After building 4 completely different preamp topologies and several variations of each (roughly 30 total design variants), I finally found "the sound." I have been searching for this sound for a while, so its a huge sigh of relief having now found it. I'm not sure what the preamp will be called, but its still a long ways off from production. This is a 2 tube per channel design, which uses a 12AU7 (same tube as used in LBA FX pedals) and a 6922. It has an external power supply with a secondary solid state regulated supply for the tube heater filaments to ensure the lowest noise possible. The preamp currently has so much gain (clean boost) that I have to consider it overkill. I'm sure it would work great for ribbon mics, but it has a bit of noise that I'll need to clear up first. One of the topologies I tried out had roughly the same amount of gain, but better control over the gain and practically no noise. While this other topology had these benefits, it didn't have "the sound." This sound I speak of is the elusive magic tone that sounds extraordinarily life-like and fluid. It sounds beautiful, with a dark background, and immense imaging. I really wanted to capture a well balanced frequency response, which in itself has proven to be no easy challenge. A very well known and highly regarded $2K+USD name-brand solid state preamp was used as a reference point for my tests. I could have easily based my testing against another tube preamp, but I felt this preamp needed to have a very fast slew rate to have the modern characteristics I was after. That said, this preamp is incredibly fast and articulate for a tube preamp. It has the big body and musical tone of a tube pre, with an incredibly fast slew rate that comes close to solid state's ultra fast response. I wanted this fast characteristic so the preamp could effectively be used on drums without sounding too mushy or vintage. It was no easy task to achieve this, which is why I went through so many different preamp designs before reaching this one.
A lot of work is still needed to completely iron out the design, but when complete this will be an all inclusive dual channel preamp solution. Input and output volume controls, front panel DI inputs, analog VU meters, phantom power, switches for low cut, phase reversal, mic/line inputs, and probably the variable & bypass-able treble roll off control from the 1401 stereo mic preamp. I hope to be able to offer this hand-wired wonder for around $3,000 USD when complete. The target release date is December 21st, 2015, but that is not yet set in stone, nor is the price. Name suggestions for this dual channel tube preamp are now being accepted. If you suggest a name that I end up using, I'll offer you a special discounted price and dibs on the first production model if you want.
About the Author
Mike Congilosi II, Inventor/Engineer