Hill IC208S Power Amp
Hill Public Address Systems produce a range of power amplifiers of different power ratings to suit most requirements. The IC208S is a slave amplifier which is designed to drive 200 watts R.M.S. continuously into a load of 8 ohms or 120 watts continuously into a 15 ohm load, it will also drive 320 watts into a 4 ohm load but a cooling fan is then required.
Because it is a slave amplifier, the only controls provided are a volume control and an an/off switch together with a mains fuse. Several versions are available with different combinations of input and output sockets and mains connector types. One can choose standard jacks or Cannon XLR connectors for the inputs and outputs and the mains can be a wired in lead, a cannon plug, a Bulgin plug or a Euro connector.
A version with a Vu meter is also available. Two amplifiers were submitted for test. One was a pre-production model fitted with cannon plugs throughout and a Vu meter. The second was a production model which differed in having a retained mains lead and no Vu meter, it is the production model to which the following test results apply.
Standard 19 inch rack format is used and only 2 units of height (approx. 3¾ inch) and 10 inches of depth are required. The main construction is of thick, black anodised aluminium extrusions which are screwed together in such a way that they are in good thermal contact with each other so that the whole case acts as the heat sink.
Most of the small electronic components are mounted on a single glass-fibre printed circuit board in the centre of the case. The components are of good quality and, apart from two resistors, are well mounted. A small transformer, which would have been a little heavy for the board to support, has been bolted directly through to the chassis for support. Some small electrolytics have been tied down to ensure they do not vibrate loose and the very 'man sized' main electrolytics have each been fixed with two separate support brackets.
Three separate fuses are fitted; two power supply fuses on the printed circuit board and a standard 20mm mains fuse in a very nice holder on the front panel.
The power supply uses two separate transformers which is less efficient than a single transformer but permits a shallow profile case to be used and also allows the same transformer to be used in the 100 watt amplifier of the range.
Output protection is by a thermally activated cut-out in the speaker circuit. The rating of the cut-out is chosen so that 200 watts of music does not activate it but it was found that 200 watts R.M.S. sine wave activated it in 9 minutes. The manufacturers are going to change the cut-out rating so that continuous sine wave at rated output does not operate it.
The thermal cut-out is more to protect speakers than to protect the amplifier and to permit high power sine wave tests to be conducted, the cut-out had to be shorted out. (Approval for this was given by the manufacturer.)
The high frequency response of the amplifier has been tailored to fall by 1dB at 15KHz and 3dB at 24KHz. This prevents high power signals outside the audio band being fed to the speaker and possibly damaging them.
|Measured Performance||RESULT||TEST CONDITION||COMMENTS|
|@ 10% total harmonic distortion|
|Power Output||441 watts||4 ohm load||exceeds manufacturers spec.|
|328 watts||8 ohm load|
|220 watts||16 ohm load|
|at onset of clipping|
|Power OP||324 watts||4 ohm|
|247 watts||8 ohm|
|144 watts||16 ohm|
|Distortion (total harmonic)||0.05%||@ 200 watts into 8 ohm||mainly 3rd harmonic|
|0.035%||@ 10 watts at 1 KHz||mainly noise|
|0.04%||@ 1 watt||mainly noise|
|Sensitivity||1.1 volts RMS||onset of clipping into 8 ohm||100K ohm input impedance|
|0.95 volts RMS||for 200 watts into 8 ohm|
|Capacitive load test||Very good||checked at 1 KHz and 10KHz||exceptional stability|
|DC offset||15mV when loaded,|
|50mV off load||O.K.|
|Noise||1 mVRMS into||this is 92dB below||this noise was mainly H.F.|
|8 ohm load||200w into 8 ohm||outside the audio band|
|Frequency response||-1dB @ 15KHz|
|-3dB @ 24KHz|
|-1dB @ 16Hz|
Short circuit operates the thermal cut-out in a fraction of a second and the amplifier works correctly immediately on resetting.
The amplifier is well constructed and of rugged design. The compact size should make it attractive to groups but care should be taken to ensure that it is not sandwiched between so many other pieces of equipment that it fails to get proper ventilation.
The option to have Cannon connectors is a good one because they should give better reliability than one can expect with jack plugs.
There is no direct form of fast acting protection which means the amplifier will give very high powers for short times before the thermal cut-out or fuses act. This could be very useful for music signals, where the peaks are very much greater than the mean. The amplifier did survive direct short circuits without damage.
The high power into 4 ohms is not specified as a continuous rating and 8 or 16 ohms should be considered as the normal load.
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