Neve 33609JD
A stereo limiter/compressor built around two identical, isolated channels, with a by-pass relay that fails safe.
Overview
The 33609JD is a stereo limiter/compressor from Neve. AMS Neve service literature documents it, with drawings dated to the late 1990s. It has two identical, isolated channels that work as mono, stereo, or independent, and units can be linked for multi-channel use. Both channels can be by-passed entirely when limiting and compression aren't wanted, and logic-controlled relays do the switching instead of mechanical contacts. An EXTernal-INTernal switch decides whether those functions run from the front panel or from a rear-panel connector. The front-panel switches are internally illuminated from the logic to show the operational mode, and external tallies come out of that same rear connector.
Collectors tend to fixate on one detail. The reset position of the relays is BY-PASS and MONO. Disconnect the power or switch the unit off and it drops into by-pass. That's deliberate. The DC power and relay logic board has a comparator monitoring the pre-regulator DC voltage. If that voltage falls below a preset level, the comparator output goes low and steals the base current from the relay driver transistor, de-energising the relays. Continuity is preserved in a power failure. That matters on a unit sitting across a studio's main output.
The audio path lives on the 11475 motherboard. It holds the attenuator control element, the 10340-C amplifier, and the limiting and compression sidechain electronics. Gain make-up works by altering the feedback in that 10340-C amplifier when the compress in switch is closed. It restores mean programme level after a low threshold point has been selected. The rear panel carries the input and output sockets, the voltage selector, and a 500mA anti-surge secondary protection fuse. Mains comes in through an IEC inlet to a front-panel POWER switch that illuminates when the unit is on, and input voltages of 100V, 120V, 220V, or 240V AC can be selected on the rear. Primary protection is a 500mA anti-surge fuse inside the body of the voltage selector itself.
Frequency response is quoted at 20Hz to 20kHz +/-0.5dB, measured at 0dBu relative to 1kHz. Noise sits at -75dBu with the input terminated in 600 ohms, by-passed IN, and compress and limit OFF. With compress IN and gain make-up at maximum, it degrades to -55dBu, which is the number anyone actually using the compressor will live with. Distortion is specified at 0.075% maximum with the unit by-passed, 0.2% maximum with compress engaged at a 6:1 ratio and gain make-up at max, and 0.45% maximum in limit with a +22dBu input. Those aren't vanishingly low figures by modern standards.
The limiting section is adjustable. Limit level runs from +4dBu to +15dBu, measured with limit recovery set at 50ms. Limit ratio is steep: with input at +10dBu increased to +20dBu, output should change by 0.1dB. Attack time is switchable between slow at 4ms and fast at 2ms, both +/-1ms. Limit recovery can be set to 50ms, 100ms, 200ms, or 800ms, plus AUTO 1 at 1500ms and AUTO 2 at 3000ms, all +/-50%. Gain make-up runs 0 to 20dB in settings that correspond with output level to within +/-0.5dB.
Compression is the gentler tool. Compress ratio is switchable across 1.5:1, 2:1, 3:1, 4:1, and 6:1. The manual spells out what each setting should do. Feed a 1kHz signal at 0dBu into a -20dBu threshold with recovery at 100ms and increase the input by 10dB. The output should rise 6.5dB at 1.5:1, 5.0dB at 2:1, 3.5dB at 3:1, 2.5dB at 4:1, and 1.5dB at 6:1. Compress recovery offers 100ms, 400ms, 800ms, and 1500ms, plus AUTO 1 at 800ms and AUTO 2 at 1500ms. Those AUTO settings are fixed time constants with an automatic label, not program-dependent behaviour.
Stereo linking of the left and right control voltages is handled by a dedicated relay on the logic board. The whole In/Out and linking scheme is driven by a chain of logic ICs, with an analogue switch selecting between front-panel and external control. A logic level from the INT/EXT switch determines which input is selected: 0V for internal, 5V for external. External input is clamped, buffered, de-bounced, and latched before it reaches the relay driver. That's more digital housekeeping than most people expect inside a piece of analogue outboard.
Physically, the 33609JD occupies 482mm (19in) of rack width at 88mm (3.26in) high and 255mm (10.03in) deep. Power consumption is listed at 5.3VA. The internal supply is built around a toroidal mains transformer (TF14201) feeding a regulator circuit that produces the +24V DC the limiter/compressor board needs. A further regulator drops that to +5V for the logic ICs. The audio circuit itself runs on a nominal +24V supply, with decoupling and smoothing components normally mounted on the motherboard.
Service documentation for this unit is unusually complete for a piece of outboard. Full switch assembly drawings exist for the compress ratio, gain make-up, compress threshold, compress recovery, limit recovery, and limit level controls, all built on Elma rotary switches with rotation stops fitted. The compress ratio switch is a two-pole six-way Elma stopped down to five positions, with resistor values of 820, 2K7, 3K3, and 15K across the contacts. The gain make-up switch is a single-pole 23-way Elma stopped to 11 positions, with the legend printed on the panel running from 0dB through +10dB to +20dB. When a switch gets noisy or a contact fails, the exact resistor values and part numbers are on record rather than reverse-engineered.
Collectors note that the 33609 family is a bus compressor and limiter rather than a channel strip. It gets bought to sit across a mix rather than to track through. The JD variant's logic-controlled relay switching, external control connector, and tallies make it the version for a facility rather than a project room. Anyone shopping for one should check the relay logic board and the voltage selector fuse holder first, since both are the parts most likely to have been abused or bypassed over the decades. Test the by-pass-on-power-loss behavior before purchase. If the comparator circuit has been tampered with, the unit may not fail safe the way the manual describes.
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