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RE: If it is high level HD.....

Posted by dave slagle on November 21, 2020 at 06:09:52:
The cone offset is a casual curiosity to me. years ago I was using lowthers and while many insisted they were underhung I found many to actually be "even hung" where the length of the voice coil equalled the physical height of the gap. The way I found this out was from having the unassembled permandur parts of a PM4A and a PM-2A where I had demagnetized it. Fully understanding that the fringing flux would be asymmetrical with more emphasis or focus occurring at the bottom of the voice coil and given the very small X-max I would typically center the coils just below the top of the gap as a best guess for centered. Understanding that air was naturally asymmetrical I often wondered about purposely "off centering" the voice coil in the gap to combat this.

Enough of the back story... I build an autoformer output amp using PP 6C33's for an early NY Noize. and the 6C33's used power toroids with additional windings as autoformers in the anodes with a negative supply in the cathode. This put the center tap of the "secondary" at ground and in a perfect world with matched DCR's and currents there would be no DC offset. Well with 400ma of anode current, variability of the 6C33 and the nature of DCR's and windings, somewhere around 100-200mv of DC offset was had at the speakers. It was enough to visibly show a displacement of the lowther cone speaker when connected. Air is asymmetrical, it generates an even order distortion (patricularly lowthers in rear loaded horns) and the phase of the drive from a 2W SE amp is clearly audible in the bass. The interesting thing about the PP 6C33 amp was it showed a similar effect with phase even though the even order distortions were minimal. My thoughts at the time had the DC offset of the driver placing the cone in the asymmetrical flux path (even order distortion generator) and depending on which way the DC offset was phased this could either add to or "cancel" the distortion of air in the rear loaded horn.

Ultimately the amp was PP and used 6C33's both of which I find have little musical character so after the show the amp was pulled off its wheels cut apart and put back into the parts bin. I do recall about a decade later looking at some underhung vintage compression drivers and noticing that the voice coils were not centered in the gap. It was a stereo pair from different vintages so I surmised the offset was indeed intentional to combat the vastly different pressures on either side of the diaphragm. This got me musing about adding a variable DC offset to tweak the voice coil positioning and that idea still remains on my to-do list.

dave