The API 535-LA is a line-amp module based on the company's venerable 325 booster card. It utilizes the same AP2503 transformer, 2510 differential amp, and 2520 op-amp that have been the DNA building blocks inside the company's consoles for decades. The module provides up to 45 dB of gain, and it can attenuate a signal to null. It sounds great, and it fits right into any number of setups — perfect for creative gain-staging, utilitarian amplification, or driving long cable runs. I found the 535-LA to be incredibly useful, more so than I initially thought it would be.
The front panel has a detented, variable input-level knob that goes from fully cut to unity gain, as well as a polarity button, a −20 dB input pad, and a smoothly variable output knob for 6—45 dB of gain. Overall gain is also affected by a three-position ratio switch that selects different taps of the output transformer. Also located on the front panel is a calibrated-level button that bypasses the input and output knobs, and shifts control to two recessed trimmers located below their respective knobs. These are set to unity gain from the factory, which is nice to have for double-checking what you're actually doing to the signal. But you could also adjust the trims to your liking to set up fully repeatable gain- staging, like on long inter-facility cable runs, for example.
The module is as solidly built as any other gear from API. Likewise, the sonic quality is right where you'd expect it to be — harmonically full and not too colored, yet not so transparently hi-fi to be invisible. It's probably an overused term, but API gear has always struck me as inherently "musical" in the sense that whatever signal you pass through it seems to pick up a little more "motion" as well as note definition. The 535-LA lives up to those characteristics.
The 535-LA was a great pairing with older mic preamps missing output controls, like a preamp stripped from an old Magnecorder PT6 tube reel-to-reel. I love the PT6 preamp; it has a ton of gain, and it sounds amazing cranked, but I usually have to run a compressor after it just to get control of its beastly output. The 535-LA was a perfect match, and I was easily able to track with the PT6 preamp wound up into full- on fuzz. It also came in handy with a Dizengoff Audio D4, which also sounds wonderful run on the hotter side. The D4 has an output trim, but it's really just a fine gain control, and using the 535-LA allowed me to get this preamp to sit right in its sweet spot.
The 535-LA also let me explore more with the gear that I use every day. Even units with onboard output controls I could leave much more wide-open when coupled with the 535-LA. I don't have a console at my studio, and I miss that type of gain-staging. The 535-LA's input control was great because I just thought of it as a fader and kept it close at hand. While 54/Tape Op#114/Gear Reviews/ the unit is designed to perfectly handle hot signals, I did occasionally run across some distortion, but only when I was being absolutely ridiculous and loose with gain-staging. While not unpleasing, I didn't find the distortion entirely useful. Most of the time, the 535-LA felt open, and it only imparted a mild but musical coloration. I never thought twice about including it in a signal path, and most of time, I could just forget about it until I needed it for adjusting gain.
Its real value was made clear when I started to pair it with gear that was far away from me — as a level control for a keyboard mixer in the corner of the room, for example, or with gear at the bottom of a rack behind me. With the 535-LA nearby, I was able to stay in my monitoring sweet spot, without rooting around on the floor or asking the musicians to stop playing so I could get them to change their signal levels. This sort of ergonomic advantage wasn't something I had considered. Having that kind of dependable, never think about it, quality of level control — always within hands reach — had an immediate streamlining effect on tracking workflow. I realized that there were several limiters that I had stopped reaching for, only because they were at the bottom of racks or their gain-staging was tweaky. Using the 535-LA at the end of the chain, I could leave the inputs and outputs nominally set for the best noise floor, and just get more clean make-up gain when I needed it.
Initially, I wondered if the 535-LA could function as a low- gain mic preamp, but it never quite delivered on that level. Even with hot sources like snare drums and loud guitars, it just didn't quite cut it, flexibility-wise. Vintage API 325 booster cards have long been a target for modification into mic preamps, with varying amounts of success/hassle. The arguments seem to fall on both sides; some think it's an easy mod, and others say it isn't worth the effort. Like all things, success probably depends on component selection and modding skills. The 535-LA is certainly not inexpensive, so I can't imagine it will draw the sort of modification attention given to older 325 cards. So you can give up the idea that this is a cheap way into an API mic preamp.
After using the 535-LA for a month, I realized that a second one would be great just for fine gain control of the mix bus, and for using it as an input or output control on some older, modified broadcast limiters, where I'd really rather leave the settings alone. For the mix bus, the 535-LA really has that API sound, and it does exhibit a tiny bit of self-compression at higher gain settings. Like running a console hot, it can add "circuit weight" to mixes, whether you sum in-the-box or you use an analog summing bus. The 535-LA seems basic and unassuming at first, but it is a seriously great-sounding and flexible tool for any studio or sound-installation that is in need of gain-staging control, line drivers, or analog-console mojo.