UTC LS-10X
A shielded input transformer that solved hum problems engineers couldn't otherwise beat.
Overview
The LS-10X is an input transformer from UTC's Linear Standard series, the company's top-tier line. The series carried a striking guarantee: response flat within ±1 dB from 20 to 20,000 cycles across the whole family. That spec appears repeatedly in UTC's own catalog pages, and it's the number that defined the series. The LS-10X takes a multiple-impedance line - 50, 200, 250, 500/600 ohms and similar - and steps it up to a 50,000-ohm grid. It's a step-up input stage for low-impedance microphones, pickups, and lines feeding a tube grid.
Shielding is what separated the LS-10X from the broader LS line. The base LS-10 handled low-impedance mike and pickup duty. The X suffix denotes the shielded variant, and UTC leaned hard on that distinction. The company's own advertising billed the LS-10X as a "tri-alloy shielded input" transformer, and the catalog copy describes it as "multiple shielded." The part exists to keep hum out of a high-impedance grid in an environment where hum is winning.
UTC's own house advertisements back this up. One period piece describes a manufacturer whose speech input and remote pickup amplifiers were unusable because of excessive hum. UTC's suggested fix was the LS-10X with its tri-alloy shield, which reportedly eliminated the problem outright. Another ad recounts a theatre amplifier installation plagued by the large external fields of dimmer reactors, where the LS-10X tri-alloy shielded input was the answer. Audio (November 1950, p.71) ran the Linear Standard line's standard pitch, describing the series as the closest approach to an ideal component in terms of uniform response, low distortion, high efficiency, thorough shielding, and dependability. The LS-10X is the member of that family built specifically for the shielding problem. Audio (June 1956, p.2) repeated the LS-10X listing in a later catalog-style placement, confirming the part stayed in the lineup across the mid-1950s.
The case is the compact LS-1 case, roughly 3⅛ inches long, 2⅝ inches wide, and 3¼ inches high, at about 3 pounds per the catalog tables. That's a small, heavy lump, which is what you'd expect from a transformer with serious core and shield mass inside. Mounting and terminal layout follow the Linear Standard pattern, though specific mounting details for the LS-10X itself are thin in the surviving documentation.
A 1988 Recording-Engineer (July 1988, p.41) session table for a Michael Jackson and Siedah Garrett duet tracking date lists a UTC-LS-10X on the bass input chain, run direct. Whether that's the same transformer or a later reissue carrying the name isn't documented here. The part's catalog life was decades earlier.
Owners and restorers should treat the LS-10X as a component, not a gadget. It does one job - clean, shielded step-up from a low-impedance line to a high-impedance grid - and it does that job in the environments where ordinary input transformers failed.
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