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This box is designed to hold your 500 series goodies, but does it deliver the goods itself?

By Phil O'Keefe

There's no denying that 500 series modules have become a big hit in the studio world. The modular design of 500 series units allows them to be plugged straight into rack frames that are designed to house multiple units and provide the I/O and power for all the modules they host, giving you the ability to reconfigure your unit or install just the types of modules you need.

sixpack main.jpg


Originally developed by API, the 500 series format has become increasingly popular over the past several years, and a wide variety of companies now offer modules of various types that are designed to work with 500 series power racks such as the Radial Engineering Workhorse SixPack. While the Radial is designed to hold your 500 series goodies, the question remains - how well does it deliver the goods?

What You Need To Know

  • As the name suggests, the SixPack is a power rack that can house six single-width (5.25" x 1.5") 500 series modules. The unit's six slots allows you a pretty decent amount of configuration options. For example, it's enough for a really outstanding stereo signal path, with two mic preamp modules, two EQ modules, and two compressor modules. Alternatively, you could load a SixPack with nothing but mic preamps, or install six compressors, or a bunch of EQ modules - the choice is yours, and this degree of flexibility no doubt has contributed to the popularity of the 500 series format.
  • The 500-series format also allows you to purchase a rack / power supply and then add channels / modules to it as you afford it, which can be a more realistic option for some people than purchasing an expensive multi-channel preamp and having to pay for it all at once. While you do have to  purchase a rack to power and house the modules, the price of the rack is spread out over the total number of modules, and the modules are generally less expensive than their full-sized rack equivalents.
  • Radial is not a member of the API VPR alliance, but their products are tested by Radial and designed to be compatible with API modules as well as those from other companies. Radial Engineering has also done extensive research into the 500 series, and has considerable specification documentation that they freely make available to other manufacturers.



  • Like all Radial gear, the SixPack is built like a tank, with a 14 gauge steel housing and a rugged powder-coated finish. An alignment tray inside the unit helps facilitate module installation. In the event you run into a slightly off-spec module in terms of size (and they're out there), this tray can be removed to allow a bit more "wiggle room" so the oversized unit can be installed. I had no trouble installing a variety of modules with the alignment tray in place. Module installation is a little tricky because the format doesn't always allow you to see what you're doing, but if you go slowly and carefully, it's really not all that hard to do.
  • A removable handle is included, and it can be placed at the top of the unit, or installed on the side, depending on how you wish to orient the SixPack. The included rubber feet can be installed in a similar manner, allowing the unit to be situated horizontally or vertically. The unit is well suited to both desktop use in the control room, or it can sit on the floor out in the studio.



  • While internal power protection features are included in the SixPack's design in case you forget, you should always turn power off before installing or swapping modules. Modules use a 15 pin card edge connector that is inserted into one of the six slots inside the SixPack. These 15 pins handle the audio I/O, power for the module, stereo link, phantom power, and all connectivity between the module and the rack's connections and power supply. If a module doesn't use a particular feature (for example, a compressor doesn't need phantom power), it's ignored.
  • The Radial Workhorse SixPack uses a 16VDC 1600mA power supply that provides an average of 266mA per slot. This is significantly more current than the original API spec calls for. The extra current provides ample power, even when using current-hungry modules such as tube preamps. It's unlikely that users will ever run into a combination of modules that the SixPack won't adequately power, even when fully loaded. In my experience with the review unit power sag due to "not enough power" was simply never an issue, even with the unit fully loaded and everything driven hard.
  • The power supply is an external "line lump" type, and it connects to the SixPack via a locking 5-pin XLR type power connector. I like that it locks into place - there's much less chance of someone accidentally disconnecting the power in mid-take. Since it uses a standard IEC power connector and is designed to work with a variety of voltages (100V-240V), using the SixPack in other countries is as simple as connecting the correct IEC power plug for the country you're visiting - no new power supply or pesky voltage adapters required.  

SixPack and Power Supply.JPG


  • Two front panel LEDs give you power and 48V phantom power-on status indication at a glance.
  • For combining stereo-ready modules, a link function is provided. You'll need to install the modules in adjacent slots. Slots 1/2, 3/4 and 5/6 can be linked with the flip of a switch.  
  • There are tons of connections and patching options on the SixPack. Each channel has an XLR input and an XLR output. Output levels are typically +4dB line level, while input level will depend on the type of module installed in the slot. If it's a mic preamp, then the XLR serves as a mic-level input connector, but if it's a compressor or EQ installed in the slot, then the input will typically function as a +4dB line level input.

