5 Ways to DIY With Discrete Opamps June 17, 2011 11:09 5 Comments

This week, I was lucky enough to receive a pair of GKL255 discrete opamps in the mail from Bearcat Audio, the newest vendor on the DIY Recording Market. (Thanks guys!) Even as I opened the parcel from Montreal, my mind was already thumbing through the projects in my DIY to do/wish list for a special place to socket these new DOAs. But first, a little overview of discrete opamps for the beginner:

An operational amplifier (opamp) is a single-ended differential amplifier. This means it takes two inputs, inverted (-) and non-inverted (+), and produces a single output which is many orders of magnitude larger than the difference between the inputs. Usually opamps are manufactured as monolithic integrated circuits (ICs); however, a few intrepid designers have created their own using discrete transistors, resistors, capicitors, etc. There are three classic DOAs that most DIYers have tried to clone, imitate, or "improve": the Melcor 1731, API 2520, and Jensen 990. Each classic opamp has it's own sound which, without getting into too much audiophoolishness, can summed up thusly:

• Melcor 1731 (1960's): Smooth highs and lows, beefy harmonics
• API 2520 (1970's-): Aggressive mids, solid lows, classic 70's coloration
• Jensen 990 (1980-): Low-distortion, fast transient response, wide open sound

1. Build an API or Jensen style mic preamp

The 2520 discrete opamp is at the heart of the "API Sound." And these days, there are a cornucopia of projects for the API-style preamp builder, including:

• Access 312 and Bicycle 312 are PCBs for 312-type preamps
• FiveFish X12, Seventh Circle A12, and Sound Skulptor MP12 are complete kits for building preamps in rack-mount enclosures
• Bearcat 412 a complete kit for building four 312-inspired pre's in a 1RU chassis 
• ClassicAPI's VP25, VP26, 312, and 312DI are 500-series replicas of API circuits

The 990 DOA was used in several very successful preamps, including Jensen's twin servo design, which utilized two Jensen opamps and transformers. Seventh Circle Audio's J99 kit is a version of the twin servo with SCA's own 990-type opamp and Lundahl and Cinemag transformers. It's also possible to create a Jensesque preamp with JLM Audio's Baby Animal Dual kits, using any 990-type DOA and Jensen transformers.

2. Concoct your own preamp design

One of the distinct pleasures of building with discrete opamps is that they have a common footprint and can often simply be swapped. This makes the basic transformer/single opamp preamp design a great platform for experimenting with new component combinations.

• JLM's Baby Animal kits provide such a platform with a pre-fab power supply and chassis so you can focus on transformer and opamp choices.
• The Bicycle 312 Hybrid PCB is also a great place to start if you want to build your units from the ground up.

3. Build a ClassicAPI "Missing Link" Console Channel

One theory about the lack of "mojo" or "warmth" in modern recordings is that we're simply not running audio through as many transformers and gain stages as they did in the days of the large-format console. Preamps, in other words, were only part of what made those consoles sound so good. ClassicAPI provides some consolation for the console-less in the form of the VC528 ST+ console channel for the 500-series/51x Alliance. The VC528 "completes" a preamp with a fader, filters, and an output amp, running the signal through two transformers and discrete opamps of your choice along the way.

5. Make a unique hybrid 1176 compressor

Igor's 1176-style compressor for the 51x Alliance/500-series, the F76 FET, provides the option to replace the original amplifier stages with DOAs. Why not be the only engineer in town--heck in the county, heck in the tri-county area--with an "API 1176"!? 

6. Upgrade the DIP8 opamps in your mixer, compressor, equalizer, converters, etc.

Until yesterday, June 16th, 2011, the sacrifice you made for using high-quality discrete opamps was not being able to use them in modern DIP8 format gear. AC Sound has begun briding this gap with a series of discrete opamps in the DIP8 format. The opamps will come in "tall" and "wide" formats and M, A, and J models (can you guess what the letters stand for?) AC Sound listed the first set of model A's on the market yesterday and plans take pre-orders the "J" and "M" models soon. Your cheap monolithic opamps had better watch their backs!