500 Series – Standards

The old saying goes something like: The audio industry loves standards! This is why we have so many! And when it comes to the 500 series, it appears that there are certainly many to choose from!

As Radial delved into the 500 series abyss, we were astounded to find that some manufacturers made products that were actually wider than a standard 1.5" module space, meaning that you could not put two modules side by side. We then discovered some that would not fit into a power rack unless you first angled the module in and wiggled it around. Some we found required more power than what is available meaning that you could not fully load up a rack without overheating the power supply. We even found some older API modules did not conform to their own general data! No wonder; if you read the history, you will discover that over 25 years, nearly half a dozen manufacturers have been involved in some way or another with the design and there has never been a single stratified standard.

In fact it took Radial nearly two years to work through all of the variations in order to come up with a series of specifications that could clearly identify powering, mechanical sizes, pin configurations and i/o signal levels. This became known as the WorkHorse Open-Source DOCument or Whos-Doc and it can be downloaded for free by anyone.

Click here for the Workhorse Open Source Document

Although the Whos-doc has not yet been adopted by the Audio Engineering Society as a standard, it has now been downloaded by a tremendous number of manufacturers - (in the hundreds) - some of which have already started to produce products following this lead. The Whos-Doc sets out mechanical specifications for the modules, proper PCB lengths for positive connection via the 15 pin card edge connector, power requirements, mix buss specifications and so on. Best of all, it is fully compatible with older API formats.

The following shows the pin assignment compatibility across the patforms:

Pins Old API Specification Current Use In Market New Radial Specification
1 Chassis Ground Chassis Ground Chassis Ground
2 Output +4dB Hot Output +4dB Hot Output +4dB (+) Phase - ¼" Tip
3 Output -2 Hot - - - Reserved for future use
4 Output Cold Output Cold Output +4dB (-) Phase - ¼" Ring
5 A- Ground A- Ground Analog Ground
6 Stereo Link Stereo Link Stereo Link
7 Input -2 Cold - - - ¼" TRS Omniport - ¼" Ring
8 Input +4dB Cold Input +4dB Cold Input +4dB (-) Phase - ¼" Ring
9 Input -2 Hot - - - ¼" TRS Omniport - ¼" Tip
10 Input +4dB Hot Input +4dB Hot Input +4dB (+) Phase - ¼" Tip
11 Gain Adjustment - - - Radial Buss Feed
12 +16VDC Supply +16VDC Supply +16VDC Supply
13 Power Ground – 0V ref Power Ground – 0V ref Power Ground – 0V ref
14 -16VDC Supply -16VDC Supply -16VDC Supply
15 +48VDC Phantom +48VDC Phantom +48VDC Phantom

Companies now employing the Whos-Doc format include:


  • Alta Moda Audio
  • BoZo Electronics
  • Burl Audio
  • Czech Music
  • DACS Ltd
  • Danfield Audio
  • Esc-Erftstadt
  • Forssell Technologies Inc
  • Grace Design
  • JLM Audio
  • Kludge Audio
  • LaChapell Audio
  • Le Quai
  • Maag Audio
  • Millennia Media, Inc.
  • S&M Audio
  • XQP

The VPR Alliance

The VPR alliance is an amalgam of manufacturers that have been approved by API so long as they follow certain criteria. This includes not selling competing power racks and having modules checked by API for mechanical compatibility. From what we have been told by API president Larry Droppa, API created the VPR Alliance as a means to ensure modules from various manufacturers will fit properly and not harm API power racks when they are connected. It is our understanding that API does not approve, test or evaluate modules in any way other than for mechanical fit.

Although Radial has requested to become a VPR member, since we produce competitive 500 series power racks, our request was denied. And even though Radial is not a member of the VPR Alliance, we believe that the intention is good and valuable from a sales & marketing perspective.