Roundup – API 500 Series Chassis

One of the most exciting hardware categories in pro audio is the
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ONE OF the most exciting hardware categories in pro audio is the 500 Series format. Originally based around a modular system developed for Aphex and API products, the 500 Series has increased in popularity to such a degree that manufacturers are retooling their standard products as modules, as well as developing new ideas for the platform.

Modular systems not only provide a way for you to create a setup that fits your immediate needs, but one that can grow as your projects do. Among the tasks that 500 Series systems excel at are voice and instrument tracking, audio processing, analog summing from a DAW, and live-performance applications. While preamps, EQs, and dynamics processors are among the most common modules, utility modules are also available, such as the spring reverb and guitar-speaker simulator from Radial Engineering, and the dynamic ladder-style filter and analog delay from Moog Music.

At first glance, a modular system may seem expensive, but they are priced in line with other hardware of this quality. Individual modules range in price from a couple hundred bucks to well over $1,000, with the price of a manufacturer’s module reflecting the product in its traditional format. In the modular format, you’re paying for the ability to assemble a custom, portable rig that shares a power supply and chassis, and may have additional features, such as integrated signal routing and mixing.

The heart of a 500 Series system is a powered rack or chassis, which is available in different shapes and configurations that support from one to 11 modules. Some of the chassis can be rackmounted for studio or stage use, while others are designed for desktop use and portability. This roundup will explain the typical features of a 500 Series rack and describe several popular models. (Prices are retail unless otherwise noted.)

Rack ’em Up The 500 Series specs, such as the slot sizes (5.25" x 1.5" for a single panel) and ±16VDC per rail are, for the most part, standardized. Behind each module is a 15-pin card that fits into a slot on the inside backwall of the rack. The pin connections must conform to the standard spec—for example, pin 1 is for chassis ground and pin 15 is for +48 VDC phantom power. However, some manufacturers use redundant pin connections in different ways.

These systems are designed for pro studio use, so they typically feature +4dB output levels using balanced I/O connections—XLR and 1/4" TRS jacks, as well as 25-pin D-sub connectors. However, rack power supplies vary among makes and models, as do the power requirements of the modules themselves, which draw a certain number of milliamps (mA) of current. As you’re shopping for a chassis, be sure the power supply provides enough power for the system of modules you plan to assemble.

Because of the growth in the 500 Series market, API launched the VPR Alliance in order to provide “complete design specifications for manufacturers interested in producing third-party modules that physically fit and electronically conform to API’s rack specifications.” This provides API a means to avoid potential warranty issues. For example, if an unapproved third-party module is used in an API rack and it causes problems or damage due to the module’s design, wiring, or power draw, it essentially voids the warranty of your API chassis. A list of compatible modules can be found at At the time of this writing, no third-party chassis have been VPR approved.

However, inclusion on the list of VPR Alliance-approved products is not a requirement in order for a module or chassis to be well within, or even surpass, the 500 Series spec.

Like all pro audio gear, hearing is believing: While price, style, and brand loyalty all play a role in the products we buy, sound and performance should be among the biggest considerations in the purchase of your modular system. Some resellers offer a try-before- you-buy deal for serious customers, allowing you to spend some time with a system before you make the final purchase. At the very least, the salespeople specializing in this product category should be able to give you expert advice in compatibility between modules and rack chassis.

A Designs


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By mounting a pair of 500 Series modules sideways, the A Designs 500HR takes up only 1RU, including internal power supply (which supplies 140mA per channel). Each slot has XLR inputs and outputs, and +48VDC phantom power is provided.

USB 500 Rack
The Aphex USB 500 Rack combines a 4-space chassis with an integrated USB 2.0 audio interface, which can be used together or independently. The interface offers 24-bit, 96kHz resolution, with support for Mac OS and iOS and Windows.

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The rear panel includes balanced TRS 1/4" analog outputs, as well as MIDI, S/PDIF, and Word Clock I/O. The front features a pair of 1/4" headphone jacks with dedicated controls, a master output knob, and buttons for Dim and Mono. As a nice touch, the rearpanel XLR inputs for the module slots can be used on their own as the front-end of your DAW, even when modules are not in the rack. The system is designed to simultaneously accept six input signals (four from the modules, two via S/PDIF) and output eight channels from the computer (four through modules, two through the TRS jacks, and stereo S/PDIF).

The module section has standard XLR I/O. Rear-panel switches link pairs of modules, or bus the signal through all four modules in series. The switches also allow you to send a DAW track through a module and back to the computer. The unit has an internal power supply yielding 220mA per module slot.

API Audio
500-6B and 500VPR
$424 STREET, $845 STREET
API offers two 500 Series racks—the 6-slot 500-6B “lunchbox” with internal power supply, and the 10-slot 500VPR, which comes with an external power supply.

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The 500-6B is a portable unit for desktop, studio, or stage use, weighs just under 10.25 lbs., and has a handle on the left side and rubber feet on the bottom. (API also sells rack ears for the unit.) Each rack slot features rear-panel XLR I/O. The unit also features a DB-25 connector for input and output, with two additional XLR pass-through inputs and outputs provided for channels 7 and 8 of the D-Sub.

The 500VPR has rear-panel XLR I/O and uses a hefty external power supply: The L200PS is a linear design with current limiting and short-circuit protection that provides 1.4 amps symmetrically. API products come with a 5-year warranty.

BAE Audio
Various chassis configurations
$425–$950 STREET
In addition to its 2-space DLB “Desktop Lunchbox” ($425 street), 6-space rackmount ($550 street) and table-top ($550 street) chassis, BAE Audio also offers an 11-space chassis ($950 street). Each model has XLR I/O and eschews PCBs for individual card connectors with shielded wiring. Steel is used throughout the chassis.

