zero||Less than 3mV.|
This is a well designed and manufactured piece of equipment with a performance far in excess of that claimed by the makers. The manufacturer's specification does not do justice to themselves or their amplifier. Very good value for money indeed.
Oscillator; purpose built sine wave generator, total harmonic distortion measured at 0.005%.
Total Harmonic Distortion Factor Meter, Sugden JE S1452.
Advance OS250 Oscilloscope with calibrator.
Resistive loads 2% tolerance, 4, 8 and 16 ohms.
Level TM3 milli-volt meter.
Power measurements are correct to +10%.
Sensitivity measurements are correct to +0.5dB.
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