This is a review and detailed measurements of the vintage Audio Research D300 Power Amplifier. It is on kind loan from a local member. The D300 came out circa 1995 and initially cost US $3,500 but then price rose to US $4,000. Quick search shows used ones going for US $1,500.The D300 is built like a tank:I am confident you could drive over it and nothing will happen to it. Even the handles are massive which is a blessing as an ultra heavy transformer sits vertically behind the front panel. I could carry the D300 by myself so it is not too heavy to be unmanageable.The back panel is as you expect:With one exception: the non-detachable power cord. What is there though seems amply sufficient as it is supple yet quite thick.Heatsinks are very thick gauge and in use, they got warm but not hot. The rest of the case got warm as well. The amp never complained no matter what I did to it in testing.Overall, if you want an understated but high quality feel, that is what you have in D300.As usual, we start with a 1 kHz tone at 5 watts into 4 ohm load:Ouch. Lots of second harmonic causing SINAD which is the sum of distortion and noise to be rather poor. As such the D300 lands well below average in our list of some 62 amplifiers tested to date:Noise performance was quite good though so it is the distortion that is the problem:32-tone test signal representing "music" showed what we already know as far as rather high distortion levels:Frequency response was excellent:As was crosstalk (one channel bleeding into the other):Let's get into the meat of the measurements with power vs THD+N starting at 4 ohm:Boy, this is worse than the dashboard indicated. Almost immediately, distortion takes over and keeps climbing. While the D300 is much more quiet, it quickly loses to an integrated home theater receiver (NAD T758)! Not good.Same story repeats for 8 ohm load:Distortion is so high at higher power levels that if we set a limit of 0.1% for THD+N, we only get 60 watts out of each channel!Relaxing that to 0.2% though gave us a lot more power and no indication of frequency mattering. This was proven yet again in our multiple sweeps:All the lines fall on top of each other indicating the amp just doesn't care what frequency you feed it. It amplifies it, adds a heap of distortion and that is that. Makes you want to cry and celebrate at the same time!There is clear excellence in the engineering in Audio Research D300. Build quality is superb as well. There must have been a conscious decision to use low amount of feedback which translated into rising distortion with power. It is a shame that capable designers follow audiophile myths to build non-performant amplifiers when they could do so much better.In this day and age, this type of performance is not competitive so I really can't recommend the D300. Then again if you want something with high build quality and can get the unit on the cheap, it makes for a decent option.--------As always, questions, comments, corrections, etc. are welcome.Ever walk through a high-end department store and get hounded by the ladies selling perfume? I know to ignore them but the pink panther I was taking for a walk there fell for it and made me buy him a fancy bottle for $100. Needless to say, I feel poor so could use some extra money. Pleaseusing : https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/