Choosing A 500-series Chassis

Not all 500‑series racks are created equal. Here’s what you need to know to make your choice.

I’m a big fan of the 500‑series modular studio outboard system. Not only is it a low‑hassle way of taking a choice selection of gear with me when working away from the studio, but in the studio its small size means I can have way more gear placed within easy reach of the listening position. Also, once you’ve invested in a rack (or ‘chassis’) to host the modules, it’s more economical than 19‑inch rack gear: module manufacturers don’t need to pay for a mains power supply or to get them approved for use in various territories, and shipping costs less. They’re not always cheap, but where there’s a 19‑inch rackmount equivalent the 500‑series version is, without exception, much more affordable. To me, then, the only real surprise about the 500 series is that it took so long to become popular!

In this issue of SOS you’ll find two articles designed to help you take your first steps in the 500 series. In the accompanying 'Choosing 500-series Modules' article, I discuss the stunning variety of 500‑series preamps, processors, effects and, for want of a better phrase, ‘utility modules’ that are available. But here I’ll explore the price of admission: the 500‑series chassis.

A 500‑series chassis needn’t be complicated: at heart, it’s a metal box, with audio inputs and outputs and a power inlet on the rear, and card‑edge connectors inside. It delivers power (and phantom power where required) to each fitted module, and audio signals to and from them. And, in essence, that’s pretty much it. But at my last count, there were at least 29 manufacturers offering 500‑series host products, and that’s not including DIY kits or out‑of‑production models. And despite there being a standard specification (set out by the VPR Alliance), not all products adhere to it strictly, and they vary considerably: they can accommodate anything from one module to 11 of them, and more if you include 500‑series mixers; there are a few different form factors; some boast a range of useful additional features; and there’s the not‑so‑small matter of the power supply.

A 500‑series chassis needn’t be complicated: at heart, it’s a metal box, with audio inputs and outputs and a power inlet on the rear, and card‑edge connectors inside.

Power Moves

The ±16V power rails of the 500 Series aren’t the limiting factor some people say they are.

I want to start there: power. Partly that’s because the type, quality and location of the power supply impact on some modules’ performance, and partly because people often seem confused about the racks’ ±16V power rails, which some believe don’t allow the same headroom as ‘proper’ professional gear.

Generally, the full professional signal level is considered to be +24dBu (equivalent to a signal voltage of 12.27V RMS, or 34.72V peak‑to‑peak) and ideally you’d want some headroom above that in the circuitry: +27dBu (allowing 3dB internal headroom) is 49V peak‑to‑peak. The 500‑series ±16V rails can deliver a theoretical maximum signal voltage of 32V peak‑to‑peak but, as audio electronics can’t normally run all the way up to the power rails, you typically only have about 30V peak‑to‑peak available for the audio. That equates to around +22.5dBu (29.26V peak to peak). Technically, then, it’s true to say that there’s slightly reduced headroom.

Yet, this limitation can easily be sidestepped: a 1:2 (or higher ratio) output transformer in a module’s line output will provide a ‘free’ 6dB of extra output level, taking the maximum output capability to +28.5dBu: full professional level, with headroom. API’s modules all use output transformers (probably one reason they settled on this voltage) as do many others. And where transformers aren’t desirable due too their size, cost or ‘character’, modern DC‑DC converters still make it practical and affordable for manufacturers to step up the ±16V rails to ±18 or ±22, or whatever they desire, even within the confines of a 500‑series module. Some do that, others don’t. But the key point is that the 500‑series’ ±16V rails aren’t inherently a problem.

Current Thinking

A more important consideration is current. The general aim is that, collectively, the modules should consume only around 75‑80 percent of the current available from the power supply. If all modules complied with the maximum specification of 130mA per module slot this would be easy to manage. But plenty consume more, and some far more.

Before you rush out to buy the cheapest rack you can find, make sure it makes available enough current to run all the modules you might want to put in it.

IGS Audio’s 10‑slot Panzer chassis is designed to run as many of their current‑hungry variable‑mu compressors as you wish: it uses an external linear power supply to make available well over three times the current specified in the VPR Alliance standards.

An extreme example is IGS Audio’s rather tasty Tubecore 500 vari‑mu compressor, which occupies two slots, runs its valves on an internal 240V supply and draws 400mA (more than three modules’ worth, going by the VPR spec) from each slot. Some modules draw less than 130mA, too. The dbx 560A VCA compressor, for instance, draws only 60mA, freeing up an extra 70mA for other modules.

Happily, it’s rare to find new racks that don’t offer substantially more current than the VPR Alliance specifies. Even so, if you plan to have multiple ‘thirsty’ modules, some racks may not be up to the job. Perhaps unsurprisingly, IGS Audio’s Panzer 10‑slot rack offers a stonking 420mA per slot, so it can safely host pretty much anything in any slot. Plenty of other options boast very generous supplies, too, with models by Fredenstein and Lindell Audio being among the most affordable.

