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API Audio 535-LA 500-Series Line Amplifier  From API

Attenuate or boost line level signals with this incredibly helpful studio tool, or drive hard to add API's signature analog tone to any instrument, line level signal, or even full mix!

API Audio


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Manufacturer's Description from API


  • Continuously variable detented input level trim
  • Polarity button
  • 20dB Input Pad
  • Calibrated Level mode
  • Output Ratio selector
  • Clip LED indicator
  • Continuously variable Output Gain
  • Traditional API fully discrete circuit design
  • Audio circuit uses the famous 2510 and 2520 Op-Amps
  • Traditional API fully discrete circuit design

Modeled after API's highly prized console-based 325 booster cards, the 535 can both amplify and attenuate incoming and outgoing signals for ultimate flexibility—all while providing API's trademark warmth and tone.

The 535 excels at adding warmth to a variety of signals. Recommended uses include:

  • Warm up digital signals of all kinds
  • Add Tone to Keyboard rigs
  • Add warmth to any mix

Audio playback devices, mic preamps, or audio processing devices that do not have their own level control will also benefit from the added tone and control of the 535-LA.

The 535-LA also includes a balanced input, a polarity switch, a 20dB pad for incoming signal attenuation and a three position toggle switch, which allows for different output gain levels from the transformer. This is designed to drive long balanced lines with low distortion just as the 325 booster card does. Output gain range is from 6dB to 45dB. The 535 utilizes the discrete 2510 and 2520 Op-Amps along with API's proprietary transformers to create the unique, warm and reliable sound that API users expect. Like all API products, the 535-LA features API's unique 5 year warranty.

If you already understand that the 535-LA is a flexible, useful tool that can add value to practically any lunchbox® it’s plugged into, you’re right! But if you’re not exactly sure how the 535 does what it does… or even what it does… no need to despair. Here’s a quick primer on why people have been asking for this unit for years, and how to make it work for you.


Actually, the 535-LA can do several different things at once.

  • It allows attenuation of input signals from devices which do not have their own output level control, or whose incoming signals would otherwise overdrive the 535-LA. (However, there may be cases where this is artistically beneficial).
  • As a line amplifier, it's capable of accepting both unbalanced and balanced signals, amplifying them, and driving the amplified, balanced signal to another input. Importantly, it allows output gain of the signal leaving the 535 to precisely match the needs of whatever unit the signal is being sent to.
  • During this process, the signal passes through the 535’s internal circuitry, which provides API’s characteristic analog warmth and subtle coloration. Devices like keyboards, media playback devices, and certainly DAWs can greatly benefit from the subtle tone enhancement.


In typical usage, the signal will enter the 535-LA from either the DB25 or XLR inputs on the back of an API lunchbox® or 500V rack. If the incoming signal level is too high, the user can engage the 20dB pad switch on the 535’s front panel. If necessary, the LEVEL trim pot at the top of the panel allows you to further reduce the signal strength and match it with the next device in the chain. Of course, if the 20dB pad has already reduced the signal below what the next device in the chain requires, you’re done with the input level attenuation, and what you really want to use next is the Output Gain knob at the bottom of the panel.

Output gain does just the opposite of the Input Level attenuator; it boosts the signal coming out of the 535 by up to 32dB, again to make sure that the signal output level leaving the 535 matches the next device in the signal chain. Conveniently, a small toggle switch on the bottom of the unit can double or even triple the effect of the Output circuit by changing the signal routing through the API transformer. Setting the switch at 1:1 results in unity gain relative to the control knob, 2:1 provides an additional +6dB of gain and 3:1 provides an additional +9dB.

This combination of a 20dB pad, Input Attenuation Trim and Output Gain control is a very powerful way to boost, cut, and fine tune the level of any signal passing through the 535-LA. Helpful tools like a polarity switch and front-panel calibration controls allow you even greater flexibility in how your 535-LA functions, and the presence of a CLIP light below the LEVEL knob gives you a handy visual indication of when the signal is clipping at either the input or output stage.

Added to all these features is the obvious bonus of having any signal processed through the 535-LA’s discrete circuitry. Enjoy the benefits of API’s world-famous sound and unquestionable reliability!

API Audio

About Manufacturer

API (Automated Processes, Inc.) is world famous for its all-discrete audio products: studio and broadcast mixing consoles, and a full line of rack mount mic preamps, equalizers and processors. 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.


Frequency Response: -0, +0.2, 30Hz-20kHz
THD+N: .006% at factory CAL Level setting (+4dBu)
Noise floor: -85dBu at factory CAL Level setting
Gain Range: -inf. to 53dB
Maximum Input level: +32 dBu, balanced (with PAD)
Maximum Output level: +32 dBu MAX, balanced (1kHz)
Input Impedance: Greater than 20,000 ohms, ~4,000 ohms w/ pad
Output Impedance: Less than 75 ohms