Recording - June 2016 - 34

B Y PA U L V N U K J R .

API 535-LA Line Amplifier
You didn't know you needed this little module... but boy, do you ever
Even considering its long and storied history building medium- and large-format recording consoles and
other studio gear, Automated Processes Incorporated
(API) is perhaps best known as the creator of the 500
Series format of modules and enclosures. This month
we will be looking at API's latest 500 Series offering,
the 535-LA. LA stands for line amplifier, and I will
posit the theory that this module, more than any
other, is the unsung hero of the API universe.
I say this based on my own personal revelation
when reviewing API's The BOX back in our
September 2014 issue. I discovered that the true
"API console sound" is not due to its mic preamps or
classic 550A EQ on their own, but how all the modules work and flow together in an API console. Each
module has its own transformer and signature API
2510 and 2520 op amps, and the magic comes
from the compounded sound of op amp and transformer into op amp and transformer, and so on. The
component that ties it all together is the integration of
the circuitry from API's 325 booster card.
This sonic signature became apparent again
when I reviewed The Channel Strip in our 2015
July issue. The mic pre, EQ, and compressor were
again rounded out by a 325 line driver at the
end of the chain. At that time, the 325 was the
secret sauce that you could not get on its own...
but here we are a year later, and I'm pleased to
introduce to you to the API 535-LA, a standalone
500 Series line amp module based on the 325.
What's inside?
The newest member of API's 500 Series makes
use of the company's standardized look and layout with a matte black metal faceplate, blue and
white lettering and two of API's proprietary knobs.
Inside is a well-laid-out, fully discrete design that uses the API
2510 and 2520 op amps and an AP2503 output transformer.
The 535-LA's front panel starts at the top with a detented
Input Level knob from -∞ (muted) to 0 dB (unity gain). API suggests that you think of it as a fader, which is a good analogy.
Below that are a trio of backlit push buttons for polarity (phase
flip), -20 dB input pad, and a Cal. Level button (see below).
Below that is a fully variable Output Gain knob with settings
of +6 dB up to +45 dB. This is rounded out by a 3-position
toggle switch that changes the output ratio of the transformer
between 1:1, 1:2, and 1:3 (the line driver setting inside The
BOX). The unit also has a single red Clip LED that's shared
between the input and output amplifier stages. If either is
pushed into overdrive (or both are), it lights up.
The Cal. Level button lets you have a preset selection of Input
Level, Output Gain, and Output Ratio accessible at the touch of
a button. The 535-LA comes from API set to unity gain and a
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RECORDING JUNE 2016

1:1 ratio. If you want to change these values,
there are two front-panel trimpots for the level and
gain and an internal jumper for the ratio.
What does it do?
Although this circuit is an essential component of the
API console sound, it's not an instrument input or a mic
preamp, so you may be wondering what it actually
does. At its simplest, it turns signals up or brings them
down. I'm not trying to be funny here; that's exactly
what a line amp does! Essentially you have a device
with a maximum throw of 53 dB of gain.
Here are its specs: frequency response of 30
Hz to 20 kHz (-0/+0.2 dB), THD+n of 0.006%
and noise floor of -85 dBu (both measured at the
factory Cal. Level setting), maximum input level of
+32 dBu balanced (with pad in), maximum output
level of +32 dBu balanced (at 1 kHz), input
impedance over 20 kΩ without pad / 4 kΩ with
pad, and output impedance under 75 Ω.

Why might I need it?
Beyond the previously-mentioned stacked API console sound,
what other benefits does the 535-LA offer? Since this is our
issue on all things DAW, I was inspired to include this review
here because this little module is a fantastic way to inject some
analog goodness into otherwise digital recordings and mixes.
In addition to being a line level mixing device that reins in
quiet signals or boosts anemic ones, it can help compensate
for balanced vs. unbalanced signal levels, and beyond that it
can be a simple DAW warmer. Since the dawn of digital
recording, many engineers have been running audio through
simple transformer boxes. They just want to hit the transformer
to impart its tightness, vibe, and color into the digital world.
The 535-LA can do that with ease, and it's capable of a full
range of sonic emotions from happy 1:1 subtle weight to 1:3
overdriven rage! This box will drive, distort, and growl beautifully, if you need it to.

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