SixPack rear.JPG


  • With some rack units, short XLR to XLR patch cables are needed to patch the output of one module into the input of whatever module you want to connect it to. You can certainly patch this way with the SixPack, but Radial includes a nifty "Feed" switch for each card slot that allows you to send the output of one slot into the next without having to use a patch cable of any kind. This is extremely handy, and for many users, this will drastically reduce the amount of patching they need to do.
  • Patching is further facilitated by the inclusion of TRS input and output jacks for each card slot. These jacks are wired in parallel with the XLR I/O, which allows for even more creative patching possibilities.
  • D-sub connectors on the unit mean it can be quickly patched into many studio patch bays, mixing consoles, and DAW setups. The connectors are DB-25 type, and the pin format follows the Tascam D-sub standard, which supports eight channels. 
  • There are two extra, or "convenience" XLR inputs on the front panel of the Radial SixPack. These can be routed to D-sub channels 7 and 8, or card slots 1 and 4, which is perfect if you're using the SixPack to host six modules in a two-channel preamp / EQ / compressor "channel strip" configuration. These extra inputs can also be routed to the rear panel channel 7 and 8 outputs. Additional rear panel XLR and TRS jacks provide access to the D-Sub I/O for channels 7 and 8.

front panel inputs.JPG


  • Another area where the SixPack differs from its competitors is the Omniport jacks. Omniport is a Radial Engineering innovation that gives you even more functionality and patching flexibility. Using a 1/4" TRS jack, Omniport can be used for things such as a side chain input for a compressor module, a send and return insert point or instrument-level input on a preamp, unbalanced I/O for the module - there's tons of possibilities. Of course, not all modules currently use Omniport, but the ones that do gain additional features that makes the Omniport a welcome addition to this rack.
  • Modules that are not designed with Omniport functionality will still work with the SixPack; the Omniport jacks are simply inactive for any slots with such modules installed.  
  • While the details are beyond the scope of this review, Radial Engineering also makes 500 series racks in other sizes, including the 3-slot Workhorse Cube and PowerStrip units, along with the 8-slot Workhorse and Workhorse WR-8, and the 10-slot Workhorse Powerhouse. 


  • There is no power switch on the Radial SixPack. Admittedly there really isn't room for one, and you can get around the lack of one fairly easily on a device like this by using a power strip or just unplugging it (after all, this kind of rack is designed to be mobile), but a switch would still be nice to have, and may be missed by some users.
  • Rack ears and desktop-insert mounting hardware is not included, but are available as extra-cost options.



I am quite impressed with the amount of thought that went into the design and specifications of this unit. As you can see from the limited amount of "limitations" I commented on, there really aren't any significant flaws to it. It's solidly-built and has thoughtful features nearly everywhere you look that make the unit easier or more flexible to use. It's a marvel of excellent design and engineering.

The switching and routing of this unit make it a breeze to use, and really minimize the need for external patch cables. When you do want to partake in a little creative signal routing and patching, there is a wealth of patch points at your disposal. While module installation is always a bit tricky with any 500 series rack, the SixPack makes it as easy as possible, while still allowing for the use of off-sized modules. The front panel XLR jacks allow you to easily patch into the Workhorse SixPack even when it's mounted in a rack or desktop without having to crawl around to the back.

The extra beefy power supply means that there's plenty of power on tap, even when using modules that demand more juice than the original API spec allows for, such as many modern tube preamp designs. Even with the SixPack fully loaded up with a variety of different modules (discrete mic pre, EQs, compressor, tube preamp, exciter, etc.), it doesn't sag or bog down.

The Radial Workhorse SixPack is not only a low-cost way to get into the world of 500-series modules, it's an extremely flexible and well-thought out one too, and for these reasons, I highly recommend it. I've been wanting a 500 series rack for quite some time now so that I could test out all the latest 500 series modules, and I've found an excellent product for this purpose, and one that I feel does a great job at delivering the goods. The Radial Workhorse SixPack is that unit, and I am so impressed with it that I have purchased the review unit. Stay tuned for some 500 series module reviews in the not too distant future.


Musician's Friend Radial Workhorse SixPack online catalog page ($550 MSRP, $449 "street")

Radial Engineering's Workhorse SixPack web page


Radial Engineering's Peter Janis introduces the SixPack to Harmony Central at NAMM 2013

Harmony Central Review Preview - Radial Workhorse SixPack

Phil\_OKeefe HC Bio Image.jpgPhil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines. 



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