F series: this series refers to pre-amplifiers designed to be used in the round back guitars, because it’s not convenient to install pre-amplifiers or to excise opening hole in the upper part of the round back guitar. Therefore, we choose to install the products in the lower part of the round back guitar, which makes everything so easy. The bottom of the product adopts wholly sealed metal shell to efficiently reduce noise, and this series of products that adopts piezo ceramics pickups is featured with large output, compact bass and invigorative alto voice and clear and loud high pitch, which exhibits the timbre featured by the excellent piezo ceramics pickup.