The 535-LA has a balanced input, a polarity switch, a -20dB pad for incoming signal attenuation and a three-position toggle switch, which switches between three different output transformer windings for three different levels of gain depicted as ratios: 1:1, 1:2, and 1:3. A 1:3 ratio means the output is three times the level of the input level.

API 535-LA

Just as the API 325 booster card, the 535-LA is designed to drive long balanced lines with low distortion. Inside, the 535 uses a discrete 2510 fixed-gain differential amplifier (no input transformer required) and then a 2520 op amp supplying up to 44dB of gain to drive one of API's large, proprietary transformers that will drive line impedances down below 75-ohms.

I liked that there are separate "Cal" or calibration pushbuttons for both the input stage and the output stage that switches the front panel gain controls over to recessed trim pots. You could calibrate a pair of 535-LAs to your specific needs--say gain up a -10db operating input signal up 11.78dB to achieve a +4dB operating level output. All this happens any time instantly at the touch of a button.

The 535 is useful anywhere signal levels need adjusting such as inputs from keyboards, audio playback devices, or from mic preamps and audio processing devices that do not have their own level control.

I tried two 535-LA Line Amplifiers in a few different configurations. First use was for a buffer amplifier for recording a Waldorf synthesizer's line level directly into Pro Tools. There was no need to use a separate pair of console channels; I just connected the synth's output to the patch bay TT inputs of the studio's 500 rack with pair of 535s installed and adjusted Input and Output levels. I liked the Input Level controls with their detents; fully CCW is -infinity and this also works as a mute button.

The Output Ratio control is handy for quickly establishing gain staging. The Waldorf can put out a hot signal so initially I used the 1:1 ratio and nearly maximum Input level setting. Later on for a direct Les Paul guitar recording, I used the 3:1 ratio and the Input Level in the middle. The Clip LED keeps an eye on for potential overloading of both the output 2520 op amp as well as the 2510 input amp. You can (purposely) overload the 535 and with the clip LEDs blazing, it is a nasty sound.

I found the 535-LA to be super quiet and transparent and solid sounding. I do wish it had a TRS input jack positioned right on the front panel but I found it a totally useful 500 module for raising to +4dB of -10dB devices, iPhones or laptop headphone outputs, home stereo gear or old tape decks or even for interfacing guitar pedals into my mix.

It's better than fixed gain "bump boxes" because you can precisely set levels for any piece of gear or use the "Cal" mode for a predetermined preset level. Awesome! I'm happy to have this pair of API 535-LA Line Amplifiers. Highly recommended!