Some racks include other handy power‑related features. Rupert Neve Designs’ R6 and R10, for instance, have a current meter on the front, so you can see how much remains available. Some others offer isolated power and overload protection for each slot. But the bottom line is this: before you rush out to buy the cheapest rack you can find, make sure it makes available enough current to run all the modules you might want to put in it now or in the future.

Rupert Neve Designs’ racks feature a handy current meter on the front.

In Or Out?

Some racks have switch‑mode power supplies, others linear ones, and there’s no inherent reason why either type can’t be designed to meet all the needs of a 500‑series rack. But the PSU’s location and/or shielding can be important. On a practical level, a rack with an integral supply is very handy for a portable recording rig, since there’s less to carry around and set up. I have an API 500‑6B HC, a six‑slot Lunchbox rack with an inbuilt supply, and it works very well. In the studio, internal supplies contribute less to the rat’s nest of cables and wall‑warts too.

One of Wes Audio’s ‘ng500 series’ modules. Note the additional connector above the 15‑pin 500‑series one. It’s used to deliver USB control signals between a DAW plug‑in and the module.On the other hand, there’s a risk that internal supplies might interfere with the audio signal paths. Modern switch mode supplies don’t emit the magnetic radiation that linear ones do, yet they vary in quality and can potentially interfere in other ways. Transformer mic preamps and valve circuits are particularly susceptible to interference, and note also that the transformers and other circuitry in many 500‑series modules aren’t fully shielded. Measures can be taken to shield the power supply, though shielding is rarely perfect.

I’ve not experienced problems with the internal supply in my API rack, and there are many happy owners of other racks at various prices that have inbuilt supplies. But if you have a rack with an integral PSU and are experiencing noise problems, try moving the sensitive/problematic modules to slots as far away from the PSU as possible (usually to the left‑most slots, viewed from the front). If you’re planning to acquire a new rack, then unless cost or practical reasons lead you elsewhere, there’s a lot to be said for buying one with an external power supply. Don’t sit an external linear PSU on top of the rack while using it, though as you’ll hear hum, but thankfullly magnetic radiation decreases rapidly with increasing distance.

Slots & Connectivity

Another potential advantage of an external supply is the space it frees up. A handful of companies, including BAE and Wes Audio, take advantage of that by making 11‑slot racks. (You could fit 11 1176‑style compressors in a measly 3U of rack space, whereas 11 full‑size 1176s would occupy 22U!) A possible downside of 11‑slot designs is that they leave barely any space on the front for niceties such as power switches or status LEDs. That said, the clever bods at JLM Audio offer a dual headphone amp to go in the half‑module space left over in their 11‑slot, 400mA per‑slot rack!

The Midas Legend L10 currently represents the keenest price per slot of any chassis, but still manages to include stereo‑linking, cascade routing and aux ins and outs.This brings us to the chewy question of how many slots you need in your rack. I prefer purchases to last me a while, and you might be surprised by how quickly you’ll fill all those spare slots, particularly if you start buying stereo or ‘doublewide’ devices. So it seems to me that unless you deliberately want a compact or lightweight and portable setup, it’s a good plan to buy something that caters for your immediate needs and then some. You can always put panel blanks in any spare slots to keep things pretty and keep the dust out. Cost can be a factor, but prices have fallen in recent years. At the bargain end of the scale you can pick up a 10‑slot Midas rack for around $300 £250 while the higher‑end 10‑slot racks cost in the region of £700‑1000 $800‑1200

For me, a three‑, six‑ or eight‑slot rack is a good portable solution. Three slots gives you enough space for a mono channel strip of mic pre, EQ and compressor, or maybe three preamps to feed your audio interface’s spare line inputs. A six‑ or eight‑slot unit could cater for two mono (or one stereo) recording channels, perhaps with a couple of spare slots. These size racks typically come with a carry handle, and often a carry case.

Eight is a particularly neat number in the audio world. DB25 D‑sub cables can carry eight channels, and any rack featuring those connectors allows you to connect to your interface with only two cables. A lot of us can access an extra eight channels on our interfaces by hooking up an ADAT expander too. Many six‑slot racks, including my API 500‑6B, have D‑subs and XLR connectors (some have jacks too), with D‑sub channels 7+8 delivering a signal to and from a spare pair of XLRs. Rack ears are often available as optional extras, allowing both six‑ and eight‑slot racks to be rackmounted.

For a more permanent fixture in the studio, I’d be looking at 10 or 11 slot racks, or perhaps a full‑rack‑width eight‑slot unit with some added functionality (more on that below). These larger racks tend to offer better value for money, allow more room for future expansion, and can host the large double‑ and triple‑width modules with ease. While some don’t feature D‑subs at all (there are more than eight channels) the form of connectivity is less important, since you should only have to plumb it in rarely.

