wharf-creek wrote:I'm just curious.....has anyone out there ever actually done the 'mono' thing according to Dynaco's instructions......and then run any legitimate tests to see just how well the process actually delivers?
Yes: I have. I was an audio technician, on and off, for years. I ran the tests using conventional bench test equipment: function generator and both harmonic and intermodulation distortion analyzers, and observed both the output waveforms and distorion waveforms on an oscilloscope. Distortion in the strapped-mono Stereo 70 starts off high, even at low output power. In fact the strapped mono Stereo 70 is substantially outperformed on the bench by ONE CHANNEL of a regular Stereo 70 until the single channel clips. Since most of the time an amplifier is not called upon to output more than a few watts, that probably explains why the strapped mono amplifier sounds bad. What little gain in output power the strapped-mono has, is far more than outweighed by its inferior performance, the very great majority of the time.
Conventional bench testing, however, does not reveal how an amplifier behaves on a transient basis. I also suspect, but have not measured, that due to the mixing of the negative feedback signals between its two amplifier channels, the strapped-mono Stereo 70 behaves and recovers from clipping worse than does one channel of a Stereo 70. If this is so, when extreme demands are made, and BOTH strapped-mono AND one-channel amplifiers clip, the strapped-mono will STILL sound worse, even when it doesn't quite clip as early as the one-channel amplifier.
What I do know from listening through real speakers in a good listening room with a listening panel: a PAIR of strapped-mono Stereo 70s is roundly outperformed by a pair of Dynaco Mark II (fifty watts rated, through a pair of EL34 output tubes, the Mark II being selected for the test to eliminate differences between the EL34 and the 6550 tube); and was also surpassed by a SINGLE regular Stereo 70 operating in its Stereo mode. There was NO ambiguity; EVERYONE said he heard those results.