Feedback issues at loud volume with full bass

studio45 wrote: Well, I stand corrected. But that means the mechanism for feedback is a bit of a mystery then. The platter definitely moves vertically, how can that set off a horizontal vibration in the needle? Is it just down to non-ideal tonearm stiffness and bearings?

While the stylus only moves horizontally when reproducing a monophonic disk recording, on stereo records the stylus moves vertically as well as horizontally. During playback, the movement of a single stylus tracking the groove is sensed independently, e.g., by two coils, each mounted diagonally opposite the relevant groove wall.^[49]

The combined stylus motion can be represented in terms of the vector sum and difference of the two stereo channels. Vertical stylus motion then carries the L − R difference signal and horizontal stylus motion carries the L + R summed signal, the latter representing the monophonic component of the signal in exactly the same manner as a purely monophonic record.

The advantages of the 45/45 system as compared to alternative systems were:

• complete compatibility with monophonic playback systems. A monophonic cartridge reproduces the monophonic component of a stereo record instead of only one of its channels. (However, many monophonic styli had such low vertical compliance that they plowed through the vertical modulation, destroying the stereo information. This led to the common recommendation never to use a mono cartridge on a stereo record.) Conversely, a stereo cartridge reproduces the lateral grooves of monophonic recording equally through both channels, rather than one channel;
• equally balanced reproduction, because each channel has equal fidelity (not the case, e.g., with a higher-fidelity laterally recorded channel and a lower-fidelity vertically recorded channel); and,
• higher fidelity in general, because the "difference" signal is usually of low amplitude and is thus less affected by the greater intrinsic distortion of hill-and-dale recording.