Boss Chorus Ensemble
A compact stompbox with a BBD heart and a filter section that most chorus pedals never bothered to include.
Overview
The owner's manual specifies an input impedance of 1 MΩ and an output impedance of 1 kΩ, with a recommended load impedance of 10 kΩ or greater. That means it plays fine with just about anything. Residual noise is quoted at −93 dBm or less, measured IHF-A with the filter control centered, referenced to 0 dBm = 0.775 V. That's quiet for a BBD pedal, and it comes straight from the circuit topology Boss chose.
The control set is what separates this one from the simpler chorus boxes. Besides the usual Effect Level, Modulation Rate, and Modulation Depth, there are two filter controls, High Filter and Low Filter. They let you trim the top and bottom of the modulated signal before it hits the amp. The pedal switch handles bypass, and the Check Indicator doubles as a battery check, which was handy when 9V batteries were the default power source. The unit runs on DC 9V via a dry battery or an optional PSA-series AC adaptor.
Inside, the Chorus Ensemble uses a Panasonic MN3007 BBD (bucket-brigade device) as the delay element, driven by an MN3101 BBD driver. The op-amps are M5218AL and M5223L types. The transistors are 2SC3378GR or 2SC2240GR, 2SC2458GR, and 2SK184GR or 2SK117GR FETs. A red LED handles the indicator. The pots are split up like this: a 50kA × 2 plus 250kC for the filter low/high section, 250kC for rate and effect level, and a 100kB for the remaining control. It's a real analog delay line, not a digital approximation.
The package is modest: 70 mm wide, 125 mm deep, and 55 mm tall in metric, or 2-3/4" × 4-15/16" × 2-3/16" in imperial. Weight is listed at 410 g / 0.9 lbs including the battery. It's a standard Boss compact footprint that fits on a crowded pedalboard. The parts list gives specific part numbers for the case, bottom base, foot base, knobs, jacks, and even the nylon washers and thumb screws.
Power options are broader than on the average pedal. Besides the standard 9V dry battery (S-006P, part number 12569103RT), Boss lists PSA-100, PSA-120, PSA-220, and PSA-240 AC adaptors as options, covering multiple regional voltages. The manual also notes an alkaline 6AM6/9V battery as an acceptable alternative. The MT Board Assy (part 7528052000, PCB 22937234RT 1/2) is the main board, and the service note says replacement MT Board Assy units include the VR Board Assy. Anyone doing a repair will want to know that. The JK Board Assy (7528054000, PCB 2292768700) and LED Board Assy (PCB 2291049600) round out the internal assemblies.
The adjustment procedure, per the manual, goes like this. Feed a 1 kHz signal at −7 dBm into the input, connect a scope to Output A, and plug Output B. Engage the effect with the pedal switch, then trim RT1 on the MT Board to get the specified waveform, with the controls set as shown in the service documentation.
Mix (May 1980, p.99) described the compact Boss chorus as a version of the "famous Boss CE-1 Chorus Ensemble," noting that it had become one of the standard signal processing devices of the industry, and that the compact model produced a similarly rich tonal character, a thicker, deeper sound comparable to a chorus of voices or a doubling effect. The same coverage noted the Rate and Depth controls could be tailored for guitar, keyboard, wind, or brass instruments, as well as vocal PA applications, and that the pedal was suited to both studio and stage use.
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