API 512c Discrete Mic/Line Pre — 500-Series Module

The distinct API sound at an extremely affordable price

API 512 preamps are most noted for their punch and clarity. The API 512c, first introduced in the ’70s, was an advancement of the original 512. Today’s 500-series API 512c preamp remains faithful to the original circuit designs of API's founder, Saul Walker. Full-featured and still hand assembled, the API 512c carefully preserves the original sound character that made API a prominent figure in the early days of recording. API innovated the modular approach to console building, plus the format and specs for the 500 series. API preamps and EQs have been used to create some of the most famous custom consoles, such as the API/De Medio console at Sunset Sound, responsible for seminal recordings by artists such as Van Halen, Crosby Stills & Nash, and the Rolling Stones, to name but a few. Offering high headroom and a wide variety of inputs and input access points, the API 512c excels on drums, percussion, guitars, and vocals.

API 512c Discrete Mic / Line Pre key features

• Mic preamp with 65dB gain
• Front- and rear-panel mic input access
• Line/instrument preamp with 45dB gain
• Front-panel line/instrument input
• LED VU meter for monitoring output level
• 20dB pad switch, applies to mic/line/instrument
• 48V switchable phantom power
• Proprietary API fully discrete circuit design
• Uses the famous API 2520 Op-Amp

Don’t forget to check out RSPE’s Rack Revolution, a virtual 500-series rack builder that lets you custom-configure the perfect 500-series lunchbox for your studio, DAW, or mobile recording rig. RSPE carries every 500-series module imaginable, including mic preamps, compressor-limiters, equalizers, and 500 power racks.

API 512c — Inside The Box

The API 512b and 512c were next-generation versions of the original 512, which was the first modular mic preamp made. What differentiated the 512 from the earlier 312 were two major factors. First, the 312 was a mic preamp card used in the early consoles, whereas the 512 was a modular design that featured all controls and functions on the front panel. Secondly, the 312 had no coupling capacitor between the output transformer and the op amp, which meant if there was any DC offset in the op amp, it could likely fry the transformer. To rectify this, API designed the 512c with a large coupling capacitor in parallel with a high-frequency capacitor, which in turn was in parallel with a resistor for transformer damping. Also, the 2520 op amp was given a servo to eliminate offset, which resulted in two things: increased headroom and the elimination of popping caused by the phase button being switched in and out of the circuit.

API 2520 Op Amp and 2503 transformer—key to the API sound

API’s 2520 Op Amp found in the 512c is given most of the credit for the API sound. Designed by API engineers, the 2520 is a high-gain, wide-band, direct-coupled amplifier specifically designed for audio applications. The circuit derives its characteristics mainly from the performance of its passive elements, being connected into a feedback loop, which provides predictable, stable performance. Furthermore, the discrete 10-transistor amplifier is encapsulated in a thermally conductive epoxy that protects against thermal shock, vibration, and humidity, assuring long life and reliable performance.

Another contributor to the API sound is the 2503 output transformer. Since the mid seventies, the API 2503 output transformer has gone through a number of iterations in an effort to get back to the original sound. The reason for the change in the sound of the output transformers was a mystery for many years—a mystery finally solved by former API owner, Paul Wolff in 1997.

According to Paul, the process for winding output transformers back in the early days was unknown to most manufacturers because they never specified it. The winding process was simply a part of the transformer maker’s procedure. They used a ribbon-like wire, where all four windings were done at once to save time and money. However, in later times, when trying to recreate the original sound of the 2503, the lacquer used to glue the windings together turned to powder, so no one could see the old process, or figure out why the old transformers sounded so good, whereas the new ones had an 80kHz peak, which extended down into audible range making the transformer sound brighter. Apparently, in the middle ’70s, and unbeknownst to most manufacturers, the ribbon wire had disappeared from use and subsequent transformers were wound from four separate spools of wire, which caused inconsistent wrapping, hence the high-frequency peak.

By 1997 the winding problem was finally identified and the experimentation to return to the former glory of the 2503 began. The solution was a type of wire called Litz wire, which comprises multiple strands of wires electrically insulated form one another, twisted together in a prescribed pattern, and then wound around the bobbin. Litz wire reduces skin effect, which causes resistance to increase at high frequencies, and proximity effect, which causes power losses at high frequencies. Litz wire is very effective up to 500kHz and ultimately produced the same effect as the ribbon wire with identical sonic results. This 10-year odyssey of discovery is why the new API 2503 transformer is virtually an exact match to the old API 2503.

In total, the elegant simplicity of 2520 discrete operational amplifier combined with the 2503 output transformer provides the core of the API sound, yielding one of the most sought-after mic preamps ever designed.

API 512c — Outside The Box

On API 512c front panel is a 7-segment LED VU meter next to a continuous gain control that provides 34dB to 65dB of low noise (-129 EIN) gain. Among the 512c’s front-panel controls are a 48V Phantom Power switch; Polarity switch, that reverses the output phase 180 degrees; -20dB Pad that controls both mic and line input; Mic or Hi-Z Instrument/Line selector, and ¼" Hi-Z Line input. Front-panel XLR and ¼" connectors combined with rear panel mic access allow for additional flexibility when installed into an API lunchbox, API 10-position 500V vertical rack, or an API console.

API Audio's VPR Alliance

The VPR Alliance is a program of standardization and consistency guidelines for approved manufacturers wishing to design products for API's 500-series rack format. The program provides complete design specifications for manufacturers interested in producing third-party modules that physically fit and electronically conform to API's rack specifications.

The creation of the VPR Alliance was encouraged by the overwhelming popularity of API's 500-series racks, including the 10-space 500V and the six-space lunchbox®, which led to a proliferation of third party modules that fit the API format. Typically installed into API racks, these third party modules had raised issues of warranty and interaction with API-manufactured modules in the same rack.

Because of the variables related to those third party modules, API had previously been forced to declare that inserting any third-party modules into a 500V or lunchbox voided the warranty on that rack. Through the VPR Alliance, API is now able to eliminate confusion as to which third-party products definitively do and do not void the API warranty. The VPR Alliance is a straightforward resolution that saves the company time and money while promoting goodwill to API customers.

API’s customers benefit from the VPR Alliance with the knowledge of exactly which products are approved for placement in API racks; customers have a wider range of module choices for their racks that have been tested by API staff for specification compliance.

An Abbreviated History of API

It all began with a dream. It was 1968. The company: Automated Processes Inc., formed by men with a vision. Best known for their now legendary "2520 amplifier", this unique amp has been and will continue to be the heart of all API discrete products. It continues to provide reliability and sonic purity unmatched by the competition. The resulting console met the needs of the music, commercial and broadcast industry.

API became the leading audio broadcast console manufacturer for radio and television networks and high profile stations. In addition, recording studios, large and small, began using API...and receiving rave reviews from engineers and producers. There are over 700 API consoles across the nation and around the world, including the three major networks. Over thirty years later, many of these consoles are still in daily use in some of the most prestigious recording and broadcast facilities in the world because, to date, there are few, if any consoles of equal sound quality at a cost-effective price.

If you don’t have it already, there’s no time like the present to get the sound of API onto your tracks. Try RSPE’s Demo 4 Free service and find out for yourself. For more information, call or chat with an RSPE representative.

This product is also available as part of the RSPE Demo 4 Free & Rental Program, click here to see the rental product page.