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Choosing A 500-series Chassis

Slot Machines By Matt Houghton
Published November 2020

SlotMachines

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

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Published November 2020