Modular Analogue Part One : API's 500-Series Format

There is no doubt that the 500-series, rack mount modular format created by API has become hugely popular in recent years, both for small and larger studio installations alike as well as musicians and even hobbyists. And why not? It is a cost-effective, modular analogue format that allows users to purchase a host rack and then populate it with any number of hundreds (thousands?) of available devices from mic and line preamps, DI’s, equalizers, dynamics and effects processors to form their own ideal (and often portable) signal paths. And it is a scalable format – one can purchase as little as a 2-slot rack (vertical or horizontal positioning), right up to 11-slots in 3ru (with an outboard power supply), so one’s setup can be as small or as large as one likes.

Beyond its obvious usefulness and cost-effectiveness (one power supply, lots of modules), the success of the 500 series format intrigues me, because it wasn’t really designed to be an open format for support and development from other manufacturers, but rather, it developed into this organically.

It can be said that API created the 500 format with the development of their original 550 equalizer module in the late 1960s (anywhere from 1967 to 1969, depending on whose talking). Also including in this format was the 560 10-band graphic EQ, the 525 compressor and the 575 oscillator module, among others (though, interestingly, these didn’t all share the same pinout). At this point API were not yet building consoles of their own, but instead manufacturing circuit modules meant to be used as building blocks by console builders (which was a quite common practice at the time – large format console manufacturers were still largely a thing of the future at that point. The more common practice, instead, was having the engineering staff of recording and broadcast studios to design and build, or commission to be built, their own consoles to best fit their unique requirements). In addition to the more comprehensive 500-series circuits mentioned above API are also well known for their 312 microphone preamplifier and 325 line amplifier cards, console building blocks, basically, which they also made during this time.

500-series modules, like their 300-series cards, could be mounted in either consoles or outboard racks (usually as a hardwired installation for processing before an FM transmitter, or to tune a room’s acoustic response, etc), but most, by far, were mounted in an actual mixing console.  Modularity in pro-audio was not a new concept in the 1960s, of course, as modular systems had been made for decades by many companies including RCA, Collins, Gates, Langevin, Western Electric, Universal Audio, Altec, and later Aphex, dbx, Valley, and ADR among many others. So API wasn’t breaking new ground with their own contributions. So why has the 500-series, in particular, become the modular standard for pro-audio today? To answer this let’s go back about 30-ish years.

In the late 1980s Paul Wolff purchased API assets from Datatronix after the brand had all but entirely disappeared, and he began the company’s resurrection by first returning to the manufacture of the beloved, discrete 550A equalizer, mostly for vintage console owners who wanted to expand their boards, but also for anyone else who wanted one. Since he wasn’t yet in a position to tackle the construction of new consoles Paul was in need of a way for potential new users to utilize the modules he was building, so with the blessing of Aphex’s Marvin Caeser (who had built nearly identical 4 and 10-slot racks for Aphex’s own modular series of signal processors), API began building 2-slot (in 1ru), 4 and 10-slot powered racks that provided mounting, power, and I/O for these modules.

For the next several years API developed and sold modules for a newly standardized 500-series format (up to this point pinout for each 500-series model could be quite different, relying on hand wiring during installation instead of a consistent pinout arrangement that could easily by manufactured with a host pcb). These modules included the 550A, the newer, four-band 550b equalizer, the 560A and later reissue 560 graphic EQ's, the 525C compressor and later reissue 525 (with new, compatible pinout compared to true originals), and, of course, the 512b microphone amplifier.

By the way, as an aside, I always wondered why I have never seen a “512” (no suffix) mic preamp for this series and after many years, just this week, I received at least a partial answer to my question when I was privileged to spend an afternoon hanging with Mike Daane, owner of West Bay Audio, a private recording facility in Dallas, TX. This studio boasts one of API’s earliest consoles (pictured at the top of this post), a rather custom, 24-channel lovely that uses mic/line preamp modules labeled “512” (well, as it turns out the one he pulled out for me to snap pics of wasn't one with its model screened on it.... oh well here are pics anyway).

Original API 512 (though this one lacks its label)

API 512 Iron and 2520 op amp

API 512 PCB view

These modules, providing both mic and line inputs to the console, are clearly original and physically smaller than typical 500-series modules, even though it has a “5xx” designation. For that matter the 528B channel modules used in API’s classic 2488 recording console are of a different size than typical 500 modules too (much larger). But I digress.... back to the article.

