Studio Tech’s Corner: API 351 Mod

Sunday, May 25th, 2014

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Years ago (95′, 96′?), so they say, Calvin acquired 5 API 325 cards from Tom Robinson of Concert Sound. You might think of the API 325 as a cousin of the legendary API 312. The 312 is a mic preamp while the 351 is line amp, but they are both based around the API 2520 operational amplifier; a mystical black box that is at the core of basically every 70’s API design.

But line amps aren’t particularly useful in the modern studio environment so Calvin got Mark Greer to convert 2 line amps into 312 mic pres. These were also all just cards pulled out of a who knows what, so Calvin also had them racked in an old radio chassis that he had.

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Fast forward 20 years or so and the custom APIs have been sitting at the top of rack for quite a while. Over time they’ve developed a low frequency hum. The line amp cards are rarely used except maybe to boost the gain of a tape delay return. And the engineers want more mic pre options. So it seemed like it was time to give the unit some love and attention.

First thing to look at was the hum. It was on all the channels, so all signs pointed to a power supply problem. Now I’d encountered this particular PS before. It is external and floats around on top of the racks and whenever I reach around for something up there it shocks me. Hard. Honestly, I hate this piece of junk, I was delighted to have an excuse to replace it. We decided to order a build-yr-own kit from JLM and install it INSIDE the chassis. Where shocking voltages belong. The card also supplies phantom power so once I installed it on the chassis, I wired up phantom for each channel. Sure enough the new PS fixed the hum. Onward.

old power supply

Out with the old


In with the new








I dug up a schematic from our files…well several schematics. It turns out we had a couple printed ones and a photocopies of hand-drawn schematics, possibly left by Mark Greer after the first mod. But hilariously (and not at all frustratingly), none of these schematics exactly matched the already modded cards. I spent a few days meticulously tracing the circuits. In the end my modded cards were kind of a combination of several schems with a couple values changed based on the gain I was looking for. This, I think, was ultimately good. I had to actually understand the circuit rather then just doing a paint-by-numbers kind of affair.

Modding the actual cards was initially nerve-wracking. It took a long time to convince myself it was okay to drill a bunch of big holes in these priceless antiques. But nothing ventured nothing gained. I stripped them all down to just the opamp, power filtering, and output transformer, and more or less followed the layout of the previous mod, cutting the same traces and drilling similarly. Because I was using a slightly different circuit, I moved this-and-that around.I think my layout is a little cleaner.

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I’ve never done this kind of mod before. It was kind of exhilarating, and definitely a confidence booster. I learned a lot about the 312 circuit, phantom power, and chassis work. Now the studio has 5 more awesome pres. They quickly seem to becoming engineer favorites. Come give them a workout.

Sam Gray

still Studio Tech at heart



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