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DIY Audio 2: JLM Dual 99v

Updated: Jul 1, 2020


Like many other engineers I want just about every piece of gear out there and often gawk at everything with a higher price point than my car. In the pursuit of not driving myself deep into debt while still picking up some awesome gear I came across the DIY pro-audio community. One piece of gear that I have always wanted to get my hands on but haven’t yet had the opportunity is the Jensen Twin-Servo preamp. If you want the real deal, The John Hardy Company (the one the only) still makes them and now Radial (who bought Jensen) also makes a 500 series module. After doing a lot of research I came across one company that makes a DIY kit in the same style (trafo in, two 990s, trafo out) all be it with a slightly different circuit. This was JLM Audio with their Dual 99v mic pre.


Ok, first up, who is JLM what do they do and why did I pick this kit? From the other side of the world (Australia) Joe Malone is the owner, designer, and all-around good guy who I bought the kit from. If you know who he is then you are probably familiar with either his Baby Animal preamps or some of his 500 series kits which is what originally brought me to the site. The Dual 99v uses the BAD design which allows for both DOAs and extra switches and pots for options such as impedance control and trim control. Let it be known the transformers are not Jensen, and it is JLM 99vs rather than the 990s which are from the original design.

Now why did I want this preamp over a 1073 kit or API 312 (both of which I have selected kits for already). I do a lot of Classical recording (now before any of you get angry as to why I would choose such a coloring preamp for classical let me explain) and I was looking for a high-gain low noise preamp that could work for ribbons as spot mics. And beyond that I do a lot of mixing with a passive summing buss, so I was looking for a preamp that could also have a range of tones for the master buss of my mixes. There enters the Jensen Twin Servo/JLM Dual 99v which does all of that (and more…)


The build itself was relatively simple considering it was a kit with all the pieces I needed for construction. The main board layout is clear but relatively dense. There also are a good number of components that are omitted or that have different values based on the different opamps used. While I was able to figure out what was needed it would have been nice to see a specific breakdown for resistor values and capacitor values for the 99v. The first board all in all probably took me about 3 hours to solder (that’s with the extra wired pots and going slow to reduce errors). But once I got to the second board I knew I wanted to get done quickly (natural lighting was starting go down and I didn’t feel like finding my led light. Now this was a mistake, for on the DI board of the second BAD I had a couple of bad solder joints, but I will get into that later. The ribbon cables were a nice addition to the BAD preamps, maid wiring take a lot less time, but wiring was probably the slowest part of the entire process. Putting three ground wires into a single pressure connector was not exactly easy when they are all twisted tightly. In fact the soldered wires were much easier to work with than the unsoldered connectors. One nice part about the kit was that it comes with a external power supply so I didn’t have to worry about mains wiring.


All in all, the build wasn’t too bad, and I had the perfect opportunity to try it out coming up. This opportunity was a concert recording at a high school. Now I said I messed up right? Cause when I turned it on for a longer bit of time I started to smell something burning… Turns out poor soldering is a bad thing as I burnt out one of my resistors as well as broke one of the 99v op amps. Yay….. After about three weeks I was finally able to get the rest of the bits I needed including a new opamp (by the way Joe was super helpful with trying figure out my noise problem which ended up being the opamp. He had plenty of ideas for what to try and fix but it ended up being my own stupidity that broke it).


Flash forward another week I finally get to use it for real. This was during a classical new music festival with some pretty top-notch performing ensembles. I originally tried it with some omni direction microphones (KM 130s if you are interested) but this is where I noticed one of its true characters. The low end on the mics are extremely present, all of sudden hearing the low wiring of the HVAC system. And without a high-pass filter I had to switch over to the D.A.V. for the stream. Later that week I got to test it as a spot mic. It truly has plenty of gain and the circuit is clean as I couldn’t hear self-noise over the room tone. And I never had to worry about headroom as it either runs on 24v rails or 33v rails (honestly can’t remember right now). Characteristically it is a truly fat (phat) sounding preamp, with a bit of a boost on the lows. I did find this useful with some ribbon mics which because of their distance the little bit of boost helped to compensate for the lack of proximity effect. I ended up keeping it on clarinet with a Beyerdynamic M160 (hyper-cardioid) and on Cello with a AKG c414 in cardioid. Most other instruments sounded better with the D.A.V. due to the lack of transformers probably.


One great aspect of the preamp is the impedance control. If you ever get the chance to work with a variable impedance preamp, try it out. Cranking it all the up just opens up the sound adding even more what I would call extension. I ended up cranking it all the way down for the cello spot because it helped to keep the low end from becoming a big issue in the mix.

Now where this preamp has really shined would be on the summing buss. I did a quick mix from a live rock performance giving the preamp lots of gain and pulling back the output and boy does it sound thick and full. Hands down every time I switched to it I felt like the mix just had a little more glue and little more texture. One thing I did notice while doing this is the preamp doesn’t have exactly the same gain settings on both channels. This probably has to do with having slightly different resistor values for the constant values. If I was to build this again I would probably find the four closest valued resistors (two gain stages, right?) for these and that would probably fix the slight inconsistencies. To my ear its only off by less than a dB so we aren’t talking catastrophic (I mean there is a trim pot anyways to match the levels).


Overall, I have to say this a cool preamp, and I am super glad to add it to my collection. You all should JLM Audio and some of the cool gear/kits they have (JLMAudio.com/shop). I wouldn’t suggest building this pre if you are a beginner but it is easy enough to figure out as far as kits go. Three hints for this pre or other pres like it: 1. Twist wires tightly, this will help reduce internal noise and also keep the runs clean. 2. Be clean on your solder job, even though some contacts look like they might almost be connect, there are a couple that definitely are not meant to be shorted. 3. Use a light while you work…… (ok this one is really more for me).




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