Spherical styli were the most commonly used geometry up to the beginning of the 1960s. Consequently, most records from the first 15 years of microgroove records have been played with a spherical stylus. This will not, however, mean that mono has to be played with a spherical stylus.
In fact, elliptical types as well as line contact types can be highly beneficial for mono records from the mid 1950s and upwards (see figures on the left). The line contact types, specifically, will ensure an improved high frequency response due to the slim shape. Also the distortion from the pinching effect, which occurs when the stylus is pushed upwards due narrow high frequency grooves, will be reduced substantially.
25µm spherical stylus used in SPU Mono and SPU CG25 Di MkII.