So things hum along accordingly till the latter part of the 1990s (around 1996 or so, I believe), and Paul sells API to live console manufacturer, ATI, and they continue to build 500 modules and the racks needed to power them as well as other rack gear and even complete consoles.

Now for a shifting of gears:  around the start of the new millennium Vince Poulos of Speck Electronics introduced a cool little 4-band parametric equalizer called the ASC. This half rack wide x 1ru tall module (which is still a current Speck product) came available with or without an output transformer, and was an impressive little performer that quickly earned many fans (including then Mercenary Audio owner and pro-audio forum mainstay, Fletcher). While I don’t know what his initial inspiration was I can vividly recall Vince’s not-too-much-later introduction of the model ASC-V, which is the ASC circuit in a 500-format module, so owners of API consoles or racks could benefit from this circuit without need and expense of a separate power supply. While I cannot say with authority that this is the first non-API product deliberately intended for use in API racks I can say, without question, that the ASC-V is the first product I was personally aware of designed thus.

I thought it was genius!

Very soon thereafter other manufacturers realized the potential benefit of following suit – by utilizing a well-known and well-revered format they could drop production costs on many of their circuits (less metalwork and no need for power supplies or I/O connectors) while expanding their potential markets to folks who may already own 500-series racks or even consoles.

“And they told two friends, and they told to friends, and so on, and so on, and so on.....” (okay, so I’m really dating myself with this quote!)

And here we are, 15-ish years and hundreds, if not a thousand or more 500-compatible products later. Though the popularity of this format has grown tremendously over the years, it has not been without its controversy. In fact, as a builder of 500-compatible products myself (Rascal Audio Two-V dual microphone amplifier) I hear, occasionally, from people who still doubt the format’s reliability, fearing that the host racks will fail. This not so much because the format or any specific rack model is truly unreliable, but because many early adopters in the manufacturing community designed circuits that are quite happy with the +/- 16vdc power rails the format provides, but didn’t really take into account the amount of current required to keep their circuit(s) happy. In fact, many people, manufacturers and consumers alike, failed to take into account the current supply needed for a given compliment of modules (plus a bit more in hand for the in-rush of current upon power up), leading them to pair said modules with a rack that was simply incapable of providing the needed current. This not only blew up the racks’ power supplies, but occasionally damaged the installed modules as well. Since most of these racks were (early on) built by API, the company suddenly found themselves with lots of unhappy owners wanting to get their racks serviced. When it was determined that the fault was not with API’s product, but the manner in which it was used (and the current demand placed upon it by modules from other manufacturers)... well, you can see there were lots of disgruntled people, many of whom feel a bit burned still.

API’s response to this problem was to create what they have come to call the VPR specification, which defines performance parameters required to be supported for use in API’s racks (if they’re going to warrant the racks they appropriately want to know that your module compliment, and therefore the current demand you’re placing on the host rack, is appropriate for the one you’re using). Interestingly, after defining this standard, I understand API revamped their own 500-series rack line for beefier power supplies all around as some of their previous offerings didn’t actually conform to the new standard.

Another problem that arises occasionally with some 500 host racks occurs when mounting transformer-balanced audio circuits right next to the rack’s built-in power transformer, causing interference/noise/degraded performance. While not always an ideal fix, this can be addressed with 1) different module placement, 2) the use of a host rack with an external power supply that can be placed farther away from the gear, 3) the use of an internal supply of the switching variety, requiring no power transformer (and therefore producing no subsequent magnetic field), or 4) the use of electronically balanced circuits that offer less interaction with power fields.

For these reasons some folks have developed a negative opinion of the 500-series format, but in truth, a little planning, purposeful placement, and a little math (particularly when it comes to current consumption of your module compliment) will render your setup quite stable, quiet, and reliable for years to come.

That about wraps the nutshell history of the 500 format.  In the next installment of this series I will proceed with something that will either make me a zero or a hero (probably both to different audiences)... In either event I believe you will find it interesting (if you're into modular hardware, that is).  You'll have to tune in to see what I mean. 

That's all for now.  Thanks for reading, and please stay tuned...

-Joel