JBL GTS600
A car amplifier rated at 200 watts per channel into 4 ohms, with a 600-watt bridged mode and a crossover you tune by swapping resistor chips.
Overview
The GTS600 is one half of a two-model car amplifier line JBL built around the GTS300 and GTS600. The spec sheet tells you which half you have: 200 watts x 2 into 4 ohms at 0.05% THD, 300 watts x 2 into 2 ohms at 0.08% THD, and 600 watts x 1 bridged into 4 ohms at 0.08% THD. Those numbers are quoted at 14.4 volts battery voltage across a 10 Hz to 50 kHz bandwidth, a wide measurement window and not a cherry-picked 1 kHz figure. The GTS300 is the 100-watt-per-channel version of the same design. Both share the same signal-to-noise rating of 100 dBA and the same frequency response spec: 10 Hz to 50 kHz at +0, -1 dB, tightening to 20 Hz to 20 kHz at +0, -0.1 dB. Damping factor is above 200.
JBL's owner's manual makes the intended buyer clear. This is a sophisticated product that the company recommends be installed by an authorized JBL dealer, and anyone going the DIY route is told to read the entire manual before starting. That's not boilerplate caution. The GTS600 has features that reward careful setup and punish sloppy wiring.
The most distinctive of those features is the crossover system. The GTS600 carries two built-in frequency-selectable crossovers: one in series with the amplifier circuitry, and a second on the preamp-level output jacks. Each can be switched to flat (full bandwidth), low-pass for subwoofer duty, or high-pass for satellite speakers. The crossover frequencies are set by resistor network "chips" inside the amplifier, a molded package of resistors across the pins. JBL sold these chips in standard values: 50 Hz (47 K, part GTM 50), 80 Hz (33 K, GTM 80), 120 Hz (20 K, GTM 120), 200 Hz (12 K, GTM 200), 250 Hz (10 K, GTM 250), 500 Hz (4.7 K, GTM 500), 2.5 kHz (1 K, GTM 2500), and 5 kHz (470 ohms, GTM 5000). If none of those suited the system, the manual invites owners to buy compatible resistor networks from a local electronics store or build their own from discrete resistors on a 14-pin DIP header. That's an unusual degree of user-serviceability for a car amp, and it made these units popular with installers who wanted precise sub/satellite integration instead of the fixed crossover a head unit offered.
The input section is just as flexible. Line-level input sensitivity runs from 100 mV to 4 V RMS with a 500 mV nominal figure, and line input impedance is 10 kOhms. Speaker-level inputs are switchable between 15 ohms and 100 kOhms via internal jumpers, with sensitivity from 200 mV to 8 V RMS (2 V nominal). JBL's "Universal Interface Circuitry" lets the GTS600 accept speaker-level signal from head units or equalizers that have no preamp outputs. The speaker-level and preamp-level inputs also feed a mixing circuit, so both can be connected at the same time. The manual pitches this as a higher-performance alternative to an FM modulator for adding CD capability to a factory cassette stereo: turn on the source you want, turn off the other, no extra switches or relays needed. The speaker-level input also carries JBL's Common Sense circuitry, which powers the amplifier on as soon as the connected high-powered head unit is switched on.
The input mode selector covers stereo, right-channel-only (driving both output channels from a single right input), and L+R summed mono. A power indicator LED on the chassis top stays lit for normal operation and blinks when the protection circuitry or muting engages.
Power requirements are 11 to 16 V DC, negative ground. The GTS600 uses two 40-amp ATC-type fuses, one more than the GTS300's single 40-amp fuse. The manual notes fuses are not included, so they have to be sourced separately. The chassis measures 17 1/4" x 11 1/4" x 2" (438 mm x 286 mm x 50.8 mm) and weighs 13 lbs 11.5 oz (6.2 kg). That is noticeably larger and heavier than the GTS300's 13 1/4" x 9 1/4" x 2" and 10 lbs 8.5 oz. Speaker-level input uses a Molex Mini-Fit Jr. connector (part #39-01-2045-P, metal pins 39-00-0039), and a wire harness is supplied.
On the speaker side, the manual is specific about load limits. In non-bridged mode the GTS600 will drive 2-ohm loads, and with a single speaker per channel virtually any conventional speaker works. In bridged mode the combined impedance of the connected speaker or speakers must be at least 4 ohms. Sustained operation below that will likely cause overheating. The manual also flatly prohibits speakers with either input terminal wired to the speaker frame or the vehicle chassis, and warns against mounting the amplifier upside down.
Wiring guidance is unusually strict for a consumer product. JBL calls for soldered connections with heat-shrink insulation, or high-quality crimp connectors set with a proper crimping tool, and recommends soldering crimp connections for extra reliability. Wire nuts, insulation-displacement connectors (ScotchLok type), and twist-and-tape joints are banned, as is electrical tape, which the manual notes will loosen with age and extreme temperatures. Routing should avoid sharp sheet metal edges and use grommets, strain reliefs, and tubing wherever wires pass through panels.
Gain adjustment follows a standard procedure. Start with the amplifier gain controls fully counterclockwise and set bass/treble or EQ controls to centered or bypassed. Then raise the source volume until distortion is just audible and back off until it disappears. If that doesn't give enough level, bring the amplifier gain up gradually until the system plays as loud as needed or the first signs of distortion appear. JBL's stated principle is that a high signal level from the source with a low gain setting on the amplifier keeps background noise low.
Period coverage of the GTS600 is thin, but it does give a price. Audio (May 1994, p.56) listed the GTS600 at $799.95 and the GTS300 at $549.95 in its amps/equalizers column, alongside a JBL GTO400 at $399.00, GTO200 at $179.00, and GTO100 at $549.10. That puts the GTS600 at the top of the JBL car amplifier range as listed in that issue.
Today the GTS600 is interesting mostly for the chip-swappable crossover and the dual-input mixing architecture, both more flexible than most period competitors. The two-fuse power requirement, the 4-ohm minimum in bridged mode, and the ban on chassis-grounded speakers are the practical gotchas to check before wiring one up. The Molex speaker-level connector means a missing harness is a real problem. The manual specifies the exact part numbers, but sourcing a 39-01-2045-P plug and 39-00-0039 pins takes some digging. The amplifier's size and weight also mean it needs a lot of mounting space.
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