If you've mastered the skills necessary to build something like a Seventh Circle Audio preamp (Tape Op #35, #54), you might want to move to something a little more challenging. Eisen Audio has just the thing. The DIY500 mkII Minimal Kit provides you with the framework to build a custom preamp for the 500-series format. The base design includes an input pad, 48-volt phantom power, and polarity reverse. There is also an instrument DI input. Included are pre-printed PCB board, metal enclosure, aluminum faceplate, control components, and hardware. It's up to you to choose (and source) the input transformer, operational amplifier, and output transformer. For those concerned, the DIY500 mkII conforms mechanically to the API VPR Alliance specifications, but does not have approval because of the variable nature of the product.
The Eisen website lists suitable transformer and op amp candidates. As of this writing, the kit is compatible with 45 discrete op amps, more than 50 unique input transformers, and at least 14 different output transformers. Assuming full compatibility between components, this means there are 31,500 possible configurations. Once you choose "the big three", check the component grid on the Eisen website to determine which resistors and capacitors your build will require. Place your orders and get ready to build.
For my tests, I wanted to build a colored preamp unlike anything I've used before. I chose a JLM14 input transformer and a JLM99v +/-16V op amp from Australia's JLM Audio,
and a CT-2385 steel-core output transformer from U.S.-based Crimson Audio. The circuit board is clearly printed and easy to understand. Other than needing to double-check the input transformer connections, I was able to build the unit in about two hours. A high-resolution photo can be seen on the Tape Op Message Board (messageboard.tapeop.com) under the Available Kits sticky topic in the DIY Gear forum.
How does my preamp sound? Well, I was toying with the idea of not submitting the review and taking my combination to market, if that gives you an impression. It's like an old-school Neve on a rampage. The low end is thick but not booming. The top is sweet without being overbearing. On vocals, this would make a radio announcer explode out of the speakers. The instrument DI is aggressive. Slap bass will move headphones off of your ears, while a plucked bass was rich, full, and clear. This build also had tons of gain, making it a great option for ribbon mics. Power chords recorded with a Royer R-121 ( Tape Op #19) and the Eisen came across as a solid block of rhythm. Using it with an AKG C 414 set to omni was a good option for drum room sounds. On acoustic guitar, a Martin with a Chameleon TS-1 (#60) came through with great finger articulation and a natural woody tone. However, the same setup with a strummed Taylor started to get a little tubby in the low mids, but still usable with a little subtractive equalization. An SM57 on a snare drum gets a built-in "twack", as if there were already a compressor on the scene. This preamp is easily on par with any top-of-the-line mic preamp I have ever used. And I am nothing but pleased with this creation. Clearly JLM and Crimson are building quality, and the two pair nicely in a preamp application. Crimson Audio also sent a pair of nickel transformers that I plan to use in another DIY project for review. Stay tuned. (DIY500 mkII Minimal Kit $207.80, www.eisenauduio.com; JLM14 input transformer $40 AUD, JLM99v op amp $75 AUD, www.jlmaudio.com; CT-2385 output transformer $40,www.crimsonaudiotransformers.com)

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