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Testbench - Amcron DC300A Dual Slave Amp


Amcron DC 300A Dual Slave Amplifier

Test Report on: Ampcron DC300A Dual Slave Amp
Date: April 1975, £502-00

The DC300A comprises two very high power slave amplifiers sharing a common power supply, it has a continuous music rating of 240 + 240 watts RMS which puts it in the class of equipment required for outdoor concerts or heavy bands in very large halls, it is designed primarily for laboratory and industrial applications and not specifically for music. Consequently, it has a bandwidth which goes down to D.C. and it is rack mounting rather than free standing.

The overall size is 19 inches long by 7 inches high by 10¼ inches deep and it must be mounted in a protective case or frame before it is suitable for group use.

The front panel has two beautiful 'man size' volume controls and an illuminated off/on switch. On the back, buried deeply between the mains transformer, electrolytics and heat sink fins, are two input jacks, two pairs of output terminals and a mains fuse and a wired-in mains lead.

Output protection of a sophisticated type is provided which includes thermal cut outs which isolate the mains if either heat sink gets too hot. To prevent this happening at full power, a completely unobstructed air space around the amplifier is required.

The unit arrived well packed with a few odds and ends of accessories; a very good handbook which included full maintenance data; and a test sheet which shows that each unit is fully tested when it leaves the factory, not just the 'go/no go' test which some manufacturers use.

Construction


The unit is designed for rack mounting and needs to be mounted in a case or frame to make it suitable for stage use. It needs to be raised clear of the floor to give sufficient ventilation and, unless protected, the electrolytics and heat sink fins would be unlikely to take life on the road without damage.

The quality of construction is good with all parts well finished and fitting correctly. Access is via the front panel. Removing this reveals three printed circuit boards, one glass fibre board holding most of the components for both amplifiers and two other boards of 'some other material' each with the eight output transistors of one channel and the associated resistors mounted on it. Components are of good quality; soldered joints are good; wiring is tidy and cable forms are held together with plastic dips.

The mains transformer is clearly labelled '120V AC' but a red ticket tied to the mains wire declares that the unit is connected for 240 V operation. This could cause no end of confusion once the ticket is lost! The fitting of an American mains plug is also of doubtful value.

RESULT TEST CONDITIONS REMARKS
Power Output 462W RMS @ 10% total harmonic distortion Manufacturer test report quoted
into 4 Ohms 364W RMS @ 1% total harmonic distortion 1KHz sine wave 320W without defining distortion level
Power Output 253W RMS @ 10% total harmonic distortion Manufacturers' test report quoted
into 8 Ohms 270W RMS @ 1% total harmonic distortion 1KHz sine wave 180W without defining distortion level
Distortion (total harmonic) Less than 0.01% 1 KHz sine wave 50W into 8 Equivalent to the limiting resolution
0.032% 1KHz, 100W into 4 of our test equipment. almost all 3rd harmonic.
Output Protection Short Circuit Current 12.7 amps 0.1 ohms load Good — agrees with handbook
Thermal cut out Worked on short circuit output after 12 secs. full drive and short circuit output Works well but can plunge you into silence for 5 mins.
Output DC offset Ch. 1; 0.5m V
Ch. 2; 2.5mV
8 ohm load
10K dummy source
Exceptionally good
Bandwidth 80KHz to 10Hz sine wave; — 3dB points Low frequency response to DC is known but this was not measured.
Signal to Noise Ration -116dB relative to 150W into 8 wideband 10k dummy source quoted value - 121dB in band 20Hz to 20KHz. Very good.
Hum & Noise 170uV.RMS noise signal across speaker Very low
Square wave Response Rise time 17.5 uS
fall time 12.5 uS
Clean without overshoot or ringing.
Capacitive load test Good 1uF load across 8 ohms; 10KHz square wave Very good. No sign of overshoot or ringing.
Sensitivity 1.7 V.RMS for 150W RMS into 8 ohms Handbook value — This is insufficient sensitivity for most pre-amps.

Conclusion


Behaviour on all electrical tests was perfect and no-one, apart from the neighbours, can have cause for complaint about performance. The points which are not ideal are:-
(i) Incorrect mains voltage cable.

(ii) Insufficient sensitivity for most pre-amplifiers.

(iii) The amount of cooling required to prevent it cutting out and leaving you in silence seemed excessive.

(iv) Accessibility of the output terminals is extremely poor.

(v) The 5A fuse in the power line blew when signal was applied. I am sure the handbook states this rating fuse should be fitted but I could not find it. Amcron, knowing human nature, have fitted a second fuse inside the equipment which will blow if the main fuse is shorted out and a fault occurs. Such foresight deserves credit.

(vi) The mains fuse is fitted with a moulded on American type plug.

This may look a long list but, apart from the mains voltage cable, they are all of only minor importance and would be dwarfed by the list of credits; if we had room to print it. This is a truly excellent piece of engineering.



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Testbench - Yamaha YTA15A Combo Amp


International Musician & Recording World - Copyright: Cover Publications Ltd, Northern & Shell Ltd.

 

International Musician - May 1975

Donated & scanned by: Mike Gorman

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