The Hafler DH-500 is a two channel audio power amplifier designed to the very highest performance stan- dards. It is available as a kit, or factory assembled. Its power rating of 255 watts per channel is very conservative, and it can deliver appreciably higher powers into impe- dances below the rated 8 ohms. You must be very cautious in the application of this amplifier, as its output capability is more than most speakers can safely tolerate. We urge you to read the “Operation” section of this manual so your speakers will be reasonably protected. The best protection is still common sense in choosing program levels which your speakers can tolerate.
The DH-500 combines power MOSFET technology with a uniquely simple and effective circuit which reduces all types of distortion at all power outputs, over the entire audio spectrum, to levels where the finest test equipment has great difficulty in measuring it. The DH-500 sets a new high in reliability and resistance to abusive operating con- ditions at such high powers. This is one of the direct ben- efits of the power MOSFET’s ability to current limit under abnormal conditions.
Where the conventional bipolar transistor must be pro- tected by special circuits from entering “thermal runa- way” when it gets hot due to heavy signals, tending to get even hotter, the MOSFET self-corrects to reduce the cur- rent, and thus the heat under the same conditions. You thus gain the dual advantages of simpler circuitry and the elimination of one of the chief causes for distorted signals imposed on the music-the sound of the actuation of com- plex protective circuits.
The speed-measured as the slew rate-of the DH-500 delivers unmatched transient linearity, revealing the most delicate nuances of the music. In refining the basic Hafler MOSFET circuit the overall open loop gain was reduced by using more local feedback in the input stage. This re- quired less compensation to stabilize the circuit, with the result that it was faster overall. With the built in bandwidth
limitation at the input, all other stages are free of any trans- ient overload.
The fully complementary, symmetrical push-pull circuit is direct coupled throughout (except at the input). It uses all silicon discrete devices in a format which is directly re- lated to the acclaimed Hafler preamplifier circuit. With its unconditional stability into virtually any load, and its enormous power capability, the result is complete freedom from listening fatigue. The longer you listen to this re- markable amplifier, the more certain you will be that you could not have made a better choice.
While modest in cost, through the elimination of frills and gadgetry, the quality is evident not only in the sound, but in the conservatively rated components. The over- sized power transformer, the conservative operating levels of the MOSFETs, the computer-grade electrolytics total- ling 40,000 microfarads, the enclosed relay, the use of film capacitors in signal circuits-all are evidence of the design efforts to achieve exceptional reliability simultaneo’usly with state of the art sonics and specifications. So rugged is the DH-500 that it can deliver over 20 amperes into a short circuit!
For those with special power needs, the DH-500 can be adapted to monophonic operation, delivering in excess of 800 watts into 8 ohms, by installing the DH-502 input bridging kit.
Those who have chosen to build this amplifier from the kit will find the assembly instructions assume no technical ability other than how to make a good solder connection. Even that has been described for those who are new to the fun of kit building. Through pre-assembly and testing of the complete signal module, the performance of the kit has already been assured, for assembly involves little more than interconnection of the power supply and control ele- ments. You can look forward to just a few hours of con- struction fun, to hear the finest audio amplifier yet pro- duced.
