Amcron DC-300A Series II Power Amplifier (HSR May 84)

Cambridge, February 1999: Russian tank commander Ivan Vladivar, preparing to storm the last bastions of democratic decency, peers warily through his field glasses. What is that heavy, square object blocking the road five hundred metres ahead? Is it the new American secret weapon, until now only whispered about behind the closed doors of the Kremlin? He focuses his binoculars more closely, and cursing heavily in Russian, signals the tanks lined up behind him to turn around. “We cannot go in there, comrades. They have blocked the road with an Amcron amplifier”.

A picture is not always worth a thousand words: the reliability and solidness of this amp are impossible to convey in a photograph, so I hope you’ll forgive the flight of fancy. Amcron is the export name used by US company Crown, and the old DC-300A has already built up a formidable reputation for quality and dependability in PA and studio monitoring work; so much so that a friend of mine cut out the Amcron logo from one of their adverts and pasted it on the front of his cheapo amplifier in the vain hope that it would be mistaken for the real thing!

What exactly do you get for your money? Before going any further, let’s just mention that it will be quite a lot of money. The Amcron is not cheap. Basically, the DC-300A is a rack mounting two channel power amplifier providing 155 watts per channel into 8 ohms, with the facility to operate in a mono ‘bridged’ mode to give 310 watts into 16 ohms. This feature can also be used to produce a balanced 70 volt line for feeding many small speakers in a building PA system, for instance.

Performance

The specifications are summarised in the box on this page. Suffice it to say that it wasn’t possible to check many of them with my equipment; it’s no use hunting for 0.001% distortion when your signal generator produces 0.01 %! Those that were measured were well within spec. I put the Amcron in place of my own (self-designed) amplifier for a couple of weeks, not expecting to hear any difference; I rarely monitor at very high levels, so an extra 5dB output is neither here nor there. What was noticeable, in fact, was a much firmer bass response from my KEF speakers.

This is due to the amplifier’s high damping factor, which works like this. If you apply a voltage to the terminals of a speaker and then take it away again, the cone will flap about for a while in a manner determined by the construction and design of the drive unit and box – damping factor is most important at bass frequencies, incidentally. Whilst the speaker cone is flapping about, it’s acting as a sort of microphone, feeding the ‘flap’ signal back to the amplifier: if this has a high damping factor, essentially a low output impedance (including the speaker wires), it absorbs these signals and stops the cone moving. In other words, the amplifier tells the speaker what to do in an accurately controlled manner.

So the Amcron sounded good – in fact the only problems were firstly, a low hum from the unit’s transformer, annoying at close quarters, and secondly, the phenomenal energy storage of its power supply capacitors. The amplifier keeps on working for a long time after it’s switched off, and all the nasty noises made by my equipment were faithfully amplified at high volume! By itself, or with the input levels turned down, the DC-300A is silent on switching on and off.

Construction

The box itself is quite small, made from extruded aluminium, but with large heatsink fins on the back. The heatsinks are joined to the chassis for maximum thermal dissipation, but good ventilation (or a fan) will still be needed in the rack to stop the built-in thermal cut-out operating at high output powers. There are eight output transistors per channel, giving tremendous overload capacity, covered by perforated metal plates.

Also protruding from the back are an enormous 1 kW transformer, and two 13000uF 70 volt electrolytic capacitors. In recognition of the American craze for sueing manufacturers who fail to warn against the most obvious misuse, these are labelled “misapplication hazardous”, and I can believe it. There are also the usual warnings against opening the amplifier, paralleling the outputs, and also an exhortation to use class 1 wiring if the speaker cables are permanently installed in a building. The output voltage could be lethal – makes you think, doesn’t it?

The instruction book continues this tradition with a closely-worded three year warranty, and various warnings and disclaimers sprinkled throughout its considerable length. Most power amplifier manuals are of the “here’s the input, there’s the output, plug it in and switch on” variety, not this one. There are full specifications with graphs and explanation of what they mean, an individually measured performance report, circuit diagram and description, and even advice on what gauge speaker wire to use!

All the connectors are on the back: inputs are via standard ¼” jack sockets, with a small slide switch for mono mode. The outputs are on terminal posts for bare wires, including 4mm sockets in their tops on the standard ¾” spacing; two dual 4mm banana plugs are supplied with the amp.

Round the front, we find a pushbutton power switch with LED indicator, and a separate level control for each channel. Associated with these are two more LEDs which are labelled ‘IOC’, or Input Output Comparator. Internal circuitry lights the LED if there is any dissimilarity between input and output; this includes clipping, operation of the protection circuitry, slew rate limiting, in fact any severe distortion. A fairly simple circuit, but a great improvement on the usual clipping indicator.

Conclusion

The Amcron DC-300A series II will cost you £776.25 including VAT, or more than £2.50 per watt. Expensive, but you get a lot for your money; this amplifier has excellent performance, is virtually indestructible in normal use, and is ruggedly and reliably constructed. Yes, it will probably still be working in 1999, invasion or no invasion!

Specifications

Output power: At least 155W average per channel into 8 ohms, both channels operating. At least 310W average into 16 ohms in mono mode.

Frequency response: DC to 20kHz +/- 0.1 dB.

Harmonic distortion: At rated output, less than 0.001% 20Hz to 400Hz, less than 0.05% at 20kHz.

Intermodulation distortion: Test frequencies 60Hz and 7kHz, power ratio 4:1, less than 0.05% between 10mW and 250mW, less than 0.01% from 250mW up to rated output.

Slew rate: 8 volts per microsecond, 16 volts per microsecond in mono.

Output impedance: Less than 7Mohms.

Damping factor: Into 8 ohms, better than 750 below 400Hz.

Input sensitivity: 1.75V unbalanced into 25k.

Hum & noise: 110dB below rated output.

Crosstalk: Better than -90dB.

Power requirements: 100, 120, 200, 220 or 240V AC +/- 10%, 500W at maximum output.

Size: 19″ wide, 7″ high, 9¾” deep.

Weight: 45 lbs.

Amcron products are distributed in the UK by HHB Hire & Sales, .