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External power supplies are included in the prices. The 2-space and 6-space supply provides 1.6A, while the supply for the 11-space model delivers 3A. Each product comes with a one-year warranty.

Compère 500
The Compère 500 is an 8-slot rack with internal power supply that provides 3A per rail. One feature that sets this unit apart from other 8-module racks is the Aux Input To Module switch. This lets you assign the front-panel XLR input to any of the eight modules, so you can, for example, compare the sound of one mic going through each of the rack’s preamps individually (hence the play on words of the product name), without interruption to phantom power.

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The Aux Output from Module switch routes the chosen slot to a dedicated rear-panel XLR output. An additional XLR jack, the Routing Section Line input, sits below it. As you would expect, each module slot has individual XLR I/O. Rear-panel switches let you internally bus a module into the one to its immediate right. Four blank panels are included with the rack, each of which has a voucher that can be redeemed towards the purchase of a Cartec module.

Chameleon Labs
The CPS-501 powers a single module in a half-rack space and includes XLR I/O. The back-panel 1/4" TRS jack, marked Special, provides a switchable send-and-return for modules that support an insert. The rack also allows you to lift pin 15 for modules that will be damaged by +48VDC.

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The CPS-501 weighs a mere 2.5 lbs. and uses an external AC power adapter. A second power-adapter port on the rear panel lets you daisy-chain up to eight units together using the included extension cable. Two CPS-501s can be combined using the company’s RM-1 ($49) rackmount kit and RM-2 ($39) dual-rackmount buckle. The unit comes with a one-year warranty.

Empirical Labs
The EL500 houses two 500 Series modules in a 1RU rack with internal power supply (with over 280mA available from both supplies). Rear-panel I/O includes XLR and TRS 1/4" input jacks and XLR and unbalanced 1/4" outputs. The high-impedance 1/4" input on the front acts as a DI and accepts an instrument-level signal. (An internal jumper adds +10dB of gain if needed to accommodate the instrument.) The left module’s slot defaults to the front-panel jack when you plug something into the DI input.

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The EL500 provides +48VDC phantom power, and you can internally link a pair of modules, such as dynamics processors. The rack comes with a 3-year limited warranty.

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$329.99 STREET
As the name suggests, the Lindell 506 holds six modules and is made from aluminum. The internal power supply provides 2.4A (400mA per module slot). Twelve gold-plated XLR connectors fill out the back-panel I/O. A neoprene gig bag is included. 

Purple Audio
The Sweet Ten
The Sweet Ten holds up to 10 modules and has a 1.3A internal, switching power supply. In addition to the rear-panel XLR I/O, each module is supplemented by balanced 1/4" I/O that utilizes pins 3 and 6 (output) and 7 and 9 (input) of the card connector. Module slot 9 has an additional XLR output designed for use with the company’s Molyn module, an 8-channel summing amp that treats slots 1 through 8 as input modules to form an 8x2 mixer.

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According to the manufacturer, the frequency bypass capacitors on each slot of The Sweet Ten improve filtering, while the chassis’s cold-rolled steel top, back, and side panels are finished in trivalent chromate to improve grounding and shielding.

Radial Engineering
Various chassis configurations
$349–$1,399 STREET
Radial has implemented a number of new elements into its rack designs, which clearly differentiates them from the VPR spec. For example, each module slot in its racks features an Omniport, a 1/4" TRS jack that can be used by manufacturers for auxiliary functions, such as a key input for a dynamics processor or a switch. Although Radial’s racks accept standard 500 Series modules, companies such as Burl Audio, Grace Design, Maag Audio, and Millennia Media are designing products with Radial’s rack specs in mind.

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Radial’s chassis configurations include the 3-module/1RU PowerStrip ($349 street), the 3-module desktop Cube ($349), the 6-module SixPack ($449 street), the 10-slot Powerhouse ($899), the Workhorse ($1,399 street), which includes an integrated mixer; and the WR-8 ($749 street), which doesn’t include the mixer. In addition to XLR and balanced 1/4" I/O and an Omniport on each channel of every Radial chassis, you’ll find a rear-panel Feed switch that sends audio to the adjacent module, and a Stereo Link switch.

The 8-slot Workshorse includes an 8x2 summing mixer with inserts, as well as monitor and main outputs on XLR and TRS 1/4" jacks, phantom power, a pair of headphone jacks, pan and level controls and an on/off button for each channel, a Mono switch, and level controls for main, monitor, and headphone outputs. (The mixer is also available as a separate module, the WM8, for $749 street.) Rear-panel D-Sub connectors offer eight channels of direct input, output, and summed output. You can connect Workhorse units together using the back-panel Expander Buss. The WR8 also holds eight modules, but it doesn’t include the mixer.

All Radial chassis include external power supplies. The Workhorse and WR8 power supplies provide 1.6A.

The RackPack500 is an 8-module rack that accepts four 500 Series modules in addition to four of SPL’s proprietary modules. Each module slot has two XLR outputs and one input. Phantom power (+48VDC) is available. The rack is just shy of 13.5 lbs. when empty, and it has a rear-panel ground-lift switch and comes with an external power supply.

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SPL also makes 4- and 8-space RackPacks for its own modules, but these chassis do not support 500 Series modules.

V4 Roadster and X4 Expander
$649, $499
The V4 Roadster ($649) is a steel 4-space desktop chassis with an internal power supply providing 425mA per slot. The V4 can also power the X4 Expander ($499), which holds four additional 500 Series modules, or the TX4 Expander, which holds four of Tonelux’s proprietary VRack modules.

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I/O is on XLR jacks throughout. The V4’s D-Sub connectors provide 8-channel I/O when either of the expander modules are used.

Gino Robair is Electronic Musician’s technical editor.