JBL GTS300

Name: JBL GTS300
Brand: JBL

A car amplifier that asks you to open it up and swap resistors before it will sound the way you want.

Overview

The GTS300 is a two-channel car amplifier from JBL's Grand Touring Series. The first thing that jumps out of the manual is the frequency response: 10 Hz to 50 kHz, flat within +0, -1 dB, and 20 Hz to 20 kHz within +0, -0.1 dB. A tenth of a decibel across the audible band is a studio-monitor claim, not a car-audio one. The power numbers back it up. JBL rated the GTS300 at 100 watts per channel into 4 ohms at 0.05% THD, 150 watts per channel into 2 ohms at 0.08% THD, and 300 watts bridged into a single 4-ohm load. Those ratings carry the same 10 Hz-50 kHz bandwidth qualifier, measured at 14.4 volts. That is the standard car-audio test condition, so the amp only delivers its numbers when the electrical system is healthy.

The crossover is what separates the GTS300 from a shelf full of anonymous car amps. Both the GTS300 and its larger sibling, the GTS600, shipped with built-in frequency-selectable crossovers - one in series with the amplifier circuitry, one feeding the preamp-level output jacks - and each can be set to flat, low-pass, or high-pass. The crossover points are set by plug-in "chips," which are just resistor networks molded into a 14-pin package. JBL sold them in fixed values: 50, 80, 120, 200, 250, 500, 2,500, and 5,000 Hz, part numbers GTM 50 through GTM 5000. If none of those suited the system, the manual invites the owner to buy compatible networks from a local electronics store or build custom values from discrete resistors on a DIP header. That kind of user serviceability essentially disappeared from car audio once DSP processors got cheap.

The rest of the feature set is aimed at people who install their own gear. Speaker-level inputs use a Molex Mini-Fit Jr. connector and include what JBL called Common Sense input circuitry, which turns the amp on automatically when a high-powered head unit feeding that input powers up. A mixing circuit ties the speaker-level and preamp-level inputs together so both can run at once. The manual pitches this as a cleaner alternative to an FM modulator for adding a CD player with its own volume control to a factory cassette stereo, switching between sources by turning one on and the other off. An input mode selector lets the amp run stereo, drive both channels from a single right-channel input, or sum left and right to mono. A top-panel LED stays lit in normal operation and blinks when protection or muting engages. Gain controls match input sensitivity to the source, and the manual is direct about why: a high signal level from the source paired with a low gain setting keeps background noise down.

Signal-to-noise is rated at 100 dBA, damping factor above 200, line-level input impedance 10k ohms with sensitivity from 100 mV to 4 V RMS (500 mV nominal). Speaker-level input impedance is selectable at 15 ohms or 100k ohms via internal jumpers, with sensitivity from 200 mV to 8 V RMS. The amp is small for its output - 13¼ by 9¼ by 2 inches, 10 pounds 8.5 ounces - and it runs on 11 to 16 V DC, negative ground, through a single 40-amp ATC fuse. The GTS600 needed two of those fuses and a considerably bigger footprint.

Installation is not casual. The manual warns that the amp must never be mounted upside down, that bridged operation below 4 ohms combined impedance will likely cause overheating, and that speakers with either input terminal tied to the speaker frame or vehicle chassis must not be used at all. Non-bridged, it will happily drive 2-ohm loads. On wiring, JBL's guidance is blunt: solder with heat-shrink tubing or use quality crimp connectors, solder the crimps if you can, and never use wire nuts, ScotchLok insulation-displacement connectors, twist-and-tape splices, or electrical tape, which the manual notes will loosen with age and temperature extremes. Fuses are not included, and the manual reminds owners to fit one in the +12V line near the battery before reconnecting the negative terminal.

The GTS line had a retail life of at least a few years. Pricing surfaced in the contemporary press put the GTS300 at $549.10 and the GTS600 at $799.95, with the larger GT0400 at $549.10 and the GT0100 and GT0200 at $179.00 and $399.00 respectively. Audio (May 1994, p.56) ran the family in its amps/equalizers coverage that year. HiFI-Stereo (August 1997, p.86) was still carrying the series three years later. The GTS300 sat in the middle of the range: well above the entry point, well below the flagship. The crossover chips are the part most likely to have gone missing over three decades, since they were small, removable, and easy to lose in a box of spare parts. Without the right chip installed, the crossover is only as useful as whatever resistor network the last owner left behind. The manual's own instructions mean a missing GTM 80 can be replaced with a trip to an electronics supplier rather than a search for an unobtainable original.