Radial’s Workhorse series led the way in adding functionality, particularly in their eight‑slot version. Their three‑slot Cube, pictured, can be mounted in a desk thanks to Radial’s angled brackets. Tree Audio’s Three In The Tree Dual: the ultimate in expensive luxury channel strips?There are a few alternative form factors to consider. Radial, for example, came up with the neat idea of angled desk‑mounting brackets for their three‑slot Workhorse Cube 500 racks. This allows you to recess the racks into a desk, tilted like the surface of a mixer, and makes some ergonomically pleasing layouts possible. Tree Audio’s eye‑wateringly expensive Three In The Trees (there’s a mono and a stereo version) are essentially luxury but ‘empty’ desktop channel strips, with metering, output fader(s) and more.

Then there’s the orientation of the modules. Personally, I prefer racks in which the modules sit vertically, since the control markings are easy to read, but A‑Designs and Fredenstein, amongst others, also offer racks that mount two or three modules horizontally in a 1U 19‑inch rackmount chassis. These are a good, space‑efficient solution if you wish to add only a few modules to an existing rackmount setup.

Sound Skulptor and Chameleon Labs both offer a modular system of single‑slot chassis, with multiple units able to share the same PSU. Perfect if there’s something only available in the 500‑series format but you have no desire to disappear down the rabbit hole!There are also quite a few racks designed to supply power and I/O to a single module, effectively turning it into a standalone device. For instance, Elysia offer their stereo 500‑series processors already loaded in their two‑slot rack. Zähl offer a similar two‑slot rack for their stereo EQ. And, perhaps most intriguingly, Sound Skulptor and Chameleon Labs offer single‑slot ‘racks’. Not only can these turn any regular 500‑series module into a standalone product, but these single‑slot hosts can also be stacked or connected with others of the same type to create a custom‑sized chassis. Both companies’ products can also run several host units from one power supply.

Added Features

For all the talk of power supplies, slots and portability, the greatest differences between racks lie in the extra functionality some of them offer. Since I’ve been discussing connectivity, let’s start there. The 500‑series’ 15‑pin edge connector has a number of ‘spare’ pins that some manufacturers have decided to take advantage of. For example, pin 3 in the original API spec provided a second, reduced‑level output, taken from before the output transformer. Similarly, pin 7 and pin 9 were used for a ‑2dBu balanced input level option.

In 2011, Radial launched their impressive eight‑slot Workhorse rack, which boasted several new features, but notably used those ‘spare’ pins for an Omniport — an extra utility socket for each channel that module designers can use for different purposes. Radial also repurposed Pin 11, originally specified for a gain trim resistor, for a mix‑bus feed; the Workhorse allows you to pan and sum the signal from all eight modules using knobs on the right‑hand front panel. It also features two headphone outputs, cascade routing (the ability to route, internally, each slot’s output to the adjacent slot’s input), insert points on the master bus, and handy guide rails to make fitting the modules less fiddly.

Speaking of pins, some companies, including Interphase Audio, TAC, JLM Audio and Audio Maintenance Limited, have embraced the 51X format, an extension of the 500‑series with two more pins (it can still host 500‑series modules) to cater for ±24V power rails.

Heritage Audio’s 4U‑high MCM8 II built on Radial’s idea of incorporating a summing bus by placing the level and pan controls in line with the channels.The cascade routing idea, adopted by several manufacturers, can be handy for configuring channel strips, even if doublewide mono devices will scupper it! Stereo linking has also proven popular: using a switch or jumper to connect two adjacent slots’ pin 6s allows two mono compressors to share the same side‑chain signal. Some manufacturers have implemented variations on the Omniport, and integral mixers and headphone amps have also surfaced elsewhere. Heritage Audio’s 4U‑high MCM‑8 II, for example, features a summing mixer with pan and output level knobs arranged above each slot, and has dual‑needle meters and master level control over on the right.

Black Lion’s innovation is as wonderful as it is simple: by building in a TT Bantam patchbay into their PBR‑8, they’ve created a rack that offers more flexible routing than any other chassis.One of my favourite ‘innovative extras’ is the TT bantam patchbay in Black Lion Audio’s eight‑slot PBR‑8. It allows the user complete freedom to change the signal path without rearranging the modules or plumbing it into a separate patchbay. Yet another innovator is Wes Audio, whose Titan has another connector above the 500‑series one, catering for two‑way communication between their ng500‑series modules and a dedicated DAW plug‑in, without leaving USB cables trailing from the modules’ front panels.

Perhaps my favourite chassis boasts many of the features I’ve described and more besides: Cranborne Audio’s 500R8. This includes a mixer, two headphone amps, and top‑class USB audio interfacing. It has insert points for each channel, and offers up to 16 channels of ADAT I/O. Their 500ADAT is similar, though lacks the USB interfacing, and offers a convenient way to add eight 500‑series modules to an interface with an ADAT port. What’s more, the two units can be connected together using ADAT, and their mix busses and cue‑mix facilities can be merged. Interestingly, Neve have recently indicated that they’ll be launching an eight‑channel 500‑series rack, with several digital A‑D/D‑A options. The future of this format really does look bright!

The Cranborne 500R8 (pictured) and its sister product the 500ADAT offer a bewildering array of features, not least a mixer and very high‑quality multichannel USB audio and ADAT interfacing.