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If you are into DIY and have some facility with a soldering iron, building one or two clones of the famed Teletronix LA2A tube compressors can be a fabulous way to burn up six months of free time. I really love all this stuff, and am endlessly searching for ways to improve my recording studio, so I decided to take the LA2A challenge. And indeed, six months later, I succeeded in my quest.

Resources

A recording engineer named Dave Jahnsen wrote an e-book detailing a point-to-point approach to building the LA2A. This was how I built mine. Unfortunately, Dave is no longer with us, although I would assume that copies of his book might be available somewhere on the web. There are also DIY circuit boards that are now made, to speed up the assembly process. The best place for all this information is one of my favorite hangouts on the web, Groupdiy.

My LA2A’s

I decided to build two, rather than a single unit. Though this was more work and double the expense, it made sense for a number of reasons. First, I wanted to use a pair in stereo to process drum kit mixes and other stereo recordings. Second, a large portion of the time spent on a project of this type is rounding up the necessary components. Once you have found them, it is just as easy to order two as one. Another big timesink is figuring out where to actually lay out components. Once this is done, working on another unit right next to it is far quicker. I would say that building the second one took half again as much time as building the first (although I built them, component for component, simultaneously). Finally, I was lucky enough to find a very cool double 3U rack unit for a few bucks at the local electronic surplus store, so that clinched it. As an added bonus, the front faceplates are hinged and open up just like the originals.

A few things I learned

If you do build using the Jahnsen book, be sure to get Chris Jenrick’s addendum, available from the groupdiy site above. You might also consider a turret board type of arrangement, rather than the point to point scheme that is used in the original. Point to point is quite messy and time consuming. 

Wire everything you can into the tube sockets before attaching them to the backplate. There are many things that can more easily be attached when the sockets are free: connections between pins on the sockets, tube heater wires, and resistors and capacitors that are directly attached to socket pins. Same thing with ground wires on the T4b, etc. Do yourself a favor and do all this before you attach them - you’ll thank yourself later.

If using a Hammond power supply without a center tap for the 6.2V heater wires, you must run a pair of 100-ohm resistors between the heater wires and ground. This will clear up some nasty buzz in the system.

The Jahnsen book has you connect the shields from the gain and gain reduction pots (R1 and R2) to ground posts on the chassis, rather than the star ground that all other audio components are grounded to. I followed this instruction, and had ground hum. As soon as I moved those connections to the star ground, the hum disappeared. In fact, once I did so, I thought I had made a mistake and disconnected the output, because the unit became so quiet. I was shocked when I passed audio through it and was blown out of my seat from the volume!

I don’t have the gain reduction circuitry working correctly with my meter yet. Everything else is working, but I can’t trim the gain reduction to zero - it stays at about -10db. More R&D will be necessary to solve this problem.

The T4B opto electronic part is available through the JBL parts department, or through Universal Audio. There is also a groundswell of action on the Tech Talk BBS about building your own T4B’s, led by DIY guru Chris Jenrick. Chris sent me 1 T4B, which I’ve used to test my compressors. It seems to work well, and it is great that he is building these things for the DIY community. However, I need a pair, so I think I will order them from Universal Audio for $140 each and be done with it.

Per Chris’ suggestion, I used Jensen JT-11P-1 and JT-10K61-1M transformers for input and output. I couldn’t find the original LA2A trafos for a decent price, and the Jensens sound great. I used the original circuit’s resistor and capacitor values, but just plugged in the Jensens instead, to good effect.

So how do they sound?

In a word, glorious. The level of beef they add to a drum kit is stunning. The low end becomes wickedly tight, and the even harmonic distortion really “warms up” the digital recording, to coin a very tired phrase. Having said that, the distortion really does not sound good on sax and other high pitched instruments - perhaps there is a problem with my compressors, though - more R&D is definitely required. 

If you are really into the creative idea of buildng your recording studio with your own hands, I highly recommend this project. It will sound endlessly better than any digital plug-in you will find, and you will have the great satisfaction of knowing you did it yourself.

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