You may have heard about the mic preamp/channel strip I've been designing if you follow this blog or the LBA Facebook feed. It has been a rough battle and at times, like yesterday morning I have felt so completely frustrated. The difficulty has been trying to find a balance between all the traits I want it to have. Low noise, High output volume (70dB minimum), wide and flat bandwidth, low distortion, and an equalizer that works well. It has been a serious challenge to meet all of these design goals, which is why I have tried every single amplifier topology I could possibly think of and even a few I couldn't (lost count, but easily +50 different designs). There was one design that I drew up a schematic for because it exhibited most of the traits, but it had a little too much noise at full volume, only had 60dB of gain, and there was significant insertion loss upon engaging the EQ. That preamp design I have thought about going back to because I could not, for the life of me, come up with something that could outperform it until last night. Finally, I feel like I achieved success. I will be doing elaborate testing today to determine its actual performance specs, but this new design exhibits all the traits and sounds absolutely fantastic! I was very pleased and then I engaged the EQ. It would have been impossible for me to have a wider smile on my face. WOW, now that's an equalizer! This is a Baxandall shelving EQ - passive of course, because that's how I roll. The bass is deep, the treble is high and its damn sexy sounding. The way that I designed this one, there is no audible insertion loss from turning on the EQ, but rather it was slightly louder. The EQ circuit has its own triode powering it (1/2 of a 12AX7). Bypass the EQ and you have one less triode in the circuit. One might think you could lose a little something good or gain a little something bad by having an additional gain stage, but that couldn't be more wrong. Since the EQ was slightly louder I wanted to dial it back, but I did it in the best way possible - negative feedback. Using just the right amount of negative feedback in the EQ circuit I was able to match the volume bypassed to active. The major benefit to using feedback this way is an extended frequency response and lower distortion, which counteracts the negative effects of having another gain stage. I feel the sound is as close to perfect as God will allow. I have decided to go with this design even if the gain is a little on the low side, which it may very well be. Upon testing later today I hope its in the 70dB ball park, but I will be satisfied if its as low as 65dB.
Here are the current features of Trinity, though some things may change:
Trinity has me really thinking big. Mixing board big! I can imaging having four Triinity-style units connected to a summing network with pan controls and a makeup amp for a very serious 8 channel tube Mixer. Believe me, that is absolutely coming up! I just need $10,000 first! lol.
About the Author
Mike Congilosi II, Inventor/Engineer